Exploring the Solar System and Beyond

Exploring the Solar System and Beyond

Cover for Exploring The Solar System and Beyong

0 0 Hubble Ultra Deep Field shot 2014

Hubble Ultra Deep Field shot 2014

Exploring the Solar System and Beyond’s goal is to fill your mind with a head start to knowing the development of planetary science in the modern Space Age with this eBook. The eBook is easy to use, refreshing and is guaranteed to be exciting! You can read this eBook without any prior knowledge of our Solar System’s past events. Many things will be in this eBook such as, Our Sun, Our Solar System, What is a Planet?, What is the Big Bang? Planets of our Solar System, Mercury, Venus, Earth, Earth’s Moon, Mars Asteroids, Meteors and Meteorites, Jupiter, Saturn, Uranus, Neptune, Dwarf Planets, Comets, Kuiper Belt and Oort Cloud, 1000 Space-Astronomy Words Dictionary and many many more!

Exploring the Solar System and Beyond will help you anywhere you go; it is a quick and easy reference tool that will take you Beyond the solar system!


Sun: Overview

Description: Color image of the sun.

Description: http://solarsystem.nasa.gov/images/spacer.gif

Plasma erupts from the sun in the shape of a massive handle.

The sun is a star, a hot ball of glowing gases at the heart of our solar system. Its influence extends far beyond the orbits of distant Neptune and Pluto. Without the sun’s intense energy and heat, there would be no life on Earth. And though it is special to us, there are billions of stars like our sun scattered across the Milky Way galaxy.

10 Need-to-Know Things About the Sun:

1. The sun is a star. A star does not have a solid surface, but is a ball of gas (92.1 percent hydrogen (H2) and 7.8 percent helium (He)) held together by its own gravity.

Description: Illustration showing Earth's tiny size compared to the sun.

Earth compared to the sun.

2. The sun is the center of our solar system and makes up 99.8% of the mass of the entire solar system.

3. If the sun were as tall as a typical front door, Earth would be about the size of a nickel.

4. Since the sun is not a solid body, different parts of the sun rotate at different rates. At the equator, the sun spins once about every 25 Earth days, but at its poles the sun rotates once on its axis every 36 days.

5. The solar atmosphere is where we see features such as sunspots and solar flares on the sun. The sun’s outer atmosphere — the corona — extends beyond the orbit of dwarf-planet Pluto.

6. The sun is orbited by eight planets, at least five dwarf planets, tens of thousands of asteroids, and hundreds of thousands to three trillion comets and icy bodies.

7. The sun does not have any rings.

8. Spacecraft are constantly increasing our understanding of the sun — from Genesis (which collected samples of the solar wind and returned the particles to Earth) to SOHO, STEREO THEMIS, and many more, which are examining the sun’s features, its interior and how it interacts with our planet.

9. Without the sun’s intense energy there would be no life on Earth.

10. The temperature at the sun’s core is about 15 million degrees Celsius (27 million degrees Fahrenheit).

Our solar system’s central star, the sun, has inspired mythological stories in cultures around the world, including those of the ancient Egyptians, the Aztecs of Mexico, Native American tribes of North America and Canada, the Chinese and many others.

Description: Illustration showing tiny earth next to the massive sun.

Approximate size of Earth compared to the Sun.

Description: Color image shows the sun looks smaller on Mars.

Sunset on Mars.

Description: Illustration shows our sun is a smaller star.

Our sun compared to other stars.

A number of ancient cultures built stone structures or modified natural rock formations to mark the motions of the sun and moon – they charted the seasons, created calendars and monitored solar and lunar eclipses. These architectural sites show evidence of deliberate alignments to astronomical phenomena: sunrises, moonrises, moonsets, even stars or planets. Many cultures believed that the Earth was immovable and the sun, other planets, and stars revolved about it. Ancient Greek astronomers and philosophers knew this geocentric concept from as early as the 6th century BCE. Now we know, of course, that all the planets orbit our lone star – the sun.

The sun is the closest star to Earth, at a mean distance from our planet of 149.60 million kilometers (92.96 million miles). This distance is known as an astronomical unit (abbreviated AU), and sets the scale for measuring distances all across the solar system. The sun, a huge sphere of mostly ionized gas, supports life on Earth. The connection and interactions between the sun and Earth drive the seasons, ocean currents, weather and climate.

About one million Earths could fit inside the sun. It is held together by gravitational attraction, producing immense pressure and temperature at its core. The sun has six regions – the core, the radiative zone, and the convective zone in the interior; the visible surface (the photosphere); the chromosphere; and the outermost region, the corona. The sun has no solid surface.

At the core, the temperature is about 15 million degrees Celsius (about 27 million degrees Fahrenheit), which is sufficient to sustain thermonuclear fusion. The energy produced in the core powers the sun and produces essentially all the heat and light we receive on Earth. Energy from the core is carried outward by radiation, which bounces around the radiative zone, taking about 170,000 years to get from the core to the convective zone. The temperature drops below 2 million degrees Celsius (3.5 million degrees Fahrenheit) in the convective zone, where large bubbles of hot plasma (a soup of ionized atoms) move upwards.

The sun’s surface – the photosphere – is a 500-kilometer-thick (300-mile-thick) region, from which most of the sun’s radiation escapes outward and is detected as the sunlight we observe here on Earth about eight minutes after it leaves the Sun. Sunspots in the photosphere are areas with strong magnetic fields that are cooler, and thus darker, than the surrounding region. Sunspot numbers fluctuate every 11 years as part of the sun’s magnetic activity cycle. Also connected to this cycle are bright solar flares and huge coronal mass ejections that blast off the sun.

The temperature of the photosphere is about 5,500 degrees Celsius (10,000 degrees Fahrenheit). Above the photosphere lie the tenuous chromosphere and the corona (crown). Visible light from these top regions is usually too weak to be seen against the brighter photosphere, but during total solar eclipses, when the Moon covers the photosphere, the chromosphere can be seen as a red rim around the sun while the corona forms a beautiful white crown with plasma streaming outward, forming the points of the crown.

Above the photosphere, temperature increases with altitude, reaching as high as 2 million degrees Celsius (3.5 million degrees Fahrenheit). The source of coronal heating has been a scientific mystery for more than 50 years. Likely solutions emerged from observations by the Solar and Heliospheric Observatory (SOHO) and the Transition Region and Coronal Explorer (TRACE) missions, but the complete answer still evades scientists. Recent missions – Hinode, Solar Terrestrial Relations Observatory (STEREO), and the Solar Dynamics Observatory (SDO) – greatly improved our knowledge of the corona, getting us still closer to the answer. They also give us an unprecedented understanding of the physics of space weather phenomena such as solar flares, coronal mass ejections, and solar energetic particles. Space weather can adversely affect our technology in space and on Earth; these missions help us to develop space weather reports.

How the Sun Got its Name:

The sun has many names in many cultures, all of them presumably pre-historic in their origins. The ancient Greeks called it Helios and the ancient Romans called it Sol, both of which derive from the same Proto-Indo-European term. Latin Sol developed as sole in Italian, sol in Portuguese and Spanish, and with the addition of an originally diminutive suffix, as soleil in French. Modern English sun evolved from the same Proto-Germanic form that today is Sonne in German and zon in Dutch, variously attested as sonne and sunne in Old and Middle English, with similar forms found in other ancient Germanic languages such as Old Norse, Old Saxon, Old High German and Gothic.

Important Dates:

150 BCE: Greek scholar Claudius Ptolemy writes the Almagest, formalizing the Earth-centered model of the solar system. The model was accepted until the 16th century.

1543: Nicolaus Copernicus publishes, On the Revolutions of the Celestial Spheres describing his heliocentric (sun-centered) model of the solar system.

1610: First observations of sunspots through a telescope made independently by Galileo Galilei and Thomas Harriot.

1645 to 1715: Sunspot activity declines to almost zero, possibly causing a Little Ice Age on Earth

1860: Eclipse observers see a massive burst of material from the sun; it is the first recorded coronal mass ejections

1994: The Ulysses spacecraft makes the first observations of the sun’s polar regions.

2004: NASA’s Genesis spacecraft returns samples of the solar wind to Earth for study.

2006: Ulysses begins its third set of data-gathering passes over the north and south poles of the sun.

2007: NASA’s double-spacecraft Solar Terrestrial Relations Observatory (STEREO) mission returns the first three-dimensional images of the sun.

2009: After more than 18 years, the Ulysses mission ends. Ulysses was the first and only spacecraft to study the sun at high solar latitudes.

2010: SDO is launched and begins observing the sun in super-high definition.

2011: The STEREO spacecraft, from their dual perspective, see the entire sun for the first time.


Our Solar System: Overview

Description: Series of illustrations showing the scale of Earth in comparison to our solar system, our galaxy, our local galactic group, local superclusters and the observable Universe. In a nutshell, it shows Earth and our solar system are very, very small in the great scheme of themes.

Earth and our solar system’s place in the Universe.

The words solar system refer to the sun and all of the objects that travel around it — planets, natural satellites such as the moon, asteroid belt, comets, and meteoroids. Our solar system is part of a spiral galaxy known as the Milky Way. The sun, the center of our solar system, holds eight planets and countless smaller objects in its orbit.

10 Need-to-Know Things About Our Solar System:

1. Our solar system is made up of the sun and everything that travels around it. This includes eight planets and their natural satellites such as Earth’s moon; dwarf planets such as Pluto and Ceres; asteroids; comets and meteoroids.

Description: Button to Launch Eyes on the Solar System Interactive

Eyes on the Solar System:

2. The sun is the center of our solar system. It contains almost all of the mass in our solar system and exerts a tremendous gravitational pull on planets and other bodies.

3. Our solar system formed about 4.6 billion years ago.

4. The four planets closest to the sun — Mercury, Venus, Earth, and Mars — are called the terrestrial planets because they have solid, rocky surfaces.

5. Two of the outer planets beyond the orbit of Mars — Jupiter and Saturn — are known as gas giants; the more distant Uranus and Neptune are called ice giants.

6. Most of the known dwarf planets exist in an icy zone beyond Neptune called the Kuiper Belt, which is also the point of origin for many comets.

7. Many objects in our solar system have atmospheres, including planets, some dwarf planets and even a couple moons.

8. Our solar system is located in the Orion Arm of the Milky Way Galaxy. There are most likely billions of other solar systems in our galaxy. And there are billions of galaxies in the Universe.

9. We measure distances in our solar system by Astronomical Units (AU). One AU is equal to the distance between the sun and the Earth, which is about 150 million km (93 million miles).

10. NASA’s twin Voyager 1 and Voyager 2 spacecraft are the first spacecraft to explore the outer reaches of our solar system.

Science is a dynamic process of questioning, hypothesizing, discovering, and changing previous ideas based on what is learned. Scientific ideas are developed through reasoning and tested against observations. Scientists assess and question each other’s work in a critical process called peer review.

Description: Color illustration showing the scale of planets in our solar system.

The solar system.

Description: Illustration of the Milky Way Galaxy

The Milky Way galaxy.

Our understanding about the Universe and our place in it has changed over time. New information can cause us to rethink what we know and reevaluate how we classify objects in order to better understand them. New ideas and perspectives can come from questioning a theory or seeing where a classification breaks down.

Defining the term planet is important, because such definitions reflect our understanding of the origins, architecture and evolution of our solar system. Over historical time, objects categorized as planets have changed. The ancient Greeks counted the Earth’s moon and sun as planets along withMercury, Venus, Mars, Jupiter, and Saturn. Earth was not considered a planet, but rather was thought to be the central object around which all the other celestial objects orbited. The first known model that placed the sun at the center of the known Universe with the Earth revolving around it was presented by Aristarchus of Samos in the third century B.C., but it was not generally accepted. It wasn’t until the 16th century that the idea was revived by Nicolaus Copernicus. By the 17th century, astronomers (aided by the invention of the telescope) realized that the sun was the celestial object around which all the planets — including Earth — orbit, and that the moon is not a planet, but a satellite (moon) of Earth. Uranus was added as a planet in 1781 and Neptune was discovered in 1846.

Ceres was discovered between Mars and Jupiter in 1801 and originally classified as a planet. But as many more objects were subsequently found in the same region, it was realized that Ceres was the first of a class of similar objects that were eventually termed asteroids (star-like) or minor planets.

Pluto, discovered in 1930, was identified as the ninth planet. But Pluto is much smaller than Mercury and is even smaller than some of the planetary moons. It is unlike the terrestrial planets (Mercury, Venus, Earth, Mars), or the gas giants (Jupiter, Saturn), or the ice giants (Uranus, Neptune). Charon, its huge satellite, is nearly half the size of Pluto and shares Pluto’s orbit. Though Pluto kept its planetary status through the 1980s, things began to change in the 1990s with some new discoveries.

Technical advances in telescopes led to better observations and improved detection of very small, very distant objects. In the early 1990s, astronomers began finding numerous icy worlds orbiting the sun in a doughnut-shaped region called the Kuiper Belt beyond the orbit of Neptune — out in Pluto’s realm. With the discovery of the Kuiper Belt and its thousands of icy bodies (known as Kuiper Belt objects, or KBOs; also called transneptunians or trans-Neptunian objects [TNOs]), it was proposed that it is more useful to think of Pluto as the biggest KBO instead of a planet. Then, in 2005, a team of astronomers announced that they had found a tenth planet — it was a KBO even larger than Pluto. People began to wonder what planethood really means. Just what is a planet, anyway? Suddenly the answer to that question didn’t seem so self-evident, and, as it turns out, there are plenty of disagreements about it.

The International Astronomical Union (IAU), a worldwide organization of astronomers, took on the challenge of classifying the newly found KBO (later named Eris). In 2006, the IAU passed a resolution that defined planet and established a new category, dwarf planet. Eris, Ceres, Pluto, and two more recently discovered KBOs named Haumea and Makemake, are the dwarf planets recognized by the IAU (as of July 2013). Pluto, Eris, Haumea, and Makemake are also classified as KBOs, and Ceres retains its asteroid label. There may be another 100 dwarf planets in the solar system and hundreds more in and just outside the Kuiper Belt.

Astronomers and planetary scientists did not unanimously agree with these definitions. To some it appeared that the classification scheme was designed to limit the number of planets; to others it was incomplete and the terms unclear. Some astronomers argued that location (context) is important, especially in understanding the formation and evolution of the solar system.

One idea is to simply define a planet as a natural object in space that is massive enough for gravity to make it approximately spherical. But some scientists objected that this simple definition does not take into account what degree of measurable roundness is needed for an object to be considered round. In fact, it is often difficult to accurately determine the shapes of some distant objects. Others argue that where an object is located or what it is made of do matter and there should not be a concern with dynamics; that is, whether or not an object sweeps up or scatters away its immediate neighbors, or holds them in stable orbits. The lively planethood debate continues.

As our knowledge deepens and expands, the more complex and intriguing the Universe appears. Researchers have found hundreds of extrasolar planets, or exoplanets, that reside outside our solar system; there may be billions of exoplanets in the Milky Way Galaxy alone, and some may be habitable (have conditions favorable to life). Whether our definitions of planetcan be applied to these newly found objects remains to be seen.


What is a Planet?

Description: Color illustration showing how Eris and Ceres are much smaller than Earth and its moon.

Earth and it’s moon.

Science is a dynamic process of questioning, hypothesizing, discovering, and changing previous ideas based on what is learned. Scientific ideas are developed through reasoning and tested against observations. Scientists assess and question each other’s work in a critical process called peer review.

Description: Color illustration showing the scale of planets in our solar system.

our solar system.

For thousands of years, people thought Earth was the center of the Universe. Our understanding about the universe and our place in it has changed over time. New information can cause us to rethink what we know and reevaluate how we classify objects in order to better understand them. New ideas and perspectives can come from questioning a theory or seeing where a classification breaks down.

Defining the term planet is important, because such definitions reflect our understanding of the origins, architecture, and evolution of our solar system. Over historical time, objects categorized as planets have changed. The ancient Greeks counted the Earth’s moon and sun as planets along with Mercury,Venus, Mars, Jupiter, and Saturn. Earth was not considered a planet, but rather was thought to be the central object around which all the other celestial objects orbited. The first known model that placed the sun at the center of the known universe with the Earth revolving around it was presented by Aristarchus of Samos in the third century BCE, but it was not generally accepted. It wasn’t until the 16th century that the idea was revived by Nicolaus Copernicus. By the 17th century, astronomers (aided by the invention of the telescope) realized that the sun was the celestial object around which all the planets – including Earth – orbit, and that the moon is not a planet, but a satellite (moon) of Earth. Uranus was added as a planet in 1781 and Neptune was discovered in 1846.

Ceres was discovered between Mars and Jupiter in 1801 and originally classified as a planet. But as many more objects were subsequently found in the same region, it was realized that Ceres was the first of a class of similar objects that were eventually termed asteroids (star-like) or minor planets.

Pluto, discovered in 1930, was identified as the ninth planet. But Pluto is much smaller than Mercury and is even smaller than some of the planetary moons. It is unlike the terrestrial planets (Mercury, Venus, Earth, Mars), or the gas giants (Jupiter, Saturn), or the ice giants (Uranus, Neptune). Charon, its huge satellite, is nearly half the size of Pluto and shares Pluto’s orbit. Though Pluto kept its planetary status through the 1980s, things began to change in the 1990s with some new discoveries.

Technical advances in telescopes led to better observations and improved detection of very small, very distant objects. In the early 1990s, astronomers began finding numerous icy worlds orbiting the sun in a doughnut-shaped region called the Kuiper Belt beyond the orbit of Neptune – out in Pluto’s realm. With the discovery of the Kuiper Belt and its thousands of icy bodies (known as Kuiper Belt objects, or KBOs; also called transneptunians), it was proposed that it is more useful to think of Pluto as the biggest KBO instead of a planet. Then, in 2005, a team of astronomers announced that they had found a tenth planet – it was a KBO even larger than Pluto. People began to wonder what planethood really means. Just what is a planet, anyway? Suddenly the answer to that question didn’t seem so self-evident, and, as it turns out, there are plenty of disagreements about it.

The International Astronomical Union (IAU), a worldwide organization of astronomers, took on the challenge of classifying the newly found KBO (later named Eris). In 2006, the IAU passed a resolution that defined planet and established a new category,dwarf planet. Eris, Ceres, Pluto, and two more recently discovered KBOs named Haumea and Makemake, are the dwarf planets recognized by the IAU (as of July 2013). Pluto, Eris, Haumea, and Makemake are also classified as KBOs, and Ceres retains its asteroid label. There may be another 100 dwarf planets in the solar system and hundreds more in and just outside the Kuiper Belt.

Astronomers and planetary scientists did not unanimously agree with these definitions. To some it appeared that the classification scheme was designed to limit the number of planets; to others it was incomplete and the terms unclear. Some astronomers argued that location (context) is important, especially in understanding the formation and evolution of the solar system.

One idea is to simply define a planet as a natural object in space that is massive enough for gravity to make it approximately spherical. But some scientists objected that this simple definition does not take into account what degree of measurable roundness is needed for an object to be considered round. In fact, it is often difficult to accurately determine the shapes of some distant objects. Others argue that where an object is located or what it is made of do matter and there should not be a concern with dynamics; that is, whether or not an object sweeps up or scatters away its immediate neighbors, or holds them in stable orbits. The lively planethood debate continues.

As our knowledge deepens and expands, the more complex and intriguing the universe appears. Researchers have found hundreds of extrasolar planets, or exoplanets, that reside outside our solar system; there may be billions of exoplanets in the Milky Way Galaxy alone, and some may be habitable (have conditions favorable to life). Whether our definitions of planet can be applied to these newly found objects remains to be seen.



Description: Universe Big Bang-1 CMB Time

The night sky presents the viewer with a picture of a calm and unchanging Universe. So the 1929 discovery by Edwin Hubble that the Universe is in fact expanding at enormous speed was revolutionary. Hubble noted that galaxies outside our own Milky Way were all moving away from us, each at a speed proportional to its distance from us. He quickly realized what this meant that there must have been an instant in time (now known to be about 14 billion years ago) when the entire Universe was contained in a single point in space. The Universe must have been born in this single violent event which came to be known as the “Big Bang.”

Astronomers combine mathematical models with observations to develop workable theories of how the Universe came to be. The mathematical underpinnings of the Big Bang theory include Albert Einstein’s general theory of relativity along with standard theories of fundamental particles. Today NASA spacecraft such as the Hubble Space Telescope and the Spitzer Space Telescope continue Edwin Hubble’s work of measuring the expansion of the Universe. One of the goals has long been to decide whether the Universe will expand forever, or whether it will someday stop, turn around, and collapse in a “Big Crunch?”

Description: Big Bang

The structure of the universe evolved from the Big Bang, as represented by WMAP’s “baby picture”, through the clumping and ignition of matter (which caused reionization) up to the present.

Background Radiation:

According to the theories of physics, if we were to look at the Universe one second after the Big Bang, what we would see is a 10-billion degree sea of neutrons, protons, electrons, anti-electrons (positrons), photons, and neutrinos. Then, as time went on, we would see the Universe cool, the neutrons either decaying into protons and electrons or combining with protons to make deuterium (an isotope of hydrogen). As it continued to cool, it would eventually reach the temperature where electrons combined with nuclei to form neutral atoms. Before this “recombination” occurred, the Universe would have been opaque because the free electrons would have caused light (photons) to scatter the way sunlight scatters from the water droplets in clouds. But when the free electrons were absorbed to form neutral atoms, the Universe suddenly became transparent. Those same photons – the afterglow of the Big Bang known as cosmic background radiation – can be observed today.

Missions Study Cosmic Background Radiation:

NASA has launched two missions to study the cosmic background radiation, taking “baby pictures” of the Universe only 400,000 years after it was born. The first of these was the Cosmic Background Explorer (COBE). In 1992, the COBE team announced that they had mapped the primordial hot and cold spots in cosmic background radiation. These spots are related to the gravitational field in the early Universe and form the seeds of the giant clusters of galaxies that stretch hundreds of millions of light years across the Universe. This work earned NASA’s Dr. John C. Mather and George F. Smoot of the University of California the 2006 Nobel Prize for Physics.

The second mission to examine the cosmic background radiation was the Wilkinson Microware Anisotropy Probe (WMAP). With greatly improved resolution compared to COBE, WMAP surveyed the entire sky, measuring temperature differences of the microwave radiation that is nearly uniformly distributed across the Universe. The picture shows a map of the sky, with hot regions in red and cooler regions in blue. By combining this evidence with theoretical models of the Universe, scientists have concluded that the Universe is “flat,” meaning that, on cosmological scales, the geometry of space satisfies the rules of Euclidean geometry (e.g., parallel lines never meet, the ratio of circle circumference to diameter is pi, etc).

A third mission, Planck, led by the European Space Agency with significant participation from NASA, was. launched in 2009. Planck is making the most accurate maps of the microwave background radiation yet. With instruments sensitive to temperature variations of a few millionths of a degree, and mapping the full sky over 9 wavelength bands, it measures the fluctuations of the temperature of the CMB with an accuracy set by fundamental astrophysical limits.

Description: Universe Fate-1 Accelerating Universe

The Universe’s “baby picture”. WMAP’s map of the temperature of the microwave background radiation shows tiny variations (of few microdegrees) in The 3K background. Hot spots show as red, cold spots as dark blue.


One problem that arose from the original COBE results, and that persists with the higher-resolution WMAP data, was that the Universe was toohomogeneous. How could pieces of the Universe that had never been in contact with each other have come to equilibrium at the very same temperature? This and other cosmological problems could be solved, however, if there had been a very short period immediately after the Big Bang where the Universe experienced an incredible burst of expansion called “inflation.” For this inflation to have taken place, the Universe at the time of the Big Bang must have been filled with an unstable form of energy whose nature is not yet known. Whatever its nature, the inflationary model predicts that this primordial energy would have been unevenly distributed in space due to a kind of quantum noise that arose when the Universe was extremely small. This pattern would have been transferred to the matter of the Universe and would show up in the photons that began streaming away freely at the moment of recombination. As a result, we would expect to see, and do see, this kind of pattern in the COBE and WMAP pictures of the Universe.

But all this leaves unanswered the question of what powered inflation. One difficulty in answering this question is that inflation was over well before recombination, and so the opacity of the Universe before recombination is, in effect, a curtain drawn over those interesting very early events. Fortunately, there is a way to observe the Universe that does not involve photons at all. Gravitational waves, the only known form of information that can reach us undistorted from the instant of the Big Bang, can carry information that we can get no other way. Two missions that are being considered by NASA, LISA and the Big Bang Observer, will look for the gravitational waves from the epoch of inflation.

Dark Energy:

During the years following Hubble and COBE, the picture of the Big Bang gradually became clearer. But in 1996, observations of very distant supernovae required a dramatic change in the picture. It had always been assumed that the matter of the Universe would slow its rate of expansion. Mass creates gravity, gravity creates pull, the pulling must slow the expansion. But supernovae observations showed that the expansion of the Universe, rather than slowing, is accelerating. Something, not like matter and not like ordinary energy, is pushing the galaxies apart. This “stuff” has been dubbed dark energy, but to give it a name is not to understand it. Whether dark energy is a type of dynamical fluid, heretofore unknown to physics, or whether it is a property of the vacuum of empty space, or whether it is some modification to general relativity is not yet known.


Mercury: Overview

Description: Black and white image of Mercury.

MESSENGER spacecraft images have revealed portions of Mercury never seen
by human eyes

Sun-scorched Mercury is only slightly larger than Earth’s moon. Like the moon, Mercury has very little atmosphere to stop impacts and it is covered with craters. Mercury’s dayside is super-heated by the sun, but at night temperatures drop hundreds of degrees below freezing. Ice may even exist in craters. Mercury’s egg-shaped orbit takes it around the sun every 88 days.

10 Need-to-Know Things About Mercury:

1. Mercury is the smallest planet in our solar system — only slightly larger than the Earth’s moon.

2. It is the closest planet to the sun at a distance of about 58 million km (36 million miles) or 0.39 AU.

3. One day on Mercury (the time it takes for Mercury to rotate or spin once) takes 59 Earth days. Mercury makes a complete orbit around the sun (a year in Mercury time) in just 88 Earth days.

Description: Color illustration of spacecraft orbiting Mercury.

MESSENGER: First to orbit Mercury

4. Mercury is a rocky planet, also known as a terrestrial planet. Mercury has a solid, cratered surface, much like Earth’s moon.

5. Mercury’s thin atmosphere, or exosphere, is composed mostly of oxygen (O2), sodium (Na), hydrogen (H2), helium (He), and potassium (K). Atoms that are blasted off the surface by the solar wind and micrometeoroid impacts create Mercury’s exosphere.

6. Mercury has no moons.

7. There are no rings around Mercury.

8. Only two spacecraft have visited this rocky planet: Mariner 10 in 1974-5 and MESSENGER, which flew past Mercury three times before going into orbit around Mercury in 2011.

9. No evidence for life has been found on Mercury. Daytime temperatures can reach 800 degrees Fahrenheit (430 degrees Celsius) and drop to -290 degrees Fahrenheit (-180 degrees Celsius) at night. It is unlikely life (as we know it) could survive on this planet.

10. Standing on Mercury’s surface at its closest point to the sun, the sun would appear more than three times larger than it does on Earth.

Mercury’s eccentric orbit takes the small planet as close as 47 million km (29 million miles) and as far as 70 million km (43 million miles) from the sun. If one could stand on the scorching surface of Mercury when it is at its closest point to the sun, the sun would appear more than three times as large as it does when viewed from Earth. Temperatures on Mercury’s surface can reach 800 degrees Fahrenheit (430 degrees Celsius). Because the planet has no atmosphere to retain that heat, nighttime temperatures on the surface can drop to -290 degrees Fahrenheit (-180 degrees Celsius).

Description: Color image showing Mercury in front of the sun.

Mercury crosses in front of the sun

Description: Color image showing massive impact basin.

Mercury’s Caloris Basin, a massive impact crater

Because Mercury is so close to the sun, it is hard to directly observe from Earth except during dawn or twilight. Mercury makes an appearance indirectly — 13 times each century, observers on Earth can watch Mercury pass across the face of the sun, an event called a transit. These rare transits fall within several days of 8 May and 10 November. The first two transits of Mercury in the 21st century occurred 7 May 2003, and 8 November 2006. The next are 9 May 2016, and 11 November 2019.

Mercury speeds around the sun every 88 days, traveling through space at nearly 50 km (31 miles) per second faster than any other planet. One Mercury solar day equals 175.97 Earth days.

Instead of an atmosphere, Mercury possesses a thin exosphere made up of atoms blasted off the surface by the solar wind and striking micrometeoroids. Because of solar radiation pressure, the atoms quickly escape into space and form a tail of neutral particles. Though Mercury’s magnetic field at the surface has just one percent the strength of Earth’s, it interacts with the magnetic field of the solar wind to episodically create intense magnetic tornadoes that funnel the fast, hot solar wind plasma down to the surface. When the ions strike the surface, they knock off neutrally charged atoms and send them on a loop high into the sky.

Mercury’s surface resembles that of Earth’s Moon, scarred by many impact craters resulting from collisions with meteoroids and comets. Very large impact basins, including Caloris (1,550 km, or 960 miles, in diameter) and Rachmaninoff (306 km, or 190 miles), were created by asteroid impacts on the planet’s surface early in the solar system’s history. While there are large areas of smooth terrain, there are also lobe-shaped scarps or cliffs, some hundreds of miles long and soaring up to a mile high, formed as the planet’s interior cooled and contracted over the billions of years since Mercury formed.

Mercury is the second densest planet after Earth, with a large metallic core having a radius of about 2,000 km (1,240 miles), about 80 percent of the planet’s radius. In 2007, researchers used ground-based radars to study the core, and found evidence that it is partly molten (liquid). Mercury’s outer shell, comparable to Earth’s outer shell (called the mantle and crust), is only about 400 km (250 miles) thick.

The first spacecraft to visit Mercury was Mariner 10, which imaged about 45 percent of the surface. NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission flew by Mercury three times in 2008-2009 and has been in orbit around the planet since 18 March 2011. Almost the entire planet has now been imaged, revealing a surface that has been shaped both by extensive volcanism and impacts.

Data from MESSENGER’s scientific instruments have provided a trove of scientific discoveries. These include the identification of a new landform known as hollows, measurements indicating that Mercury has a remarkably high abundance of the volatile elements sulfur and potassium, and the discoveries that Mercury’s magnetic field is offset relative to the planet’s equator and that the planet has a highly unusual internal structure. In 1991, astronomers on Earth using radar observations showed that Mercury may have water ice at its north and south poles inside deep craters. MESSENGER observations have shown that the materials identified by radar are present only in regions of permanent shadow, consistent with the idea that they are cold enough to preserve water ice, despite the extreme high temperatures experienced by sunlit parts of the planet.

How Mercury Got its Name:

Mercury is appropriately named for the swiftest of the ancient Roman gods. Mercury, the god of commerce, is the Roman counterpart to the ancient Greek god Hermes, the messenger of the gods.

Important Dates:

1631: Thomas Harriott and Galileo Galilei observe Mercury with the newly invented telescope.

1631: Pierre Gassendi uses a telescope to watch from Earth as Mercury crosses the face of the sun.

1965: Incorrectly believing for centuries that the same side of Mercury always faces the sun, astronomers find that the planet rotates three times for every two orbits.

1974-1975: Mariner 10 photographs roughly half of Mercury’s surface in three flybys.

1991: Scientists using Earth-based radar find signs of ice locked in permanently shadowed areas of craters in Mercury’s polar regions.

2008-2009: MESSENGER observes Mercury during three flybys.

2011: MESSENGER begins its orbital mission at Mercury, yielding a treasure trove of images, compositional data and scientific discoveries.


Venus: Overview

Description: Color image showing Venus topography

Magellan spacecraft radar data enabled scientists to penetrate Venus’ thick clouds and create simulated views of the surface.

Venus is a dim world of intense heat and volcanic activity. Similar in structure and size to Earth, Venus’ thick, toxic atmosphere traps heat in a runaway ‘greenhouse effect.’ The scorched world has temperatures hot enough to melt lead. Glimpses below the clouds reveal volcanoes and deformed mountains. Venus spins slowly in the opposite direction of most planets.

10 Need-to-Know Things About Venus:

1. Venus is only a little smaller than Earth.

2. Venus is the second closest planet to the sun at a distance of about 108 million km (67 million miles) or 0.72 AU.

Description: Color illustration of spacecraft orbiting Venus.

Magellan: Mapping Venus

3. One day on Venus lasts as long as 243 Earth days (the time it takes for Venus to rotate or spin once). Venus makes a complete orbit around the sun (a year in Venusian time) in 225 Earth days.

4. Venus is a rocky planet, also known as a terrestrial planet. Venus’ solid surface is a cratered and volcanic landscape.

5. Venus’ thick and toxic atmosphere is made up mostly of carbon dioxide (CO2) and nitrogen (N2), with clouds of sulfuric acid (H2SO4) droplets.

6. Venus has no moons.

7. There are no rings around Venus.

8. More than 40 spacecraft have explored Venus. The Magellan mission in the early 1990s mapped 98 percent of the planet’s surface.

9. No evidence for life has been found on Venus. The planet’s extreme high temperatures of almost 480 degrees Celsius (900 degrees Fahrenheit) make it seem an unlikely place for for life as we know it.

10. Venus spins backwards (retrograde rotation) when compared to the other planets. This means that the sun rises in the west and sets in the east on Venus.

Venus and Earth are similar in size, mass, density, composition, and gravity. There, however, the similarities end. Venus is covered by a thick, rapidly spinning atmosphere, creating a scorched world with temperatures hot enough to melt lead and surface pressure 90 times that of Earth (similar to the bottom of a swimming pool 1-1/2 miles deep). Because of its proximity to Earth and the way its clouds reflect sunlight, Venus appears to be the brightest planet in the sky.

Description: Color image showing Venus cloaked by brownish clouds.

Venus is cloaked in thick, perpetual clouds.

Description: Color image of Venus in front of the sun.

Venus crossing the face of the Sun as seen from Earth orbit.

Description: Color image showing heat spots on a volcano.

Venus is a volcanic world.

We cannot normally see through Venus’ thick atmosphere, but NASA’sMagellan mission during the early 1990s used radar to image 98 percent of the surface, and the Galileo spacecraft used infrared mapping to view both the surface and mid-level cloud structure as it passed by Venus on the way toJupiter. In 2010, infrared surface images by the European Space Agency’sVenus Express provided evidence for recent volcanism within the past several hundred thousand years. Indeed, Venus may be volcanically active today.

Like Mercury, Venus can be seen periodically passing across the face of the sun. These “transits” of Venus occur in pairs with more than a century separating each pair. Transits occurred in 1631, 1639; 1761, 1769; and 1874, 1882. On 8 June 2004, astronomers worldwide watched the tiny dot of Venus crawl across the sun; and on 6 June 2012, the second in this pair of transits occurred. The next transit is 11 December 2117. Observing these transits helps us understand the capabilities and limitations of techniques used to find and characterize planets around other stars.

Venus’ atmosphere consists mainly of carbon dioxide, with clouds of sulfuric acid droplets. Only trace amounts of water have been detected in the atmosphere. The thick atmosphere traps the sun’s heat, resulting in surface temperatures higher than 470 degrees Celsius (880 degrees Fahrenheit). The few probes that have landed on Venus have not survived longer than 2 hours in the intense heat. Sulfur compounds are abundant in Venus’ clouds; the corrosive chemistry and dense, moving atmosphere cause significant surface weathering and erosion.

The Venusian year (orbital period) is about 225 Earth days long, while the planet’s rotation period is 243 Earth days, making a Venus day about 117 Earth days long. Venus rotates retrograde (east to west) compared with Earth’s prograde (west to east) rotation. Seen from Venus, the sun would rise in the west and set in the east. As Venus moves forward in its solar orbit while slowly rotating backwards on its axis, the top level of cloud layers zips around the planet every four Earth days, driven by hurricane-force winds traveling at about 360 kilometers (224 miles) per hour. Speeds within the clouds decrease with cloud height, and at the surface are estimated to be just a few kilometers per hour. How this atmospheric “super-rotation” forms and is maintained continues to be a topic of scientific investigation.

Atmospheric lightning bursts, long suspected by scientists, were confirmed in 2007 by the European Venus Express orbiter. On Earth, Jupiter, and Saturn, lightning is associated with water clouds, but on Venus, it is associated with sulfuric acid clouds.

Craters smaller than 1.5 to 2 kilometers (0.9 to 1.2 miles) across do not exist on Venus, because small meteors burn up in the dense atmosphere before they can reach the surface. It is thought that Venus was completely resurfaced by volcanic activity 300 to 500 million years ago. More than 1,000 volcanoes or volcanic centers larger than 20 kilometers (12 miles) in diameter dot the surface. Volcanic flows have produced long, sinuous channels extending for hundreds of kilometers. Venus has two large highland areas – Ishtar Terra, about the size of Australia, in the north polar region; and Aphrodite Terra, about the size of South America, straddling the equator and extending for almost 10,000 kilometers (6,000 miles). Maxwell Montes, the highest mountain on Venus and comparable to Mount Everest on Earth, is at the eastern edge of Ishtar Terra.

Venus has an iron core that is approximately 3,000 kilometers (1,900 miles) in radius. Venus has no global magnetic field – though its core iron content is similar to that of Earth, Venus rotates too slowly to generate the type of magnetic field that Earth has.

How Venus Got its Name:

Venus is named for the ancient Roman goddess of love and beauty. (Venus is the Roman counterpart to the Greek goddess Aphrodite.) It is believed Venus was named for the most beautiful of the ancient gods because it shone the brightest of the five planets known to ancient astronomers. Other civilizations have named it for their god or goddess of love/war as well.

Important Dates:

650 BCE: Mayan astronomers make detailed observations of Venus, leading to a highly accurate calendar.

1761-1769: Two European expeditions to watch Venus cross in front of the sun lead to the first good estimate of the sun’s distance from Earth.

1962: NASA’s Mariner 2 reaches Venus and reveals the planet’s extreme surface temperatures. It is the first spacecraft to send back information from another planet.

1970: The Soviet Union’s Venera 7 sends back 23 minutes of data from the surface of Venus. It is the first spacecraft to successfully land on another planet.

1990-1994: NASA’s Magellan spacecraft, in orbit around Venus, uses radar to map 98 percent of the planet’s surface.

2005: The European Space Agency launches Venus Express to study the atmosphere and surface. The orbiter reached Venus in April 2006, and will study the planet through at least 2014. Japan’s Akatsuki (“Dawn”) orbiter is en route to Venus, scheduled to arrive in 2015. Combining the Venus Express and Akatsuki datasets should greatly enhance our knowledge of the planet.


Earth: Overview

Description: Color image showing the full disk of Earth.

A true-color NASA satellite mosaic of Earth.

Earth, our home planet, is the only planet in our solar system known to harbor life – life that is incredibly diverse. All the things we need to survive exist under a thin layer of atmosphere that separates us from the cold, airless void of space.

10 Need-to-Know Things About Earth:

1. If the sun were as tall as a typical front door, Earth would be the size of a nickel.

2. Earth is the third planet from the sun at a distance of about 150 million km (93 million miles) or one AU.

Description: Color image of spacecraft orbiting Earth.

Spacecraft study Earth from above.

3. One day on Earth takes 24 hours (this is the time it takes the Earth to rotate or spin once). Earth makes a complete orbit around the sun (a year in Earth time) in about 365 days.

4. Earth is a rocky planet, also known as a terrestrial planet, with a solid and dynamic surface of mountains, valleys, canyons, plains and so much more. What makes Earth different from the other terrestrial planets is that it is also an ocean planet: 70 percent of the Earth’s surface is covered in oceans.

5. The Earth’s atmosphere is made up of 78 percent nitrogen (N2), 21 percent oxygen (O2) and 1 percent other ingredients — the perfect balance for us to breathe and live. Many planets have atmospheres, but only Earth’s is breathable.

6. Earth has one moon. Another name for a moon is satellite.

7. Earth has no rings.

8. Many orbiting spacecraft study the Earth from above as a whole system and together aid in understanding our home planet.

9. Earth is the perfect place for life.

10. Earth’s atmosphere protects us from incoming meteoroids, most of which break up in our atmosphere before they can strike the surface as meteorites.

Earth, our home planet, is the only planet in our solar system known to harbor life – life that is incredibly diverse. All the things we need to survive exist under a thin layer of atmosphere that separates us from the cold, airless void of space.

Description: Color image of Earth and the Moon from the Space Shuttle.

Earth and its moon from the space shuttle.

Description: Color image of Earth and its moon.

Earth and it’s moon as seen by a departing spacecraft.

Description: Color image of Earth with the shadow of the moon on it.

Here is what the Earth looks like during a solar eclipse.

Earth is made up of complex, interactive systems that create a constantly changing world that we are striving to understand. From the vantage point of space, we are able to observe our planet globally, using sensitive instruments to understand the delicate balance among its oceans, air, land, and life. NASA satellite observations help study and predict weather, drought, pollution, climate change, and many other phenomena that affect the environment, economy, and society.

Earth is the third planet from the sun and the fifth largest in the solar system. Earth’s diameter is just a few hundred kilometers larger than that of Venus. The four seasons are a result of Earth’s axis of rotation being tilted 23.45? degrees with respect to the plane of Earth’s orbit around the sun. During part of the year, the northern hemisphere is tilted toward the sun and the southern hemisphere is tilted away, producing summer in the north and winter in the south. Six months later, the situation is reversed. When spring and fall begin, both hemispheres receive roughly equal amounts of solar illumination.

Earth’s global ocean, which covers nearly 70 percent of the planet’s surface, has an average depth of about 4 kilometers (2.5? miles). Fresh water exists in the liquid phase only within a narrow temperature span – 0 to 100 degrees Celsius (32 to 212 degrees Fahrenheit). This span is especially narrow when contrasted with the full range of temperatures found within the solar system. The presence and distribution of water vapor in the atmosphere is responsible for much of Earth’s weather.

Near the surface, an atmosphere that consists of 78 percent nitrogen, 21 percent oxygen, and 1 percent other ingredients envelops us. The atmosphere affects Earth’s long-term climate and short-term local weather, shields us from much of the harmful radiation coming from the sun, and protects us from meteors as well – most of which burn up before they can strike the surface as meteorites.

Our planet’s rapid rotation and molten nickel-iron core give rise to a magnetic field, which the solar wind distorts into a teardrop shape in space. (The solar wind is a stream of charged particles continuously ejected from the sun.) When charged particles from the solar wind become trapped in Earth’s magnetic field, they collide with air molecules above our planet’s magnetic poles. These air molecules then begin to glow, and are known as the aurorae – the northern and southern lights.

Earth’s lithosphere, which includes the crust (both continental and oceanic) and the upper mantle, is divided into huge plates that are constantly moving. For example, the North American plate moves west over the Pacific Ocean basin, roughly at a rate equal to the growth of our fingernails. Earthquakes result when plates grind past one another, ride up over one another, collide to make mountains, or split and separate. Unifying centuries of Earth sciences studies, the theory of motion of lithospheric plates was developed within only the last 47 years.

How Earth Got its Name:

The name Earth is at least 1,000 years old. All of the planets, except for Earth, were named after Greek and Roman gods and goddesses. However, the name Earth is an English/German word, which simply means the ground: eor(th)e and ertha(Old English) and erde (German).

Important Dates:

1960: NASA launches the Television Infrared Observation Satellite (TIROS), the first weather satellite.

1972: The Earth Resources Technology Satellite 1 (renamed Landsat 1) is launched.

1987: NASA’s Airborne Antarctic Ozone Experiment helps determine the cause of the Antarctic ozone hole.

1992: TOPEX/Poseidon, a U.S.-France mission, begins measuring sea-surface height. Jason 1 continues in 2001.

1997: TOPEX/Poseidon captures the onset of one of the largest El Niño events of the 20th century.

1997: The U.S.-Japan Tropical Rainfall Measuring Mission is launched to provide 3-D maps of storm structure.

1999: Quick Scatterometer (QuikScat) launches in June to measure ocean surface wind velocity; in December the Active Cavity Irradiance Monitor Satellite launches to monitor the total amount of the sun’s energy reaching Earth.

1999-2006: A series of satellites is launched to provide global observations of the Earth system: Terra (land, oceans, atmosphere), Aqua (water cycle), Aura (atmospheric chemistry)CloudSat (clouds), and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation mission (aerosols, clouds).

2006: The Antarctic ozone hole was the largest yet observed.

2007: Arctic sea ice reaches the all-time minimum since satellite records began.

2008: The third U.S.-France mission to measure sea-level height, Ocean Surface Topography Mission/Jason 2, is launched, doubling global data coverage.

2009: NASA and Japan release the most accurate topographic map of Earth.

2011: NASA launches Aquarius, its first instrument to measure the salinity of the global oceans.


Earth’s Moon: Overview

Description: Color image of the full disk of the Moon.

The Galileo spacecraft sent back this image of the Moon as it headed into the outer solar
system. The distinct bright ray crater at the bottom of the image is the Tycho impact basin.

Our moon makes Earth a more livable planet by moderating our home planet’s wobble on its axis, leading to a relatively stable climate, and creating a tidal rhythm that has guided humans for thousands of years. The moon was likely formed after a Mars-sized body collided with Earth and the debris formed into the most prominent feature in our night sky.

10 Need-to-Know Things About Earth’s Moon:

1. If the sun were as tall as a typical front door, Earth would be the size of a nickel and the moon would the size of a green pea.

2. The moon is Earth’s satellite and orbits the Earth at a distance of about 384 thousand km (239 thousand miles) or 0.00257 AU.

Description: Color image of astronaut walking on the moon.

Twelve human beings have walked on the moon.

3. The moon makes a complete orbit around Earth in 27 Earth days and rotates or spins at that same rate, or in that same amount of time. This causes the moon to keep the same side or face towards Earth during the course of its orbit.

4. The moon is a rocky, solid-surface body, with much of its surface cratered and pitted from impacts.

5. The moon has a very thin and tenuous (weak) atmosphere, called an exosphere.

6. The moon has no moons.

7. The moon has no rings.

8. More than 100 spacecraft been launched to explore the moon. It is the only celestial a body beyond Earth that has been visited by human beings (The Apollo Program).

9. The moon’s weak atmosphere and its lack of liquid water cannot support life as we know it.

10. Surface features that create the face known as the “Man in the moon” are impact basins on the moon that are filled with dark basalt rocks.

The regular daily and monthly rhythms of Earth’s only natural satellite, the moon, have guided timekeepers for thousands of years. Its influence on Earth’s cycles, notably tides, has been charted by many cultures in many ages. The moon moderates Earth’s wobble on its axis, leading to a relatively stable climate over billions of years. From Earth, we always see the same face of the moon because the moon is spinning on its axis at the same speed that it is going around Earth (that is, it is in synchronous rotation with Earth).

Description: Color illustration shows ice concentrations in a lunar crater.

Ice may make up as much as 22% of the surface material
in Shackleton crater.

Description: Color image of astronaut next to giant moon boulder.

An astronaut next to a huge, split boulder on the moon.

Description: Black and white of lunar crater.

Rugged Copernicus crater.

The light areas of the moon are known as the highlands. The dark features, called maria (Latin for seas), are impact basins that were filled with lava between 4.2 and 1.2 billion years ago. These light and dark areas represent rocks of different composition and ages, which provide evidence for how the early crust may have crystallized from a lunar magma ocean. The craters themselves, which have been preserved for billions of years, provide an impact history for the moon and other bodies in the inner solar system.

The leading theory of the moon’s origin is that a Mars-sized body collided with Earth approximately 4.5 billion years ago, and the resulting debris from both Earth and the impactor accumulated to form our natural satellite. The newly formed moon was in a molten state. Within about 100 million years, most of the global “magma ocean” had crystallized, with less-dense rocks floating upward and eventually forming the lunar crust. The early moon may have developed an internal dynamo, the mechanism for global magnetic fields for terrestrial planets.

Since the ancient time of volcanism, the arid, lifeless moon has remained nearly unchanged. With too sparse an atmosphere to impede impacts, a steady rain of asteroids, meteoroids, and comets strikes the surface. Over billions of years, the surface has been ground up into fragments ranging from huge boulders to powder. Nearly the entire moon is covered by a rubble pile of charcoal-gray, powdery dust and rocky debris called the lunar regolith. Beneath is a region of fractured bedrock referred to as the megaregolith.

The moon was first visited by the U.S.S.R.’s Luna 1 and Luna 2 in 1959, and a number of U.S. and U.S.S.R. robotic spacecraft followed. The U.S. sent three classes of robotic missions to prepare the way for human exploration: the Rangers (1961-1965) were impact probes, the Lunar Orbiters (1966-1967) mapped the surface to find landing sites, and the Surveyors (1966-1968) were soft landers. The first human landing on the moon was on 20 July 1969. During the Apollo missions of 1969-1972, 12 American astronauts walked on the moon and used a Lunar Roving Vehicle to travel on the surface and extend their studies of soil mechanics, meteoroids, lunar ranging, magnetic fields, and solar wind. The Apollo astronauts brought back 382 kilograms (842 pounds) of rock and soil to Earth for study.

After a long hiatus, lunar exploration resumed in the 1990s with the U.S. robotic missions Clementine and Lunar Propspector. Results from both missions suggested that water ice might be present at the lunar poles, but a controlled impact of the Prospector spacecraft produced no observable water.

The European Space Agency was first in the new millennium with SMART-1in 2003, followed by Kaguya (Japan), Chang’e-1 (China), and Chandrayaan-1(India) in 2007-2008. The U.S. began a new series of robotic lunar missions with the joint launch of the [LINK||MISSION:LRO||Lunar Reconnaissance Orbiter]] (LRO) and Lunar Crater Observation and Sensing Satellite (LCROSS) in 2009. In 2011, a pair of repurposed spacecraft began the ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the moon’s Interaction with the Sun) mission. In 2012, the Gravity Recovery and Interior Laboratory (GRAIL) twin spacecraft studied the moon’s gravity field and produced the highest-resolution gravity field map of any celestial body.

How the Moon Got its Name:

Earth’s only natural satellite is simply called the moon because people didn’t know other moons existed until Galileo Galilei discovered four moons orbiting Jupiter in 1610. Other moons in our solar system are given names so they won’t be confused with each other. We call them moons because, like our own, they are natural satellites orbiting a solar system body (which in turn is orbiting a star).

Important Dates:

1610: Galileo Galilei is the first to use a telescope to make scientific observations of the moon.

1959-1976: The U.S.S.R.’s Luna program of 17 robotic missions achieves many “firsts” and three sample returns.

1961-1968: The U.S. Ranger, Lunar Orbiter, and Surveyor robotic missions pave the way for Apollo human lunar landings.

1969: Astronaut Neil Armstrong is the first human to walk on the moon’s surface.

1994-1999: Clementine and Lunar Prospector data suggest that water ice may exist at the lunar poles.

2003: The European Space Agency’s SMART-1 lunar orbiter inventories key chemical elements.

2007-2008: Japan’s second lunar spacecraft, Kaguya, and China’s first lunar spacecraft, Chang’e 1, both begin one-year missions orbiting the moon; India’s Chandrayaan-1 soon follows in lunar orbit.

2008: The NASA Lunar Science Institute is formed to help lead NASA’s research activities related to lunar exploration goals.

2009: NASA’s LRO and LCROSS launch together, beginning the U.S. return to lunar exploration. In October, LCROSS was directed to impact a permanently shadowed region near the lunar south pole, resulting in the discovery of water ice.

2011: Twin GRAIL spacecraft launch to map the interior of the moon from crust to core, and NASA begins the ARTEMIS mission to study the moon’s interior and surface composition.


Mars: Overview

Description: Color image of a thin atmosphere over reddish Mars. Link to MAVEN toolkit.

Mars lost much of its atmosphere over time. Where did the atmosphere–and the water–go?
The MAVEN mission’s hunt for answers will help us understand when and for how long Mars
might have had an environment that could have supported microbial life in its ancient past.

Mars is a cold desert world. It is half the diameter of Earth and has the same amount of dry land. Like Earth, Mars has seasons, polar ice caps, volcanoes, canyons and weather, but its atmosphere is too thin for liquid water to exist for long on the surface. There are signs of ancient floods on Mars, but evidence for water now exists mainly in icy soil and thin clouds.

Description: Link to Eyes on the Solar System simulator

Explore Mars in 3D

10 Need-to-Know Things About Mars:

1. If the sun were as tall as a typical front door, Earth would be the size of a nickel, and Mars would be about as big as an aspirin tablet.

2. Mars orbits our sun, a star. Mars is the fourth planet from the sun at a distance of about 228 million km (142 million miles) or 1.52 AU.

3. One day on Mars takes just a little over 24 hours (the time it takes for Mars to rotate or spin once). Mars makes a complete orbit around the sun (a year in Martian time) in 687 Earth days.

4. Mars is a rocky planet, also known as a terrestrial planet. Mars’ solid surface has been altered by volcanoes, impacts, crustal movement, and atmospheric effects such as dust storms.

5. Mars has a thin atmosphere made up mostly of carbon dioxide (CO2), nitrogen (N2) and argon (Ar).

6. Mars has two moons named Phobos and Deimos.

7. There are no rings around Mars.

8. More than 40 spacecraft have been launched for Mars, from flybys and orbiters to rovers and landers that touched surface of the Red Planet. The first true Mars mission success was Mariner 4 in 1965.

9. At this time in the planet’s history, Mars’ surface cannot support life as we know it. A key science goal is determining Mars’ past and future potential for life.

10. Mars is known as the Red Planet because iron minerals in the Martian soil oxidize, or rust, causing the soil — and the dusty atmosphere — to look red.

Though details of Mars’ surface are difficult to see from Earth, telescope observations show seasonally changing features and white patches at the poles. For decades, people speculated that bright and dark areas on Mars were patches of vegetation, Mars was a likely place for advanced life forms, and water might exist in the polar caps. When the Mariner 4 spacecraft flew by Mars in 1965, photographs of a bleak, cratered surface shocked many – Mars seemed to be a dead planet. Later missions, however, showed that Mars is a complex planet and holds many mysteries yet to be solved. Chief among them is whether Mars ever had the right conditions to support small life forms called microbes.

Description: Orbital image of ice-laced dunes.

Mars is not all red dust and rocks.

Description: Color image showing giant canyon on Mars.

Valles Marineris is more than 3,000 km long and 8 km deep.

Description: Color image of giant dust storm on Mars.

Close-up image of a dust storm on Mars

Mars is a rocky body about half the size of Earth. As with the other terrestrial planets – Mercury, Venus, and Earth – volcanoes, impact craters, crustal movement, and atmospheric conditions such as dust storms have altered the surface of Mars.

Mars has two small moons, Phobos and Deimos, that may be captured asteroids. Potato-shaped, they have too little mass for gravity to make them spherical. Phobos, the innermost moon, is heavily cratered, with deep grooves on its surface.

Like Earth, Mars experiences seasons due to the tilt of its rotational axis. Mars’ orbit is about 1.5 times farther from the sun than Earth’s and is slightly elliptical, so its distance from the sun changes. That affects the length of Martian seasons, which vary in length. The polar ice caps on Mars grow and recede with the seasons. Layered areas near the poles suggest that the planet’s climate has changed more than once. Volcanism in the highlands and plains was active more than 3 billion years ago. Some of the giant shield volcanoes are younger, having formed between 1 and 2 billion years ago. Mars has the largest volcano in the solar system, Olympus Mons, as well as a spectacular equatorial canyon system, Valles Marineris.

Mars has no global magnetic field today. However, NASA’s Mars Global Surveyor orbiter found that areas of the Martian crust in the southern hemisphere are highly magnetized, indicating traces of a magnetic field from 4 billion years ago that remain.

Scientists believe that Mars experienced huge floods about 3.5 billion years ago. Though we do not know where the ancient flood water came from, how long it lasted, or where it went, recent missions to Mars have uncovered intriguing hints. In 2002, NASA’s Mars Odyssey orbiter detected hydrogen-rich polar deposits, indicating large quantities of water ice close to the surface. Further observations found hydrogen in other areas as well. If water ice permeated the entire planet, Mars could have substantial subsurface layers of frozen water. In 2004, Mars Exploration Rover Opportunity found structures and minerals indicating that liquid water once existed at its landing site. The rover’s twin, Spirit, also found the signature of ancient water near its landing site, halfway around Mars from Opportunity’s location.

The cold temperatures and thin atmosphere on Mars do not allow liquid water to exist at the surface for long. The quantity of water required to carve Mars’ great channels and flood plains is not evident today. Unraveling the story of water on Mars is important to unlocking its climate history, which will help us understand the evolution of all the planets. Water is an essential ingredient for life as we know it. Evidence of long-term past or present water on Mars holds clues about whether Mars could ever have been a habitat for life.

In 2008, NASA’s Phoenix Mars lander was the first mission to touch water ice in the Martian arctic. Phoenix also observed precipitation (snow falling from clouds), as confirmed by Mars Reconnaissance Orbiter. Soil chemistry experiments led scientists to believe that the Phoenix landing site had a wetter and warmer climate in the recent past (the last few million years). NASA’s Mars Science Laboratory mission, with its large rover Curiosity, is examining Martian rocks and soil at Gale Crater, looking for minerals that formed in water, signs of subsurface water, and carbon-based molecules called organics, the chemical building blocks of life. That information will reveal more about the present and past habitability of Mars, as well as whether humans could survive on Mars some day.

How Mars Got its Name:

Mars was named by the Romans for their god of war because of its red, bloodlike color. Other civilizations also named this planet from this attribute; for example, the Egyptians named it “Her Desher,” meaning “the red one.”

Important Dates:

1877: Asaph Hall discovers the two moons of Mars, Phobos and Deimos.

1965: NASA’s Mariner 4 sends back 22 photos of Mars, the world’s first close-up photos of a planet beyond Earth.

1976: Viking 1 and 2 land on the surface of Mars.

1997: Mars Pathfinder lands and dispatches Sojourner, the first wheeled rover to explore the surface of another planet.

2002: Mars Odyssey begins its mission to make global observations and find buried water ice on Mars.

2004: Twin Mars Exploration Rovers named Spirit and Opportunity find strong evidence that Mars once had long-term liquid water on the surface.

2006: Mars Reconnaissance Orbiter begins returning high-resolution images as it studies the history of water on Mars and seasonal changes.

2008: Phoenix finds signs of possible habitability, including the occasional presence of liquid water and potentially favorable soil chemistry.

2012: NASA’s Mars rover Curiosity lands in Gale Crater and finds conditions once suited for ancient microbial life on Mars.


Asteroids: Overview

Description: Black and white image of asteroid Eros.

This picture of Eros, the first of an asteroid taken from an orbiting spacecraft, is a mosaic of four
images obtained by NASA’s NEAR mission immediately after the spacecraft’s insertion into orbit.

Asteroids are rocky, airless worlds that orbit our sun, but are too small to be called planets. Tens of thousands of these minor planets are gathered in the main asteroid belt, a vast doughnut-shaped ring between the orbits of Mars and Jupiter. Asteroids that pass close to Earth are called near-earth objects.

10 Need-to-Know Things About Asteroids:

1. If all of the asteroids were combined into a ball, they would still be much smaller than Earth’s moon. If the sun was as tall as a typical front door, Earth would be the size of a nickel, the moon would be about as big as a green pea and Ceres (the largest object in the main asteroid belt) would be as small as a sesame seed.

Description: Link to Dawn Mission

Dawn: First to orbit an asteroid.

2. Most Asteroids orbit our sun, a star, in a region of space between the orbits of Mars and Jupiter known as the Asteroid Belt.

3. Days and years vary by asteroid. A day on asteroid Ida, for example, takes only 4.6 hours (the time it takes to rotate or spin once). Ida makes a complete orbit around the sun (a year in this asteroid’s time) in 4.8 Earth years.

4. Asteroids are solid, rocky and irregular bodies.

5. Asteroids do not have atmospheres.

6. More than 150 asteroids are known to have a small companion moon (some have two moons). The first discovery of an asteroid-moon system was of asteroid Ida and its moon Dactyl in 1993.

7. One asteroid, named Chariklo, is known to have two dense and narrow rings.

8. More than 10 spacecraft have explored asteroids. NEAR Shoemaker even landed on an asteroid (Eros). The Dawn mission is the first mission to orbit (2011) a main belt asteroid (Vesta).

9. Asteroids cannot support life as we know it.

10. Ceres, the first and largest asteroid to be discovered (1801 by Giuseppe Piazzi) and the closest dwarf planet to the sun, encompasses over one-third of the estimated total mass of all the asteroids in the asteroid belt.

Asteroids, sometimes called minor planets, are rocky remnants left over from the early formation of our solar system about 4.6 billion years ago.

Most of this ancient space rubble can be found orbiting the sun between Mars and Jupiter within the main asteroid belt. Asteroids range in size from Ceres – the largest at about 950 kilometers (590 miles) in diameter and also identified as adwarf planet- to bodies that are less than 1 kilometer (0.6 mile) across. The total mass of all the asteroids combined is less than that of Earth’s Moon.

Editor’s note: Even with more than one-half million asteroids known (and there are probably many more), they are still much more widely separated than sometimes seen in Hollywood movies: on average, their separation is in excess of 1-3 million km (depending on how one calculates it).

Description: black and white image of spherical asteroid.

A mosaic of the best views of the giant asteroid Vesta.

Description: Color image of an asteroid and its tiny moon.

The Galileo spacecraft found asteroids can have moons.

Description: Illustration that shows the locations of many asteroids.

A snapshot of near-Earth asteroids.

Most asteroids are irregularly shaped, though a few are nearly spherical, and they are often pitted or cratered. As they revolve around the sun in elliptical orbits, the asteroids also rotate, sometimes quite erratically, tumbling as they go. More than 150 asteroids are known to have a small companion moon (some have two moons). There are also binary (double) asteroids, in which two rocky bodies of roughly equal size orbit each other, as well as triple asteroid systems.

The three broad composition classes of asteroids are C-, S-, and M-types. The C-type (chondrite) asteroids are most common, probably consist of clay and silicate rocks, and are dark in appearance. They are among the most ancient objects in the solar system. The S-types (“stony”) are made up of silicate materials and nickel-iron. The M-types are metallic (nickel-iron). The asteroids’ compositional differences are related to how far from the sun they formed. Some experienced high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava to the surface. Only one such asteroid, Vesta, survives to this day.

Jupiter’s massive gravity and occasional close encounters with Mars or another object change the asteroids’ orbits, knocking them out of the main belt and hurling them into space in all directions across the orbits of the other planets. Stray asteroids and asteroid fragments slammed into Earth and the other planets in the past, playing a major role in altering the geological history of the planets and in the evolution of life on Earth.

Scientists continuously monitor Earth-crossing asteroids, whose paths intersect Earth’s orbit, and near-Earth asteroids that approach Earth’s orbital distance to within about 45 million kilometers (28 million miles) and may pose an impact danger. Radar is a valuable tool in detecting and monitoring potential impact hazards. By reflecting transmitted signals off objects, images and other information can be derived from the echoes. Scientists can learn a great deal about an asteroid’s orbit, rotation, size, shape, and metal concentration.

Several missions have flown by and observed asteroids. The Galileospacecraft flew by asteroids Gaspra in 1991 and Ida in 1993; the Near-Earth Asteroid Rendezvous (NEAR-Shoemaker) mission studied asteroids Mathilde and Eros; and the Rosetta mission encountered Steins in 2008 and Lutetia in 2010. Deep Space 1 and Stardust both had close encounters with asteroids.

In 2005, the Japanese spacecraft Hayabusa landed on the near-Earth asteroid Itokawa and attempted to collect samples. On June 3, 2010, Hayabusa successfully returned to Earth a small amount of asteroid dust now being studied by scientists.

NASA’s Dawn spacecraft, launched in 2007, orbited and explored asteroid Vesta for over a year. Once it left in September 2012, it headed towards dwarf planet Ceres, with a planned arrival of 2015. Vesta and Ceres are two of the largest surviving protoplanet bodies that almost became planets. By studying them with the same complement of instruments on board the same spacecraft, scientists will be able to compare and contrast the different evolutionary path each object took to help understand the early solar system overall.

Asteroid Classifications:

Main asteroid belt: The majority of known asteroids orbit within the asteroid belt between Mars and Jupiter, generally with not very elongated orbits. The belt is estimated to contain between 1.1 and 1.9 million asteroids larger than 1 kilometer (0.6 mile) in diameter, and millions of smaller ones. Early in the history of the solar system, the gravity of newly formed Jupiter brought an end to the formation of planetary bodies in this region and caused the small bodies to collide with one another, fragmenting them into the asteroids we observe today.

Trojans: These asteroids share an orbit with a larger planet, but do not collide with it because they gather around two special places in the orbit (called the L4 and L5 Lagrangian points). There, the gravitational pull from the sun and the planet are balanced by a trojan’s tendency to otherwise fly out of the orbit. The Jupiter trojans form the most significant population of trojan asteroids. It is thought that they are as numerous as the asteroids in the asteroid belt. There are Mars and Neptune trojans, and NASA announced the discovery of an Earth trojan in 2011.

Near-Earth asteroids: These objects have orbits that pass close by that of Earth. Asteroids that actually cross Earth’s orbital path are known as Earth-crossers. As of June 19, 2013, 10,003 near-Earth asteroids are known and the number over 1 kilometer in diameter is thought to be 861, with 1,409 classified as potentially hazardous asteroids – those that could pose a threat to Earth.

How Asteroids Get Their Names:

The International Astronomical Union’s Committee on Small Body Nomenclature.is a little less strict when it comes to naming asteroids than other IAU naming committees. So out there orbiting the sun we have giant space rocks named for Mr. Spock (a cat named for the character of “Star Trek” fame), rock musician Frank Zappa, regular guys like Phil Davis, and more somber tributes such as the seven asteroids named for the crew of the Space Shuttle Columbia killed in 2003. Asteroids are also named for places and a variety of other things. (The IAU discourages naming asteroids for pets, so Mr. Spock stands alone).

Asteroids are also given a number, for example (99942) Apophis. The Harvard Smithsonian Center for Astrophysics keeps a fairly current list of asteroid names.

Important Dates:

1801: Giuseppe Piazzi discovers the first and largest asteroid, Ceres, orbiting between Mars and Jupiter.

1898: Gustav Witt discovers Eros, one of the largest near-Earth asteroids.

1991-1994: The Galileo spacecraft takes the first close-up images of an asteroid (Gaspra) and discovers the first moon (later named Dactyl) orbiting an asteroid (Ida).

1997-2000 : The NEAR Shoemaker spacecraft flies by Mathilde and orbits and lands on Eros.

1998: NASA establishes the Near Earth Object Program Office to detect, track and characterize potentially hazardous asteroids and comets that could approach Earth.

2006: Japan’s Hayabusa becomes the first spacecraft to land on, collect samples and take off from an asteroid.

2006: Ceres attains a new classification — dwarf planet — but retains its distinction as the largest known asteroid.

2007: The Dawn spacecraft is launched on its journey to the asteroid belt to study Vesta and Ceres.

2008: The European spacecraft Rosetta, on its way to study a comet in 2014, flies by and photographs asteroid Steins, a type of asteroid composed of silicates and basalts.

2010: Japan’s Hayabusa returns its asteroid sample to Earth.

2010: Rosetta flies by asteroid Lutetia, revealing a primitive survivor from the violent birth of our solar system.

2011-2012: Dawn studies Vesta. Dawn is the first spacecraft to orbit a main-belt asteroid and continues on to dwarf planet Ceres in 2015.


Meteors and Meteorites: Overview

Description: Color image of meteorite on Mars.

NASA’s Opportunity rover found this meteorite on Mars. It is about the size of a basketball.

Little chunks of rock and debris in space are called meteoroids. They become meteors — or shooting stars — when they fall through a planet’s atmosphere; leaving a bright trail as they are heated to incandescence by the friction of the atmosphere. Pieces that survive the journey and hit the ground are called meteorites.

10 Need-to-Know Things About Meteors and Meteorites:

1. Meteoroids become meteors — or shooting stars — when they interact with a planet’s atmosphere and cause a streak of light in the sky. Debris that makes it to the surface of a planet from meteoroids are called meteorites.

2. Meteorites may vary in size from tiny grains to large boulders. One of the largest meteorite found on Earth is the Hoba meteorite from southwest Africa, which weighs roughly 54,000 kg (119,000 pounds).

3. Meteor showers are usually named after a star or constellation which is close to the radiant (the position from which the meteor appears to come).

4. Meteors and meteorites begin as meteoroids, which are little chunks of rock and debris in space.

5. Most meteorites are either iron, stony or stony-iron.

6. Meteorites may look very much like Earth rocks, or they may have a burned appearance. Some may have depressioned (thumbprint-like), roughened or smooth exteriors.

7. Many of the meteor showers are associated with comets. The Leonids are associated with comet Tempel-Tuttle; Aquarids and Orionids with comet Halley, and the Taurids with comet Encke.

8. When comets come around the sun, they leave a dusty trail. Every year the Earth passes through the comet trails, which allows the debris to enter our atmosphere where it burns up and creates fiery and colorful streaks (meteors) in the sky.

9. Leonid MAC (an airborne mission that took flight during the years 1998 – 2002) studied the interaction of meteoroids with the Earth’s atmosphere.

10. Meteoroids, meteors and meteorites cannot support life. However, they may have provided the Earth with a source of amino acids: the building blocks of life.

Shooting stars, or meteors, are bits of interplanetary material falling through Earth’s atmosphere and heated to incandescence by friction. These objects are called meteoroids as they are hurtling through space, becoming meteors for the few seconds they streak across the sky and create glowing trails.

Description: Color image of massive impact crater in the Arizona desert.

Barringer is one of the best-preserved impact craters on Earth.

Description: Color image of meteorite on Martian sands.

An iron meteorite on Mars.

Description: Color image of streaks of light against the night sky.

A burst of Perseid meteors.

Scientists estimate that 44 tonnes (44,000 kilograms, about 48.5 tons) of meteoritic material falls on the Earth each day. Several meteors per hour can usually be seen on any given night. Sometimes the number increases dramatically – these events are termed meteor showers. Some occur annually or at regular intervals as the Earth passes through the trail of dusty debris left by a comet. Meteor showers are usually named after a star or constellation that is close to where the meteors appear in the sky. Perhaps the most famous are the Perseids, which peak around 12 August every year. Every Perseid meteor is a tiny piece of the comet Swift-Tuttle, which swings by the Sun every 135 years. Other meteor showers and their associated comets are the Leonids (Tempel-Tuttle), the Aquarids and Orionids (Halley), and the Taurids (Encke). Most comet dust in meteor showers burns up in the atmosphere before reaching the ground; some dust is captured by high-altitude aircraft and analyzed in NASA laboratories.

Chunks of rock and metal from asteroids and other planetary bodies that survive their journey through the atmosphere and fall to the ground are called meteorites. Most meteorites found on Earth are pebble to fist size, but some are larger than a build-ing. Early Earth experienced many large meteorite impacts that caused extensive destruction.

One of the most intact impact craters is the Barringer Meteorite Crater in Arizona, about 1 kilometer (0.6 mile) across, formed by the impact of a piece of iron-nickel metal approximately 50 meters (164 feet) in diameter. It is only 50,000 years old and so well preserved that it has been used to study impact processes. Since this feature was recognized as an impact crater in the 1920s, about 170 impact craters have been identified on Earth.

A very large asteroid impact 65 million years ago, which created the 300-kilometer-wide (180-mile-wide) Chicxulub crater on the Yucatan Peninsula, is thought to have contributed to the extinction of about 75 percent of marine and land animals on Earth at the time, including the dinosaurs.

Well-documented stories of meteorite-caused injury or death are rare. In the first known case of an extraterrestrial object to have injured a human being in the U.S., Ann Hodges of Sylacauga, Alabama, was severely bruised by a 3.6-kilogram (8-pound) stony meteorite that crashed through her roof in November 1954.

Meteorites may resemble Earth rocks, but they usually have a burnedexterior. This fusion crust is formed as the meteorite is melted by friction as it passes through the atmosphere. There are three major types of meteorites: the “irons,” the “stones,” and the stony-irons. Although the majority of meteorites that fall to Earth are stony, more of the meteorites that are discovered long after they fall are irons – these heavy objects are easier to distinguish from Earth rocks than stony meteorites. Meteorites also fall on other solar system bodies. Mars Exploration Rover Opportunity found the first meteorite of any type on another planet when it discovered an iron-nickel meteorite about the size of a basketball on Mars in 2005, and then found a much larger and heavier iron-nickel meteorite in 2009 in the same region. In all, Opportunity has discovered six meteorites during its travels on Mars.

More than 50,000 meteorites have been found on Earth. Of these, 99.8 percent come from asteroids. Evidence for an asteroid origin includes orbits calculated from photographic observations of meteorite falls projected back to the asteroid belt; spectra of several classes of meteorites match those of some asteroid classes; and they are very old, 4.5 to 4.6 billion years. However, we can only match one group of meteorites to a specific asteroid – the eucrite, diogenite, and howardite igneous meteorites come from the third-largest asteroid, Vesta. Asteroids and the meteorites that fall to Earth are not pieces of a planet that broke apart, but instead are the original diverse materials from which the planets formed. The study of meteorites tells us much about the earliest conditions and processes during the formation and earliest history of the solar system, such as the age and composition of solids, the nature of the organic matter, the temperatures achieved at the surface and interiors of asteroids, and the degree to which materials were shocked by impacts.

The remaining 0.2 percent of meteorites is split roughly equally between meteorites from Mars and the moon. The over 60 known martian meteorites were blasted off Mars by meteoroid impacts. All are igneous rocks crystallized from magma. The rocks are very much like Earth rocks with some distinctive com-positions that indicate martian origin. The nearly 80 lunar meteorites are similar in mineralogy and composition to Apollo mission moon rocks, but distinct enough to show that they have come from other parts of the moon. Studies of lunar and martian meteorites complement studies of Apollo Moon rocks and the robotic exploration of Mars.

Important Dates:

4.55 billion years ago: Formation age of most meteorites, taken to be the age of the solar system.

65 million years ago: Chicxulub impact leads to the death of 75 percent of the animals on Earth, including the dinosaurs.

50,000 years: Age of Barringer Meteorite Crater in Arizona.

1478 BCE: First recorded observation of meteors.

1794: Ernst Friedrich Chladni publishes the first book on meteorites, in which he proposes that they have an extraterrestrial origin.

1908: (Tunguska), 1947 (Sikote Alin), 1969 (Allende and Murchison), 1976 (Jilin) – Important 20th-century meteorite falls.

1969: Discovery of meteorites in a small area of Antarctica leads to annual expeditions by U.S. and Japanese teams.

1982-1983: Meteorites from the moon and Mars are identified in Antarctic collections.

1996: A team of NASA scientists suggests that martian mete-orite ALH84001 may contain evidence of microfossils from Mars, a still-controversial claim.

2005: NASA’s Mars Exploration Rover Opportunity finds a basketball-size iron-nickel meteorite on Mars.

2009 : Opportunity finds another iron-nickel meteorite on Mars.


Jupiter: Overview

Description: A true-color image of Jupiter taken by the Cassini spacecraft. The Galilean moon Europa casts a shadow on the planet's cloud tops.

The Galilean moon Europa casts a shadow on the planet’s cloud tops.

The most massive planet in our solar system — with dozens of moons and an enormous magnetic field — Jupiter forms a kind of miniature solar system. It resembles a star in composition, but did not grow big enough to ignite. The planet’s swirling cloud stripes are punctuated by massive storms such as the Great Red Spot, which has raged for hundreds of years.

10 Need-to-Know Things About Jupiter:

1. If the sun were as tall as a typical front door, the Earth would be the size of a nickel and Jupiter would be about as big as a basketball.

Description: Link to Juno mission profile

Juno: Next generation Jupiter explorer.

2. Jupiter orbits our sun, a star. Jupiter is the fifth planet from the sun at a distance of about 778 million km (484 million miles) or 5.2 AU.

3. One day on Jupiter takes about 10 hours (the time it takes for Jupiter to rotate or spin once). Jupiter makes a complete orbit around the sun (a year in Jovian time) in about 12 Earth years (4,333 Earth days).

4. Jupiter is a gas-giant planet and therefore does not have a solid surface. However, it is predicted that Jupiter has an inner, solid core about the size of the Earth.

5. Jupiter’s atmosphere is made up mostly of hydrogen (H2) and helium (He).

6. Jupiter has 50 known moons, with an additional 17 moons awaiting confirmation of their discovery — that is a total of 67 moons.

7. Jupiter has a faint ring system that was discovered in 1979 by the Voyager 2 mission.

8. Many missions have visited Jupiter and its system of moons. The Juno mission will arrive at Jupiter in 2016.

9. Jupiter cannot support life as we know it. However, some of Jupiter’s moons have oceans underneath their crusts that might support life.

10. Jupiter’s Great Red Spot is a gigantic storm (about the size of two to three Earths) that has been raging for hundreds of years.

Jupiter is the largest and most massive planet in our solar system, containing more than twice the amount of material of the other bodies orbiting our sun combined. Most of the material left over after the formation of the sun went to Jupiter, forming a type of planet called a gas giant.

Description: Image of dark marks on the clouds of Jupiter.

Dark spots mark impact sites of comet fragments on Jupiter.

Description: Color image of Io in front of Jupiter.

A montage of New Horizons images of Jupiter and its volcanic moon Io.

Description: Color image of Jupiter's rings

Jupiter’s faint ring system.

Jupiter’s appearance is a tapestry of colorful cloud bands and spots. Most visible clouds are composed of ammonia and ammonia compounds, with unknown chemicals providing color. Jupiter’s fast rotation – spinning once every 10 hours – creates strong jet streams, smearing its clouds into bands across the planet.

With no solid surface to slow them down, Jupiter’s spots can persist for many years. The Great Red Spot, a swirling oval of clouds twice as wide as Earth, has been observed on the giant planet for more than 300 years. More recently, three smaller ovals merged to form the Little Red Spot, about half the size of its larger cousin. Scientists do not yet know if these ovals and planet-circling bands are shallow or deeply rooted to the interior.

The composition of Jupiter’s atmosphere is similar to that of the sun — mostly hydrogen and helium. Deep in the atmosphere, pressure and temperature increase, compressing the hydrogen gas into a liquid. This gives Jupiter the largest ocean in the solar system – an ocean made of hydrogen instead of water. Scientists think that, at depths perhaps halfway to the planet’s center, the pressure becomes so great that electrons are squeezed off the hydrogen atoms, making the liquid electrically conducting. Jupiter’s fast rotation is thought to drive electrical currents in this region, generating the planet’s powerful magnetic field. It is still unclear if, deeper down, Jupiter has a central core of solid material.

The Jovian magnetosphere is the region of space influenced by Jupiter’s powerful magnetic field. It balloons 1 to 3 million kilometers (600,000 to 2 million miles) toward the sun and tapers into a windsock-shaped tail extending more than 1 billion kilometers (600 million miles) behind Jupiter, as far as Saturn’s orbit. The magnetic field rotates with the planet and sweeps up particles that have an electric charge. Near the planet, the magnetic field traps a swarm of charged particles and accelerates them to very high energies, creating intense radiation that bombards the innermost moons and can damage spacecraft.

With four large moons and many smaller moons, Jupiter forms a kind of miniature solar system. Newly discovered moons of Jupiter are reported by astronomers and acknowledged with a temporary designation by the International Astronomical Union; once their orbits are confirmed, they are included in Jupiter’s large moon count. Not including the “temporary” moons, Jupiter has 50 total.

Jupiter’s four largest moons – Io, Europa, Ganymede, and Callisto – were first observed by the astronomer Galileo Galilei in 1610 using an early version of the telescope. These four moons are known today as the Galilean satellites. Galileo would be astonished at what we have learned about these moons, largely from the NASA mission named for him: Io is the most volcanically active body in the solar system; Ganymede is the largest moon in the solar system and the only moon known to have its own magnetic field; and a liquid-water ocean with the ingredients for life may lie beneath the frozen crust of Europa, making it a tempting place to explore.

Discovered in 1979 by NASA’s Voyager 1 spacecraft, Jupiter’s rings were a surprise, as they are composed of small, dark particles and are difficult (but not impossible) to see except when backlit by the sun. Data from the Galileo spacecraft indicate that Jupiter’s ring system may be formed by dust kicked up as interplanetary meteoroids smash into the giant planet’s small innermost moons.

In December 1995, NASA’s Galileo spacecraft dropped a probe into one of the dry, hot spots of Jupiter’s atmosphere. The probe made the first direct measurements of the planet’s composition and winds. Galileo studied Jupiter and its largest moons until 2003. Beginning in 2016, NASA’s Juno spacecraft will conduct an in-depth investigation of the planet’s atmosphere, deep structure, and magnetosphere for clues to its origin and evolution.

How Jupiter Got its Name:

The largest and most massive of the planets was named Zeus by the Greeks and Jupiter by the Romans; he was the most important deity in both pantheons.

Important Dates:

1610: Galileo Galilei makes the first detailed observations of Jupiter.

1973: Pioneer 10 becomes the first spacecraft to cross the asteroid belt and fly past Jupiter.

1979: Voyager 1 and 2 discover Jupiter’s faint rings, several new moons and volcanic activity on Io’s surface.

1994: Astronomers observe as pieces of comet Shoemaker-Levy 9 collide with Jupiter’s southern hemisphere.

1995-2003: The Galileo spacecraft drops a probe into Jupiter’s atmosphere and conducts extended observations of Jupiter and its moons and rings.

2007: Images taken by NASA’s New Horizons spacecraft, on the way to Pluto, show new perspectives on Jupiter’s atmospheric storms, the rings, volcanic Io, and icy Europa.

2009: On 20 July, almost exactly 15 years after fragments of comet Shoemaker-Levy slammed into Jupiter, a comet or asteroid crashes into the giant planet’s southern hemisphere.

2011: Juno launches to examine Jupiter’s chemistry, atmosphere, interior structure, and magnetosphere.


Saturn: Overview

Description: Color image show part of Saturn and its rings and Earth as a pale blue dot in the background.

In this rare image taken on 19 July 2013, the wide-angle camera on NASA’s Cassini spacecraft has captured Saturn’s rings and our planet Earth and its moon in the same frame.

Adorned with thousands of beautiful ringlets, Saturn is unique among the planets. All four gas giant planets have rings — made of chunks of ice and rock — but none are as spectacular or as complicated as Saturn’s. Like the other gas giants, Saturn is mostly a massive ball of hydrogen and helium.

10 Need-to-Know Things About Saturn:

1. If the sun were as tall as a typical front door, the Earth would be the size of a nickel and Saturn would be about as big as a basketball.

Description: Link to Cassini mission

Cassini: Long-lived Saturn Explorer

2. Saturn orbits our sun, a star. Saturn is the sixth planet from the sun at a distance of about 1.4 billion km (886 million miles) or 9.5 AU.

3. One day on Saturn takes 10.7 hours (the time it takes for Saturn to rotate or spin once). Saturn makes a complete orbit around the sun (a year in Saturnian time) in 29 Earth years.

4. Saturn is a gas-giant planet and does not have a solid surface.

5. Saturn’s atmosphere is made up mostly of hydrogen (H2) and helium (He).

6. Saturn has 53 known moons with an additional 9 moons awaiting confirmation of their discovery.

7. Saturn has the most spectacular ring system of all our solar system’s planets. It is made up of seven rings with several gaps and divisions between them.

8. Five missions have been sent to Saturn. Since 2004, Cassini has been exploring Saturn, its moons and rings.

9. Saturn cannot support life as we know it. However, some of Saturn’s moons have conditions that might support life.

10. When Galileo Galilei looked at Saturn through a telescope in the 1600s, he noticed strange objects on each side of the planet and drew in his notes a triple-bodied planet system and then later a planet with arms or handles. Thehandles turned out to be the rings of Saturn.

Saturn was the most distant of the five planets known to the ancients. In 1610, Italian astronomer Galileo Galilei was the first to gaze at Saturn through a telescope. To his surprise, he saw a pair of objects on either side of the planet. He sketched them as separate spheres, thinking that Saturn was triple-bodied. Continuing his observations over the next few years, Galileo drew the lateral bodies as arms or handles attached to Saturn. In 1659, Dutch astronomer Christiaan Huygens, using a more powerful telescope than Galileo’s, proposed that Saturn was surrounded by a thin, flat ring. In 1675, Italian-born astronomer Jean-Dominique Cassini discovered a “division” between what are now called the A and B rings. It is now known that the gravitational influence of Saturn’s moon Mimas is responsible for the Cassini Division, which is 4,800 kilometers (3,000 miles) wide.

Description: Color image of Saturn's rings.

Natural color view of Saturn’s rings.

Description: Color image of small moon in front of Saturn.

Icy Dione enriched by the tranquil gold and blue hues of Saturn

Description: Color image showing top of Saturn.

A bizarre six-sided feature encircling the north pole of Saturn.

Like Jupiter, Saturn is made mostly of hydrogen and helium. Its volume is 755 times greater than that of Earth. Winds in the upper atmosphere reach 500 meters (1,600?feet) per second in the equatorial region. In contrast, the strongest hurricane-force winds on Earth top out at about 110 meters (360 feet) per second. These super-fast winds, combined with heat rising from within the planet’s interior, cause the yellow and gold bands visible in the atmosphere.

In the early 1980s, NASA’s two Voyager spacecraft revealed that Saturn’s rings are made mostly of water ice, and they imaged “braided” rings, ringlets, and “spokes” – dark features in the rings that form and initially circle the planet at different rates from that of the surrounding ring material. Saturn’s ring system extends hundreds of thousands of kilometers from the planet, yet the vertical height is typically about 10 meters (30 feet) in the main rings. During Saturn’s equinox in autumn 2009, when sunlight illuminated the rings edge-on, Cassini spacecraft images showed vertical formations in some of the rings; the particles seem to pile up in bumps or ridges more than 3 kilometers (2 miles) tall.

Saturn’s largest satellite, Titan, is a bit bigger than the planet Mercury. (Titan is the second-largest moon in the solar system; only Jupiter’s moon Ganymede is bigger.) Titan is shrouded in a thick, nitrogen-rich atmosphere that might be similar to what Earth’s was like long ago. Further study of this moon promises to reveal much about planetary formation and, perhaps, about the early days of Earth. Saturn also has many smaller “icy” satellites. From Enceladus, which shows evidence of recent (and ongoing) surface changes, to Iapetus, with one hemisphere darker than asphalt and the other as bright as snow, each of Saturn’s satellites is unique.

At Saturn’s center is a dense core of rock, ice, water, and other compounds made solid by the intense pressure and heat. It is enveloped by liquid metallic hydrogen, inside a layer of liquid hydrogen – similar to Jupiter but considerably smaller. Saturn’s magnetic field is smaller than Jupiter’s but still 578 times as powerful as Earth’s. Saturn, the rings, and many of the satellites lie totally within Saturn’s enormous magnetosphere, the region of space in which the behavior of electrically charged particles is influenced more by Saturn’s magnetic field than by the solar wind. Aurorae occur when charged particles spiral into a planet’s atmosphere along magnetic field lines. On Earth, these charged particles come from the solar wind. Cassini showed that at least some of Saturn’s aurorae are like Jupiter’s and are largely unaffected by the solar wind.

The next chapter in our knowledge of Saturn is being written right now by the Cassini-Huygens mission. The Huygens probe descended through Titan’s atmosphere in January 2005, collecting data on the atmosphere and surface. The Cassini spacecraft, orbiting Saturn since 2004, continues to explore the planet and its moons, rings, and magnetosphere. The Cassini Equinox Mission studied the rings during Saturn’s autumnal equinox, when the Sun was shining directly on the equator, through 2010. Now the spacecraft is seeking to make exciting new discoveries in a second extended mission called the Cassini Solstice Mission, which continues until September 2017.

How Saturn Got its Name:

Saturn is named for the Roman god of agriculture. The Greek equivalent was Cronos, father of Zeus/Jupiter. Other civilizations have given different names to Saturn, which is the farthest planet from Earth that can be observed by the unaided human eye.

Important Dates:

1610: Galileo Galilei reports seeing odd appendages on either side of Saturn; he did not realize he was viewing Saturn’s rings.

1979: Pioneer 11 is the first spacecraft to reach Saturn, flying within 22,000 kilometers (13,700 miles) of the cloud tops.

1981: Using Saturn’s powerful gravity as an interplanetary slingshot, Voyager 2 is placed on a path toward Uranus, then Neptune, then out of the solar system.

1994: The Hubble Space Telescope finds evidence of surface features beneath the hazy atmosphere of Titan.

2004: After a seven-year journey, Cassini-Huygens becomes the first spacecraft to orbit Saturn.

2005: The Huygens probe successfully lands on Titan, returning images of the complex surface.

2005-2008: The Cassini spacecraft continues to return high-resolution images of the Saturn system. Mission discoveries include evidence for liquid hydrocarbon lakes of methane and ethane on Titan, a new radiation belt around Saturn, new rings and moons, and icy jets and geysers at the south polar region of the moon Enceladus.

2008-2010: Cassini’s mission is extended for two years and designated the Cassini Equinox Mission.

2010-2017: Cassini’s mission is extended for seven years and designated the Cassini Solstice Mission.


Uranus: Overview

Description: Color image of Uranus with small moon in front of it.

Uranus’ moon Ariel (white dot) and its shadow (black dot) were caught crossing the face of
Uranus in this Hubble Space Telescope image.

Uranus is the only giant planet whose equator is nearly at right angles to its orbit. A collision with an Earth-sized object may explain the unique tilt. Nearly a twin in size to Neptune, Uranus has more methane in its mainly hydrogen and helium atmosphere than Jupiter or Saturn. Methane gives Uranus its blue tint.

10 Need-to-Know Things About Uranus:

1. If the sun were as tall as a typical front door, Earth would be the size of a nickel and Uranus would be about as big as a baseball.

Description: Color image of planet with faint rings. Links to larger image.

Uranus: The Sideways Planet

2. Uranus orbits our sun, a star. Uranus is the seventh planet from the sun at a distance of about 2.9 billion km (1.8 billion miles) or 19.19 AU.

3. One day on Uranus takes about 17 hours (the time it takes for Uranus to rotate or spin once). Uranus makes a complete orbit around the sun (a year in Uranian time) in about 84 Earth years.

4. Uranus is an ice giant. Most (80 percent or more) of the planet’s mass is made up of a hot dense fluid of “icy” materials – water (H2O), methane (CH4). and ammonia (NH3) – above a small rocky core.

5. Uranus has an atmosphere which is mostly made up of hydrogen (H2) and helium (He), with a small amount of methane (CH4).

6. Uranus has 27 moons. Uranus’ moons are named after characters from the works of William Shakespeare and Alexander Pope.

7. Uranus has faint rings. The inner rings are narrow and dark and the outer rings are brightly colored.

8. Voyager 2 is the only spacecraft to have visited Uranus.

9. Uranus cannot support life as we know it.

10. Like Venus, Uranus has a retrograde rotation (east to west). Unlike any of the other planets, Uranus rotates on its side, which means it spins horizontally.

The first planet found with the aid of a telescope, Uranus was discovered in 1781 by astronomer William Herschel, although he originally thought it was a comet or star. The seventh planet from the Sun is so distant that it takes 84 years to complete one orbit.

Description: Color image of bluish Uranus.

Bands and a new dark spot in Uranus’ atmosphere.

Description: Color image shows Uranus tilted on its side.

Infrared image of the dark side of the rings.

Description: Black and white image shows Uranus and several moons.

Uranus and several of its moons in near-infrared.

Like Venus, Uranus rotates east to west. Uranus’ rotation axis is tilted almost parallel to its orbital plane, so Uranus appears to be rotating on its side. This situation may be the result of a collision with a planet-sized body early in the planet’s history, which apparently radically changed Uranus’ rotation. Because of Uranus’ unusual orientation, the planet experiences extreme variations in sunlight during each 20-year-long season.

Voyager 2, the only spacecraft to visit Uranus, imaged a bland-looking sphere in 1986. When Voyager flew by, the south pole of Uranus pointed almost directly at the sun because Uranus was near its southern summer solstice, with the southern hemisphere bathed in continuous sunlight and the northern hemisphere radiating heat into the blackness of space.

Uranus reached equinox in December 2007, when it was fully illuminated as the sun passed over the planet’s equator. By 2028, the north pole will point directly at the sun, a reversal of the situation when Voyager flew by. Equinox also brings ring-plane crossing, when Uranus’ rings appear to move more and more edge-on as seen from Earth.

The Hubble Space Telescope and the Keck Observatory in Hawaii captured detailed images of Uranus as the planet approached equinox. While Voyager 2 saw only a few discrete clouds, more recent observations reveal that Uranus exhibits dynamic clouds as it approaches equinox, including rapidly evolving bright features and a new Great Dark Spot like those seen on Neptune.

Uranus is one of the two ice giants of the outer solar system (the other is Neptune). The atmosphere is mostly hydrogen and helium, with a small amount of methane and traces of water and ammonia. Uranus gets its blue-green color from methane gas in the atmosphere. Sunlight passes through the atmosphere and is reflected back out by Uranus’ cloud tops. Methane gas absorbs the red portion of the light, resulting in a blue-green color. The bulk (80 percent or more) of the mass of Uranus is contained in an extended liquid core consisting mostly of icy materials (water, methane, and ammonia).

For nearly a quarter of the Uranian year (equal to 84 Earth years), the sun shines directly over each pole, plunging the other half of the planet into a long, dark winter.

While magnetic fields are typically in alignment with a planet’s rotation, Uranus’ magnetic field is tipped over: the magnetic axis is tilted nearly 60 degrees from the planet’s axis of rotation, and is also offset from the center of the planet by one-third of the planet’s radius. The magnetic fields of both Uranus and Neptune are very irregular.

Uranus has two sets of rings. The inner system of nine rings, discovered in 1977, consists mostly of narrow, dark rings. Voyager 2 found two additional inner rings. An outer system of two more-distant rings was discovered in Hubble Space Telescope images in 2003. In 2006, Hubble and Keck observations showed that the outer rings are brightly colored. Uranus has 27 known moons, named for characters from the works of William Shakespeare or Alexander Pope. Miranda is the strangest-looking Uranian moon: its complex surface may indicate partial melting of the interior, with icy material drifting to the surface.

How Uranus Got its Name:

William Herschel tried unsuccessfully to name his discovery Georgian Sidus after King George III; but instead the planet was named for Uranus, the Greek god of the sky.

Important Dates:

1781: Astronomer William Herschel discovers Uranus.

1787-1851: Four Uranian moons are discovered and named Titania, Oberon, Ariel, and Umbriel.

1948: Another moon, Miranda, is discovered.

1977: Scientists discover nine faint rings of Uranus while observing a distant star pass behind the planet.

1986: Voyager 2 discovers 10 moons and two additional rings during its historic flyby.

1997-2005: Astronomers discover more tiny moons.

2003-2005: The Hubble Space Telescope images two delicate rings far from the planet and two new moons.

2007: Uranus reaches equinox.


Neptune: Overview

Description: Voyager 2 captured this image of Neptune in 1989.

Voyager 2 captured this image of Neptune in 1989.

Dark, cold and whipped by supersonic winds, Neptune is the last of the hydrogen and helium gas giants in our solar system. More than 30 times as far from the sun as Earth, the planet takes almost 165 Earth years to orbit our sun. In 2011 Neptune completed its first orbit since its discovery in 1846.

10 Need-to-Know Things About Neptune:

1. If the sun were as tall as a typical front door, the Earth would be the size of a nickel and Neptune would be about as big as a baseball.

Description: Color image of clouds on Neptune.

Vertical relief in Neptune’s bright cloud streaks.

2. Neptune orbits our sun, a star. Neptune is the eighth planet from the sun at a distance of about 4.5 billion km (2.8 billion miles) or 30.07 AU.

3. One day on Neptune takes about 16 hours (the time it takes for Neptune to rotate or spin once). Neptune makes a complete orbit around the sun (a year in Neptunian time) in about 165 Earth years (60,190 Earth days).

4. Neptune is a sister ice giant to Uranus. Neptune is mostly made of a very thick, very hot combination of water (H2O), ammonia (NH3), and methane (CH4) over a possible heavier, approximately Earth-sized, solid core.

5. Neptune’s atmosphere is made up mostly of hydrogen (H2), helium (He) and methane (CH4).

6. Neptune has 13 confirmed moons (and 1 more awaiting official confirmation of discovery). Neptune’s moons are named after various sea gods and nymphs in Greek mythology.

7. Neptune has six rings.

8. Voyager 2 is the only spacecraft to have visited Neptune.

9. Neptune cannot support life as we know it.

10. At times during the course of Neptune’s orbit, dwarf planet Pluto is actually closer to the sun, and us, than Neptune. This is due to the unusual elliptical (egg) shape of Pluto’s orbit.

The ice giant Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky. (Galileo had recorded it as a fixed star during observations with his small telescope in 1612 and 1613.) When Uranus didn’t travel exactly as astronomers expected it to, a French mathematician, Urbain Joseph Le Verrier, proposed the position and mass of another as yet unknown planet that could cause the observed changes to Uranus’ orbit. After being ignored by French astronomers, Le Verrier sent his predictions to Johann Gottfried Galle at the Berlin Observatory, who found Neptune on his first night of searching in 1846. Seventeen days later, its largest moon, Triton, was also discovered.

Description: Color image of clouds on Neptune.

Vertical relief in Neptune’s bright cloud streaks.

Description: Black and white image showing thin rings at Neptune.

Voyager 2 image of Neptune’s rings.

Description: Black and white image showing the crescent slivers of Neptune and Triton.

Dramatic view of the crescents of Neptune and Triton.

Nearly 4.5 billion kilometers (2.8 billion miles) from the Sun, Neptune orbits the Sun once every 165 years. It is invisible to the naked eye because of its extreme distance from Earth. Interestingly, the highly eccentric orbit of the dwarf planet Pluto brings Pluto inside Neptune’s orbit for a 20-year period out of every 248 Earth years. Pluto can never crash into Neptune, though, because for every three laps Neptune takes around the Sun, Pluto makes two. This repeating pattern prevents close approaches of the two bodies.

The main axis of Neptune’s magnetic field is tipped over by about 47 degrees compared with the planet’s rotation axis. Like Uranus, whose magnetic axis is tilted about 60 degrees from the axis of rotation, Neptune’s magnetosphere undergoes wild variations during each rotation because of this misalignment. The magnetic field of Neptune is about 27 times more powerful than that of Earth.

Neptune’s atmosphere extends to great depths, gradually merging into water and other melted ices over a heavier, approximately Earth-size solid core. Neptune’s blue color is the result of methane in the atmosphere. Uranus’ blue-green color is also the result of atmospheric methane, but Neptune is a more vivid, brighter blue, so there must be an unknown component that causes the more intense color.

Despite its great distance and low energy input from the Sun, Neptune’s winds can be three times stronger than Jupiter’s and nine times stronger than Earth’s. In 1989, Voyager 2 tracked a large, oval-shaped, dark storm in Neptune’s southern hemisphere. This “Great Dark Spot” was large enough to contain the entire Earth, spun counterclockwise, and moved westward at almost 1,200 kilometers (750 miles) per hour. Subsequent images taken by the Hubble Space Telescope showed no sign of this Great Dark Spot, but did reveal the appearance and then fading of two other Great Dark Spots over the last decade. Voyager 2 also imaged clouds casting shadows on a lower cloud deck, enabling scientists to visually measure the altitude differences between the upper and lower cloud decks.

Neptune has six known rings. Voyager 2’s observations confirmed that these unusual rings are not uniform but have four thick regions (clumps of dust) called arcs. The rings are thought to be relatively young and short-lived.

Neptune has 13 known moons, six of which were discovered by Voyager 2. A 14th tiny, very dim, moon was discovered in 2013 and awaits official recognition. Triton, Neptune’s largest moon, orbits the planet in the opposite direction compared with the rest of the moons, suggesting that it may have been captured by Neptune in the distant past. Triton is extremely cold – temperatures on its surface are about -235 degrees Celsius (-391 degrees Fahrenheit). Despite this deep freeze at Triton, Voyager 2 discovered geysers spewing icy material upward more than 8 kilometers (5 miles). Triton’s thin atmosphere, also discovered by Voyager, has been detected from Earth several times since, and is growing warmer – although scientists do not yet know why.

How Neptune Got its Name:

Neptune was predicted by John Couch Adams and Urbain Le Verrier. The men independently accounted for the irregularities in the motion of Uranus by correctly predicting the orbital elements of a trans-Uranian planet. Using the predicted parameters of Le Verrier (Adams never published his predictions), Johann Galle discovered the planet in 1846. Galle wanted to name the planet for Le Verrier, but that was not acceptable to the international astronomical community. Instead, this planet is named for the Roman god of the sea.

Important Dates:

1846: Using mathematical calculations, astronomers discover Neptune, increasing the number of known planets to eight. Neptune’s largest moon, Triton, is found the same year.

1984: Astronomers find evidence for the existence of a ring system around Neptune.

1989: Voyager 2 becomes the first and only spacecraft to visit Neptune, passing about 4,800 km (2,983 miles) above the planet’s north pole.

1998: Scientists using telescopes on Earth and in space image Neptune’s rings and ring arcs for the first time.

2003: Using improved observing techniques, astronomers discover five new moons orbiting Neptune.

2005: Scientists using the Keck Observatory image the outer rings and find that some of the ring arcs have deteriorated.

2011: Neptune completes its first 165-year orbit of the sun since its discovery in 1846.

2013: A scientist studying Neptune’s ring arcs in archival Hubble Space Telescope images finds a previously unknown 14th moon of Neptune, provisionally designated S/2004 N 1.


Dwarf Planets: Overview

Description: Illustration showing dwarf planet sizes compared to Earth.

dwarf planets Eris, Pluto and Ceres in comparison to Earth. Pluto’s moon
Charon also is shown.

Dwarf planets are round and orbit the Sun just like the eight major planets. But unlike planets, dwarf planets are not able to clear their orbital path so there are no similar objects at roughly the same distance from the Sun. A dwarf planet is much smaller than a planet (smaller even than Earth’s moon), but it is not a moon. Pluto is the best known of the dwarf planets.

10 Need-to-Know Things About Dwarf Planets:

1. If the sun were as tall as a typical front door, Earth would be the size of a nickel and dwarf planets Pluto and Eris, for example, would each be about the size of the head of a pin.

Description: Color illustration showing how much smaller than Earth and the moon.

Known dwarf planets are smaller than Earth’s moon.

2. Dwarf planets orbit our sun, a star. Most are located in the Kuiper Belt, a region of icy objects beyond the orbit of Neptune. Pluto, one of the largest and most famous dwarf planets, is about 5.9 billion km (3.7 billion miles) or 39.48 AU away from the sun. Dwarf planet Ceres is in the main asteroid belt between Mars and Jupiter.

3. Days and years vary on dwarf planets. One day on Ceres, for example, takes about nine hours (the time it takes for Ceres to rotate or spin once). Ceres makes a complete orbit around the sun (a year in Ceresian time) in about 4.60 Earth years.

4. Dwarf planets are solid rocky and/or icy bodies, The amount of rock vs. ice depends on their location in the solar system.

5. Many, but not all dwarf planets have moons.

6. There are no known rings around dwarf planets.

7. Dwarf planets Pluto and Eris have tenuous (thin) atmospheres that expand when they come closer to the sun and collapse as they move farther away. It is possible dwarf planet Ceres has an atmosphere.

8. The first mission to a dwarf planet is Dawn (to Ceres).

9. Dwarf planets cannot support life as we know it.

10. Pluto was considered a planet until 2006. The discovery of a similar-sized worlds deeper in the distant Kuiper Belt sparked a debate that resulted in a new official definition of a planet that did not include Pluto.


Comets: Overview

Description: Color image of comet against a background of stars.

Comet C/2001 Q4 (NEAT)

Comets are cosmic snowballs of frozen gases, rock and dust roughly the size of a small town. When a comet’s orbit brings it close to the sun, it heats up and spews dust and gases into a giant glowing head larger than most planets. The dust and gases form a tail that stretches away from the sun for millions of kilometers.

10 Need-to-Know Things About Comets:

1. If the sun were as tall as a typical front door, Earth would be the size of a nickel, dwarf planet Pluto would be the size of a head of a pin and the largest Kuiper Belt comet (about 100 km across, which is about one twentieth the size of Pluto) would only be about the size of a grain of dust.

Description: Link to Rosetta mission to a comet

Rosetta: Comet Chaser

2. Short-period comets (comets that orbit the sun in less than 200 years) reside in the icy region known as the Kuiper Belt beyond the orbit of Neptune from about 30 to 55 AU. Long-period comets (comets with long, unpredictable orbits) originate in the far-off reaches of the Oort Cloud, which is five thousand to 100 thousand AUs from the sun.

3. Days on comets vary. One day on comet Halley varies between 2.2 to 7.4 Earth days (the time it takes for comet Halley to rotate or spin once). Comet Halley makes a complete orbit around the sun (a year in this comet’s time) in 76 Earth years.

4. Comets are cosmic snowballs of frozen gases, rock and dust.

5. A comet warms up as it nears the sun and develops an atmosphere, or coma. The coma may be hundreds of thousands of kilometers in diameter.

6. Comets do not have moons.

7. Comets do not have rings.

8. More than 20 missions have explored comets from a variety of viewpoints.

9. Comets may not be able to support life themselves, but they may have brought water and organic compounds — the building blocks of life — through collisions with Earth and other bodies in our solar system.

10. Comet Halley makes an appearance in the Bayeux Tapestry from the year 1066, which chronicles the overthrow of King Harold by William the Conqueror at the Battle of Hastings.

In the distant past, people were both awed and alarmed by comets, perceiving them as long-haired stars that appeared in the sky unannounced and unpredictably. Chinese astronomers kept extensive records for centuries, including illustrations of characteristic types of comet tails, times of cometary appearances and disappearances, and celestial positions. These historic comet annals have proven to be a valuable resource for later astronomers.

Description: Color image of comet and massive tail over the ocean.

Comet McNaught setting over the Pacific Ocean.

Description: Black and white image of a comet with surface details labeled.

Details of comet Tempel 1.

Description: Color image showing gas and material erupting from comet nucleus.

Comet Halley in 1986.

We now know that comets are leftovers from the dawn of our solar system around 4.6 billion years ago, and consist mostly of ice coated with dark organic material. They have been referred to as “dirty snowballs.” They may yield important clues about the formation of our solar system. Comets may have brought water and organic compounds, the building blocks of life, to the early Earth and other parts of the solar system.

As theorized by astronomer Gerard Kuiper in 1951, a disc-like belt of icy bodies exists beyond Neptune, where a population of dark comets orbits thesun in the realm of Pluto. These icy objects, occasionally pushed by gravity into orbits bringing them closer to the sun, become the so-called short-period comets. Taking less than 200 years to orbit the sun, in many cases their appearance is predictable because they have passed by before. Less predictable are long-period comets, many of which arrive from a region called the Oort Cloud about 100,000 astronomical units (that is,?100,000 times the distance between Earth and the Sun) from the Sun. These Oort Cloud comets can take as long as 30 million years to complete one trip around the Sun.

Each comet has a tiny frozen part, called a nucleus, often no larger than a few kilometers across. The nucleus contains icy chunks, frozen gases with bits of embedded dust. A comet warms up as it nears the Sun and develops an atmosphere, or coma. The sun’s heat causes the comet’s ices to change to gases so the coma gets larger. The coma may extend hundreds of thousands of kilometers. The pressure of sunlight and high-speed solar particles (solar wind) can blow the coma dust and gas away from the Ssun, sometimes forming a long, bright tail. Comets actually have two tails – a dust tail and an ion (gas) tail.

Most comets travel a safe distance from the sun – comet Halley comes no closer than 89 million kilometers (55 million miles). However, some comets, called sungrazers, crash straight into the Sun or get so close that they break up and evaporate.

Scientists have long wanted to study comets in some detail, tantalized by the few 1986 images of comet Halley’s nucleus. NASA’s Deep Space 1spacecraft flew by comet Borrelly in 2001 and photographed its nucleus, which is about 8 kilometers (5 miles) long.

NASA’s Stardust mission successfully flew within 236 kilometers (147 miles) of the nucleus of Comet Wild 2 in January 2004, collecting cometary particles and interstellar dust for a sample return to Earth in 2006. The photographs taken during this close flyby of a comet nucleus show jets of dust and a rugged, textured surface. Analysis of the Stardust samples suggests that comets may be more complex than originally thought. Minerals formed near the Sun or other stars were found in the samples, suggesting that materials from the inner regions of the solar system traveled to the outer regions where comets formed.

Another NASA mission, Deep Impact, consisted of a flyby spacecraft and an impactor. In July 2005, the impactor was released into the path of the nucleus of comet Tempel 1 in a planned collision, which vaporized the impactor and ejected massive amounts of fine, powdery material from beneath the comet’s surface. En route to impact, the impactor camera imaged the comet in increasing detail. Two cameras and a spectrometer on the flyby spacecraft recorded the dramatic excavation that helped determine the interior composition and structure of the nucleus.

After their successful primary missions, the Deep Impact spacecraft and the Stardust spacecraft were still healthy and were retargeted for additional cometary flybys. Deep Impact’s mission, EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation), comprised two projects: the Deep Impact Extended Investigation (DIXI), which encountered comet Hartley 2 in November 2010, and the Extrasolar Planet Observation and Characterization (EPOCh) investigation, which searched for Earth-size planets around other stars on route to Hartley 2. NASA returned to comet Tempel 1 in 2011, when the Stardust New Exploration of Tempel 1 (NExT) mission observed changes in the nucleus since Deep Impact’s 2005 encounter.

How Comets Get Their Names:

Comet naming can be complicated. Comets are generally named for their discoverer — either a person or a spacecraft. This International Astronomical Union guideline was developed only in the last century. For example, comet Shoemaker-Levy 9 was so named because it was the ninth short-periodic comet discovered by Eugene and Carolyn Shoemaker and David Levy. Since spacecraft are very effective at spotting comets many comets have LINEAR, SOHO or WISE in their names.

Important Dates:

1070-1080: The comet later designated Halley’s Comet is pictured in the Bayeux Tapestry, a chronicle of the Battle of Hastings of 1066.

1449-1450: Astronomers make one of the first known efforts to record the paths of comets across the night sky.

1705: Edmond Halley publishes that the comets of 1531, 1607, and 1682 are the same object and predicts its return in 1758. The comet arrives on schedule and is later named Halley’s Comet.

1986: An international fleet of five spacecraft converges on comet Halley as it makes its regular (about every 76 years) pass through the inner solar system.

1994: In the first observed planetary impact by a comet, awed scientists watch as fragments of comet Shoemaker-Levy 9 smash into Jupiter’s atmosphere.

2001: Deep Space 1 flies by and photographs comet Borrelly.

2004: NASA’s Stardust spacecraft collects dust samples from comet Wild 2 and images the nucleus.

2005: The Deep Impact impactor collides with comet Tempel 1 to enable scientists to study the interior of the nucleus.

2006: The Stardust sample return capsule lands in Utah carrying cometary particles and interstellar dust.

2009: Scientists announce that the amino acid glycine, a building block of life, was collected by the Stardust spacecraft from comet Wild 2.

2010: The Deep Impact spacecraft studies its second cometary target, Hartley 2, a small, hyperactive comet.

2011: The Stardust spacecraft encounters Tempel 1 and captures views of the Deep Impact impact site, the opposite side of the nucleus, and evolution on the comet’s surface.


Kuiper Belt & Oort Cloud: Overview

Description: Artist's concept of Eris and its moon. The sun is in the distance.

Eris and its moon. The sun is in the distance.

The Kuiper Belt is a disc-shaped region of icy objects beyond the orbit of Neptune — billions of kilometers from our sun. Pluto and Eris are the best known of these icy worlds. There may be hundreds more of these ice dwarfs out there. The Kuiper Belt and even more distant Oort Cloud are believed to be the home of comets that orbit our sun.

10 Need-To-Know Things About the Regions Beyond Neptune:

1. The Kuiper Belt and the Oort Cloud are regions of space. The known icy worlds and comets in both regions are much smaller than Earth’s moon.

Description: Color illustration of New Horizons at Pluto. Links to mission page.

New Horizons: Kuiper Belt Explorer

2. The Kuiper Belt and the Oort Cloud surround our sun, a star. The Kuiper Belt is a doughnut-shaped ring, extending just beyond the orbit of Neptune from about 30 to 55 AU. The Oort Cloud is a spherical shell, occupying space at a distance between five thousand and 100 thousand AU.

3. Long-period comets (which take more than 200 years to orbit the sun) come from the Oort Cloud. Short-period comets (which take less than 200 years to orbit the Sun) originate in the Kuiper Belt.

4. There may be are hundreds of thousands of icy bodies larger than 100 km (62 miles) and an estimated trillion or more comets within the Kuiper Belt. The Oort Cloud may contain more than a trillion icy bodies.

5. Some dwarf planets within the Kuiper Belt have thin atmospheres that collapse when their orbit carries them farthest from the sun.

6. Several dwarf planets in the Kuiper Belt have tiny moons.

7. The are no known rings around worlds in either region of space.

8. The first mission to the Kuiper Belt is New Horizons. New Horizons will reach Pluto in 2015.

9. Neither region of space is capable of supporting life as we know it.

10. Both the Kuiper Belt and the Oort Cloud are named for the astronomers who predicted their existence during the 1950s: Gerard Kuiper and Jan Oort.

In 1950, Dutch astronomer Jan Oort proposed that certain comets come from a vast, extremely distant, spherical shell of icy bodies surrounding the solar system. This giant swarm of objects is now named the Oort Cloud, occupying space at a distance between 5,000 and 100,000 astronomical units. (One astronomical unit, or AU, is the mean distance of Earth from the sun: about 150 million km or 93 million miles.) The outer extent of the Oort Cloud is believed to be in the region of space where the sun’s gravitational influence is weaker than the influence of nearby stars.

The Oort Cloud probably contains 0.1 to 2 trillion icy bodies in solar orbit. Occasionally, giant molecular clouds, stars passing nearby, or tidal interactions with the Milky Way’s disc disturb the orbits of some of these bodies in the outer region of the Oort Cloud, causing the object to fall into the inner solar system as a so-called long-period comet. These comets have very large, eccentric orbits and take thousands of years to circle the sun. In recorded history, they are observed in the inner solar system only once.

In contrast, short-period comets take less than 200 years to orbit the sun and they travel approximately in the plane in which most of the planets orbit. They are presumed to come from a disc-shaped region beyond Neptune called the Kuiper Belt, named for astronomer Gerard Kuiper. (It is sometimes called the Edgeworth-Kuiper Belt, recognizing the independent and earlier discussion by Kenneth Edgeworth.) The objects in the Oort Cloud and in the Kuiper Belt are presumed to be remnants from the formation of the solar system about 4.6 billion years ago.

The Kuiper Belt extends from about 30 to 55 AU and is probably populated with hundreds of thousands of icy bodies larger than 100 km (62 miles) across and an estimated trillion or more comets.

Description: Color image of Eris and it's moon Dysnomia.

The discovery of Eris — which is similar in size to Pluto — caused scientists to
reconsider the definition of a planet.

In 1992, astronomers detected a faint speck of light from an object about 42 AU from the sun — the first time a Kuiper Belt object (or KBO for short) had been sighted. More than 1,300 KBOs have been identified since 1992. (They are sometimes called Edgeworth-Kuiper Belt objects, and they are sometimes called transneptunian objects or TNOs for short.)

Because KBOs are so distant, their sizes are difficult to measure. The calculated diameter of a KBO depends on assumptions about how reflective the object’s surface is. With infrared observations by the Spitzer Space Telescope, most of the largest KBOs have known sizes.

One of the most unusual KBOs is Haumea, which is a part of a collisional family orbiting the sun. The parent body, Haumea, apparently collided with another object that was roughly half its size. The impact blasted large icy chunks away and sent Haumea reeling, causing it to spin end-over-end every four hours. It spins so fast that it has pulled itself into the shape of a squashed American football. Haumea and two small moons — Hi’iaka and Namaka — make up the family.

In March 2004, a team of astronomers announced the discovery of a planet-like transneptunian object orbiting the sun at an extreme distance, in one of the coldest known regions of our solar system. The object (2003VB12), since named Sedna for an Inuit goddess who lives at the bottom of the frigid Arctic ocean, approaches the sun only briefly during its 10,500-year solar orbit. It never enters the Kuiper Belt, whose outer boundary region lies at about 55 AU — instead, Sedna travels in a long, elliptical orbit between 76 and nearly 1,000 AU from the sun. Since Sedna’s orbit takes it to such an extreme distance, its discoverers have suggested that it is the first observed body belonging to the inner Oort Cloud.

In July 2005, a team of scientists announced the discovery of a KBO that was initially thought to be about 10 percent larger than Pluto. The object, temporarily designated 2003UB313 and later named Eris, orbits the sun about once every 560 years, its distance varying from about 38 to 98 AU. (For comparison, Pluto travels from 29 to 49 AU in its solar orbit.) Eris has a small moon named Dysnomia. More recent measurements show it to be slightly smaller than Pluto.

The discovery of Eris — orbiting the sun and similar in size to Pluto (which was then designated the ninth planet) — forced astronomers to consider whether Eris should be classified as the tenth planet. Instead, in 2006, the International Astronomical Union created a new class of objects called dwarf planets, and placed Pluto, Eris and the asteroid Ceres in this category.

While no spacecraft has yet traveled to the Kuiper Belt, NASA’s New Horizons spacecraft is scheduled to arrive at Pluto in 2015. The New Horizons mission team hopes to study one or more KBOs after its Pluto mission is complete.

How the Kuiper Belt and Oort Cloud Got Their Names:

Both distant regions are named for the astronomers who predicted their existence — Gerard Kuiper and Jan Oort. Objects discovered in the Kuiper Belt get their names from diverse mythologies. Eris is named for the Greek goddess of discord and strife. Haumea is named for a Hawaiian goddess of fertility and childbirth. Comets from both regions are generally named for the person who discovered them.

Important Dates:

1943: Astronomer Kenneth Edgeworth suggests that a reservoir of comets and larger bodies resides beyond the planets.

1950: Astronomer Jan Oort theorizes that a vast population of comets may exist in a huge cloud on the distant edges of our solar system.

1951: Astronomer Gerard Kuiper predicts the existence of a belt of icy objects just beyond the orbit of Neptune.

1992: After five years of searching, astronomers David Jewitt and Jane Luu discover the first KBO, 1992QB1.

2002: Scientists using the 48-inch Oschin telescope at Palomar Observatory find Quaoar, the first large KBO hundreds of kilometers in diameter. This object was photographed in 1980, but was not noticed in those images.

2004: Astronomers using the 48-inch Oschin telescope announce the discovery of Sedna (2003VB12).

2005:Astronomers announce the discovery of 2003UB313. This object, later named Eris, is slightly larger than Pluto.

2008: The Kuiper Belt object provisionally known as 2005FY9 (“Easterbunny”) is recognized in July as a dwarf planet and named Makemake (pronounced MAHkeh-MAHkeh) after the Polynesian (Rapa Nui) creation god. In September, 2003EL61 (“Santa”) was designated a dwarf planet and given the name Haumea after the Hawaiian goddess of fertility and childbirth.


Beyond Our Solar System: Overview

Description: Annotated image of the Milky Way galaxy.

There are billions of stars and countless solar systems in our galaxy.

Our star and its planets are a tiny part of the Milky Way galaxy. The Milky Way is a huge city of stars, so big that even at the speed of light, it would take 100,000 years to cross it. All the stars in the night sky, including our sun, are just some of the residents of this galaxy. Beyond our own galaxy lies a vast expanse of galaxies.

10 Need-to-Know Things About the Universe:

1. Our universe is expanding. About 14 billion years ago, scientists believe the universe was compressed into a single point in space.

Description: Link to Voyager 1 interstellar mission.

Voyager 1: First to Interstellar Space.

2. There are at least 100,000,000,000 galaxies in the universe. A galaxy is full of stars: Our sun is just one of at least 100,000,000,000 stars in our own Milky Way galaxy, and each of those stars could have their own planetary system.

3. About 68 percent of the universe is made of dark energy. Dark matter makes up about 27 percent. The rest – everything on Earth, everything ever observed with all of our instruments, all normal matter adds up to less than 5 percent of the universe.

4. We now know that our universe has a foamy structure. The galaxies that comprise the observable universe are concentrated in vast sheets and filaments surrounding enormous cosmic voids.

5. The Milky Way galaxy is in the Local Group, a neighborhood of about 30 galaxies. Our nearest major neighboring galaxy is called Andromeda.

6. More than 1,700 extrasolar planets (or exoplanets) planets have been confirmed. There are thousands of potential exoplanets await discovery confirmation.

7. Other planetary systems could have the potential for life, but no signs have yet been found beyond Earth.

8. Two-thirds of the galaxies within the Universe are spiral-shaped like our Milky Way galaxy. The have elliptical shapes, and a few have unusual shapes like toothpicks or rings.

9. The Hubble Space Telescope observed a tiny patch of sky (one-tenth the diameter of the moon) for 11.6 days and found approximately 10,000 galaxies of all sizes, shapes and colors.

10. Black holes are not empty spaces in the Universe. A black hole is a great amount of matter packed into a very small area, which results in a gravitational field so strong that nothing — not even light — can escape.

Description: Color image of spiral galaxy.

Spiral galaxy NGC 7331 looks similar to our Milky Way.

Description: Color illustration of cosmic dust cloud.

gas pillars in the Eagle Nebula (M16).

Description: Color image of distant star.

The first planet discovered beyond our solar system orbits the star 51 Pegasi.

Description: Color image showing hundres of galaxies in a single frame.

Several hundred galaxies are visible in this image.

Our sun is one of at least 100 billion stars in the Milky Way, a spiral galaxy about 100,000 light years across. The stars are arranged in a pinwheel pattern with four major arms, and we live about two-thirds of the way up one of them. Many if not most of the stars host their own families of planets. More than a thousand of these extrasolar (or exoplanets) have been discovered and thousands more are awaiting confirmation.

All of the stars in the Milky Way orbit a supermassive black hole at the galaxy’s center, which is estimated to be some 4 million times as massive as our sun. Fortunately, it is a safe distance of around 28,000 light years away Earth. The Milky Way zips alonga galactic orbit at an average speed of about 514,000 miles per hour (828,000 km/hr). It takes about 230 million years for our solar system to make one revolution around the galactic center.

Beyond Our Galaxy

The Milky Way is part of the Local Group, a neighborhood about 10 million light years across, consisting of more than 30 galaxies that are gravitationally bound to each other. Aside from our galaxy, the most massive one in this group is Andromeda, which appears to be on course to collide with the Milky Way in about 4 billion years.

Scientists studying galaxies observed that the stars in the outer parts are orbiting the galactic centers just as quickly as the stars further in, a violation of Newton’s well-established laws of gravitation. They deduced that something other than the stars and clouds of gas and dust known to comprise galaxies was providing extra gravity –

lots of it. They calculated that there must be five times as much of this mysterious dark matter, detectable only by its gravitational pull, as there is of the matter we already knew about.

The Local Group is only one of many, many clusters of galaxies, and they are all moving away from each other as more and more space comes into being between them. This means the universe, itself, is expanding. That discovery is what led to the theory of the Big Bang origin of the universe.

Scientists expected that the gravitational attraction of everything in the universe would put the brakes on the rate of expansion, and eventually the expansion would stop or even reverse. But in the 1990s, scientists discovered that the expansion is actually getting faster. The force responsible for this surprising acceleration was dubbed dark energy. No one is sure what it is, but one possibility is that it is energy contained within the very vacuum of space.

Since matter and energy are equivalent (as expressed in Einstein’s famous equation, E=MC2) scientists have been able to calculate that whatever dark energy is, it comprises about 68 percent of everything in the universe. Dark matter accounts for another 27 percent, leaving only five percent for protons, neutrons, electrons and photons – in other words, everything we see and understand.

Scientists calculate that there are at least 100 billion galaxies in the observable universe, each one brimming with stars. On a very large scale, they form a bubbly structure, in which vast sheets and filaments of galaxies surround gargantuan voids.


1000 Space-Astronomy Words Dictionary

abiotic : abiotic describes conditions and factors that are the nonliving elements of an ecosystem. examples of abiotic factors include climate, air currents, temperature, moisture, light, and soil type.

ablation : a process by where the atmosphere melts away and removes the surface material of an incoming meteorite.

abort motor : the abort motor generates 1,800,000 newtons (400,000 pounds) of thrust in a fraction of a second to rapidly move the crew to safety during a launch pad or in-flight emergency.

absolute magnitude : also known as absolute visual magnitude, relates to measuring a heavenly object’s brightness when viewed from 10 parsec or 32+ light years.

absolute zero : the international community agreed to define absolute zero as equivalent to -273.15°c on the celsius scale or-459.67°f on the fahrenheit scale. it is the theoretical temperature entropy reaches its minimum value.

absorb : absorb means to take in. light energy that is absorbed is not given off, it is taken in by the object that absorbs the light. as a result, the object may become warmer.

absorption lines : are a dark feature in the spectrum of a star formed by cooler gases in a star’s outer layer.

absorption spectrum : an absorption spectrum is a spectrum, broken by a specific pattern of dark lines or bands, observed when light passes through a gas. the absorption pattern is unique and can be used to identify the gas.

acceleration : acceleration is any change in speed or velocity (when an object speeds up, slows down, or changes direction). acceleration can be described as positive or negative (e.g., speeding up is positive acceleration, slowing down is negative acceleration).

accredited : an educational institution which has been recognized as maintaining standards that qualify graduates for admission to higher, or more specialized, institutions or professional practice.

accretion : the process by where dust and gas accumulated into larger bodies such as stars and planets.

accretion disk : a disk of gas that accumulates around a center of gravitational attraction, such as a white dwarf, neutron star, or black hole. as the gas spirals in, it becomes hot and emits light or even x-radiation.

accretion disks : arise when material, usually gases, are transferred from one celestial object to another. there are two places astronomers find accretion disks, binary star systems and galactic nuclei.

achondrite : a stone meteorite that contains no chondrules.

achromatic lens : is a combination of lenses made of different glass. these bring two wavelengths into focus (normally red & blue) on the same plane. achromatic lenses are used to take chromatic aberrations away from images.

active galactic nuclei : is a region in the center of a galaxy that has a higher than normal brightness. it is a class of galaxies that emit a large amount of energy from their center more than ordinary galaxies.

active optics : technology developed in the 80’s for reflecting telescopes. the construction enables telescopes to move 8 meter primary mirrors. as the name suggests, it works by “actively” adjusting the telescope mirrors.

adaptive optics : technology used to improve performance of optical systems through the reduction of rapidly changing optical distortion. it is used to remove atmospheric distortion through the use of astronomical telescopes and laser communication.

aerodynamic heating : aerodynamic heating is the heating of a solid body produced as air or other fluid passes over the body.

aerodynamics : aerodynamics is the study of how efficiently air flows around an object.

airfoil : an airfoil is a surface, such as a wing or propeller, designed to aid in lifting and controlling an aircraft by means of air currents.

airy disk : named after george airy, it is the central spot in a diffraction pattern of a stars image in focus in a telescope.

albedo : is the ratio that light is reflected by a planet or satellite to that received by it. it is the ratio of total-reflected light.

albedo feature : a dark or light marking on the surface of an object that may or may not be a geological or topographical feature.

altazimuth mount : is a two-axis mount used to support and rotate an instrument in two common perpendicular axes, vertical (altitude) and horizontal (azimuth).

altitude : is the height of anything above given a planetary reference plane. in astronomy the angular distance of a heavenly body above the horizon.

amplitude : amplitude is a measure of the height of a sound wave which determines the sound’s volume.

anaglyph : is a composite picture printed in two colors to produce a 3d image viewed through eye glasses having lenses of the same colors.

andromeda galaxy : is a spiral galaxy that is nearly two and a half million light years away in the constellation andromeda.

angular size : is the angle between two lines of sight to its two opposite sides. it is a measure of how large an object actually appears to be.

anisotropic : anisotropic materials have a crystalline structure where the arrangement of atoms along one axis is different than that of another axis. optically anisotropic materials rotate polarized light as it passes through them.

anisotropy : is the state of being directionally dependent. the property of being anisotropic and having a different value when measured in different directions.

annular eclipse : a solar eclipse in which the moon covers all but the bright ring around the circumference of the sun. when sun and moon are exactly in line, because the moon is smaller, the sun appears as a bright ring (annulus) surrounding the moon.

anthrosphere : the anthrosphere is the part of the environment that is created or modified by humans for use in human activities and human habitats.

antimatter : is matter composed of anti-particles

antipodal point : a point that is on the direct opposite side of a planet.

apastron : the point of greatest separation of two stars, such as in a binary star system.

aperture : is a hole, gap, or slit and any other small opening. diameter of the objective of a telescope.

aphelion : is the point of orbit of a planet or comet which is farthest away from the sun.

apogee : is the point in the orbit of an object (moon, satellite, etc…) orbiting the earth that is at the greatest distance from the center of the earth.

apparent magnitude : is the measure of brightness of a celestial body as seen from earth as seen without atmosphere.

apparition : is the appearance or time when a comet is visible such as halley’s comet.

Archeoastronomy : the study of how people of the past “understand phenomena in the sky and how those phenomena affect their cultures.” branch of archaeology that deals with use by prehistoric civilizations of astronomical techniques to establish seasons or cycle of the year, as evidenced in megaliths and other ritual structures.

arcminute : is a unit of angular distance equal to a 60th of a degree.

arcsecond : a 60th part of a minute of an arcminute.

arteries : arteries are blood vessels that carry blood away from the heart.

artificial satellite : an artificial satellite is a manufactured object that continuously orbits earth or some other body in space.

asterism : is a group of starts. also a pattern of stars seen from earth which is not part of an established constellation.

asteroid : a rocky space object which can be from a few hundred feet to several hundred km wide. most asteroids in our solar system orbit the sun in a belt between mars and jupiter.

asteroid belt : is the region in space between the orbits of mars and jupiter where most asteroids are located.

asteroids : are any of thousands of smaller bodies or planetoids that orbit around the sun. they range in size from 1.6 miles to 480 miles.

astrochemistry : the branch of science that explores the chemical interactions between dust and gas interspersed between the stars.

astrometry : is the branch of astronomy dealing with the measurement of the positions and motions of heavenly bodies.

astronaut : an astronaut is a person who is trained to travel into space.

astronomer : an expert in the study of the sun, moon, stars, planets, and other space bodies.

astronomical unit : the approximate distance from the sun to the earth which is equal to 150,000,000 kilometers.

astronomical unit (au) : a unit of length which is equal to the mean distance of the earth from the sun.

astronomy : is the science that deals with the material universe beyond the earth’s atmosphere. natural science engaged with the study of celestial objects.

astrophotography : a specialized branch of photography that captures images of astronomical objects and large portions of the night sky.

astrophysics : the branch of astronomy that deals with the physics of stellar phenomena.

atmosphere : a layer of gases surrounding a planet, moon, or star. the earth’s atmosphere is 120 miles thick and is composed mainly of nitrogen, oxygen, carbon dioxide, and a few other trace gases.

atmosphere : the layers of gases which surround a star, like our sun, or a planet, like our earth.

atom : atoms are the smallest part of an element that maintains the chemical properties of that element.

attitude : attitude describes the position of a spacecraft relative to the direction of motion.

attitude control motor : the attitude control motor helps to stabilize and reorient the orion crew module before the crew module is released from the abort system to begin its controlled descent.

aurora : is a radiant emission from the upper atmosphere that occurs intermittently over the middle and high altitudes of both hemispheres. they appear in the form of luminous bands, streamers, or the like. this is caused by the constant bombardment of the atmosphere by charged particles attracted by earth’s magnetic lines.

aurora australis : also known as the southern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the southern hemisphere. it is caused by charged particles from the sun as they interact with the earth’s magnetic field. known as the aurora borealis in the northern hemisphere.

aurora australis or southern lights : are dynamic displays of light that appear in the antarctic skies in winter. they are nature’s light show. it is the name given to light emitted by atoms, molecules, and ions that have been excited by energetic charged particles. common colors are pale green and pink in spiral curtains, arcs and streamers.

aurora borealis : also knows as northern polar lights are natural occurring light display in the heavens in the northern hemisphere. they are nature’s light show. it is the name given to light emitted by atoms, molecules, and ions that have been excited by energetic charged particles. common colors are pale green and pink in spiral curtains, arcs and streamers.

auto guider : is a tool used in astrophotography to track celestial objects that are photographed from drifting away from the field of view.

autumnal equinox : is the time that signals the end of the summer months and the beginning of winter. it is when the sun passes the equator.

averted vision : a technique to view faint objects using peripheral vision. you do not look at the object directly, but just off to the side, you do this while concentrating on the object.

axis : is the line which an object rotates. a straight line about which a body or geometric object rotates or may be conceived to rotate.

axis : an imaginary straight line around which an object rotates.

azimuth : is the arc of the horizon measured clockwise from the south point, in astronomy, or from the north point, in navigation, to the point where a vertical circle through a given heavenly body intersects the horizon.

bacteria : living things that have only one cell and are so small they can only be seen with a microscope.

ballast : material inside a vehicle that give it additional mass for stability or other purposes. this material gives the vehicle more weight, a force pulling down toward the center of the earth. ballast is sometimes designed to be disposable.

bar : a unit of measure of atmospheric pressure. one bar is equal to 0.987 atmospheres, 1.02 kg/cm2, 100 kilopascal, and 14.5 lbs/square inch.

barlow lens named after its creator peter barlow : is a removable lens that can be attached to the eyepiece of a telescope and improves magnification.

barnard’s star : is a red dwarf star that is six light years away from earth. it is also known as “barnard’s runaway star”.

barred spiral galaxy : is a spiral galaxy that has a centric bar-shaped configuration made-up of stars.

Baryon : is a proton, neutron, or any elementary particle that decays into a set of particles that includes a proton.

basalt : a dark, fine-grained volcanic rock.

Baseline : is a line that serves as a basis for measurement, calculation, or location. a line between two points or telescopes of an interferometer.

bernoulli’s principle : bernoulli’s principle states that as the velocity of a fluid (such as air) increases, the pressure exerted by that fluid decreases.

big bang theory : a theory which states that the universe began to expand after a super powerful explosion of concentrated matter and energy.

binary : a system of two stars that revolve around a common center of gravity.

binary star : is a star system composed of two stars that orbit a common center. the primary star is brightest; the secondary is referred to as the companion star.

binoculars : are optical devices providing good depth effect for both eyes. this consists of two small telescopes fitted side by side.

biome : a biome is a major biotic community characterized by distinct climate and dominant forms of flora and fauna.

biosphere : the biosphere is the region of the earth’s surface and atmosphere where living organisms exist.

biosphere 2 : biosphere 2 is a model of earth’s biosphere located north of tucson, az.

birefringence : birefringence is a process where light of different polarizations travels at different speeds in different directions through a transparent medium. birefringence is also called double refraction.

black hole : theoretically a massive object formed at the beginning of the universe or by a gravitational collapse of a star exploding as a supernova. the gravitational field is intense that no electromagnetic radiation can escape.

blazar : is a compact quasar. an active galaxy with very active and highly variable radio, electromagnetic, and optical emissions.

blink comparator : is used by astronomers, it is an optical instrument used to detect differences in two photographs of the same object by rapidly switching between the two, one picture at a time.

blood vessels : blood vessels are tubes that carry blood. they include arteries, veins, and the capillaries that connect them.

blueshift : is a shift toward shorter wavelengths on the spectral lines of a celestial object. this is caused by the movement of the object toward the object.

blur : to make less clear, to run together.

bok globule : is small interstellar clouds of very cold gas and dust that are thick. because of the thickness, they are totally opaque to visible light; however, they can be studied using infrared and radio techniques.

bolide : is a fireball. a large brilliant meteor that explodes.

bolt : a flash of lightning.

bose-einstein condensate : also known as superatom. a phase of matter in which all bosons in a given physical system have been cooled to a temperature near absolute zero and enter the same quantum state.

boulder : a very large piece of rock.

brittle : easy to break or crack.

brown dwarf : is a cold and dark star too small to initiate nuclear reactions that generate heat and light.

buckyball : is a natural occurring type of carbon recognized as c60. the molecular structure looks like the geodesic domes designed by buckminster fuller.

bulge : the generally spherical and central region of a spiral galaxy.

buoyancy : buoyancy is an upward force on an object in a fluid, e.g., when you float in a pool or the ocean or a balloon floats in air

caldera : a type of volcanic crater that is extremely large, usually formed by the collapse of a volcanic cone or by a violent volcanic explosion. crater lake is one example of a caldera on earth.

camber : camber is the difference between the and bottom curves of an airfoil. airplane wings tend to have a longer curve on the upper than the lower wing surfaces (although the opposite is true of supersonic jets).

cannibal coronal mass ejections : are fast moving solar eruptions that overtake and often absorb their slower moving kin.

capillaries : capillaries are the thinnest blood vessels. nutrients and gases can pass through capillary walls. capillaries connect veins and arteries.

carbon dioxide : carbon dioxide is a colorless, odorless, non-poisonous gas that is a normal part of earth’s atmosphere. carbon dioxide is a product of fossil fuel combustion.

carbon star : is a cool, red giant having a spectrum with strong bands of carbon compounds.

carbonaceous chondrites : are recognized as a group of chondritic meteorites composed of at least 7 recognized groups.

cassegrain telescope : is a reflecting telescope in which the light, passing through a central opening in the primary mirror, is brought into focus a short distance behind it by a secondary mirror.

cataclysmic variable : are stars that invariably increase in brightness and decrease to a nearly dormant state.

catadioptric telescope : is a telescope that uses a combination of mirrors and lenses to increase the focal length of the telescope while allowing it to be folded into a more convenient and compact size.

catena : a series or chain of craters.

cavus : a hollow, irregular depression.

celestial equator : an imaginary line that divides the celestial sphere into a northern and southern hemisphere.

celestial pole : are two points in which the extended axis of the earth cuts the celestial sphere and about which the stars seem to revolve.

celestial poles : the north and south poles of the celestial sphere.

celestial sphere : is an imaginary spherical shell formed by the sky represented as an infinite sphere. the observer’s position is the given center of the sphere.

celsius : a scale on a thermometer where the interval between the boiling point and the freezing point of water is divided into 100 degrees. freezing point is represented by 0 degrees and boiling point is represented by 100 degrees. the same scale is also utilized in centigrade.

cepheid variable : is a variable star in which changes in brightness are due to alternate contractions and expansions in volume.

chandrasekhar limit : named after indian astrophysicist subrahmanyan chandrasekhar, is the mass limit above which a star has too much mass to become a white dwarf after gravitational collapse.

chaos : a distinctive area of broken terrain.

charge-coupled device (ccd) : is a silicon chip used to detect light. a more efficient device at collecting light than regular film.

charged particles : electrons, protons, ions.

charles messier : french astronomer recognized for publishing the astronomical catalogue that consist of nebulae, star clusters that later become known as “103 messier objects.”

chasma : another name used to describe a canyon.

chlorophyll : chlorophyll is a green chemical in plant cells that allows plants to use light energy to make food.

chondrite : is a stony meteorite containing chodrules. unchanged meteorites due to melting.

chondrule : small, glassy spheres commonly found in meteorites.

chromosphere : is a layer of the sun’s atmosphere. a gaseous envelope that surrounds the sun outside the photosphere from which large quantities of hydrogen and other gases erupt from.

Circumpolar : circumpolar stars are permanently above the horizon from a given observing point on earth; that is to say, they never set. at earth’s geographical north pole (90° north latitude), all stars in the sky are circumpolar. on earth’s equator, no stars are circumpolar.

circumpolar star : a star that never sets but always stays above the horizon. this depends on the location of the observer. the further south you go the fewer stars will be circumpolar. polaris, the north star, is circumpolar in most of the northern hemisphere.

circumstellar disk : a torus or ring-shaped accumulation of gas, dust, or other debris in orbit around a star in different phases of its life cycle.

clay : clay is soil with very small particles.

climate : climate is the long-term weather pattern of an area, including temperature, precipitation, and wind.

clock drive : is a mechanism that causes an equatorial telescope to revolve about its polar axis so that it keeps the same star in its field of view.

cloud : a cloud is a large collection of very tiny droplets of water or ice crystals in the atmosphere.

coated optics : are optical elements that have refracting and reflecting surfaces coated with one or more coatings of dielectric or metallic material.

collapsar : is a gravitationally collapsed star.

collapse : to fall down or fall to pieces.

Collimation : perfectly aligning a telescope’s optics.

coma : an area of dust or gas surrounding the nucleus of a comet.

coma berenices : is a constellation in the northern sky near boötes and leo that contains a prominent cluster of galaxies and the north pole of the milky way.

comet : frozen masses of gas and dust which have a definite orbit through the solar system.

comet nucleus : the solid, central part of a comet, also known as a “dirty snowball.” it is made of rock, dust, and frozen gases.

composite materials : composite materials are two or more different materials that are combined together. the combined materials do not lose their individual properties. the properties of the product are a combination of the properties of each material.

compression : compression is the process of molecules being pressed closer together.

compression wave : a compression wave is a wave that is propagated by the compression of molecules in a substance.

condensation : condensation is the process of a gas changing to a liquid.

conduction : conduction is the transfer of heat between two solid objects that are touching.

conductors : conductors are materials that easily transfer heat or electricity.

conjunction : a moment when two or more objects appear close together in the sky.

conjunction : an event that occurs when two or more celestial objects appear close close together in the sky.

constellation : is any of various groups of stars to which definite names have been given, as ursa major, ursa minor, boötes, cancer, orion.

constellation : the definition of constellation you may be familiar with is a group of stars that can be connected together to form a pattern such as the ôbig dipper.ö nasaæs mission which includes building a new vehicle, the orion, capable of going to the moon for lunar exploration and research, is called the constellation program.

constraint : any limit or restriction given for the design process is called a constraint.

contracts : when the heart contracts, or gets smaller, the heart muscle is squeezing a larger space within the heart into a smaller space.

controlled descent : a controlled descent is a landing where the speed and direction of the fall is modified.

convection : the transfer of heat by the circulation or movement of heated parts of a liquid or gas.

convection : convection is the transfer of heat between flowing gases or liquids.

coordinate system : a coordinate system is a grid placed on a map to help quickly locate specific locations.

core : the central region of a planet, star, and galaxy.

corona : the outer part of the sun’s atmosphere. the corona is visible from earth during a total solar eclipse. it is the bright glow seen in most solar eclipse photos.

coronagraph : is an instrument for observing and photographing the sun’s corona, consisting of a telescope fitted with lenses, filters, and diaphragms that simulate an eclipse.

coronagraph mask : is a circular shaped instrument designed to block light from a star’s disk. this allows the area close to the target to be studied.

coronal mass ejection (cme) : is a large-scale solar event involving an ejection of hot plasma that may accelerate charged particles and travel as far as the earth’s orbit, preceded by a shock front that may create a magnetic storm on earth

cosmic microwave background : microwave radiation that permeates the universe and represents the still cooling heat generated in the universe.

cosmic ray : a radiation of high penetrating power that originates in outer space and consists partly of high-energy atomic nuclei.

cosmic string : a tube-like configuration of energy that is believed to have existed in the early universe. a cosmic string would have a thickness smaller than a trillionth of an inch but its length would extend from one end of the visible universe to the other.

cosmogony : the study of celestial systems, including the solar system, stars, galaxies, and galactic clusters.

cosmological : having to do with the study of the history, structure, and changes in the universe.

cosmological constant : is a term introduced by einstein into his field equations of general relativity to permit a stationary, nonexpanding universe

cosmologist : a cosmologist is a scientist or astronomer who studies large scale structures and dynamics of the universe, including the origins of the universe.

cosmology : is a branch of astronomy that deals with the general structure and evolution of the universe.

cosmonaut : an astronaut from the former soviet union or the current republic of russia.

cosmos : the world or universe seen as an orderly, harmonious system.

covalent bond : a covalent bond is a chemical bond formed between two atoms by sharing electrons between the atoms.

crater : a bowl-shaped depression formed by the impact of an asteroid or meteoroid. also the depression around the opening of a volcano.

crater : a hole caused by an object hitting the surface of a planet or moon.

crescent : a lunar or planetary phase wherein less than half the surface is illuminated.

criteria : criteria are rules guiding the design process, such as size, type of material, or dollar limit to build the model.

critical density : the density of a pure element or compound at a critical point. density of the universe that provide enough gravity to bring the expansion to halt.

cross-polarizers : cross-polarizers are created when two pieces of polarizing film are placed at 90 angles to each other.

crust : is the outermost geological thin layer of an asteroid, moon, or planet.

cryosphere : the cryosphere is the part of earth’s system that includes water in its frozen state. earth’s cryosphere includes snow, sea ice, lake ice, glaciers, permafrost, ice caps, and ice sheets.

cryovolcanism : is an icy volcano. when water and other liquids or vapor-phase volatiles, together with gas-driven solid fragments, onto the surface of a planet or moon due to internal heating.

crystal lattice : a crystal lattice is the structure of an ionic solid in which orderly

damocloid : an elliptical shaped asteroid with a comet-like orbit. it is rare and named after asteroid 5335 damocles, the first of its kind discovered.

dark adaptation : the ability of the human eye to adjust seeing dim objects in the dark.

dark energy : negative gravity that plays a role in the acceleration in the expansion of the universe.

dark matter : dark matter is the name given to the amount of mass whose existence is deduced from the analysis of galaxy rotation curves but which until now has escaped all detection. there are many theories about dark matter, but the subject is still a mystery.

dark nebula : dust grains that appear as clouds and is thick enough to shade light from stars in the background.

debris : broken, scattered remains; rubble; pieces of rubbish or litter.

debris disk : a ring-shaped circumstellar disk of dust and debris in orbit around a star. debris disks can be created as the next phase in planetary system development following the protoplanetary disk phase. they can also be formed by collisions between planetesimals.

decaying orbit : a path around an object which decreases in size with time. for example, when a satellite enters a decaying orbit above earth, its orbit size decreases to the point that it enters earth’s atmosphere where it burns up.

decibel : the decibel (db) is a unit of sound intensity

declination : angular distance of an object in the sky, above or below the celestial equator.

decompose : to decompose is to break down. the process of decomposing is the breaking down of dead plants and animals into tiny pieces. when these pieces mix with dirt they form soil.

deep-sky objects : objects that are located beyond the solar system, usually consisting of galaxies, nebulae, stars, and star clusters.

degree angular scale interferometer (dasi) : used to measure temperature and polarization in the cosmic microwave background. this is located at nsf amundsen-scott south pole station.

denison olmsted : american physicist and astronomer born in hartford, connecticut. he is attributed for founding meteor science. he demonstrated that meteors are cosmic in origin and not an atmospheric phenomenon.

density : the density (d) of a material or object is a measure of how tightly the matter within it is packed together, and is given by the ratio of its mass (m) to its volume (v), or d = m/v. it is typically expressed in kilograms per cubic meter (kg/m¦) or grams per cubic centimeter (g/cm¦) or grams per milliliter (g/ml).

deposit : to deposit is to drop off. wind and water often carry sand and mud particles from one place and later deposit them somewhere else. what they drop off is called a deposit (so deposit is both a verb and a noun).

design process : the design process is a series of steps in designing and refining/improving something. steps of the design process 1. identify the problem 2. identify criteria and constraints 3. brainstorm possible solutions 4. generate ideas 5. explore possibilities 6. select a design 7. build a model or prototype 8. refine the design, repeating steps 1-8

detect : to discover something which is hidden or unknown.

deuterium : an isotope of hydrogen with one proton and one neutron in the nucleus having an atomic weight of 2.014.

diffraction : spreading out of light as it passes the edge of an obstacle.

diffraction grating : a diffraction grating is a surface with many closely spaced parallel grooves or splits in it which splits and diffracts light to produce the light’s spectrum.

dirt : dirt is made of small particles formed from the breakdown of rocks.

disk : the surface of the sun or other celestial body projected against the sky.

dobsonian telescope : a telescope with a stable altazimuth mount that rotates easily.

doppler effect : the change in wavelength of sound or light emitted by an object in relation to an observer’s position. an object approaching the observer will have a shorter wavelength (blue) while an object moving away will have a longer (red) wavelength. the doppler effect is used to estimate an object’s speed and direction.

doppler shift : a shift in an object’s spectrum due to a change in the wavelength of light that occurs when an object is moving toward or away from earth.

double asteroid : two asteroids that orbit around each other and linked by the gravity between them.

double star : grouping of two stars. the grouping may look distinct, where the stars appear close together, or physical, such as a binary system.

drag : drag is the resistance on an object to movement through a fluid. it is a force that slows an object down. for example, swimmers and submarines experience drag as they move through water and birds and aircraft experience drag as they move through air.

drop test : an experiment that measures speed, velocity or acceleration of a falling object, or the results of a fall or the impact of the fall, is called a drop test.

dust : minute particles floating in space.

dwarf galaxy : small galaxy that contain a few million stars, it is the most common kind of galaxy in the universe.

dwarf planet : a celestial body orbiting the sun that is massive enough to be rounded by its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite. it has to have sufficient mass to overcome rigid body forces and achieve hydrostatic equilibrium. pluto is considered to be a dwarf planet.

dwarf star : a smaller star. any star of average to low brightness, mass, and size.

earth orbiting system : the earth orbiting system, or eos, is a series of satellites that orbit earth and collect various types of data.

earth system : the earth system is a unified system comprised of six spheres

eccentric : deviation from a circle, applied when describing the shape of an orbit.

eccentricity : the measure of how an object’s orbit differs from a perfect circle. eccentricity defines the shape of an object’s orbit.

eclipse : the total or partial blocking of one celestial body by another.

eclipsing binary : binary star with an orbital plane oriented so that one star passes in front of the other, thus completely or partially blocking the light from the other star during each orbital period.

ecliptic : the great circle formed by the intersection of the plane of the earth’s orbit with the celestial sphere; the apparent annual path of the sun in the heavens.

edwin hubble : american astronomer who pioneered the understanding of the universe. he showed that other galaxies existed, specifically the milky way. born in marshfield, missouri then later moved to chicago at the age of 9. young edwin hubble had always been fascinated with science. he attended oxford university on a rhodes scholarship and studied law. he later realized that his true passion was astronomy; in 1917 hubble received his doctorate in astronomy from the university of chicago.

ejecta : material from beneath the surface of a body such as a moon or planet that is ejected by an impact from a meteor and distributed on the surface. ejecta usually appear lighter in color than the surrounding surface.

electromagnetic radiation : radiation that travels through space at the speed of light, and increases the interplay of oscillating and magnetic fields. the radiation has a wavelength and frequency.

electromagnetic spectrum : the range of all kinds of wavelengths of electromagnetic radiation. these include short to long wavelength gamma rays, x-rays, ultra-violet, optical, infrared and radio waves.

electromagnetic wave : a wave of electric and magnetic energy that is generated when an electric charge is accelerated.

electron : negatively charged elementary particle found outside, but is attached to, the nucleus of an atom.

electron flux : rate of flow of electrons through a reference surface.

electron volt : a unit of energy equal to the energy gained by an electron that falls through a potential difference of one volt.

element : fundamental unit of matter consisting of fixed number of protons. number of neutrons and electrons may vary.

ellipse : an oval shape. johannes kepler discovered the orbits of planets are elliptical in shape and not circular.

elliptical : shaped like an egg that has ends which are equal.

elliptical galaxy : a galaxy whose structure is shaped like an ellipse and is smooth and lacks complex structures such as spiral arms.

elongation : the angular separation of an object from the sun.

emission : discharge of electromagnetic radiation from an object.

emission nebula : cloud of hot gas being illumined from within by the radiation of energetic, young stars.

emission spectrum : an emission spectrum is a spectrum of bright lines or bands of light of specific wavelength which are emitted when a gaseous element is exposed to high energy. each element has its own unique pattern of bands.

emit : to emit is to give off. leds give off, or emit, light. colored leds emit very specific wavelengths of light.

energy : (see also thermal energy) energy is the ability to do work, and there are several different forms of energy (e.g., kinetic, potential, thermal, sound, light, chemical, etc.). while energy may be transformed from one form to another, the total energy remains the same within a closed system.

engineers : engineers use math and science to design new tools and devices to solve practical problems.

environment : everything that surrounds anything.

ephemeris : table that identifies the positions of astronomical objects at certain intervals.

equatorial mount : a telescope mount in which one axis lies parallel to earth’s rotational axis; the motion of the telescope about this axis compensates for earth’s rotation.

equinox : two points in which the sun crosses the celestial equator in its yearly path in the sky. equinoxes signal the start of spring and autumn seasons that occur on or near march 21 and september 22, respectively.

escape velocity : speed required for something or an object, to be free of the gravitational pull of a planet or other body.

evaporation : evaporation is the process of changing from a liquid to gas.

evening star : venus, when it appears in the evening sky.

event horizon : an invisible boundary around a black hole from which nothing can escape the gravitational pull, not even light.

evolved star : a star that is near the end of its life cycle where most of its fuel has been used up. at this point the star begins to loose mass in the form of stellar wind.

exit pupil : image of the objective lens or primary mirror of a telescope formed on the eye side of the eyepiece.

exobiologist : a person who studies the origin, development, and distribution of ‘living’ systems that may exist outside of earth.

expands : when the heart expands, or gets larger, the heart muscles stretch and the spaces within the heart spread out.

extinct : an extinct animal or plant is one that has died out.

extinction : the apparent dimming of star or planet when low on the horizon due to absorption by the earth’s atmosphere.

extragalactic : beyond the milky way galaxy.

extrasolar : beyond the sun.

extraterrestrial : beyond earth.

eye relief : the distance between the eyeball and the lens nearest the eye of an eyepiece at which an observer can clearly see the entire field of view

eyepiece : a magnifying lens used to view the image produced by a telescope’s primary lens or mirror.

faculae : bright patches that are visible on the sun’s surface, or photosphere.

fahrenheit : a scale on a thermometer where the freezing point of water is represented by 32 degrees and the boiling point is represented by 212 degrees.

far ultraviolet : ultraviolet radiation with the shortest wavelengths.

fermi gamma-ray space telescope : fermi is a space telescope that consists of two parts

field of view : the area of the sky visible through a telescope or binoculars.

filament : a strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the sun.

filter : a device that transmits light of only certain wavelengths. used by astronomers to observe view specific wavelengths and to minimize the light of exceptionally bright objects.

finder : a small, wide-field telescope attached to a larger telescope. the finder is used to help point the larger telescope to the desired viewing location.

finder scope : a small, low-powered telescope attached to a larger telescope that helps the observer locate objects in the sky.

fireball : a very bright meteor.

first quarter : phase of the moon a quarter of the way around its orbit from new moon. eastern portion is visibly bright during this phase.

flare : the sudden, violent outburst of energy from a star’s surface.

flare star : a faint red star that appears to change in brightness due to explosions on its surface.

fluid : a fluid is a substance that will flow. when a substance flows, the particles in the fluid can move past one another. both liquids and gases are fluids.

fluid shift : while in space, fluids in the body move from the lower part of the body toward the head. this movement is fluid shift.

fluorescent light : a fluorescent light uses an electric current to heat gas particles inside a specially coated glass tube. when the particles hit the sides of the tube, a glowing light is produced.

focal length : distance from a lens or mirror to the point it draws light to a focus.

focal ratio : the ratio of the focal length of a lens or mirror to its diameter.

focus : point at which rays of light passing through a lens meet.

focuser : the device on a telescope that holds an eyepiece and moves to allow an observer to bring light to a sharp focus.

force : a force is whatever can cause an object with mass to accelerate (change its direction or speed). force may be expressed with both magnitude (speed or velocity) and direction making it a vector quantity.

forced convection : forced convection occurs when a pump or other mechanism moves a heated fluid.

fork mount : an equatorial mount in which the telescope swings in declination between the two prongs of a fork.

fossil fuel : fossil fuel is a general term for crude oil, coal, natural gas, or heavy oils. these fuels are created by exposure to heat and pressure in the earth’s crust over hundreds of millions of years.

fragment : a broken piece of something.

freezing point : the freezing point of a liquid is the temperature at which the liquid changes state from a liquid to a solid.

frequency : the number of wave crests or troughs that pass a particular point in a given interval of time (usually one second); usually expressed in hertz (cycles per second)

frequency (v) : frequency is the number of waves that pass a fixed point in a given period of time. the frequency of electromagnetic radiation is measured in hertz (hz) which is defined as the number of waves per second.

fuel : anything that is burned to give heat or power.

full moon : phase of the moon when it is halfway around its orbit from new moon and opposite the sun in the sky; the full disk is illuminated.

fusion : a nuclear reaction in which an element with small atoms fuses to form an element with larger atoms, releasing large amounts of energy.

g : the force of earthæs gravity.

galactic disk : disk of a spiral galaxy.

galactic halo : the name given to the spherical region surrounding the center, or nucleus of a galaxy.

galactic nucleus : central region of a galaxy. contains a high density of stars and gas and a super massive black hole.

galactic plane : projection of the milky way’s disk on the sky.

galaxy : a giant collection of gas, dust, and millions or billions of stars.

galaxy cluster : gravitationally bound assemblage of dozens to thousands of galaxies.

galilean moons : the name given to jupiter’s four largest moons, io, europa, callisto & ganymede. they were discovered independently by galileo galilei and simon marius.

galilean moons/satellites : jupiter’s four largest moons

gamma rays : the highest energy, shortest wavelength form of electromagnetic radiation.

gamma-ray burst : short and intense burst of high energy radiation emanating from the distant universe.

gantry : nasaæs gantry is a large apparatus in hampton, virginia that was built to test apollo space capsules. now it helps researchers test the orion space capsule (see figure 3).

gap : an empty space.

gas : a form of matter which is not a liquid or a solid. a gas will spread out to fill up all of the space that is open to it.

gas giant : planets made primarily of gas, these include jupiter, saturn, uranus, and neptune.

general relativity : theory of relativity governing accelerated motion that describes gravity as a curvature of space-time.

geosphere : the geosphere is the solid portion of earth and the processes that shape earth’s surface.

geosynchronous : an orbit in which a satellite’s rate of revolution matches the earth’s rate of rotation. this allows the satellite to stay over the same site on the earth’s surface at all times.

geosynchronous orbit : a geosynchronous orbit is a satellite orbit at approximately 35,800 kilometers above the equator in which objects travel at the same speed as earth. objects in this orbit remain stationary in reference to earth.

german equatorial mount : mount in which the declination axis sits on top of the polar axis, with the telescope on one end of the declination axis and a counterweight on the other.

giant molecular cloud : interstellar clouds of cold gas and dust that contain tens or hundreds of thousands of solar masses.

giant molecular cloud (gmc) : massive clouds of gas in interstellar space composed primarily of hydrogen molecules. these clouds have enough mass to produce thousands of stars and are frequently the sites of new star formation.

gibbous : the phase of the moon between first quarter and last quarter, when the moon appears more than half illuminated.

glacial advance : glacial advance is an increase in the thickness and area of a glacier. this term also describes the time period it takes for the increase in glacial thickness to occur.

glacial retreat : glacial retreat occurs when backward melting at the front of a glacier takes place at a rate exceeding forward motion.

global climate change : global climate change is the long-term fluctuations in temperature, precipitation, wind, and all other aspects of earth’s climate.

globular cluster : a roughly spherical congregation of hundreds of thousands of stars; most globular clusters consist of old stars and exist in a galaxy’s halo.

granulation : a pattern of small cells that can be seen on the surface of the sun. they are caused by the convective motions of the hot gases inside the sun.

gravitational field : the volume over which an object exerts a gravitational pull.

gravitational force : see gravity

gravitational lens : a concentration of matter such as a galaxy or cluster of galaxies that bends light rays from a background object. gravitational lensing results in duplicate images of distant objects.

gravitational pull : the attraction that one object has for another object due to the invisible force of gravity.

gravity : gravity is a force between objects based on their masses and the distance between the objects. the force of gravity on the moon is less than the force of gravity on earth because the moon has only 1/6 the mass of earth. earth’s gravity is described as 1g.

gravity or gravitational waves : weak, wavelike disturbances which represent the radiation related to the gravitational force; produced when massive bodies are accelerated or otherwise disturbed.

greenhouse effect : an increase in temperature caused when incoming solar radiation is passed but outgoing thermal radiation is blocked by the atmosphere. carbon dioxide and water vapor are two of the major gases responsible for this effect.

greenhouse gases : greenhouse gases are gases that contribute to the warming of the earth’s atmosphere by reflecting solar radiation from earth’s surface. carbon dioxide, ozone, and water vapor are examples of greenhouse gases.

ground truth : ground truthing is a validation process where a person on the ground (or sometimes in an airplane) makes a measurement of the same phenomenon a satellite is measuring, at the same time the satellite is measuring it. the two answers are compared to evaluate how well the satellite instrument is performing. the actual measurements taken on earth are called “ground truth.”

gyroscope : a heavy wheel or disk mounted so that its axis can turn freely in one or more directions. a spinning gyroscope tends to resist change in the direction of its axis.

habitable zone : zone around a star in which a planet can maintain liquid on its surface.

habitat : the place in which an organism lives and obtains the materials it needs in order to survive.

halo : outer region of a galaxy, contains globular clusters, a few stray stars, and dark matter.

heat : heat is the amount of thermal energy absorbed, released, or transferred by a material. this is typically expressed as q, and is measured in joules (j).

heliacal rising : the period of time when an object, such as a star, is briefly seen in the eastern sky before dawn and is no longer hidden from the glare of the sun.

heliocentric : having the sun as a center, such as a heliocentric solar system.

heliopause : the point in space at which the solar wind meets the interstellar medium or solar wind from other stars.

heliosphere : a vast region around the sun dominated by the solar wind.

helium : second lightest element, consists of two protons, two neutrons and two electrons. eight percent of the atoms in the universe are helium.

hertz : a unit of frequency equal to one cycle per second.

hertzsprung-russell diagram : a diagram that plots luminosity against temperature for a group of stars.

hiad : (hypersonic inflatable aerodynamic decelerators) hiads are inflatable heat shield structures made up of incredibly strong, yet flexible fabric designed to maintain shape and withstand head during atmospheric reentry.

highlands : highlands are mountainous regions of land.

hii region : an area filled with clouds of ionized hydrogen; the ionization is usually caused by radiation from newborn stars.

hill : a hill is raised above the surrounding land, but is smaller than a mountain.

hubble law : the principle that a distant galaxy’s recessional velocity is proportional to its distance from earth

hubble space telescope (hst) : the hubble space telescope makes its observations from above earth’s atmosphere. the telescope orbits 600 kilometers (375 miles) above earth, working around the clock. it was originally designed in the 1970s and launched in 1990. the telescope is named for astronomer edwin hubble.

hubble’s law : the law of physics that states that the farther a galaxy is from us, the faster it is moving away from us.

humus : humus is part of the soil that is made up of decayed organic materials.

hurricane : a very, very strong windstorm where the wind blows in circles at more than 46 kilometers per hour. heavy rains often come with the winds.

hydrazine : colorless liquid which burns quickly and used as rocket and missile fuel.

hydrogen : the simplest and lightest element; usually consists of just a single proton and electron; about 90 percent of the atoms in the universe are hydrogen.

hydrosphere : the hydrosphere is the part of earth that is composed of water.

hydrostatic equilibrium : a state that occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. hydrostatic equillibrium is responsible for keeping stars from imploding and for giving planets their spherical shape.

hypered film : film that has been treated, usually with gas, to enhance its response to low light levels.

hypergalaxy : a system consisting of a spiral galaxy surrounded by several dwarf white galaxies, often ellipticals. our galaxy and the andromeda galaxy are examples of hypergalaxies.

ice : a term used to describe water or a number of gases such as methane or ammonia when in a solid state.

ice age : an ice age is a cycle cold period marked by periods of glacial advance with episodes of glacial retreat.

ice core : ice cores are cylinders of ice obtained by drilling into a glacier.

ice sheet : an ice sheet is the layer of ice covering a large land mass, notably antarctica and greenland. ice sheets form from the compression of snow as new snow builds on of it.

igneous rock : rock formed by the solidification of magma.

impact craters : craters which are the result of a collision between a large body, such as a planet or satellite, and a smaller body such as an asteroid or meteorite.

incandescent light : an incandescent light produces light when a metal wire inside a glass bulb is heated and gets hot enough to glow. the glass bulb contains an inert gas that does not react with the filament

inclination : angle between a planet’s orbit and the ecliptic place; angle between a satellite’s orbit and its host planet’s rotational plane.

inclination : inclination is the angle between a reference plane and another plane or axis of direction. for an artificial satellite, the reference plane is the equator. the inclination of a satellite’s orbit is the angle that the orbit crosses the equator. if a satellite has a 0¦ inclination then it would be orbiting over the equator. if a satellite has a 90¦ inclination, then its orbit is perpendicular to the equator and it would pass over the poles.

inertia : inertia is the tendency of an object to continue doing what it is doing, either moving or resting, unless acted on by an outside force. the inertia of an object is related to its mass (the greater the mass, the greater the inertia).

inferior conjunction : the configuration of an inferior planet when it lies between the sun and earth.

inferior planet : a planet that orbits between the earth and the sun. mercury and venus are the only two inferior planets in our solar system.

inflation : a brief and extraordinarily rapid period of expansion a fraction of a second after the big bang.

infrared : a form of light with slightly lower energy than visible light but with greater energy than radio waves.

infrared thermometer : an infrared thermometer is a tool that measures the heat being given off by an object.

infrared waves : electromagnetic radiation with long wavelengths which is found in the invisible part of the spectrum. human beings experience infrared waves as heat.

insulator : an insulator is a material or substance that does not conduct heat, sound, or electricity easily.

intensity : sound intensity is the amount of energy transferred by a sound wave per unit time.

interacting galaxies : galaxies caught in each other’s gravitational embrace, often results in galactic mergers or extreme star formation.

interference or interferometric fringes : a wave-like pattern resulting from the successful combination of two beams of light which amplifies the light.

interferometer : a system of two or more widely separated telescopes that achieves the resolving power of a much larger telescope.

interferometry : the technique of using two or more widely separated telescopes to achieve the resolving power of a much larger telescope.

intergalactic : space between the galaxies.

international astronomical union (iau) : an international organization that unites national astronomical societies from around the world and acts as the internationally recognized authority for assigning designations to celestial bodies and their surface features.

international space station : a global cooperative program between the united states, russia, canada, japan, and europe, for the joint development, operation, and utilization of a permanently habitat in space close to low-earth orbit.

interplanetary : space between the planets.

interplanetary magnetic field : the magnetic field carried along with the solar wind.

interstellar : space between the stars of a galaxy.

interstellar medium : gas and dust located between the stars.

ion : an electrically charged particle. ions may be negatively or positively charged.

ionic : ionic compounds are compounds in which the atoms are held together by ionic bonds. an ionic bond is a chemical bond in which one atom loses one or more electrons to form a positive ion and another atom gains one or more electrons to form a negative ion. the force of attraction between the positive and negative ions forms the bond.

ionization : process an atom gains or loses electrons.

ionized gas : gas heated to a state where it contains ions and free-floating electrons. also referred to as plasma.

ionosphere : an atmospheric layer with a high concentration of ions and free electrons.

iron meteorite : a meteorite that is composed mainly of iron mixed with smaller amounts of nickel.

irregular galaxy : galaxy without a clearly defined spiral or elliptical shape.

irregular satellite : a satellite that orbits a planet far away with an orbit that is eccentric and inclined. they also tend to have retrograde orbits. irregular satellites are believed to have been captured by the planet’s gravity rather than being formed along with the planet.

isotope : forms of an element wherein all atoms have the same number of protons but different numbers of neutrons.

isotropic : isotropic materials have a crystalline structure where atoms are arranged in the same way along each axis.

jansky : a unit used in radio astronomy to indicate the flux density (the rate of flow of radio waves) of electromagnetic radiation received from outer space. a typical radio source has a spectral flux density of roughly 1 jy. the jansky was named to honor karl gothe jansky who developed radio astronomy in 1932.

jet : a narrow stream of gas or particles ejected from an accretion disk surrounding a star or black hole.

jet stream : a high-speed, wandering wind current in the upper troposphere that blows from west to east and affects weather

joule (j) : the joule is a unit of energy. one joule is the energy expended when 1 newton of force is applied to move an object a distance of 1 meter.

jovian planet : a planet with the same attributes of jupiter (gas giant).

jpl (jet propulsion laboratory) : the lead u.s. center for robotic exploration of the solar system located in pasadena, california; jpl spacecraft have visited recognized planets with the exception of pluto.

kelvin : a scale for measuring temperature where 0 kelvin is equal to -273.16 degrees celsius. zero kelvin is referred to as absolute zero, the point at which all motion within molecules comes to a stop.

kepler’s first law : a planet orbits the sun in an ellipse with the sun at one focus.

kepler’s second law : a ray directed from the sun to a planet sweeps out equal areas in equal times.

kepler’s third law : the square of the period of a planet’s orbit is proportional to the cube of that planet’s semi major axis; the constant of proportionality is the same for all planets.

kilogram : 1000grams. a kilogram equals 2.2 pounds.

kilometer : 1000 meters. a kilometer equals 0.6214 miles.

kiloparsec : 1000 parsecs. a parsec equals 3.26 light years.

kirkwood gaps : regions in the main belt of asteroids where few or no asteroids are found. they were named after the scientist who first noticed them.

kuiper belt : a region in the outer solar system beyond neptune’s orbit that contains billions of small, icy bodies; pluto is the largest known kuiper belt object.

l chondrite : a chondrite (a stony meteorite containing small, round, silicate granules called chondrules) that has a low amount of iron.

lagrange point : one of five locations in space relative to two bodies where less massive body can maintain a stable orbit around a common center of mass.

large magellanic cloud : irregular galaxy that orbits the milky way galaxy.

last quarter : phase of the moon three quarters of the way around its orbit from the new moon, the western side is lit.

latent heat : latent heat is heat energy that is released or absorbed by a substance when it changes from one phase to another.

latitude : latitude is the number of degrees north or south of the equator. the equator is 0¦n or s, and the north and south poles are 90¦ n and 90¦ s respectively. to visualize this think of earth as a circle divided into 360¦.

launch abort system, or las : the launch abort system offers a safe, reliable method of moving the entire crew out of danger in the event of an emergency on the launch pad or during the climb to earth orbit.

lens : curved piece of glass that brings light to a focus.

lenticular galaxy : a galaxy possessing a large bulge and small disk.

libration : the small oscillations in the moon’s motion that allow earth-based observers to see slightly more than half the moon’s surface.

lift : lift is an upward force resulting from pressure differences (e.g. different pressures on the and bottom of a bird’s or an aircraft’s wing moving through a fluid) due to the air above the wing traveling faster than the air below the wing, because the upper surface is longer than the lower surface.

light bank : a light bank is a group of lights that are connected. a solid state lighting module, or sslm, is a light bank made up of rows of light-emitting diodes.

light pollution : light, typically from artificial sources, that reaches the night sky, obscuring the view of faint astronomical objects.

light year : an astronomical unit of measure equal to the distance light travels in a year, approximately 5.8 trillion miles.

light-gathering power : the ability of a telescope to collect light; the larger a telescope’s aperture, the greater its light-gathering power.

light-year : the distance light travels in one year, equivalent to approximately 5.9 trillion miles (9.5 trillion km).

limb : edge of a celestial object.

limiting magnitude : the apparent magnitude of the faintest objects that can be seen given the local observing conditions and any telescope, film, or other detector you may be using.

liner galaxy : a low-ionization nuclear emission-line region galaxy belongs to a common class of otherwise normal galaxies that display low-ionization line emissions near their central regions.

loam : loam is soil made up of a mixture of sand, silt, and clay.

local group : the galaxy cluster containing 35 galaxies to which the milky way galaxy belongs.

local supercluster : the galaxy supercluster to which the local group belongs; it spreads over 100 million light-years and boasts the virgo cluster as its dominant member.

longitude : longitude is the number of degrees east or west of the prime meridian, the 0¦ e or w line going through greenwich, england and the north and south poles. the prime meridian divides the globe into eastern and western hemispheres. it runs between the poles through the pacific ocean on the side of the globe opposite england. this line is called the international date line. degrees longitude are 0¦ e or w at the prime meridian and 180¦ e or w at the international date line.

longitudinal wave : a longitudinal wave is a wave whose particles vibrate parallel to the direction the wave is traveling.

long-period comet : comets that have orbital periods greater than 200 years.

lowlands : the lowlands are sections of ground lower than the surrounding area. a valley is an example of lowlands.

luminosity : the total amount of light that an object radiates.

luna : luna is the official name of earth’s moon.

lunar eclipse : a phenomenon caused by the earth passing between the sun and moon.

lunar module : the section of the apollo spacecraft designed to land on the moon.

lunar month : the period of one complete revolution of the moon around earth, 29.5 days.

lunar rover : the car-like vehicle used by apollo astronauts while exploring the moon’s surface.

lunation : the time between two successive new moons; approximately 29.5 days.

luster : luster is a way a mineral reflects light from its surface. words like shiny and dull describe the luster of a mineral.

magellanic clouds : two small, irregular galaxies found just outside our own milky way galaxy. the magellanic clouds are visible in the skies of the southern hemisphere.

magnetic field : the area in which an attractive or repelling force exists between two magnets or in association with the element iron. the earth’s magnetic field is thought to be due to the liquid iron-nickel which is in its core. this magnetic field protects earth from constant bombardment by high-energy charged particles.

magnetic pole : either of two limited regions in a magnet at which the magnet’s field is most intense.

magnetograph : a recording magnetometer used for recording variations in the earth’s magnetic field.

magnetometer : an instrument that measures the intensity of earth’s magnetic field.

magnetopause : the boundary space between the earth’s magnetosphere and interplanetary space (40,000 miles / 65,000 km) above the earth, marked by an abrupt decrease in the earth’s magnetic induction.

magnetosphere : the dynamic region around a planet where the magnetic field traps and controls the movement of charged particles from the solar wind.

magnitude : the measurement of an object’s brightness; the lower the number, the brighter the object.

main belt : the area between mars and jupiter where most of the asteroids in our solar system are found.

main sequence : the band of stars on a hertzsprung-russell diagram stretching from the upper left to the lower right; stars spend most of their lives in the main sequence phase, in which they are fusing hydrogen into helium in their cores.

major planet : a name used to describe any planet that is considerably larger and more massive than the earth, and contains large quantities of hydrogen and helium. jupiter and neptune are examples of major planets.

maksutov telescope : a catadioptric telescope that uses a deeply curved meniscus lens as the correcting plate. (see catadrioptric telescope)

mantle : the portion of a planet’s interior above the core but below the crust.

mare : dark and smooth area on the surface of the moon or on a planet.

mass : a measure of the total amount of matter within an object.

mass loss : the loss of mass by a star during its evolution; some of the causes of mass loss include stellar winds, bipolar outflows, and the ejection of material in a planetary nebula or supernova.

matter : anything which has mass and occupies space.

megaparsec : one million parsecs, equivalent to 3.26 million light-years.

melting point : the melting point of a solid is the temperature at which the solid changes state from a solid to a liquid.

meridian : imaginary circle on the celestial sphere that connects the zenith to the north, or south, celestial pole.

messier catalog : a catalog of 107 bright deep-sky objects that belong to a catalog compiled by french astronomer charles messier in the 1700s.

metal : a term used by astronomers to describe all elements except hydrogen and helium, as in “the universe is composed of hydrogen, helium and traces of metals”. this astronomical definition is quite different from the traditional chemistry definition of a metal.

meteor : a flash of light that occurs when a meteoroid burns up in earth’s atmosphere, also known as shooting star.

meteor shower : an event where a large number of meteors enter the earth’s atmosphere from the same direction in space at nearly the same time. most meteor showers take place when the earth passes through the debris left behind by a comet.

meteor storm : rare events that occur when earth encounters dense regions within a meteor stream. such encounters can increase normal meteor rates by more than 1,000 meteors per minute.

meteorite : rock from space that survives as it passes through the earth’s atmosphere and falls to the ground.

meteorite : fragments of material that fall from space and impact on other larger space bodies.

meteoroid : small rock that orbits the sun.

metric ton : 1000 kilograms. a metric ton equals 2,204 pounds.

microgravity : the condition of microgravity (a small amount of gravity) exists when objects are in free fall, like the space shuttle and other objects orbiting the earth. the objects would actually fall to the earth if they weren’t moving very quickly in a different direction.

microlensing : effect of gravity from a small astronomical body focusing light rays, similar to lenses.

micrometeoroid : very small pieces of matter which are encountered in space.

micron : one millionth of a meter.

microwave : electromagnetic radiation which has a long wavelength (between 1 mm and 30 cm). microwaves can be used to study the universe, communicate with satellites in orbit around earth, and cook popcorn.

milky way : spiral galaxy containing our solar system. it can be observed by the naked eye as a faint luminous band stretching across the heavens, containing approximately a trillion stars, most of which are too distant to be seen individually.

millibar : a measure of atmospheric pressure equal to 1/1000 of a bar. standard sea-level pressure on earth is about 1013 millibars.

millisecond pulsar : neutron star rotates hundreds of times per second, which typically accretes matter from a stellar companion.

minor planet : rocky body that orbits the sun; also recognized as an asteroid.

mirror : piece of glass coated with a highly reflective material.

mixture : a mixture is a physical combination of two or more substances. each substance in a mixture retains its own physical and chemical properties.

model : a model represents something else. a model might be a drawing or a 3-d object. models are smaller than the original object or made out of less expensive materials than the actual object. many iterations (repetitions or versions) of model are often needed before the actual object (for instance, the orion spacecraft) can be built. the first version is often a drawing scaled down to fit on a piece of paper. the next may be a series of structures made out of paper, cardboard, plastic, or other readily available materials.

module : a part of a set that can be arranged together in different ways.

molecular cloud : an interstellar cloud of molecular hydrogen containing trace amounts of other molecules such as carbon monoxide and ammonia.

molecule : combination of two or more atoms that represent the smallest part of a compound that has the chemical properties of that compound.

monomer : a monomer is a small molecule that is linked with large numbers of other small molecules to form a chain or a network (polymer).

moon : smaller body orbiting a larger body; often refers to earth’s moon.

morning star : venus, when it appears in the morning sky.

mountain : a mountain is any place on earth that rises sharply and is well above its surroundings.

multicultural astronomy : the variety of ways cultures of the past and present have observed, recorded, interpreted, and made use of astronomy to structure their lives, and in some cases satisfy their curiosity about the universe.

multiple star system : gravity bound system in which two or more stars orbit a common center of mass.

muses-c : the muses-c mission will investigate an asteroid known as an earth-approaching type. through this mission, the institute of space and astronautical science (isas) in japan intends to establish the technology to bring back samples of an asteroid’s surface to earth.

mylar : a tough polyester material used as an insulator.

mythology : old stories that usually explain how something came to be.

naked eye : something visible without the aid of binoculars or a telescope.

nasa : the national aeronautics and space administration which oversees the space program in the united states.

near-infrared : light from the part of the infrared band of the electromagnetic spectrum closest to the visible range.

nebula : a cloud of interstellar gas and dust; some nebulae represent stellar nurseries, others represent stellar graveyards.

nebula : a low density cloud of gas and dust in which a star is born.

net force : the net force is the total of all of the forces acting on an object. these forces are vectors, which means they have direction as well as magnitude.

neutrino : a subatomic particle produced in nuclear reactions and in supernovae that very rarely interacts with matter; neutrinos have no electrical charge and travel at or very close to the speed of light.

neutron : a subatomic particle with no electric charge that resides in an atomic nucleus; it has about the same mass as a proton.

neutron star : a neutron star is the type of star formed when a massive star explodes as a supernova, leaving behind an ultra dense core.

new moon : the phase in which the moon is in the same direction as the sun in earth’s sky, so it is unilluminated and invisible.

newtonian fluid : a newtonian fluid is a fluid that reacts the same way no matter how much stress, or force, is applied to it.

newtonian telescope : a reflecting telescope in which a flat secondary mirror (called the diagonal) in the center of the tube reflects light to a focus outside the tube.

newton’s first law of motion : a body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.

newton’s second law of motion : for an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed; the constant of proportionality is the inertial mass of the body.

newton’s third law of motion : in a system where no external forces are present, every action force is always opposed by an equal and opposite reaction.

ngc : new general catalogue, a 19th-century compendium of deep-sky objects such as galaxies, globular clusters, and nebulae.

ngc objects : deep-sky objects such as galaxies, globular clusters, and nebulae included in the new general catalogue.

non-newtonian fluid : a non-newtonian fluid is a fluid that changes behavior depending on the amount of stress, or force, applied to it.

north celestial pole : the point in the sky to which earth’s geographical north pole points.

nova : an explosion on the surface of a white dwarf that is accreting matter from a companion star, which causes the system to temporarily brighten by a factor of several hundred to several thousand.

nuclear fusion : the process by which two atomic nuclei combine to form a heavier atomic nucleus; this is the energy source that causes most stars to shine.

nucleosynthesis : the creation of heavy elements from lighter ones by nuclear fusion.

nucleus : the central region of an atom, comet, or galaxy.

nutrients : nutrients are substances that an organism (plant or animal) needs in order to survive and grow.

ob association : loose grouping of o and b stars, which are the most luminous, most massive, and shortest-lived stars.

objective : telescopes primary lens or mirror that gathers light and brings it to a focus.

oblateness : a measure of flattening at the poles of a planet or other celestial body.

obliquity : the angle between the plane of the earth’s orbit and that of the earth’s equator, equal to 23°27′; the inclination of the earth’s equator.

occultation : the passage of one object in front of a smaller one, temporarily obscuring all or part of the background object from view.

omega : 1. the ratio of the density of the universe to the critical density 2. the 24th letter of the greek alphabet.

omega centauri : massive globular cluster in the southern constellation centaurus located about 17,000 light-years from earth; also known as ngc 5139.

omega nebula : also known as the swan nebula, m17, ngc 6618, the horseshoe nebula, and the lobster nebula. one of the milky way’s numerous stellar nurseries; the omega nebula is about 5,000 light-years from earth and can be seen in the constellation of sagittarius the archer.

oort cloud : a huge cloud which is thought to surround our solar system and reach over halfway to the nearest star. comets originate in the oort cloud.

open cluster : system containing a few dozen to a few thousand stars that formed from the same stellar nursery.

opposition : best time to observe a planet. the moment a planet far from the sun than earth appears opposite the sun in the sky.

optical double : two stars at different distances that lie along nearly the same line of sight and thus appear close together.

optics : study of light and its properties; lenses or mirrors.

orbit : curved path, usually elliptical in shape, an object follows around a bigger object or a common center of mass.

orbital decay : orbital decay is the reduction in altitude of a satellite’s orbit caused by gravity and drag from the atmosphere.

orbital period : orbital period is the time it takes a satellite to complete one orbit.

orion : nasa’s orion crew exploration vehicle will replace the space shuttle after it is retired. orion is the flagship of nasa’s programs for space exploration beyond low earth orbit and a key element of nasa’s constellation program to explore the moon, mars and beyond.

o-type star : a hot, massive blue star that emits strongly at ultraviolet wavelengths and has a surface temperature of roughly between 28,000 to 40,000 kelvin’s.

outgassing : release of gas from rocky body.

oxygen : oxygen is a gas found in air that cells need to live (represented by o2).

pahs : polycyclic aromatic hydrocarbons (pahs). a class of stable organic molecules. flat molecules made of carbon and hydrogen atoms. these are common and highly carcinogenic. it is one of the by-products of combustion from automobiles and airplanes.

panchromatic : sensitive to light of all colors in the visible spectrum.

parallax : shift of a nearby object against a fixed background due to the movement of the observer. astronomers observe the parallax of stars to measure the distances of these same stars.

parsec : distance an object would have to be from earth so that its parallax when seen from two points separated by 1 au is equivalent to one arcsecond, equivalent to 3.26 light years.

particle : a very, very tiny piece of matter such as an electron, proton, or neutron found inside of an atom.

parts per million by volume (ppmv) : parts per million by volume is the measure of the concentration of a gas within the atmosphere. in this case, it measures the concentration of carbon dioxide in the air.

patera : a shallow crater with a scalloped and complex edge; saucer shaped volcanic structure.

patera : a shallow crater with a complex, scalloped edge.

payload : cargo which is carried on the space shuttle.

payload bay : the main body of the space shuttle where the payload, or cargo, is stored.

penumbra : outer filament region of a sunspot. lighter region of a sunspot surrounding the umbra (dark center).

penumbral eclipse : when the moon passes into the outer ring of earth’s shadow, causing a slight shading in the moon’s appearance.

periastron : location in an objects orbit where it is closest to the star it orbits.

perigee : the point in the orbit of the moon or other satellite at which it is closest to the earth.

perigree : position of a satellite’s orbit when it is closest to earth.

perihelion : position of an object, or body, when it is closest to the sun.

period : measured interval a regular event takes place.

periodic comet : comet that has been observed to circle, orbit, the sun more than once.

permafrost : permafrost is any soil or rock that is frozen throughout the year.

perturb : to cause a planet or satellite to deviate from a theoretically regular orbital motion.

ph : ph is a measure of the concentration of hydrogen ion (h+) in a substance which determines if a solution is acidic or basic.

phase : cycle of changes in the appearance of a moon or a planet.

phase change : a phase change occurs when matter changes from one state to another. this is any combination of changing from a solid to a liquid to a gas.

photometer : an instrument that measures light emitted by an object.

photometry : degree and measurement of light intensities.

photon : a particle of light composed of a minute quantity of electromagnetic energy.

photons : single waves of light.

photosphere : visible surface of the sun.

photosynthesis : the process by which plants use carbon dioxide, nutrients, and sunlight to produce food.

photovoltaic : when light energy or emissions are converted into electricity.

physical map : a physical map shows the location of major landforms such as mountains, plains, and deserts. it also shows country borders and major cities.

physicist : a person who studies physics.

physics : the science of matter and energy, and of interactions between the two. a person who studies physics is called a physicist.

pixel : short name for “picture element.” individual light detectors on a ccd chip.

plains : vast, flat areas with low elevation.

planck scale : a unit of measurement scientists utilize to describe the universe. one unit (length) of planck is 10^-33 centimeters.

planck’s constant (h) : planck’s constant is a physical constant relating the energy of a photon to its frequency. the value of this constant is approximately 6.626 x 10-34 jouleòsecond.

planemo : a large planet or planetary body that does not orbit a star. planemos instead wander cold and alone through the cosmos. it is believed that most planemos once orbited their mother star but were ejected from the star system by gravitational interaction with another massive object.

planet : a gaseous, rocky body that orbits a star.

planetary nebula : gas ejected by dying, low mass stars that appear as glowing shells.

planetesimal : a solid object that is believed to exist in protoplanetary disks and in debris disks. planetesimals are formed from small dust grains that collide and stick together and are the building blocks that eventually form planets in new planetary systems.

planetesimals : asteroid sized bodies in a new planetary system that collide and form larger bodies.

planisphere : map of the sky in two-dimensions with an adjustable overlay and shows a part of the sky that is visible anytime of the night or year.

planitia : a low plain.

planum : a high plain or plateau.

plasma : gas heated to a state wherein it contains ions and free floating electrons.

plasmasphere : area of cold and high density plasma above the ionosphere.

plate tectonics : theory describing the possibility on how earth’s crust is broken into plates, suggesting that those plates move thru and across earth’s surface.

polar cap : icy region of a planet, specifically the north and south pole.

polar orbit : a polar orbit is an orbit in which the satellite passes over the north and south poles on each orbit, and eventually passes over all points on earth.

polarization : when the direction of electric or magnetic fields in an electromagnetic wave changes in a regular pattern.

polarized light : polarized light waves are vibrating in one direction as they pass through or are reflected by certain media.

pole : the point at either end of the invisible line known as the axis. planets have a south pole and a north pole.

political map : a political map shows the location of cities and the borders of countries and states.

polymer : a polymer is long or large molecule consisting of a chain or network formed by chemically bonding many repeating units, or monomers, together.

position angle : direction in the heavens one celestial object from another, measured eastward from due north.

power : capability of a telescope or binoculars to increase the size of an object that is far away.

poynting-robertson effect : interplanetary particles that are dragged and is caused by its interaction with solar radiation. this causes particles to lose momentum in their orbit and is drawn towards the sun.

precession : periodic change in the direction of an objects axis caused by the gravitational influence from another body.

precipitation : precipitation is water in the atmosphere that falls to earth as rain, snow, hail, sleet, or freezing rain.

pressure : pressure is the force per unit area. even though you can’t feel it, air has pressure. air molecules move continuously. the more times they bump into each other or a surface, the greater the pressure. air pressure is increased by

primary lens : main lens of a telescope that gathers light bringing the object into focus.

primary mirror : main mirror of a telescope that gathers and reflects light to bring the object in focus.

prime meridian : the line of longitude that runs through greenwich, england.

prism : a piece of glass that breaks white light into it’s basic colors, it is wedge shaped.

probes : unmanned spacecraft which are launched into space in order to collect data about the solar system and beyond. space probes are not necessarily designed to return to earth.

prograde : same direction a planet rotates. an object that move, or looks like it moves in the same direction of solar system bodies or moons.

prograde orbit : in reference to a satellite, a prograde orbit means that the satellite orbits the planet in the same direction as the planet’s rotation. a planet is said to have a prograde orbit if the direction of its orbit is the same as that of the majority of other planets in the system.

projectile : a projectile is any object that is thrown or otherwise launched. it is affected by earth’s gravity. a projectile may start at a given height and move toward the ground in an arc. regardless of its path, a projectile will follow these rules

prominence : a massive eruption of gas streaming off the surface of the sun towards the corona.

proper motion : annual movement of a star across the sky.

proton : subatomic particle that is found in an atom’s nucleus and possesses a positive electric charge.

protoplanet : gas, dust, and rocks that gradually becomes a whole planet.

protoplanetary disk : disk of gas and dust surrounding a new planet; planets that form through the collision of particles inside the disk.

protostar : cloud of hot, dense gas and dust that gravitationally collapses to form a star.

prototype : a prototype is an original or model on which something is based.

proxima centauri : nearest star to the sun at 4.2 light years away.

pulsar : a rotating neutron star that showers earth with regular pulses of electromagnetic radiation.

pulse rate : a pulse rate is how many times a heart beats in one minute.

quadrature : a point in the orbit of a superior planet where it appears at right angles to the sun as seem from earth.

quadrillion : a number represented in the u.s. with a 1 followed by 15 zeros, in the u.k., 1 followed by 24 zeros.

qualitative data : qualitative data are sets of information that describe attributes.

quantitative data : quantitative data are values that can be counted or measured.

quantum mechanics : law in physics describing the behavior of matter at the atomic and subatomic level.

quasar : a distant energy source which gives off vast amounts of radiation, including radio waves and x-rays.

quasi-stellar object : sometimes also called quasi-stellar source, this is a star-like object with a large redshift that gives off a strong source of radio waves. they are highly luminous and presumed to be extragalactic.

radial velocity : acceleration of an object going away from or headed towards an observer.

radiant : location in the sky where meteors belonging to a meteor shower appear to come from. 2. very bright and shining.

radiation : radiation is the transfer of heat through space.

radiation belt : regions of charged particles in a magnetosphere.

radiation pressure : amount of pressure applied on a surface by electromagnetic radiation or light.

radio galaxy : galaxy that emanates a large amount of radio waves.

radio telescope : designed to observe radio waves coming from space.

radio waves : a type of electromagnetic radiation which has the lowest frequency, the longest wavelength, and is produced by charged particles moving back and forth. radio waves are not blocked by clouds in the earth’s atmosphere.

radiometer : instrument to measure total energy or power from an object in the form of radiation, especially infrared radiation.

rarefaction : rarefaction is the process of molecules becoming more spread out (as opposed to compression).

recycled : used materials that are made into new products are called recycled materials. recycling reduces the waste of useful materials, and it reduces the need for new materials. recycling sometimes reduces energy, costs, and pollution.

red dwarf : smaller star with a low mass, cooler, and less luminous than the sun.

red giant : cool star nearing the end of its cycle. these have expanded up a hundred times the diameter of the sun.

red supergiant : cool star nearing the end of its cycle. these have expanded from a hundred to a thousand times the diameter of the sun.

redshift : multiplication of wavelength of light coming from an object due to its motion away from earth; expansion of the universe; strong gravitational field.

reduced gravity : reduced gravity is less gravity than normally experienced on earth, or less than 1g.

re-entry : the return of a spacecraft into earth’s atmosphere.

reflect : reflected light is light that hits a surface and bounces off.

reflection nebula : gas and dust clouds made visible due to the dust reflection from the light of nearby star.

reflector : telescope using curved mirrors to gather light.

refract : when light is refracted it is bent. light refracts when it passes from one material to another.

refractor : telescope using a glass lens to gather light.

regolith : a fine dust called regolith covers the moon. regolith is created when micrometeoroids bombard the moon’s surface, breaking up moon rocks. (the vowels are pronounced like those in “lego” and “miss.”)

regular satellite : a satellite that orbits close to a planet in a nearly circular, equatorial orbit. regular satellites are believed to have been formed at the same time as the planet, unlike irregular satellites which are believed to have been captured by the planet’s gravity.

rehydrate : to rehydrate is to put water back into the body that is removed during spaceflight or after exercise on earth.

relativity : theory in physics developed by albert eistein. describes measurement made by two observers who are in relative motion.

relief map : a relief map shows the variation in land heights. the different heights are shown as lines or different colors.

resolution (resolving power) : a camera or a telescopes ability to capture fine details of a subject.

resonance : a state in which an orbiting object is subject to periodic gravitational perturbations by another.

reticle : using two fine wires as part of a grid attached to part of the focal plane or a telescope eyepiece. this is used to locate the position and size of a celestial object.

retrograde : viewing objects that move or appear be moving in the opposite direction of a solar system bodies.

retrograde motion : the phenomenon where a celestial body appears to slow down, stop, them move in the opposite direction. this motion is caused when the earth overtakes the body in its orbit.

retrograde orbit : the orbit of a satellite where the satellite travels in a direction opposite to that direction of the planet’s rotation.

reusable : objects or materials that can be used again are called reusable. the objects may be used for the same purpose or different purposes.

reusable launch vehicle (rlv) : a spacecraft that may be reused on successive missions. a single stage to orbit spacecraft.

revolution : the circling of a smaller object around a larger object.

revolve : to move in an orbit or circle around something.

ribonucleic acid : nucleic acid containing genetic information.

rich clusters : galaxy clusters with high population densities.

rich-field telescope : designed to show a larger field of view at low magnification.

right ascension : angular distance of a celestial object located east of the vernal equinox; outer space sphere equivalent to longitude.

ring galaxy : a galaxy that has a ring-like appearance. the ring usually contains luminous blue stars. ring galaxies are believed to have been formed by collisions with other galaxies.

roche limit : the smallest distance from a planet or other body at which purely gravitational forces can hold together a satellite or secondary body of the same mean density as the primary. at a lesser distance the tidal forces of the primary would break up the secondary.

rotate : to turn around a center point, or axis, like a wheel turns on a bicycle.

rotation : spin of an asteroid, planet, star, moon, or galaxy on its central axis.

rotation period : measurable interval an asteroid, planet, star, moon, or galaxy completes one rotation.

r-value : the r-value measures a material’s insulating properties.

sand : sand is rock material that has been eroded into small grains.

satellite : small body or object that goes (orbit) around a planet or asteroid.

saturated solution : a saturated solution contains the maximum amount of solute that can be dissolved in a given amount of solvent at a specified temperature.

scale : scale is the ratio of the length in a drawing, or model, to the length of the real object. if you scale an object, you size or measure it proportionately.

scale factor : scale factor is the ratio of any two corresponding lengths in two similar geometric figures.

scarp : cliffs created by erosion and fault movement.

scattered : going in many different directions.

schmidt camera : catadioptric telescope used as a camera to photograph wide-angle pictures of the sky.

schmidt-cassegrain telescope : small telescope wherein light passes through a correcting lens located at the front of the telescope; it then reflects off a primary mirror back to a secondary mirror, which then directs the light through a hole in the primary and out the back of the scope; this is a popular telescope for backyard observers.

scientists : scientists use systematic methods to study the world around them. they use an organized approach to observe and study the world. they ask questions, look for patterns, and try to find general rules for the natural world.

secondary mirror : a small mirror used in a telescope that redirects light gathered by a primary mirror.

sediments : sediments are particles that have been deposited by some natural process, such as blowing wind or moving water.

seeing : state of observing phenomena created by earth’s atmosphere that blurs images of astronomical objects.

self-healing material : self-healing materials are able to repair damage by closing the gap around a penetrating object.

semimajor axis : average distance an orbiting body has from its main body.

seti : “search for extra-terrestrial intelligence”

seyfert galaxy : galaxy with a bright nucleus coupled with spectral emission lines, first discovered by carl seyfert in 1943.

shell star : a type of star which is believed to be surrounded by a thin envelope of gas, which is often indicated by bright emission lines in its spectrum.

shepherd satellite : a satellite that constrains the extent of a planetary ring through gravitational forces. also known as a shepherd moon.

shock wave : powerful wave emanating from a sudden change in density, temperature, or pressure traveling through a medium faster than sound travels on that same medium.

short-period comet : a comet that orbits less than 200 years.

sidereal : relating to or measured in association with the stars.

sidereal month : the average period of revolution of the moon around the earth in reference to a fixed star, equal to 27 days, 7 hours, 43 minutes in units of mean solar time.

sidereal period : the period of revolution of a planet around the sun or a satellite around its primary.

sidereal year : amount of time a body revolves around another with respect to the stars.

siderostat : a movable flat mirror that reflects light from a celestial object to a given location.

silicon : a non-metallic chemical element.

silt : silt is made of particles smaller than sand.

similar : geometric shapes are similar if their corresponding sides are proportional and corresponding angles are equal.

simulation : a simulation is something that substitutes for the real thing. for instance, flight simulators are mockups for pilots to practice so they do not crash real aircraft under different weather and equipment emergencies. a simulation can have many qualities of the authentic experience without all the expenses and dangers. a model simulates

singularity : an area wherein space and time are infinitely distorted.

small magellanic cloud : an irregular and small galaxy orbiting the milky way galaxy.

small solar system body : a term defined in 2006 by the international astronomical union to describe objects in the solar system that are neither planets or dwarf planets. these include most of the asteroids, comets, and other small bodies in the solar system.

soil : soil is a mixture of minerals, weathered rocks, and decayed plant and animal material.

solar : having to do with the sun.

solar cycle : the approximately 11-year quasi-periodic variation in frequency or number of solar active events.

solar eclipse : when the moon passes between the earth and the sun.

solar filter : a safety precaution, a filter used to block almost all the suns light when being viewed.

solar flares : a magnetic storm on the sun’s surface which shows up as a sudden increase in brightness.

solar irradiance : radiant energy given by the sun over all wavelengths that falls each moment on one square meter of earth’s atmosphere.

solar mass : amount of mass contained in the sun, equivalent to 330,000 times to that of earth.

solar nebula : the cloud of dust and gas out of which the solar system was believed to have formed about 5 billion years ago.

solar prominences : gases trapped at the edge of the sun which appear to shoot outward from the sun’s surface.

solar radiation : solar radiation refers to energy that travels in rays or waves and originates from the sun.

solar system : the solar system includes the sun, the planets, and other bodies that revolve around the sun including comets.

solar wind : streams of gas particles flowing out from the sun.

solid rocket boosters : solid rocket boosters use a propellant/fuel in solid form. two solid rocket boosters enable the shuttle to reach earth orbit.

solstice : two points on the celestial sphere wherein the sun is farthest north or south of the equator.

solute : the solute is the dissolved component of a solution. the solute is usually, but not always, present in a smaller amount than the solvent.

solution : a solution is a mixture in which the components are evenly mixed so that every part of the mixture is the same as any other.

solvent : the solvent is the component of a solution that dissolves one or more solutes.

sound : sound is a form of energy produced and transmitted by vibrating objects.

sound wave : a sound wave is a series of compressions and rarefactions traveling through a substance.

south celestial pole : point in the sky earth’s south pole points.

space probe : an unmanned research craft sent into space.

space weathering : process of changing the surface of an object in space by impacts from small meteors, cosmic rays, and even the solar wind.

space-time : when the three dimensions of space come together with one dimension of time wherein the events can be exactly calculated.

special relativity : theory of relativity applied concerning uniform motion. it proposes that the equivalence of mass and energy and differs from newtonian physics only when speeds approach that of light.

specific heat : different materials require different amounts of heat to produce similar changes in their temperatures. in other words, materials have different specific heat capacities, often called, specific heat. the specific heat capacity of a material is the amount of energy it takes to raise the temperature of 1 gram of the material 1 degree celsius. specific heat capacity can be measured in joules per gram per degree celsius (j/g ¦c).

spectra : plural of spectrum. radiant source energy.

spectral class : classification of stars based on its spectrum as dictated by the surface temperature.

spectral line : specific wavelength of light that corresponds to the energy exchange of an atom or molecule.

spectrograph : the image of the electromagnetic spectrum produced by a spectroscope.

spectrograph/spectrometer : instrument coupled to a telescope that records the spectrum of an astronomical object.

spectroheliograph : instrument to photograph the sun on a single wavelength of light.

spectrometer : a spectrometer is a tool that helps scientists study comets. spectrometers can be found on satellites, rockets, airplanes, and telescopes. a spectrometer works like our eyes, but it breaks light into colors like a rainbow.

spectroscope : an instrument which separates visible light into its various wavelengths. each wavelength corresponds to a specific color in the spectrum.

spectroscopy : spectroscopy refers to the science and practice of using spectrometers and spectroscopes and of analyzing spectra.

spectrum : a band of colors which forms when visible light passes through a prism. the band ranges in color from violet (shorter wavelength) to red (longer wavelength).

speed : speed is the time it takes an object to travel a certain distance. speed equals distance divided by time or s = d / t.

speed of light : light travels through a vacuum at 186,000 miles per second, or 300,000 km per second. distance light travels in a unit of time through a specific substance.

speed of light (c) : the speed of light is the speed of electromagnetic radiation in a perfect vacuum. the speed of light is the same for all frequencies of electromagnetic radiation, 3.0×10 8 m/s.

spicules : supersonic jet about 300 miles(500 km) in diameter found in the chromosphere of the sun.

spiral arm : concentration of young stars, gas and dust that are finds its way out of the nucleus of a spiral galaxy.

spiral galaxy : spiral shaped system composed of stars, gas clouds, and dust, numbering in the billions.

standard candle : in astronomy, refers to an object known for its brightness and is sometimes used to determine distances.

star : sphere of hot gas held together by gravity and emanates brightness by itself; common stars utilize nuclear fusion from its core to generate energy.

star atlas : collection of maps using a coordinate system to mark positions of astronomical objects, stars, galaxies, and nebula.

star cluster : a large grouping of stars, from a few dozen to a few hundred thousand, that are bound together by their mutual gravitational attraction.

star hopping : techniques using familiar patterns of stars to hop from one part of the sky to another; this is done through the use of a telescope and the naked eye.

star party : gathering of friends and other like-minded people to observe the night sky.

starburst galaxy : galaxy going through a high rate of star formation.

states of matter : states of matter are the distinct forms of matter. three common states of matter are solid, liquid, and gas.

steady state theory : the theory that suggests the universe is expanding but exists in a constant, unchanging state in the large scale. the theory states that new matter is being continually being created to fill the gaps left by expansion. this theory has been abandoned by most astronomers in favor of the big bang theory.

stellar evolution : process, that include changes a star goes through during its existence.

stellar wind : the ejection of gas from the surface of a star. many different types of stars, including our sun, have stellar winds. the stellar wind of our sun is also known as the solar wind. a star’s stellar wind is strongest near the end of its life when it has consumed most of its fuel.

sterocomparator : device that allows astronomers to view two separate images of the same region in the sky at the same time.

stone meteorite : meteorite resembling a terrestrial rock made of similar materials.

stony iron : a meteorite that contains regions resembling both a stone meteorite and an iron meteorite.

sublimate : transition of solid substance that is evaporated into a gas without reaching the liquid phase.

summer : season in the northern hemisphere that commences around june 21.

sunspot : a magnetic storm on the the sun’s surface which appears as a dark area. a sunspot is approximately 1500 degrees celsius cooler than it’s surrounding material. the number of sunspots we see on the sun at any given time appears to cycle every 11 years.

sunspot cycle : cycle that averages eleven years at which the number of sunspots decreases and increases.

sun-synchronous polar orbit : a sun-synchronous polar orbit is a special kind of polar orbit. when traveling in this orbit, a satellite not only travels over the north and south poles, but it passes over the same part of earth at roughly the same time each day. tracking and data relay satellite or tdrs – tdrs is a system of nine geosynchronous communications satellites. they are used to communicate from earth to orbiting satellites, the space shuttle, and the international space station.

supercluster : huge congregation of galaxy clusters that span hundreds and millions of light years away.

superfluid : state of matter exhibiting frictionless flow. liquid helium is the one element that produces this when cooled to absolute zero.

supergiant : the stage in a star’s evolution where the core contracts and the star swells to about five hundreds times its original size. the star’s temperature drops, giving it a red color.

superior conjunction : constitution of an inferior planet when it lies on the far side of the sun.

superior planet : planets that are farther from the sun than earth

superluminal motion : movement that looks to be faster than the speed of light.

supermassive black hole : black hole located at the center of a galaxy containing millions or billions of solar masses.

supernova : an explosion of a star that causes the star to shine millions of times brighter than usual.

supernova remnant : growing cloud of gas that is the outer layers of star that just exploded.

symmetrical : when an object is balanced, or equal on both sides, it has symmetry and is called symmetrical.

synchronous rotation : identical rate of rotation of a satellite or moon to the main and bigger object it orbits.

synchrotron emission : electromagnetic field from high-energy electrons that are moving in a given magnetic field.

synodic period : interval between points of opposition in a superior planet.

tail : the comet’s tail forms when the comet travels near the sun. the tail always points away from the sun.

tectonic activity : a shifting of an object’s surface due to changes in the material underlying the surface.

tektite : a small, glassy material formed by the impact of a large body, usually a meteor or asteroid. tektites are commonly found at the sites of meteor craters.

telescope : any of various devices, sometimes made with an arrangement of lenses, mirrors, or both, used to detect and observe distant objects by their emission, transmission, reflection, or other interaction with invisible radiation.

telescopes : telescopes are tools that help you see objects that are far away.

temperature : temperature is a measure of the average heat or thermal energy of the particles in a substance.

tera (trillion) : american use, one followed by 12 zeros

terminator : boundary of a planet or moon separating the lighted from the unlighted sides.

terra : terra is the official name of earth.

terrestrial : related to the earth.

terrestrial planet : small and rocky planet which includes mercury, venus, earth, and venus.

texture : texture is the way something feels. for instance, sand feels rough (has a grainy texture) while smaller mud particles have a smoother or slimy texture.

thermal energy : thermal energy is the energy of movement of the molecules within a substance. the higher the temperature, the faster the molecules move, thus temperature can be used as a measure of thermal energy.

thermal protection system : a thermal protection system is a combination of materials used to insulate and reduce the amount of heat transferred to a spacecraft as it enters an atmosphere.

thermal radiation : electromagnetic radiation coming from an object that is not at absolute zero.

thermometer : an instrument for measuring temperature.

thrust : thrust is a force that propels an object. thrust must be greater than drag for an object to move forward.

tidal force : disparity in gravitational force between two points on an object caused by the gravity of another object; this leads to a deformation of an object.

tidal heating : frictional heating of a satellite’s interior due to flexure caused by the gravitational pull of its parent planet and/or other neighboring satellites.

tides : distortion of a body caused by the gravitational influence on another body.

trajectory : trajectory is the path that an object takes moving through space.

transit : passage of a smaller body in front of a larger body. passage of a celestial body across an observer’s meridian.

trans-neptunian object (tno) : any one of a number of celestial objects that orbit the sun at a distance beyond the orbit of the planet neptune.

trans-neptunion object : object in our solar system lying beyond the orbit of neptune.

transparency : clarity of the sky.

trapezium : open cluster of young stars, protostars, gas, and dust in the orion nebula that feature four stars forming a the trapezium.

tremolite : common mineral in metamorphic rocks, made up mainly of calcium and magnesium.

trend : a trend is a general direction of movement. on a graph, the trend is the overall direction (either increase or decrease) of the values graphed.

trojan : asteroid lying in or near the lagrange points 60 degrees for or aft jupiter along the planet’s orbit.

tropical year : time earth revolves around the sun in relation to the vernal equinox.

true field of view : angle of sky viewed through an eyepiece attached to a telescope.

type ii quasars : a quasar enshrouded in gas and dust that emits very little visible light, however, is easily seen in the infrared and x-ray region of the electromagnetic spectrum

type ii supernova : the explosion of a massive star that occurs when its core runs out of nuclear fuel; these explosions leave behind a neutron star or a black hole

type la supernova : the explosion of a white dwarf that occurs when it accretes enough mass from a companion star to go above the chandrasekhar limit.

uib : unidentified infrared bands

ultraviolet : ultraviolet wavelengths are shorter than visible light. shorter waves have more energy. ultraviolet radiation can burn and cause skin cancer.

ultraviolet light or radiation : radiation with a higher amount of energy than visible light, not as much as x-rays.

ultraviolet rays : invisible electromagnetic radiation which is comprised of very short wavelengths. human beings get a sunburn from the ultraviolet rays emitted by the sun.

umbra : perfect and whole shadow of an opaque body, like a planet, wherein direct light from the source of brightness is totally reduced. 2) area of complete darkness on the shadow made by an eclipse.

universal time : also recognized as greenwich mean time, forming the basis in all civil time keeping. local time centered in greenwich, england.

universal time (ut) : also known as greenwich mean time, this is local time on the greenwich meridian. universal time is used by astronomers as a standard measure of time.

universe : the vast expanse of space which contains all of the matter and energy in existence.

ut : short for universal time

uv (ultraviolet) : short for ultra violet.

valley : a valley is a place that is naturally lower than the surrounding land. valleys are often located between mountains or hills.

van allen belts : dual belts of charged particles from a solar wind trapped in earth’s magnetic field above the atmosphere. radiation zone of charged particles surrounding earth. shape of van allen belts is determined by earth’s magnetic field.

variable : a letter that represents a group of numbers is called a variable. variables are italicized. thatæs how you can tell them apart from units such as grams (g). an example of some variables used in this activity

variable star : star with varying luminosity.

vector : a vector is a variable (something that can change/vary) that is composed of both an amount and a direction. an example of a vector is velocity. what makes velocity different than speed is the direction of travel. wind velocity is a way vectors are used in everyday life. when the weather report states the wind is 40 kilometers per hour (25 miles per hour) out of the west, the wind’s velocity, not speed, is what is being reported.

vector quantity : a vector quantity is any force that has both size and direction. for example, speed becomes a vector once you give it a direction.

vehicle : something used to carry people and things over land or in space.

veins : veins are blood vessels that carry blood back to the heart.

velocity : the speed and the direction of travel of an object is the objectæs velocity. velocity is similar to speed, but whereas an example of speed would be, ôthe wind was blowing at 40 miles per hour,ö velocity would be expressed as ô40 miles per hour from the se.ö direction becomes important when dealing with navigation of boats, aircraft, wind and water currents, etc.

vernal equinox : time of the year when the sun moves across the celestial equator towards the north, usually around march 21.

vignetting : decreased illumination over an image plane in a camera or in some cases a telescope, this causes a distortion close to the edge of an image.

virgo cluster : 2,500 known galaxies near the north galactic pole of the constellation virgo that is 60 million light years from earth.

visible : able to be seen.

visible light : wavelengths in the electromagnetic spectrum that the human eye can see.

visual magnitude : a scale used by astronomers to measure the brightness of a star or other celestial object. visual magnitude measures only the visible light from the object. on this scale, bright objects have a lower number than dim objects.

voids : big regions of empty space found amidst galaxy clusters and superclusters.

volatiles : chemical compounds that become gaseous at very low temperatures.

volcanism : volcanic activity.

volcano : an opening in a planet’s surface through which hot liquid rock is thrown up.

volume : the volume of an object is how much space it occupies, and it is typically expressed in milliliters (ml), cubic centimeters (cm or cc), liters (l) or cubic meters (m3).

waning/waxing : interval between full and new moon

water cycle : the water cycle is one part of the earth system. it involves the movement of water from earth’s surface to the atmosphere and back through the processes of evaporation, condensation, and precipitation.

water vapor : water which is in the form of a gas.

wavelength : wavelength is the distance between two crests or two troughs on a wave. light is classified by its wavelength. wavelength is usually measured in meters.

weather : weather is the current state of the atmosphere, measured in terms of temperature, pressure, humidity, wind speed and direction, cloudiness and precipitation.

weathering : weathering is a process through which rocks or other materials are broken down. wind, moving water (rivers and waves) and glaciers all cause weathering.

weight : weight is the force of gravity on an object. here on earth, it means how hard the earth pulls down on objects. because the moon is smaller than the earth, the moon wouldn’t pull down on an object as hard and it would weigh a lot less there. however, since the object would still be made up of the same number and type of particles, its mass and density would be the same on the earth and on the moon.

weightless : having little or no weight; not feeling the effects of gravity.

white dwarf : dense remains of an intermediate mass star like the sun that has collapsed and is the same size as earth.

winter : season in the northern hemisphere that begins december 21.

wolf-rayet star : luminous and hot star having temperatures reaching 90,000 kelvins.

x ray star : bright object emitting x rays as a primary component of its radiation.

x-class flares : most energetic kind of solar flares and the brightest.

x-ray astronomy : the field of astronomy that studies celestial objects by the x-rays they emit.

x-ray star : a bright celestial object that gives off x-rays as a major portion of its radiation.

x-rays : type of electromagnetic radiation that is like light but has a shorter wavelength capable of penetrating solid objects and ionizing gases.

yellow dwarf : ordinary star, like the sun and is at its stable point in its transformation.

zenith : point on the celestial sphere directly above an observer.

zenith hourly rate : meteorites expected to be viewed per hour during a meteor shower, where the meteor showers radiance is at an observer’s zenith.

zodiac : imaginary belt across the sky wherein the solar system can always be found.

zodiacal light : a faint cone of light that can sometimes be seen above the horizon after sunset or before sunrise. zodiacal light is caused by sunlight reflecting off small particles of material in the plane of the solar system.

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