The Cosmos

SUN
A Ball of Hot Gas

We start our journey through the Solar System at the center. Here we find a gigantic ball of gas so massive that the immense pressure has ignited a fusion reaction. It is an average-sized yellow star known as the Sun. It is one of the most common type of star in the universe. Here in this cosmic furnace, hydrogen atoms under unimaginable pressures are being fused into helium atoms, releasing a tremendous amount of energy. Scientists believe that this reaction has been taking place for almost 5 billion years, and will likely continue for another 5 billion years. At that time, the Sun's hydrogen supply will have been depleted, and heavier elements will begin to fuse. This will cause the star to swell to the size of a red giant, consuming most if the inner planets in the process. Eventually the Sun will end its life as a cold lightless body known as a black dwarf.

Solar Facts

The Sun is the largest body in the Solar System. It has a diameter of over 863,482 miles (1,390,000 km). The nuclear reactions taking place within the Sun drive its surface temperature to over 10,832° F (6,000° C). Solar radiation is measured in wavelengths that include radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays. Like all other bodies in the Solar System, the Sun rotates on its axis. Unlike the others, its equatorial region rotates faster that its Polar Regions. The equatorial region rotates once every 27 days, while the Polar Regions rotate every 34 days.

Features of the Sun

Although the light from the Sun seems constant to us, the Sun is far from stable. Magnetic storms and anomalies cause a number of unique phenomena. Dark spots can sometimes been seen on its surface when viewing through a solar filter to reduce the light. Known as sunspots, they are actually parts of the surface that are several hundred degrees cooler than the surrounding areas. Tremendous flames of hot gas called solar flares and prominences shoot out several hundred thousand miles into space. A stream of ionized particles known as the solar wind blows out into the far reaches of the Solar System at speeds of up to a million miles per hour. The Sun provides us with a unique opportunity to study what makes a star work. It is the only star in the universe that is close enough for us to see its surface.

Mercury

Messenger of the Gods

The next stop on our tour is a small, barren world called Mercury. It is the closest planet to the Sun and the smallest planet in the Solar System. Mercury was named after the ancient Roman messenger of the gods because it moved across the sky much faster than the other planets. In fact, Mercury completes one orbit around the Sun in only 88 days! The planet has a very slow rotational period. It takes 58 days for Mercury to make a complete rotation on its axis. Because of its close proximity to the Sun, it can only be observed in the sky in the east just before sunrise or in the west just after sunset. It never appears more than 27 degrees away from the Sun.

A Hot and Cold Barren World

Mercury has almost no atmosphere. Scientists believe that a thicker atmosphere may have existed at one time, but was stripped away by the strong solar winds. The atmosphere on Earth helps to maintain a relatively uniform temperature around the planet. By contrast, the temperature on Mercury ranges from 800° F (427° C) during the day, to -279° F (-173° C) at night. It is indeed a very inhospitable world.

Features of Mercury

Mercury was first visited by NASA's Mariner 10 space probe in 1974. In a series of three flybys, the probe managed to map most of the planet's surface. When the photos were received on Earth, the features might easily have been mistaken for the Moon. Mercury's surface, however, is almost completely covered with craters. The moon has more open areas known as maria, which are mostly crater-free. Mercury has only one major open area known as the Caloris Basin. These open areas are thought to have been formed by more recent volcanic flows, which may have covered the older craters. The surface is also marked by steep cliffs, also known as rupes, which rise as much as 6300 feet (1920 m) above the valley floors.

In 2008, a second spacecraft passed close to Mercury. This spacecraft was named MESSENGER, which stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging. MESSENGER brought high resolution cameras to the planet and revealed many new features and new terrain not previously seen on the Mariner mission. MESSENGER is scheduled to enter orbit around Mercury in March 2011, where it will spend a full year mapping the planet's surface. New images from MESSENGER show that Mercury's surface is more homogeneous, ancient, and more heavily cratered, unlike the surfaces of the Moon and Mars.

  VENUS
Goddess of Love and Beauty

A Cloud Covered Greenhouse

Contrary to its outward appearance, Venus is a very inhospitable world. The planet is covered by a thick atmosphere composed primarily of carbon dioxide. It contains almost no water vapor. The atmosphere is so thick in fact, that the pressure at the surface of the planet is 92 times that of Earth. Carbon dioxide allows solar radiation in, but prevents it from escaping back into space. This works very much like a greenhouse, and because of this "greenhouse effect", temperatures on the surface rise to over 900° F (482° C). To make things worse, the thick layer of clouds in the atmosphere is composed of sulfuric acid droplets. There is no water on Venus, but pools of strong sulfuric acid may be found. Beneath its innocent and delicate wispy cloud tops, however, Venus could easily be mistaken for Hades, the underworld.

Features of Venus

Until recently, the dense cloud cover on Venus prevented every attempt to get a look at its surface. The U.S. Mariner spacecraft could only send back pictures of a world shrouded in clouds. The real breakthroughs came with Pioneer Venus in 1978, Magellan in 1990, and with two Soviet space probes in 1982. Pioneer Venus was able to do some low resolution mapping of the surface. The two Soviet Venera probes landed on the surface, but could only send back a few poorly aimed pictures before they were destroyed by the intense temperature and pressure. Nevertheless, they did show a barren rocky terrain. Magellan used high definition radar to map the planet in detail, and revealed a relatively young surface. It consists of plains covered by lava flows, and highland regions formed by geological activity. Thousands of volcanoes and numerous impact craters dot the surface. Venus is a very active planet undergoing constant change.

A Blue Miracle

As we leave behind the dense clouds of Venus, we next encounter a truly remarkable sight. As we approach the third planet from the Sun, we see a shiny blue sphere of unimaginable beauty, partnered with a singular, large rocky Moon. A delicate veil of thin white clouds encompasses this blue globe. The blue color of this unusual world is caused by liquid water, an rare sight in the Solar System. In fact, this truly unique gift is known to exist only on this one planet. The blue spaces are broken by rocky shapes covered with a color we have never seen before. The beautiful browns of the land masses are painted with various shades of green. These are life forms. This planet is home to living organisms, making it potentially unique in the entire universe. This is the Terran system, comprised of the planet Earth and its lonely Moon.

The Wonder of life

The Terran system is home to the only planet in the universe known to contain the miracle of life. And life on the Earth exists in a myriad of forms and varieties. It is believed that life first started in the Earth about 3.5 billion years ago. These early life forms were simple, single-celled organisms that lived in water. Early life forms were able to capture the energy of the sun through a process known as photosynthesis. Through the miracle of cellular mutation and environmental change, life has evolved in to unimaginably complex forms. Today there are millions of species of animals and plants inhabiting the Earth. One of these creatures even developed self awareness, the intelligence to question its own existence, and the driving curiosity to explore its environment. That creature is man.

The road to modern life forms has been a rocky one. Because our Solar System is populated by a variety of small bodies such as asteroids and comets, there have been many times in Earth's past when life almost didn't survive. In fact, many scientists believe that there have been five mass extinction events since life first exploded on Earth. The last of these events occurred about 65 million years ago when an asteroid larger than mount Everest smashed in to the planet. The impact occurred in an area known as Chicxulub, near the eastern coast of Mexico. That impact and the resulting environmental changes are believed to have wiped out more than half of all species on the planet, including the mighty dinosaurs. Some scientists believe that this event paved the way for the mammals to take control of the planet, which eventually led to the development of modern humans.

The Goldilocks Zone

The Terran system is locates in an area of the Solar System that is not too hot and not too cold. Astronomers refer to this area as the Goldilocks zone, or the habitable zone. This thin ring around the Sun begins just beyond the orbit of Venus and extends to just before the orbit of Mars. This is the area where water can exist in its liquid form. And water is the most important factor in supporting life as we know it. If the Earth was too close to the sun, the water and oceans on the planet would boil away. If it were too far, all of the liquid water on the planet would freeze solid. Fortunately, the Terran system exists in this zone where everything is just right. It is because of this that our planet has been blessed with liquid water and life.

The Double Planets

The Earth's neighbor in space is the Moon. It it the planet's only natural satellite. It is very large in comparison to its parent planet, about a quarter as large. This leads some astronomers to consider the Terran system a double planet system. However, since the common center of mass around which both bodies revolve is located beneath the surface of the Earth, the Terran system is officially labeled as a planet-satellite system. Leading theories suggests that the Moon was formed when a small planet the size of Mars smashed into the early Earth about 4.5 billion years ago. The iron cores of the two planets merged, giving the Earth a larger metal core than the other terrestrial planets. A large portion of the crust and mantles of the two planets was thrown out into space, where it formed a ring of material. This ring of material eventually coalesced to form the Moon.

When the Moon first formed, it was much closer to the Earth. It only took 20 days to orbit the planet instead of the 28 days it takes today. At this time, a day on Earth was only 18 hours long. Since then, the Earth's rotation has slowed down and the Moon has been slowly drifting away. Today, the Moon moves about 1.6 inches farther away each year.

The Moon plays a very important role in the Terran system. Its gravity helps to keep the Earth stable on its axis. Without it, the axis of the Earth would wobble wildly, making it difficult for life to exist. The Moon is also responsible for the ocean tides on Earth. Its gravity pulls on the water, causing a slight elliptical bulge. As the Earth rotates through this bulge, the sea levels rise and fall by several feet. Many scientists believe that life may not have been possible on Earth if there was no Moon.

The Blue Planet

Our first stop in the Terran system is a small planet that could only be described as the most beautiful sight in the entire solar system. Sparkling before us is a bright blue gem of dazzling complexity. Delicate white clouds dance above its surface of brilliant blues, greens, and browns. This is the planet Earth. It is the third planet from the Sun, and it is our home. Only slightly larger than Venus, Earth is the largest of the four terrestrial, or rocky inner planets in the Solar System. It is also the densest of the four.

There are two very important factors that distinguish this planet from all others in the Solar System. First, it is the only planet known to contain liquid water on its surface. It is the availability of this water that gives rise to Earth's second unique feature. It is the only known planet to support life!

Planet of Life

The surface of Earth is literally covered with water. Nearly 3/4 of the planet's surface is liquid water. Water is also found in abundance as ice in the planet's poles and as water vapor in the atmosphere. Most of this water was thought to have been deposited by billions of years of volcanic activity. Some scientists believe that water may also have been brought to Earth by early comets. It is this water, combined with Earth's perfect distance from the Sun, that has given rise to the Solar System's only known life forms.

The atmosphere also played an important factor, providing the right mixture of gases and helping to protect the surface from meteor impacts. Because of these perfect conditions, Earth has evolved an unimaginable diversity of millions of different life forms. Life on Earth has adapted to all types of environments, including the land, oceans, and air. At least one of these has become self-aware and has begun to explore its surroundings. We humans have become the first form of life to leave their planetary home and explore the universe.

Features of Earth

Earth is a rocky world that is still very much alive and active. Volcanic activity continually creates new land while earthquakes reshape the landscape. Earth's surface is constantly changing. The crust is divided up into several "plates" (somewhat like a cracked eggshell) that float on a sea of molten rock. Where these plates collide, the land is thrust upwards to create enormous mountain ranges. The tallest of these mountains, Mt. Everest, rises 29,028 feet (8850 m) above sea level. In some places, one plate sinks underneath another forming deep valleys and trenches. The deepest of these, the Marianas trench, plunges seven miles (11 km) beneath the sea.

Uneven heating of Earth's atmosphere, combined with its rotation causes complex weather patterns. Wind and rain gradually erode the mountains and create vast sandy plains. Even though the planet was bombarded by asteroids and meteors throughout its early history, erosion has wiped out most of the craters. Some can still be found, however. Meteor crater in Arizona is over a mile (1.6 km) wide and almost 600 feet (183 m) deep. It is believed that large meteors may have caused mass extinctions throughout Earth's history. One of these may have been responsible for wiping out the dinosaurs. Although the planet may seem calm and tranquil now, its surface is highly active and ever changing on the wider geologic scale of time.

Luna, Goddess of the Moon

As we finish exploring the wonders of Earth, we turn our attention to a large, cratered object orbiting only 238,800 miles (384,400 km) away. This is Earth's moon, and it is the planet's only natural satellite. Although we call it simply "the Moon", it is associated with the Roman goddess Luna, who was goddess of the hunt and of the Moon. It is the sixth largest moon in the Solar System, and has been Earth's partner for most of the planet's known history. Although it has been lighting our dark nights for so long, its origins are mostly unknown. Some believe it was formed when a gigantic asteroid smashed into the Earth. The resulting impact flung molten material far out into space where it cooled and formed the Moon we see today. Others believe it was a wandering planetoid captured by the Earth's gravitational pull. Wherever it came from, it has fascinated mankind for centuries. The Moon is the only planetary body whose surface can be seen from Earth with the naked eye.

Footsteps in the Lunar Sand

Our fascination with the Moon came to an exciting climax in 1969. On July 20, Neil Armstrong became the first human being to step onto an alien world. The crew of Apollo 11 left their footsteps behind as a tribute to the spirit of human exploration. Five other Apollo missions soon followed. By the time these missions had ended, a total of ten men had walked on the Moon's surface. Their tasks included surveying, mapping, setting up experiments, and collecting samples of rock and sand. The Moon's surface is now littered with evidence of our visits. The remains of 6 lunar landers can be found along with various scientific experiment packages and several American flags. We now know a great deal about our partner in space. Recent unmanned missions have continued mapping the surface and have even detected the presence of water ice on the surface. The presence of water is important if we are to ever establish a permanent base on the Moon.

Features of the Moon

Perhaps the most prominent features of the Moon are its numerous craters. Its surface has been bombarded continually throughout its history. Since the Moon has no measurable atmosphere and no liquid water, there is no erosion taking place. This has preserved the many thousands of craters on its surface.

Another prominent feature on the Moon are the smooth areas called maria. The maria are believed to have been formed by more recent lava flows which have covered over the older craters. There are also many mountain ranges and rift valleys on the Moon's surface. All of these features have combined to form a unique facial feature known as the "Man in the Moon" that can be seen if one knows what to look for. This feature is visible every night of the year because the Moon always keeps its same face towards the Earth. This is because its period of rotation and revolution are the same; 29 days, 12 hours and 44 minutes.

This cycle is known as the lunar month As the Moon orbits the Earth, it appears to change shape as more or less of its sunlit side is visible. When the Moon is on the opposite side of the Earth as the Sun, it appears as a full moon. When it is on the same side, it is invisible, also known as a new moon. Since it is so close to the Earth, the Moon's gravity pulls on our oceans and creates the tides.

One other item in Jupiter's long list of oddities is its magnetic field. It is so strong that is actually extends past the orbit of Saturn. In fact, it is 4000 times stronger than Earth's magnetic field. Even stranger still is the fact that Jupiter's rotation is so fast - only 9.8 Earth hours - that the planet actually bulges out in the center. A close examination of any Jupiter photo will reveal that the planet is more oval shaped than round.

God of the Harvest

As we first enter the Saturnian system, we are presented with a sight of sheer beauty. It is a giant gas planet with a delicate system of silky rings. This is the planet Saturn. It is the sixth planet from the Sun and is the second largest in the Solar System. Saturn was named after the Roman god of agriculture and the harvest. His Greek counterpart was Chronus, the youngest of the Titans.

Many consider Saturn to be the most beautiful object in the Solar System. It is one of the brightest objects in the night sky, and has been known since prehistoric times. Galileo was the first person to observe Saturn through a telescope in 1610. He noticed the planet's odd shape, but was unable to figure out the significance of what he had seen. In 1659, Christiaan Huygens was able to correctly identify the odd shape as Saturn's rings. Since then, Saturn has been visited by three Spacecraft including Pioneer 11, Voyagers 1 and 2, and Cassini. The Cassini probe arrived at Saturn in 2004 and gave us spectacular new views of the planet and its moons. Saturn is surrounded by a small army of 60 known moons second only to Jupiter in number. Most of these moons are small, but seven of them are large enough to warrant further investigation.

The Ringed Planet

Without a doubt the most striking feature of Saturn is its rings. Unlike the rings of Jupiter, which were not discovered until the Voyager 1 encounter in 1979, Saturn's rings have been known to exist since 1659. This is due to the fact that the ring system is quite large and bright. It can easily be seen from Earth with a small telescope. The view of Saturn's rings from Earth actually changes from year to year. This phenomenon occurrs because the tilt of Saturn's rings changes as it orbits the Sun. Because the rings are less than a mile in thickness, they seem to disappear completely every few years when they appear on edge as seen from Earth. This change in the ring system's orientation causes the brightness, or apparent magnitude of the planet to change as well.

Saturn has three main ring bands. They are identified as the A, B, and C-rings. A large gap can be seen between the A and B rings. This is known as the Cassini division, and was named after Giovanni Cassini, the discoverer the gap as well as four of Saturn's largest moons. It wasn't until the Voyager encounters with Saturn in 1980 and 1981 that the true mystery of Saturn's rings was unveiled. The ring system is really composed of seven rings with extremely complex structures. The larger rings are actually composed of many smaller ringlets. The Voyager probes also found strange radial, spoke-like structures in the rings. These are believed to be caused by dust particles. Saturn's innermost ring, the F-ring, appears to be knotted or braided. Astronomers are not sure yet what causes this strange phenomenon.

The Voyager probes revealed the existence of small moons in and near the rings. They are called "shepherding satellites" because they are believed to help keep the rings in place. Without the gravitation of these moons, the ring particles might fly off into space and the rings would be lost.

The origin of Saturn's rings is not well understood. They appear to be composed of millions of tiny particles of ice and rock, each in its own orbit around the planet. The size of these particles ranges from as small as a dust grain to as large as an iceberg. Astronomers believe that the rings could be the debris of one or more moons that were pulverized by asteroid or comet impacts.

Features of Saturn

Even though the ring system is Saturn's most striking feature, the planet is also interesting in a number of other ways. Saturn is a gas giant with a chemical composition very similar to that of Jupiter. It is composed of about 75% hydrogen and 25% helium, with traces of methane, water, and ammonia. Astronomers believe that Saturn's interior is also similar to that of Jupiter, with a core of rock and ice, surrounded by a layer of liquid metallic hydrogen. There is no solid surface on Saturn. Instead, the atmospheric gases get denser and thicker as they approach the planet's center. Saturn is also the least dense of all the planets. Its specific gravity is only 0.7, which is less than that of water. If Saturn could be placed into a giant container of water, it would actually float!

Saturn shares many other features with its close cousin, Jupiter, such as high winds and cloud banding. Winds at the equator can reach an unbelievable 1,100 miles per hour. The horizontal bands on Saturn are much fainter than those on Jupiter. The Hubble space telescope has also revealed that storms occasionally erupt in Saturn's turbulent atmosphere. Like most of the other gas giant planets, Saturn has a strong magnetic field. Like Jupiter, Saturn is visibly flattened at its poles. This is caused by the planet's rapid rotation of only 10 hours, 39 minutes. In July of 2004, the Cassini probe is scheduled to arrive at Saturn. It will orbit the planet and send back detailed images and data. It will also drop the Huygens probe into the atmosphere of Saturn's moon Titan. There it will land and sample the composition of Titan's thick atmosphere and send back images of the surface.

URANUS

Wife of Cronus

The next planetary encounter on our tour brings us to the domain of the ice giants. These planets differ from the gas giants because they are composed primarily of frozen water, ammonia, and methane rather than hydrogen and helium. Here we find a large, blue planet with little or no surface features. It shines like a blue green gem in the heavens. This is the planet Uranus, and it is the seventh planet from the Sun. Uranus was named after the god of the heavens in Greek mythology. According to myth, Uranus was the son and mate of Gaia the father of Cronus (Saturn) and of the Cyclopes and Titans. The planet Uranus was the first planet to be discovered in modern times. The British astronomer Sir William Herschel found it with his telescope in 1781. Before this, it had been assumed to be a star. Herschel also discovered Uranus' two largest moons, Oberon and Titania. We now know of 27 moons in orbit around Uranus. They represent a wide range of sizes and shapes. Uranus has only been visited by one spacecraft. This was the extended Voyager 2 mission. That encounter took place on January 24, 1986. Nearly everything we know about Uranus was learned at this time.

The Sideways Planet

Uranus is different from any other planet in the Solar System in that its axis of rotation is tilted 98 degrees. It is nearly parallel to the ecliptic. Uranus appears to be tipped on its side. This has some interesting effects on the planet. The Sun shines alternately on the poles and equatorial regions of the planet rather than constantly shining on its equatorial regions, as on Earth. Astronomers are not sure what accounts for this strange position. One theory suggests that a massive collision with another planet-sized body may have knocked Uranus on its side. This collision is believed to have taken place very early in the Solar System's history. Another strange fact about Uranus is that its magnetic field is tilted at a 60-degree angle to its axis of rotation. This angle is only 12 degrees on Earth. An interesting effect of Uranus' sideways tilt is that the tail of the planet's magnetic field is twisted like a corkscrew.

Features of Uranus

When Voyager 2 arrived at Uranus, astronomers were expecting to find complex cloud structures similar to those on Jupiter and Saturn. But when the images finally arrived, nothing was visible but a featureless blue-green sphere. False-color images of the planet later showed that banded structures of clouds were present but were extremely faint. They are partially obscured by the overlying layer of methane. Uranus is the third of the gas giants from the Sun. It is believed to be composed of about 83% hydrogen, 15% helium, 2% methane, and trace amounts of acetylene and other hydrocarbons. It is the methane in the planet's upper atmosphere that gives it a blue-green color. This is because the methane absorbs red light. High winds in the middle latitudes of Uranus blow in the direction of the planet's rotation at up to 360 miles (580 km) per hour. Studies have shown somewhat slower winds blowing in the opposite direction at the planet's equator. Astronomers once thought that beneath these layers of clouds was a massive ocean of super-pressurized liquid ammonia and water. But data sent back from Voyager 2 seem to indicate that this is probably not the case. The internal structure of Uranus is probably very similar to that of Jupiter and Saturn, but without the rocky core and liquid metallic hydrogen center. Instead, there is probably a less-dense collection of rocky material at the core. In 1977, Earth-based astronomers discovered that Uranus, like all of the other gas giant planets, has a system of rings. A total of nine rings were identified from Earth. When Voyager 2 visited Uranus in 1986, two more rings were discovered. Two of Uranus' moons, Cordelia and Ophelia, act as shepherding moons for one of the rings. Voyager 2 also discovered that Uranus has a magnetic field. It is tilted 60 degrees from the planet's rotational axis, and is about the same intensity as the magnetic field on Earth. The magnetic field on Earth and the other terrestrial planets is probably caused by electrical currents produced in the planets' molten core. Since Uranus has no molten core, astronomers are not sure what is responsible for creating the magnetic field.

NEPTUNE

God of the Seas

Leaving Uranus and its moons behind us, we next encounter Uranus's sister planet, Neptune, and its system of moons. Neptune is nearly the same size as Uranus, and its composition is extremely similar. It is also the outermost of the two ice giants. Neptune was named after the Roman god of the seas. It was discovered in 1846 by Johann Gottfried Galle of the Berlin Observatory. Neptune has 13 known moons, most of which were not discovered until after Voyager 2's encounter with the planet in 1989. Most of these moons are quite tiny and are probably captured asteroids. The largest of these moons however, has a diameter of 1,680 miles (2,700 kilometers).

The Big Blue Spot

Since Neptune was known to be so similar to Uranus, astronomers were expecting to find another bland and featureless world when Voyager 2 arrived there in August of 1989. Much to the surprise of everyone on the Voyager team, Neptune would turn out to be anything but bland. The most startling feature to be discovered was a giant blue spot in the planet's southern hemisphere. It was very similar in appearance to the famous red spot on Jupiter, and about half the size. In 1994, observations of Neptune by the Hubble space telescope have revealed that the giant blue spot has disappeared. Recently, a new blue spot was observed in the planet's northern hemisphere. Astronomers are not sure why the spot on Neptune was relatively short-lived, while the great red spot on Jupiter has bee present for hundreds of years. This indicates that the Neptune's atmosphere is highly active and prone to sudden and severe changes. Several smaller spots were also noted, as was a small, fast-moving, cloud that astronomers named "scooter".

Features of Neptune

Nearly everything we know about Neptune was learned from the Voyager 2 encounter. Like Uranus, Neptune's blue color is the result of red light being absorbed by methane gas in the planet's upper atmosphere. Neptune's composition is believed to be very similar to that of Uranus. It probably has a small, rocky core about the size of the Earth. This core is thought to be surrounded by an extremely thick layer gases composed mainly of hydrogen and helium with a little methane. Besides the mysterious dark and light spots were other fascinating discoveries. Long, white clouds were seen floating in the planet's upper atmosphere. Neptune has the fastest winds in the Solar System, reaching speeds of 1,200 miles (2,000 kilometers) per hour. Voyager 2 also revealed Neptune's system of delicate rings. There are 4 narrow rings, which are extremely faint. Astronomers think they may be composed of dust particles formed from meteorites smashing into the planet's moons. Like Uranus, Neptune's magnetic field is tilted 47 degrees from its rotational axis. Astronomers believe this is caused by internal forces deep within the planet.

Pluto & Charon

God of the Underworld

As we leave behind us the system of Neptune and its moons, we leave the world of the gas giants and approach the extreme outer limits of our solar system. This region is inhabited by many small, rocky worlds. This region is known as the Kuiper belt, and two of its most famous inhabitants are the planet Pluto and its single moon, Charon. The darkness and isolation of these two worlds has likened them to the underworld. In fact, Pluto was named after the Roman god of the underworld. Charon was named after the mythological figure who ferried the dead across the river Styx to Hades. Pluto was discovered in 1930 by Clyde W. Tombaugh at Lowell Observatory in Arizona. It was much too small to be seen by most telescopes of the time. Astronomers first theorized that a ninth planet might exist when they observed that something seemed to be exerting a gravitational pull on the planet Neptune. An in-depth sky survey finally turned up the tiny planet. Pluto is so far away from the Sun that it takes 248 Earth years to complete one orbit. The temperature here averages -382° F (-230° C).

The Twin Planets

For many decades, Pluto was thought to be alone in its dark corner of our solar system. But in 1978, Jim Christy discovered that Pluto had a companion. This moon, Charon, is more than half the size of its parent planet. In fact, many astronomers once referred to Pluto and Charon as "twin planets" because they are so close to each other in size. Because Charon is so large, it does not actually orbit around Pluto. Rather, the two bodies actually orbit around a common center of gravity somewhere between them. Pluto and Charon are also tidally locked in a synchronous orbit. Pluto's rotational period is 6.3 days. It takes 6.3 days for Charon to make one revolution around Pluto. Thus, the two bodies continuously face each other. In 2005, two additional moons were discovered by the Hubble Space Telescope. Named Nix and Hydra, these tiny moons orbit Pluto at three times the distance as Charon. The orbit of Pluto is so highly eccentric that is actually crosses inside the orbit of Neptune. Because of this, Pluto is actually closer to the Sun than Neptune for 20 years of its 248-year orbit. The 17-degree inclination of Pluto's orbit means that it there is no chance that it will eventually collide with Neptune.

In 2005, two new moons were discovered by the Hubble Space Telescope Pluto Companion Search Team on. Once the orbits were confirmed, the moons were assigned the names Nix and Hyrda. The names were chosen in part because the initials (NH) refer to the New Horizons mission, which is scheduled to orbit Pluto in the year 2015. In Greek mythology, Hydra was the 9 headed serpent who guarded the waters of the underworld and Nix was the goddess of the night. Two additional moons were discovered in 2011 and in 2012, bringing the total to five.

Features of Pluto and Charon

Very little is known about surface of Pluto and Charon. They are so tiny and so far away that even the Hubble space telescope has trouble resolving any surface features on their surfaces. With a diameter of only 1412 miles (2274 km), Pluto is not considered to be a planet. Instead, it belongs to a relatively new category of objects known as dwarf planets. Until recently, no one really knew why this small, rocky  object could be found among the giant gas planets of the outer solar system. But recent discoveries of many similar objects have answered that question. There may be hundreds, even thousands of Pluto-like objects lurking in the outer Solar System near the Kuiper belt. Between 1985 and 1990, the Earth was aligned with the orbits of Pluto and Charon in such a way that a series of eclipses would take place. Astronomers were able to take advantage of this situation to accurately determine the size of both Pluto and Charon, and to create the first image of Pluto showing some of its larger surface features. This revealed that Pluto is the second most contrasty body in the Solar System. Astronomers are hoping that a future space probe may one day be able to show us the surface of this strange and distant world. But until then, every new discovery will bring with it a long list of new questions.

The Lost Planet

For years,many astronomers questioned the status of Pluto as one of the nine planets of the Solar System. They believed that Pluto and Charon could be asteroids or comets that somehow got locked into an orbit around the Sun. Soon after the discovery of the Kuiper belt, most astronomers began to think that these two bodies were actually rogue Kuiper objects and should not longer be classified as planets. The debate raged on for decades, supported by the public's fondness for their favorite planet. Finally, on August 24, 2006, the International Astronomical Union (IAU) decided to redefine the definition of a planet. This decision officially reclassified Pluto and Charon as dwarf planets. Only 76 years after its discovery, Pluto was quietly added to the list of minor planets and given the number 134340. It was the end of an era as the Solar System's ninth planet surrendered its title. Today, we know that many similar objects can be found in this region of space. Some are even larger and farther away than Pluto. The future holds the possibility of many exciting new discoveries as we continue to probe the darkness in search of Pluto's cousins.

MARS

The Red Planet

As we leave behind the blue marble of Earth, we next encounter a smaller rocky world that is totally barren. This is the fourth planet from the Sun, and it is a a bright orange-red world pockmarked with deep scars and enormous mountain peaks. Its red color is the result of millions of years of iron oxidation. This planet is rusty. It is capped on both ends by bright sheets of white ice, and is accompanied by two tiny, lonesome, oddly-shaped bodies. This is the planet Mars, and its two moons, Phobos and Deimos.

Two Tiny Moons

Mars is the last of the four terrestrial inner planets of the Solar System. It is about half the size of Earth and much less dense. Its red-orange color is caused by iron oxide, commonly known as rust. The Martian system contains two tiny moons named Phobos and Deimos. They were discovered in 1877 by Asaph Hall. These little moons look more like asteroids, and most astronomers believe that they may have actually been asteroids that were captured by the gravity of Mars. Both moons orbit very close to the planet. Phobos orbits so close, in fact, that its orbit is slowly decaying. Is it moving closer to the planet each year and will one day eventually crash into the surface.

Past Life

Because scientists believe that liquid water may have once existed on the surface of Mars, it is possible that life may have once existed there as well. Recent missions to the red planet have confirmed that water once flowed across the planet. It is also believed that the atmosphere of Mars was once much thicker than it is today. It is possible that some type of microscopic life forms may have existed on the planet during these ancient times. Many robotic spacecraft have attempted to verify evidence of past life on the planet, but so far this evidence has remained elusive. Future missions will examine the planet in more detail, but it may require a manned mission to the red planet to decide once and for all if life did or did not exist on Mars.

Exploring the Martian System

Because many scientists believe that life may have once existed on Mars, it has been explored more than any other body in the Solar System. A total of 38 missions have been launched to the Red planet, but due to various technical problems, only 19 of these missions succeeded. With a success rate of only 50%, many people have labeled this phenomenon as the Mars curse. Most of the failures occurred in the early years of the Russian Mars program. The United Statues has had better luck, with 13 of 18 missions succeeding in reaching Mars. But even this shows a success rate of only 72%. Fortunately, six out of seven attempts to land on the surface were successful.

The early missions to Mars consisted of a simply flyby of the planet. These missions collected data about the planet's magnetic field and atmospheric structure. The first close-up images of another world were sent from Mariner 4 in 1965. The low resolution images sent back from the spacecraft showed a barren world pockmarked with craters. There was no direct evidence of life and no sign of the famous canals that had been speculated by famous astronomer Percival Lowell. Later missions did, however, find what appeared to be dried stream beds, indicating that water may have once flowed across the surface of Mars at some time in the distant past.

The Viking mission was the first to land on the surface. On July 20, 1976, Viking 1 made a successful landing on Mars and captured the first images of the surface of an alien world. These images showed a barren, rocky terrain totally devoid of life. Later that year, on September 3, Viking 2 landed successfully and sent back similar images of the surface. Both landers took samples of the Martian soil to test for life. All tests were negative. It seemed Mars was a lifeless orb after all.

Since the Mariner and Viking missions, many other spacecraft have visited the Mars. Recent successes include the Mars Global Surveyor, Mars Pathfinder, Mars Odyssey Orbiter, the Phoenix lander, and the now famous Spirit and Opportunity rovers. These new missions have given us our most detailed views yet of the red planet. The Mars Reconnaissance Orbiter spent two years mapping and photographing the surface of the planet in detail. Spirit and Opportunity exceeded their intended life spans by several years and sent back conclusive evidence that liquid water once existed on the surface of Mars. They also revealed many unexpected phenomenon, such as large dust devils wandering across the surface of the planet.

Since the question of life still has not been answered conclusively, many future missions to Mars have been planned. The Mars Science Laboratory will be launched in 2011. This roving vehicle will be larger than Spirit and Opportunity and will perform much more advanced experiments. It will be five times heavier and carry ten times the number of experiments as its predecessors in an attempt to finally determine if live ever existed on the red plant.

It is quite possible that the question of life may only be answered by a manned mission to the Mars. Human beings may be able to find evidence that remains hidden to the robotic spacecraft. NASA plans to send people to Mars eventually, but the high cost and technical challenges may delay such a mission for many more years or even decades.

Dirty Snowballs

In our travels through the Solar System, we may be lucky enough to encounter what appear to be giant balls of ice. These are the comets. Some astronomers have referred to comets as "dirty snowballs" or "icy mud balls" because they are composed mainly of ice with dust and fragments of rock. The ice can be both water ice and frozen gases. Astronomers believe that comets may be composed of the very material that formed the Solar System. Although most of the smaller objects in our solar system represent very recent discoveries, comets have been well known since ancient times. The Chinese have records of comets that date back to 260 B.C. This is because comets are the only small bodies in the Solar System that can be seen with the naked eye. Comets that are in orbit around the Sun can become quite a breathtaking sight.

A Comet's "Tail"

Comets are actually invisible until they begin to get close to the Sun. As they begin to heat up, an amazing transformation takes place. The dust and gases frozen within the comet begin to expand and burst forth at explosive velocities. The solid part of the comet is called the nucleus, while the envelope of dust and gas around it is known as the coma. Solar winds cause the material in the coma to trail behind the comet for a much as a million miles. As the Sun illuminates this material, it begins to glow brightly. This forms the famous tail of the comet. Comets and their tails can usually be seen from Earth and can be quite bright if conditions are right. Some comets may have as many as three separate tails. One will be composed mainly of hydrogen gas, and is invisible to the eye. Another tail of dust glows bright white, while a third tail of plasma usually will take on a thin, blue glow. As the Earth passes through these dust trails left behind by comets, the dust enters the atmosphere and creates meteor showers. Some comets are in an orbit that brings them near the Sun at regular intervals. These are known as periodic comets. Periodic comets lose much of their material every time they near the Sun. Eventually, after all of this material is lost, they will cease to become active and wander the Solar System as a dark ball of rock and dust. Comet Halley is probably the most famous example of a Periodic Comet. Halley makes its appearance every 76 years.

Visitors From Beyond

The sudden appearance of these mysterious objects in ancient times was often viewed as a bad omen and a warning of disaster to come. We know that most of the comets are found in a dense layer at the very edge of our solar system. Astronomers call this the Oort cloud. They believe that gravity from the occasional passing of stars or other objects can knock some of the comets out of the Oort cloud and send them on a journey to the inner solar system. Comets can, and have, struck the Earth. In June of 1908, something exploded high in the atmosphere over Tunguska in Siberia. The explosion had the force of 1,000 Hiroshima bombs and leveled trees for hundreds of miles. The lack of any traces of meteorite fragments have led some scientists to believe that this may have been a small comet that exploded on impact with the atmosphere. A comet impact may also have been responsible for the extinction of the dinosaurs, and many astronomers now believe that ancient comet impacts may have brought most of the water to our planet. Although the possibility exists that the Earth could be hit by a large comet again in the future, the odds of this event happening in our lifetime is more than a million to one. For now, comets will simply continue to be objects of wonder and amazement in the night skies.

Cosmic Leftovers

As we leave the red planet and its moons behind us, we next encounter a strange collection of small planet-like objects. This is the asteroid belt. These asteroids, also known as planetoids, range in size from several hundred miles to several hundred feet. At least 30,000 of these giant rocks are known to be floating in their own individual orbits between Mars and Jupiter. Scientists are not sure why there are asteroids here instead of a planet. Some believe that a large, rocky planet may have existed here in the past. This planet may have then been blasted to pieces by a cataclysmic impact. Others think that the asteroids are composed of the leftover material from which the Solar System was originally formed. The largest of these asteroids, Ceres, is over 630 miles in diameter. It was discovered by Giuseppe Piazzi in 1801.

Bullets in the Sky

Not all asteroids are confined to the asteroid belt. There are several hundred known asteroids that regularly cross the orbit of the Earth. They are known as Earth-crossing asteroids. They range in size from several miles to the size of a small pebble. Sometimes these objects get close enough to the Earth that they are pulled in by its gravity. Outside the Earth's atmosphere, these objects are known as meteroids. They can enter the atmosphere at speeds of up to 45,000 miles per hour. Once a meteroid enters the atmosphere, it is known as a meteor. Most meteors burn up in the atmosphere before they can reach the ground. The bright, glowing streak they leave in the sky is cause by hot ionized gases formed from the heat of friction. Occasionally, a large meteor will survive its trip through the atmosphere to reach the ground. A meteor that has fallen to the ground is called a meteorite. Meteorites can be found in most parts of the world, and are generally composed of stone, iron, or a combination of the two. Several times during the year, hundreds of meteors can be seen in a single night. This event is known as a meteor shower and happens when the Earth passes through the debris left behind by a comet. Most of this debris is the size of dust or sand. Some particles can be a little larger, about the size of a small pebble. These larger particles can cause extremely bright meteors known as fireballs.

The Doomsday Legacy

The Earth-crossing asteroids present some reason for concern. Several hundred of these have been discovered with a diameter of over a mile. At the extremely high speed with which these objects travel, even small asteroids can cause a great deal of damage. Barringer Crater in Arizona is nearly a mile across and was formed by a meteor only 150 feet in diameter. Most scientists believe that the dinosaurs were killed when a asteroid about 10 miles in diameter plowed into the Earth. This impact left a crater nearly 200 miles in diameter. Millions of tons of dust and dirt would have been thrown into the atmosphere, cutting off the light from the Sun. With no sunlight, the plants would die. With no food, the animals would also die off. Scientists believe that several mass extinctions like this may have taken place throughout the Earth's long history. It is believed that there are several hundred Earth-crossing asteroids yet to be discovered. Both professional and amateur astronomers are combing the skies to try to find these elusive objects. Is Earth in danger? The answer is yes. Our planet has been hit by asteroids in the past and it will most definitely be hit again in the future. But don't worry too much. The odds of an event like this happening in our lifetime are a million to one. And chances are that if an asteroid were discovered to be on a collision course with Earth, we would be able to use our technology to stop it. Movies like Deep Impact and Armageddon may not be too far fetched in the not-too-distant future.

The Great Beyond

Not long after we cross the orbit of Pluto, we begin to escape the effects of the Sun. Here we enter interstellar space. From this point on, we will likely not encounter another object until we reach the next nearest star system. The distances between the stars is so incredibly vast, that it would take us 4 years traveling at the speed of light just to reach the closest star to the Sun. The Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2 spacecraft are about to be the first man-made objects to leave the Solar System. These spacecraft have cleared the known boundaries of the Solar System and are searching for the heliopause, which marks the end of the Solar wind and the beginning of interstellar space. Both Spacecraft are expected to continue to operate for the next 25 to 30 years, sending back data on magnetic fields and interstellar particles.

A Guaranteed Solid Gold Hit

In the event that there is other intelligent life out there somewhere, NASA included with both Voyager spacecraft an audio disk entitled "Sounds of Earth" This 12-inch copper disk features greetings from the people of Earth in 60 languages, music from several different cultures, and natural sounds such as ocean surf, thunder, chirping birds, whale song, and others. It could be called "Earth's Greatest Hits". The disk also contains electronic information that an advanced civilization could convert to pictures, diagrams, and printed messages, including a message from President Carter. Both the Pioneer and the Voyager spacecraft also carried a plaque showing the location of our solar system in relation to 14 pulsars and the center of the Milky Way galaxy. These spacecraft may indeed be our ambassadors to the unknown beyond.

The End of the Road

As we pass the outer limits of the Solar System, we are faced with a vast emptiness. Beyond our home system lies the great mysteries of deep space. Here, the distances are so great as to boggle the mind. The nearest star is four light years away. That means that it would take four years to get there if you could travel at the speed of light, which is 186,000 miles per second. We can only look out into that emptiness with our telescopes and be humbled at the wonder of it all. This concludes our Tour of the Solar System.