Mercury – “Messenger of the Gods”
M ercury
This is the 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 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, which rise as
much as 6300 feet (1920 m) above the valley floors.
Statistics for Mercury
Diameter ...... 3031 miles (4880 km)
Number of Satellites ...... 0
Mean Distance from the Sun ...... 35,974,272 miles (57,910,000 km)
Rotational Period ...... 58.65 days
Orbital Period ...... 87.97 days
Orbital Eccentricity ...... 0.206
Orbital Inclination ...... 7.004 degrees
Inclination of Axis ...... 0 degrees
Mean Surface Temperature ...... 354° F (179° C)
Main Atmospheric Component ...... Helium
Atmospheric Depth ...... negligible
Apparent Magnitude ...... -1.9
Mercury's Atmospheric composition
Helium42%
Sodium42%
Oxygen 15%
Other 1%
Detection Method : Naked Eye
Year Discovered : Prehistory
Mass : 0.055 Earths
Mean orbital velocity (km/sec) 47.88
Venus – “Goddess of Love and Beauty“
Our next stop on the tour brings us to a planet completely covered by wispy white clouds. This is the planet Venus, and it is the
second planet from beauty. It is also known as the jewel of the sky, and as the morning star. Venus is so bright in the sky that it has
been known since prehistoric times. Venus is sometimes referred to as Earth's sister planet, as both are nearly the same size and
have nearly the same mass. Venus differs from all other bodies in the Solar System because it rotates from east to west. It is, in a
sense, upside down. Another odd fact about this planet is that its day is longer than its year! Venus takes 225 Earth days to make
one complete revolution around the Sun, while it takes 243 days for Venus to rotate once on its axis.
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.
Statistics for Venus
Diameter ...... 7518 miles (12,103 km)
Number of Satellites ...... 0
Mean Distance from the Sun ...... 67,214,920 miles (108,200,000 km)
Rotational Period ...... 243.0 days
Orbital Period ...... 224.7 days
Orbital Eccentricity ...... 0.007
Orbital Inclination ...... 3.39 degrees
Inclination of Axis ...... 3 degrees
Mean Surface Temperature ...... 899° F (482° C)
Main Atmospheric Component ...... Carbon Dioxide
Atmospheric Pressure ...... 92 bars
Atmospheric Depth ...... 100 miles (161 km)
Apparent Magnitude ...... -4.4
Venus Atmospheric composition
Carbon dioxide : 96%
Nitrogen : 3+%
Trace amounts of: Sulfur dioxide, water vapor,
carbon monoxide, argon, helium, neon,
hydrogen chloride, and hydrogen fluoride.
Detection Method: Human Eye
Year Discovered: Prehistory
Mass: 0.814 Earths
Orbital Velocity: 126, 077 km/h
Earth – “The Blue Planet“
As we leave the shrouded mysteries of Venus behind us, we find ourselves presented with what 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 next stop on our tour, 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 planets. 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 Water, 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 and Earthquakes
frequently 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 largest animals that ever lived 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.
Statistics for Earth
Diameter ...... 7924 miles (12,756 km)
Number of Named Satellites ...... 1
Mean Distance from the Sun ...... 92,933,000 miles (149,600,000 km)
Rotational Period ...... 23 hrs, 56 min
Orbital Period ...... 365.2 days
Orbital Eccentricity ...... 0.017
Orbital Inclination ...... 0 degrees
Inclination of Axis ...... 23.45 degrees
Mean Surface Temperature ...... 59° F (15° C)
Main Atmospheric Component ...... Nitrogen
Atmospheric Pressure ...... 1.013 bars
Atmospheric Depth ...... 120 miles (193 km)
Atmosphere
Composition 78.08% Nitrogen (N2)
20.95% Oxygen (O2)
0.93% Argon
0.038% Carbon dioxide
About 1% water vapor (varies with climate)
Mass 5.9736 × 10^24 kg
Average orbital speed 29.783 km/s
107,218 km/h
Mars – “God of War”
M
ars
With the Earth system well behind us, we next encounter a beautiful, bright red world. This is the planet Mars, and it is the fourth
planet from the Sun. It was named after the Roman god of war because its color reminded observers of a bloody Roman battlefield.
Early astronomers believed they could see faint, dark lines crossing the surface of the planet. Some believed these to be artificially
built canals. This gave rise to some of the early speculation of intelligent life on Mars. These lines were later found to be an optical
illusion. Later unmanned missions to the planet found no evidence of the so-called canals. Still, there is perhaps no other body in the
Solar System that has intrigued us more than Mars. To this very day, it is still one of the most likely candidates for extraterrestrial
life. In fact, meteorites recently discovered in Antarctica are believed to have originated from the red planet. These ancient Martian
rocks may contain fossils of tiny bacteria. If this turns out to be true, it will be the first evidence that life evolved on other planets
besides Earth.
The Red Planet
The bright red color of Mars has earned it the name of red planet. This unique color is caused by the chemistry of its surface. Mars
does have a thin atmosphere, which helps cause iron compounds to oxidize. It is this iron oxide in the rocks and sand that gives the
planet it reddish-brown color. It could very appropriately be called the "rusty" planet. We have learned a great deal about Mars in
the last few decades. Several unmanned space missions have been sent to explore the red planet. Several Mariner spacecraft
orbited the planet in the mid and late 1960's. In 1975, two Viking spacecraft landed on the surface, sending back the first color
pictures of this alien world. Recently, the Mars Pathfinder mission visited the planet. A small roving vehicle known as the Sojourner
Rover explored the surface sending back the sharpest pictures to date. Scientists continue to be fascinated with Mars because of its
similarities to Earth. Some have even suggested that we may be able to colonize the planet in the not too distant future. If the planet
does turn out to be completely lifeless, it may be a candidate for a process known as terraforming. This process would involve using
various chemical and biological means to make the planet warmer and the atmosphere more hospitable for human life.
Features of Mars
Mars is a small, rocky world about half the size of Earth. As small as it is, however, it is a planet of extremes. Mars is the home of the
largest known volcano in the Solar System. Olympus Mons is an extinct volcanic crater that rises 15 miles above the Martian surface.
That is over twice the height of the tallest mountain on Earth, Mt. Everest. It is a scar so large it can be seen from Earth. Another
gigantic feature of the red planet is Valles Marinares. This is the Martian equivalent of the Grand Canyon. This one is much bigger,
however. Valles Marinares stretches 3000 miles (4829 km) along the surface of Mars and is 435 miles (700 km) wide at its widest
point. If this canyon existed on Earth, it would stretch across the continental United States from coast to coast! During the Martian
winters, the poles of the planet are covered with large sheets of water ice. Several features that resemble riverbeds suggest that
there may have been running water on Mars at one time. The thin atmosphere of Mars is composed mainly of carbon dioxide. Winds
of up to 300 miles per hour whip up enormous dust storms that constantly erode and reshape the surface of the planet. With an
average surface temperature of 60 degrees below zero, it may not seem very hospitable. But with its thin atmosphere and
abundance of water, Mars remains the most likely candidate for life anywhere in the Solar System. A future manned mission to the
planet is in the works. Only then will we be able to tell if the red planet is really a dead planet.
Statistics for Mars
Diameter ...... 4220 miles (6794 km)
Number of Named Satellites ...... 2
Mean Distance from the Sun ...... 141,050,000 miles (227,940,000 km)
Rotational Period ...... 24.62 hours
Orbital Period ...... 687 days
Orbital Eccentricity ...... 0.093
Orbital Inclination ...... 1.85 degrees
Inclination of Axis ...... 23.98 degrees
Mean Surface Temperature ...... -81° F (-63° C)
Main Atmospheric Component ...... Carbon Dioxide
Atmospheric Pressure ...... 0.007 bars
Atmospheric Depth ...... 100 miles (161 km)
Apparent Magnitude ...... -2.01
The Moons of Mars
Name






Radius (km) Distance (km)
Year
Discovered
Phobos 13.5x10.8x9.4
9,380
1877
Deimos
23,460
1877
7.5x6.1x5.5
Carbon dioxide: 95.32 percent
Nitrogen: 2.7 percent
Argon: 1.6 percent
Oxygen: 0.13 percent
Carbon monoxide: 0.08 percent
Also, minor amounts of: water, nitrogen oxide, neon, hydrogen-deuterium-oxygen, krypton and xenon
Detection Method : Naked Eye
Year Discovered : 1500’s
Mass : 0.107 Earths
Mean orbital velocity (km/sec) 24.13
Jupiter – “King of the Gods”
Ju
piter
As we enter the Jovian system, we are presented with a truly awesome sight. It is a gaseous planet of enormous proportions. This is
the planet Jupiter. It is the fifth planet from the Sun and is the largest. In fact, Jupiter is almost twice the size of all of the other
planets combined. Over one thousand Earths could fit inside if it were hollow. Because of its gigantic size, Jupiter was named after
the king of the gods in Roman mythology. In Greek mythology, he was known as Zeus, and was famous for hurling lightning bolts at
earthbound mortals. Jupiter is the fourth brightest object in the sky, so it has been known to exist since prehistoric times. But it was
Galileo who discovered Jupiter's four largest moons, Callisto, Europa, Ganymede, and Io, in 1610. We now know of 63 moons in orbit
around Jupiter, ranging in size from just a few miles across to nearly Earth-sized. The Jovian system is like a miniature solar system
within a solar system. With so many satellites tugging on Jupiter with their tidal forces, these moons are actually causing Jupiter's
rotation to slow down. These same tidal forces are also pushing the moons farther away from the massive planet.
The Famous Red Spot
One of the most famous features of Jupiter is the giant red spot. This spot is actually a gigantic storm system. It is a high-pressure
region where the winds blow in a counter-clockwise direction at over 250 miles (402 km) per hour. The cloud tops are much higher
and colder than the surrounding clouds. This accounts for the spot's darker color. With a diameter of more than 16,000 miles
(25,000 km), this storm is twice the size of the Earth! Many spots and colorful bands appear and disappear in Jupiter's turbulent
atmosphere, but the great red spot has been know to exist for more than 300 years. Astronomers do not yet know what forces have
kept this storm together for so long. And nobody knows just how much longer it will last. Spots of a different kind were seen in 1994
when a comet named Shoemaker-Levy 9 smashed into the planet. The comet had broken into 21 separate fragments as a result of a
close encounter with Jupiter's massive gravitational forces. On July 16 1994, for seven days, these fragments bombarded the giant
planet. The impacts left monstrous black scars on the Jupiter's surface. Explosive plumes several times the size of Earth were
observed, and the dark scars remained visible on the planet for nearly a year. It was one of the most watched astronomical events in
history, and it proved once and for all that comets do hit planets.
Features of Jupiter
Jupiter has amazed and intrigued astronomers for centuries. It has been visited by a total of six spacecraft, including the current
Galileo mission. In 1995, Galileo dropped a probe into Jupiter's atmosphere. The probe took measurements and gave us our first
clues as to what was inside Jupiter's thick atmosphere. With this data, and data collected on previous missions, we now know that
Jupiter is essentially a gigantic ball of gas. The planet is composed mainly of hydrogen, with helium and traces amounts of other
gases. One surprising find was that there is much less water in Jupiter's atmosphere than previously expected. Astronomers believe
that Jupiter has no solid surface. Instead the gases get thicker and denser as they get closer to the planet's center. The extreme
pressure at Jupiter's core compresses the hydrogen into a rare liquid metallic state. The planet may also have a small, rocky core
about 10 times the size of Earth. Jupiter's outer atmosphere is extremely dynamic. Colorful bands and spots can be see rotating and
changing regularly. The colors in the bands are believed to be the result of chemical reactions in the atmosphere. The Galileo probe
measured wind speeds exceeding 400 miles (643 km) per hour. A big surprise revealed by the Voyager 1 spacecraft was that Jupiter
has rings, much like Saturn. But unlike Saturn's rings, Jupiter's rings are very delicate and simple. These rings are divided into three
main segments, and were not known to exist prior to Voyager's discovery because they are too thin to be seen from Earth. 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.
Statistics for Jupiter
Diameter...... 88,823 miles (142,984 km)
Number of Named Satellites...... 62
Mean Distance from the Sun...... 482,546,000 miles (778,330,000 km)
Rotational Period...... 9.84 hours
Orbital Period...... 4333 days
Orbital Eccentricity...... 0.0483
Orbital Inclination...... 1.308 degrees
Inclination of Axis...... 3.12 degrees
Mean Cloud Temperature...... -185° F (-121° C)
Main Atmospheric Component...... Hydrogen
Atmospheric Pressure...... 0.7 bars
Atmospheric Depth...... 100 miles (161 km)
Apparent Magnitude...... -2.7
Jupiter's composition is mainly hydrogen and helium. In contrast to planetary bodies covered with a hard surface crust
(the Earth, for example), the jovian surface is gaseous-liquid, rendering the boundary between the atmosphere and the
planet itself almost indistinguishable. Below the roughly 1000-kilometer-thick atmosphere, a layer of liquid hydrogen
extends to a depth of 20,000 kilometers. Even deeper, it is believed that there is a layer of liquid metallic hydrogen at a
pressure of 3 million bars. The planet core is believed to comprise iron-nickel alloy, rock, etc., at a temperature
estimated to exceed 20,000C.
Detection Method : Normal Sight
Year Discovered : Prehistory
Mass (kg) : 1.900e+27
Mean orbital velocity (km/sec) : 13.07
Saturn – “God of the Harvest”
Sa
turn
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 Kronos, the youngest of the Titans. Many
consider Saturn to be the most beautiful object in the Solar System. It is one of the more bright 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 is caused by the Earth passing through the plane of Saturn's rings. Every few years, the rings are seen edge-on and
actually seem to disappear completely. This is because the rings are only slightly more than a mile thick. 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 sea 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.
Statistics for Saturn
Diameter
Number of Named Satellites
Mean Distance from the Sun
Rotational Period
Orbital Period
Orbital Eccentricity
Orbital Inclination
Inclination of Axis
Mean Cloud Temperature
Main Atmospheric Component
Atmospheric Pressure
Atmospheric Depth
Apparent Magnitude
...... 74,565 miles (120,536 km)
...... 33
...... 884,740,000 miles (1,429,400,000 km)
...... 10.25 days
...... 29.46 years
...... 0.056
...... 2.488 degrees
...... 26.73 degrees
...... -193° F (-125° C)
...... Hydrogen
...... 1.4 bars
...... 90 miles (145 km)
...... 0.67
Saturn's Atmosphere
Composition
~96% Hydrogen (H2) ~3% Helium ~0.4% Methane ~0.01% Ammonia ~0.01%
Hydrogen deuteride (HD) ~0.000 7% Ethane
Mass
95.152 Earths
Average orbital speed
\
9.69 km/s
Uranus – “Father of Saturn”
The next planetary encounter on our tour brings us to yet another gas giant system. Here we find a giant blue planet with little or no
surface features. It shines like a blue gem in the heaves. 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.
Statistics for Uranus
Discovered by ...... William Herschel
Year of Discovery ...... 1781
Diameter ...... 31,755 miles (51,118 km)
Number of Named Satellites ...... 27
Mean Distance from the Sun ...... 1,783,487,000 miles (2,870,990,000 km)
Rotational Period ...... 17.3 hours
Orbital Period ...... 84 years
Orbital Eccentricity ...... 0.047
Orbital Inclination ...... 0.774 degrees
Inclination of Axis ...... 97.86 degrees
Mean Cloud Temperature ...... -315° F (-193° C)
Main Atmospheric Component ...... Hydrogen
Atmospheric Pressure ...... 1.2 bars
Apparent Magnitude ...... 5.52
Uranus Atmosphere
Composition
~83 %Hydrogen ~15% Helium ~2.3% Methane
Mass: 14.536 Earths
Average orbital speed 6.81 km/s
Neptune – “God of the Seas”
Nep
tune
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 great gas 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.
Statistics for Neptune
Discovered by ...... Johann Gottfried Galle
Year of Discovery ...... 1846
Diameter ...... 30,744 miles (49,492 km)
Number of Named Satellites ...... 13
Mean Distance from the Sun ...... 2,798,116,000 miles (4,504,300,000 km)
Rotational Period ...... 15.8 hours
Orbital Period ...... 165 years
Orbital Eccentricity ...... 0.009
Orbital Inclination ...... 1.774 degrees
Inclination of Axis ...... 29.31 degrees
Mean Cloud Temperature ...... -315° F (-193° C)
Main Atmospheric Component ...... Hydrogen
Atmospheric Pressure ...... 2 bars
Apparent Magnitude ...... 7.84
Neptune's Atmospheric Composition
Composition 80%
Hydrogen
19%
Helium
1.5%
Methane
~0.00015% Ethane
Mass
17.147 Earths
Average orbital speed 5.43 km/s
Pluto – “God of the Underworld”
Pluto &
Charon
Hubble photo of Pluto
& its moon Charon
Recent Hubble image of
Pluto & its four moons
Digitally enhanced Hubble
image of Pluto surface
(NASA/JPL)
(NASA/JPL)
(NASA/JPL)
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.
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.
Statistics for Pluto
Discovered by
Year of Discovery
Diameter
Number of Named Satellites
...... Clyde W. Tombaugh
...... 1930
...... 1412 miles (2274 km)
...... 3
Mean Distance from the Sun ...... 3,673,537,000 miles (5,913,520,000 km)
Rotational Period
Orbital Period
Orbital Eccentricity
Orbital Inclination
Inclination of Axis
Mean Surface Temperature
...... 6.3 days
...... 248 years
...... 0.250
...... 17.14 degrees
...... 122.5 degrees
...... -382° F (-230° C)
Main Atmospheric Component ...... Methane
Apparent Magnitude ...... 15.12
Composition:
nitrogen, methane
Mass: 0.0021 Earths
average orbital speed 4.666 km/s