The Far Planets

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The farther we penetrate the unknown, the vaster and more marvelous it becomes.

—Charles A. Lindbergh, 1974

The far planets are Jupiter, Saturn, Uranus, Neptune, and Pluto. They lie far from the Sun, in the coldest and darkest part of the solar system.

In ancient times people noticed that some lights in the sky followed odd paths around the heavens. The Greeks called them asteres planetos, or wandering stars. Later they would be called planets. The ancients could see only two of the far planets in the nighttime sky—Jupiter and Saturn.

Jupiter was named for the mythical Roman god of light and sky. He was the supreme god also known as Jove or dies pater (shining father). His counterpart in Greek mythology was named Zeus. Saturn was named after the god of agriculture, who was also Jupiter's father. His Greek counterpart was called Kronos.

Following the invention of the telescope, three more of the far planets were discovered—Uranus, Neptune, and Pluto. Uranus was named for the father of the god Saturn. Neptune was the god of the sea and Jupiter's brother in Roman mythology. Pluto was named after the Greek god of the underworld.

When the Space Age began humans sent robotic spacecraft to investigate the far planets. They returned images of strange and marvelous worlds composed of gas and slush instead of rock. Many new moons were revealed. Some of these moons are covered with ice and have atmospheres. There could be liquid water beneath that ice teeming with life. This possibility is particularly appealing to space scientists and to all people who wander if we are alone in the universe.


It took three centuries for humans to uncover the far planets in our solar system. In the 1600s the telescope opened up new opportunities for observation. People learned that Jupiter and Saturn had moons and that Saturn had rings. The telescope also showed that wandering stars were not stars at all, because they did not generate their own light, but reflected light from the Sun.

No new planets were discovered during the 1600s. The far planets were still too distant and fuzzy to be recognized for what they were. Uranus was discovered in the late 1700s. Another century passed before the discovery of Neptune. Pluto, discovered in 1930 was the last planet found in the solar system.

Astronomers categorize planets based on geology and composition. Mercury, Venus, Earth, and Mars are called the terrestrial planets, because they are made of rock and metal. They have solid surfaces on which spacecraft could land. Jupiter, Saturn, Uranus, and Neptune are called the gas giants. Some scientists think they may have solid cores, but the exterior of these planets consists of huge clouds of gas. These planets are also known as the Jovian planets (after Jove or Jupiter). All of them have ring systems.

Pluto is in a class by itself. It is a small ice world. Astronomers argue whether it is even a planet at all. Some believe that it is a dormant comet. Nevertheless, Pluto has been classified as a planet for decades now, and will likely continue to be so.


Jupiter is the fifth planet from the Sun and the largest planet in this solar system. It takes the planet nearly twelve years to make one orbit around the Sun. Jupiter is eleven times larger than Earth. The planet is bright enough to be seen with the naked eye and appears yellowish from Earth. Jupiter is similar in composition to a small star and has an incredibly powerful magnetic field that stretches out millions of miles. The poles experience dazzling auroras many times more powerful and bright than the Northern Lights on Earth.

Jupiter's atmosphere is 90 percent hydrogen. The other 10 percent is mostly helium, with a tiny bit of methane, water, and ammonia. Its sky is streaked with clouds and often with lightning. A gigantic hurricane-like storm has raged on the planet for hundreds of years, if not longer. It is a cold high-pressure area that is two to three times wider than Earth. The storm is nicknamed Jupiter's Great Red Spot. The red color is probably due to the presence of certain chemicals within the storm.

Scientists believe that Jupiter's surface is not solid, but slushy. The planet has dozens of moons. They are named for the lovers and children of Jupiter or Zeus. Jupiter also has a very light ring of material that orbits the planet.

Galileo Is First to Discover Jupiter's Moons

On January 7, 1610, the Italian astronomer Galileo Galilei (1564–1642) was looking through his homemade telescope and discovered four celestial objects near Jupiter. At first he thought they were stars. After watching them for a week Galileo realized that they were satellites in orbit around Jupiter. Two months later Galileo published his findings in Sidereus Nuncius (Starry Messenger).

That same year the German Astronomer Simon Marius published a book called Mundus Iovialis (The Jovian World) in which he claimed that he discovered the satellites before Galileo. Marius did not provide any observational data in his book, and Galileo was better respected. The credit was given to Galileo.

In his book Marius proposed the names Io, Europa, Ganymede, and Callisto for the satellites. In Greek mythology these characters were lovers of Zeus. Marius said that fellow astronomer Johannes Kepler (1571–1630) suggested the names to him. Galileo referred to the moons as the Medician stars (to honor the family that ruled his Italian province) and numbered them from one to four. This naming convention was used for two centuries.

Renaming Jupiter's Moons

During the 1800s astronomers decided that a numbering system was too complicated for the moons of planets. More and more of the satellites were being discovered as telescopes improved. It was decided to name moons after literary characters from myths, legends, plays and poems. Galileo's Medician moons were renamed Io, Europa, Ganymede, and Callisto as Marius had suggested.

More Jupiter Moons

The number of known satellites around Jupiter increased slowly until the twentieth century. Many of Jupiter's named moons were discovered in 1979 and 2000. In 2003 astronomers at an observatory atop Mauna Kea in Hawaii spotted twenty-three previously unknown moons around Jupiter. As of March 2004 these moons have not yet received names from the International Astronomical Union (IAU).


Saturn is the sixth planet from the Sun, and the second largest planet in our solar system. It takes 29.5 Earth years to orbit around the Sun. Saturn's atmosphere is mostly hydrogen, with a little helium and methane. It is a hazy yellow color. The planet is very windy, with wind speeds reaching 1,000 miles per hour.

Saturn is very flat at the poles. The planet is surrounded by many thin rings of orbiting material that circle near its equator. Saturn has dozens of moons. They are named after various characters from Greek and Roman mythology (mainly Saturn's siblings, the titans) and after giants from Gallic, Inuit, and Norse legends.

Galileo Sees Saturn's Handles

In 1610, when Galileo first saw Saturn through his telescope, its rings appeared to him to be two dim stars on either side of the planet. He described these stars as "handles." In 1612 Galileo reported that he could no longer see the dim stars. Much to his amazement, they had disappeared.

In the following years other astronomers saw the strange shapes around Saturn. They were variously described as ears or arms extending from the planet's surface. It would take an improvement in telescopic power before their true nature was revealed.

Huygens Finds a Moon and a Ring

Christiaan Huygens (1629–1695) was a Dutch astronomer who became famous for his observations of Saturn. He and his brother Constantyn built new and more powerful telescopes that were greatly admired by astronomers of the time.

On March 25, 1655 Christiaan Huygens discovered a satellite around Saturn. This turned out to be the planet's largest moon. In 1656 Huygens wrote about his discovery in De Saturni Luna Observatio Nova (The Discovery of a Moon of Saturn). Huygens referred to his discovery as simply Saturn's Moon. Later, it would be called Titan.

Huygens also figured out that the mysterious shapes near Saturn were not stars, arms, or ears, but a ring of material around the planet. Huygens mistakenly thought the ring was one solid object. In 1659 he published his observations in Systema Saturnium (The Saturn System).

Ring Plane Crossings

Huygens explained that the ring around Saturn was difficult to view, because it is very thin. Every fourteen to fifteen years the Earth moved into the same plane as the ring. If you tried to observe Saturn from Earth at this time, Huygens said, you would be viewing the outer edge of the ring head-on, making it virtually invisible. This explained why Galileo was unable to see the handles around Saturn in 1612. It was a year in which Earth passed through Saturn's ring plane.

There are several planetary alignments that cause Saturn's rings to be invisible to Earth observers. One of these is when Earth passes into the Saturn ring place. A similar effect occurs when the Sun passes through Saturn's ring plane and when the Sun and Earth are on opposite sides of the ring plane. The next Saturn ring plane passage will occur in August and September of 2009. Throughout history ring crossings have been the best times to discover new moons around Saturn.

More Moons of Saturn

Giovanni Cassini (1625–1712) was born in Italy but lived in France. He was the first director of the Royal Observatory in Paris. During the late 1600s he discovered four more of Saturn's moons. The first two he observed during the ring crossing of 1671–1672. The second pair he discovered just prior to the ring crossing of 1685.

Over the next three centuries many more Saturn moons were discovered. The latest discovery occurred in September 2000 when astronomers at the Mauna Kea observatory in Hawaii found eight previously unknown moons around Saturn.

Cassini and Saturn's Rings

During the 1600s Giovanni Cassini discovered a major gap in the ring around Saturn. This proved that the structure was not one solid object as Huygens had thought. The gap would later be called the Cassini Division. Cassini believed that Saturn's rings were composed of millions of small particles. This view was shared by French astronomer Jean Chapelain (1595–1674). However, it was not generally accepted until the eighteenth century.

Modern astronomers believe that the rings are composed of chunks of ice. These chunks range in size from tiny particles to icebergs as large as automobiles. Saturn's ring system is actually many ringlets of different sizes nestled within each other.


Uranus is the seventh planet from the Sun, with the third largest diameter in the solar system. It was named for the father of the god Saturn in Roman mythology.

Uranus looks featureless through even the most powerful telescopes. Scientists believe the planet is shrouded in clouds that hide it from view. The presence of methane in the upper atmosphere is believed to account for the planet's light blue-green color. It takes eighty-four Earth years for Uranus to orbit around the sun. Uranus is unique in the solar system, because its axis is tilted so far from its orbital plane. The planet lies on its "side" as it orbits with a pole pointed toward the Sun.

Uranus has nearly two dozen moons. They are named after characters from the plays of William Shakespeare (1564–1616) and from the poem The Rape of the Lock by Alexander Pope (1688–1744).

Herschel Discovers Uranus and Two of Its Moons

The astronomer Frederic William Herschel (1738–1822) was born in Germany, but lived and worked in Britain. He dropped his first name and was commonly known as William Herschel. On March 13, 1781, he was searching the sky with his telescope when he discovered Uranus. Herschel wanted to name the planet "Georgium Sidus" in honor of King George III of England. However, the name Uranus was selected from ancient mythology.

A few years later, in 1787, Herschel was the first to spot satellites around the planet. He discovered the two largest moons named Titania and Oberon.

More Moons

During the mid-1800s two more moons were discovered around Uranus by the British astronomer William Lassell (1799–1880). It was another century before the next moon was found. During the 1980s and 1990s more than a dozen new moons were added to the list. The most recent moon discovery took place in 2001 by astronomers at the Cerro Tololo Inter-American Observatory in Chile.


Neptune is usually the eighth planet from the Sun. (Occasionally the erratic orbit of Pluto brings that planet in closer than Neptune). Neptune was named after the mythical Roman god of the sea. The planet is very far from Earth and extremely difficult to observe. The planet has a distinctive bluish hue when viewed through a telescope. It orbits around the sun once in 165 Earth years.

Neptune has several moons. They are named after characters associated with Neptune or Poseidon (his Greek counterpart) or other sea-related individuals in ancient mythology.

Neptune's Controversial Discovery

Neptune's discovery is a twisted tale of mathematics, bureaucrats, and international competition. Following the discovery of Uranus in 1781, astronomers watched that planet for several decades. They were puzzled because its orbit did not follow the path expected. Some astronomers began to suspect that there might be another planet beyond Uranus. The effect of its gravity would explain the irregularities that astronomers saw in the orbit of Uranus.

During the 1840s two scientists used mathematics to figure out where this mystery planet could be located. Their names were Urbain Le Verrier (1811–1877) of France and John Couch Adams (1819–1892) of Britain. Adams presented his theory to the director of the Cambridge Observatory in England. For some reason the director did not pursue the matter and look for the unknown planet. Le Verrier submitted his theory to the Berlin Observatory. It was directed by Johann Galle (1812–1910). On the night of September 23, 1846, Galle used Le Verrier's notes to locate the planet in the sky.

When Le Verrier and Galle publicized the discovery, the director of the Cambridge Observatory complained that Adams had described the location of the planet months before Le Verrier. It turned into a heated argument between France and England. Astronomers decided to split the credit for the planet's discovery between Adams and Le Verrier. Galle is considered the first to observe the planet. However, a review of Galileo's notes from the 1600s revealed that Galileo actually spotted the planet centuries before, but thought it was a fixed star.

Lassell Discovers a Moon Around Neptune

Only weeks after Neptune's discovery its first moon was discovered. In early October of 1846 an amateur British astronomer named William Lassell (1799–1880) spotted the moon. It was named Triton after the sea-god son of Poseidon. The name was suggested by the French astronomer Camille Flammarion (1842–1925).

More Moons of Neptune

During the 1940s another moon around Neptune was discovered from a ground-based telescope. In 1989 images from the spacecraft Voyager 2 revealed six previously unknown moons. In 2003 astronomers with the National Research Council of Canada and the Harvard-Smithsonian Center for Astrophysics discovered three more moons using telescopes at an observatory atop Mauna Kea in Hawaii and the Cerro Tololo Inter-American Observatory in Chile. As of March 2004 these moons had not yet been assigned names by the International Astronomical Union (IAU).


Pluto is the smallest planet in the solar system and usually the farthest planet from the sun. It is named after the mythical Greek god of the underworld. It takes Pluto 248 Earth years to circle the sun. Pluto's orbit is very erratic. For twenty of those 248 years the planet orbits closer to the Sun than does Neptune. This last occurred between 1979 and 1999.

Pluto is believed to be a dark and icy world. It has been observed and photographed only from great distances. No spacecraft have ever been near the planet.

Pluto has only one known moon, Charon (pronounced "Karen"). It is named after a character in Greek mythology who ferried the souls of the dead across the river Styx in the underworld.

Tombaugh Discovers Pluto

Clyde Tombaugh (1906–1997) is credited with discovering the planet Pluto. Tombaugh made the discovery while working at the Lowell observatory in Flagstaff, Arizona. This famous observatory was founded in the 1890s by Percival Lowell. For years Lowell had searched for a planet believed by some astronomers to lie beyond Neptune. Following Lowell's death the observatory continued the search. On February 18, 1930, Tombaugh found Pluto after diligently photographing the sky for many nights and studying the photographs for objects that changed position relative to the fixed stars.

The Naming of Pluto

Lowell's widow wanted to name the planet after her late husband. This was not allowed, because it would have broken the tradition of using names from Greek and Roman mythology. The name Pluto was finally selected from many suggestions made by the public.

Pluto was the Greek god of the underworld, and able to make himself invisible. The name seemed appropriate for the darkest planet in the solar system that had been so difficult to find. Also, the first two letters of the name matched the initials of Percival Lowell. The name Pluto was originally suggested by an 11-year-old British girl named Venetia Burney.

Christy Discovers Pluto's Moon

On June 22, 1978, Pluto's moon Charon was discovered by James Christy at the U.S. Naval Observatory in Washington, DC. Christy was studying photographs of the planet when he noticed an odd shape in some of the images. After comparing photographs he realized that the shape moved over time compared with Pluto and the fixed stars. When the discovery was made public Christy suggested the name that was assigned to the moon.


The far planets have not been as popular as the Moon and Mars in science fiction stories. One of the first mentions of Jupiter occurs in an 1894 story called A Journey in Other Worlds. In this story Jupiter is similar to a prehistoric Earth. Skeleton Men of Jupiter was an unfinished story written by Edgar Rice Burroughs. It appeared in print during the 1940s. The story referred to a Jupiter-like world called "Sasoom," inhabited by creatures that looked like human skeletons. In Arthur C. Clarke's 1968 novel, 2001: A Space Odyssey, the exploration of the solar system reaches Saturn, while in the Stanley Kubrick film of the same name, much of the story takes place near Jupiter.

Advances in telescopes and astronomy made it clear that Jupiter and the other far planets were gaseous worlds without solid surfaces. This made them much less appealing as home worlds for aliens. During the 1990s scientists learned that larger moons in the outer solar system may have thick atmospheres and some organic chemicals in their composition. This increases the chances that life could exist there. These moons became popular home worlds for sea-faring creatures in science fiction stories.


During the early 1970s the United States began a series of interplanetary missions designed to explore the far planets. The first of these missions was aptly named Pioneer.

Pioneer spacecraft were the first to investigate Jupiter and Saturn. The missions were managed by NASA's Ames Research Center in Mountain View, California, for the agency's Office of Space Science. The two spacecraft involved were called Pioneer 10 and Pioneer 11. The total cost of their mission was approximately $350 million.

On each spacecraft was mounted a 6-inch by 9-inch metal plaque with greetings from Earth. The plaque included illustrations of a human man and woman, the spacecraft's silhouette, and some mathematical, chemical, and astronomical data represented in binary code symbols. An image of the solar system at the bottom of the plaque shows a Pioneer spacecraft leaving Earth and passing between Jupiter and Saturn on its way out of the solar system. The scientists who designed the plaque hoped that the images and symbols would serve as a viable means of communication, should any intelligent life form happen to encounter the spacecraft.

Pioneer 10

On March 3, 1972, Pioneer 10 was launched atop an Atlas-Centaur rocket from Cape Kennedy in Florida. It was the first mission ever sent to the outer solar system. Ultimately it became the first human-made object to leave the solar system for interstellar space.

Pioneer 10 was the first spacecraft to travel through the asteroid belt between Mars and Jupiter. Scientists had feared that this would be a dangerous area of space. They learned that the asteroids in the belt are spread far apart and do not pose a significant hazard to spacecraft flying through.

In December 1973 Pioneer 10 was the first spacecraft to investigate Jupiter. Its closest approach came within 124,000 miles of the planet. Pioneer 10 carried various instruments to study the solar wind, magnetic fields, cosmic radiation and dust, and hydrogen concentrations in space. Its Jupiter studies focused on the planet's magnetic effects, radio waves, and atmosphere. The atmospheres of Jupiter's satellites (particularly Io) were also investigated.

On June 13, 1983, Pioneer 10 became the first human-made object to leave the solar system. Over the years the instruments aboard the spacecraft began to fail or were turned off by NASA to conserve power. In 1997 NASA ceased routine tracking of the spacecraft due to budget reasons. The spacecraft was the most distant human-made object in space until February 1998, when it was passed by an even faster spacecraft called Voyager 1. NASA last detected a signal from Pioneer 10 in January 2003. It was approximately 7.6 billion miles away from Earth.

As of March 2004 Pioneer 10 is on its way out of the solar system. It is headed in the general direction of a star called Aldebaran that forms the eye of the constellation Taurus (The Bull). It will take the spacecraft nearly 2 million years to reach the star.

Pioneer 11

On April 5, 1973, the Pioneer 11 spacecraft was launched into space by an Atlas-Centaur rocket. A year and a half later it flew by Jupiter on its way to Saturn. The spacecraft approached within 21,000 miles of Jupiter. It was the first spacecraft to observe the planet's polar regions. It also returned detailed images of the Great Red Spot. Like its sister spacecraft, Pioneer 11 investigated solar and cosmic phenomena and interplanetary and planetary magnetic fields during its journey.

In September 1979 Pioneer 11 flew within 13,000 miles of Saturn and returned the first close-up pictures of the planet and its rings. It continued past the planet toward the edge of the solar system. In 1995 routine missions operations were ended and NASA received its last transmission from the spacecraft. By the end of that year, Pioneer 11 was approximately 4 billion miles from Earth.

Pioneer and Plutonium

The Pioneer spacecraft were built with special power systems based on radioisotope thermoelectric generators (RTGs). RTGs generate electricity from the heat released during the natural radioactive decay of a plutonium pellet. Although sending plutonium into space is controversial, NASA has used the power source on all of its missions to the far planets. The planets are too far from the sun to make solar power a feasible and reliable choice for these spacecraft.


In 1977 NASA began another bold mission to investigate Jupiter and Saturn. The program was called Voyager and included twin robotic spacecraft named Voyager 1 and Voyager 2. An illustration of a Voyager spacecraft is shown in Figure 8.1.

The various instruments on board were designed to detect and measure the solar wind and other charged particles, cosmic radiation, magnetic field intensities, and plasma waves. The original five-year Voyager mission was so successful that it was extended to include flybys of Uranus and Neptune. The total cost of Voyager's planetary explorations was $865 million.

The Missions

Both spacecraft were launched into space atop Titan rockets. Voyager 2 was the first to launch, on August 20, 1977. It was followed on September 5, 1977, by Voyager 1. Both spacecraft traveled for two years to fly by Jupiter. They made many scientific observations as they passed Jupiter and continued on to Saturn. Voyager 1 was on a faster trajectory than Voyager 2 and reached the planet first. Voyager 2 was directed on to fly by Uranus and Neptune. It was the first spacecraft to do so.

The Voyager spacecraft proved to be so hardy after completing their planetary journeys that they were sent on a new mission called the Voyager Interstellar Mission (VIM). The purpose of the VIM is to use the instruments on the spacecraft to explore the outermost edge of the heliosphere. This is the region of space dominated by energy effects from the Sun.

Planetary Achievements

The Voyager missions were two of the most successful in NASA's history. The spacecraft revealed a number of discoveries about the gas giants in the outer solar system:

  • Jupiter, Uranus, and Neptune have faint ring systems.
  • Jupiter has a complicated atmosphere in which lightning storms and aurora are common.
  • Jupiter's moon Io has active volcanoes.
  • Jupiter's moon Europa has a smooth surface composed of water ice.
  • The radiation levels experienced during the Jupiter fly by were 1,000 times stronger than what is lethal to humans.
  • Saturn's rings are comprised of thousands of strands (ringlets).
  • Saturn's ringlets are not as uniform and separate as expected. Some are kinked or braided together. Additional gaps between rings were discovered.
  • Saturn's weather is relatively tame compared to that on Jupiter.
  • Saturn's largest moon, Titan, has a dense smoggy atmosphere that contains nitrogen and carbon-containing compounds.
  • Saturn's moon Mimas has a massive impact crater.
  • Neptune's moon Triton has a thin atmosphere.

The mission also uncovered twenty-three previously unknown moons (three around Jupiter, four around Saturn, ten around Uranus, and six around Neptune). The discovery of water ice on the surface of Jupiter's moon Europa was particularly exciting, because it raised the possibility that there was liquid water underneath.

Voyager Interstellar Mission

In February 1998 Voyager 1 became the most distant human-made object in space when it reached a distance of 6.5 billion miles from the Sun, surpassing the record of Pioneer 10. It continues to travel at a speed of approximately 335 million miles per year. Voyager 2 is a little slower than its sister ship.

As of March 2004 Voyager 1 was nearly 8.5 billion miles from the Sun. Voyager 2 was roughly 6.7 billion miles away. Both have been traveling through space for more than twenty-five years. Some of the redundant instruments on board the spacecraft have been turned off to conserve power. According to NASA, both spacecraft should continue to function until at least 2020. They have enough propellant to last that long and should continue to generate adequate electrical power to run their scientific instruments.

Voyager 1 is closest to reaching the edge of the heliosphere. This edge is called the termination shock and it marks the location where the solar wind slows down due to pressure from interstellar gases. NASA scientists hope that both spacecraft will be able to transmit data as they cross the termination shock into the vast unknown of interstellar space.

Messages from Earth

The Voyager spacecraft carry written and recorded messages from Earth, in case they come across any intelligent life. Attached to each spacecraft is a 12-inch gold-plated copper disk inside a protective aluminum case. The cover of the protective case has symbolic instructions for playing the disc and a diagram of Earth's location in the solar system carved into it. The disks contain recorded greetings in fifty-five different languages and various other sounds, including bits of music and natural and human-made sounds. There are 115 images encoded in analog form on the disks of various Earth scenes. These include pictures of people, objects, and places from around the world. The disks carried printed messages from President Jimmy Carter and the United Nations Secretary General.


NASA's Galileo mission was the first to put a spacecraft in orbit around one of the far planets. The $1.4 billion mission to Jupiter included a scientific probe that left the orbiter and plunged into the planet's atmosphere. See Figure 8.2 for a diagram of the spacecraft including the descent probe. The mission was operated by NASA's Jet Propulsion Laboratory in Pasadena, California.

On October 18, 1989, the space shuttle Atlantis lifted off from Kennedy Space Center in Florida on mission STS-34 with the Galileo spacecraft on board. The shuttle astronauts released Galileo in Earth orbit, then the craft used its two-stage inertial upper stage rocket to boost itself toward Venus.

The spacecraft swung by Venus once and Earth twice as part of "gravity assist" maneuvers. These are maneuvers in which a spacecraft flies in close enough to a planet to get a boost from the orbital momentum of a planet traveling around the Sun. NASA compares a gravity assist to throwing a ping pong ball to skim along the top of one of the moving blades of an electric fan. The blades circle the fan's motor at a high rate of speed. The ping pong ball gets close enough to the blade to pick up momentum and shoot off in a different direction. Using gravity assists during space flight saves on fuel. This is particularly important for long journeys to the outer solar system.

By July 1995 Galileo was nearing Jupiter. It released the probe, which was about four feet in diameter and three feet long, and the probe began a five-month plunge toward the planet. On December 7, 1995, the orbiter was in position when the probe began its final descent at more than 105,000 miles per hour. For nearly an hour the heavily protected probe transmitted data about Jupiter's atmosphere, temperature, and weather. It was finally destroyed by the intense heat and pressure surrounding the planet. It had penetrated 124 miles into the violent atmosphere.

The orbiter spacecraft spent the next eight years in orbit around Jupiter. It conducted numerous flybys of the moons Europa, Ganymede, and Callisto, using its eleven scientific instruments to collect data about radiation, magnetic fields, charged particles, and cosmic dust.

The Galileo orbiter was originally designed for a two-year mission. It ended up lasting for fourteen years. In September 2003 NASA scientists destroyed the spacecraft by purposely plunging it into Jupiter's atmosphere. The orbiter was running low on propellant. The scientists feared that it could run out of fuel and crash into one of Jupiter's moons. This could contaminate environments that might possibly contain water and life forms.

The Galileo mission was hugely successful. The spacecraft traveled more than 2.8 billion miles during its long journey. It flew by two asteroids, Gaspra and Ida, on its way to the planet and watched Comet Shoemaker-Levy 9 impact Jupiter while it was in orbit there. Galileo captured thousands of detailed images of the planet and its largest moons and collected a wealth of data about these celestial objects.

Major findings attributed to, or confirmed by, the Galileo mission include:

  • There is an intense radiation belt around Jupiter
  • The surface of the moon Io is constantly being reshaped by heavy volcanic activity
  • There is evidence of liquid water oceans beneath the icy surface of the moon Europa and possibly Callisto
  • The moon Ganymede has its own magnetosphere and probably its own magnetic field
  • Ganymede is heavily cratered from impacts of comets and asteroids and has icy plains, mountains, and basins likely caused by geologic forces
  • Ganymede has a thin ionosphere (electrically charged atmosphere)
  • Ganymede, Europa, and Io all appear to have metallic cores

The possibility of liquid water is considered very strong for the moon Europa and somewhat likely for the moon Callisto. These conclusions are based on analysis of geological and magnetic data collected by Galileo.


In 1997 NASA collaborated with the European Space Agency (ESA) and the Italian space agency to launch the Cassini mission to Saturn. The spacecraft is depicted in Figure 8.3. It was designed to orbit the planet for four years and release a probe into the atmosphere of Titan, Saturn's largest moon. Cassini is scheduled to arrive at Saturn in July 2004. If successful it will be the first spacecraft ever to orbit the planet.

On October 15, 1997, the spacecraft was launched atop a Titan IV rocket from the Kennedy Space Center in Florida. Over the next three years it received two gravity assists from Venus and one each from Earth and Jupiter.

The Cassini orbiter is equipped with twelve scientific instruments. It also carries the Huygens probe with six instruments of its own. The probe, provided for the mission by the ESA, is to be released during Cassini's first orbit around Saturn and will penetrate the thick cloud cover that hides the moon Titan. The probe will sample Titan's atmosphere and provide the first photographs ever of its surface.

The orbiter will circle Saturn for detailed study of the planet, its rings, and moons. Specific mission objectives are to investigate Saturn's magnetosphere and atmosphere, determine the structure and behavior of its rings, and characterize the composition, weather, and geological history of its moons (particularly Titan). Titan is of particular interest to astronomers, because its surface may be covered with some kind of liquid.


As of March 2004 NASA's next mission to a far planet is expected to be the Jupiter Icy Moons Orbiter (JIMO). This mission will focus on the three large moons of Jupiter named Europa, Ganymede, and Callisto. The Voyager and Galileo spacecraft provided tantalizing evidence that these three moons may have oceans of liquid water beneath their icy surfaces. The JIMO project is expected to launch sometime in 2012.