The Far Planets and Beyond

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

—Charles A. Lindbergh Jr., Autobiography of Values (1978)

Beyond Mars lie the far planets: Jupiter, Saturn, Uranus, and Neptune. Even though they are a great distance from the Sun, they are not even close to the edge of the solar system. Beyond Neptune is a large icy area called the Kuiper Belt that extends outward seven billion miles. Within it there are untold numbers of celestial bodies orbiting the Sun. One of these Kuiper Belt Objects is Pluto, formerly a full-fledged planet, but now considered a dwarf planet. Figure 8.1 shows the relative locations of the far planets and Pluto. They are far from the Sun, in a cold and dark 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 (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, Uranus, Neptune, and Pluto were discovered. 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 wonder if life extends beyond Earth.

THREE CENTURIES OF DISCOVERY

It took three centuries for humans to uncover the far planets in the 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 was discovered in 1930.

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. 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 a different story. It is a small ice world. For decades astronomers argued whether it was even a planet. In 2006 the debate was ended by a decision from the International Astronomical Union (IAU), the body responsible for naming celestial objects. On August 24, 2006, the IAU proclaimed that Pluto will from now on be called a dwarf planet. By official definition, a dwarf planet is planetlike in that it orbits the Sun and has

sufficient mass and self-gravity to be nearly round in shape. However, the IAU explains in the press release “IAU 2006 General Assembly: Result of the IAU Resolution Votes” (August 24, 2006, http://www.iau.org/iau0603.414.0.html) that unlike a planet, a dwarf planet “has not cleared the neighbourhood around its orbit.”

JUPITER

Jupiter is the fifth planet from the Sun and the largest planet in the solar system. It takes the planet nearly twelve Earth 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. It 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 aurora borealis (northern lights) on Earth.

Jupiter’s atmosphere is 90% hydrogen, and the remaining 10% is mostly helium, with traces 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 the Great Red Spot. The red color is probably due to the presence of certain chemical elements 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 thin 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, he realized 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 (1573–1624) published 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

In the years since Galileo’s discovery, other observers have discovered many smaller moons around Jupiter. The pace of these discoveries accelerated greatly in the late twentieth and early twenty-first centuries as better equipment was developed. In 2003 astronomers at an observatory atop Mauna Kea in Hawaii spotted twenty-three previously unknown moons around Jupiter. These moons have been named after the lovers, favorites, or descendants of Zeus in accordance with IAU guidelines. NASA notes in “Planetary Satellite Discovery Circum-stances” (December 28, 2007, http://ssd.jpl.nasa.gov/ ?sat_discovery) that Jupiter has sixty-two known moons.

SATURN

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

Saturn is flat at the poles. The planet is surrounded by several 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.

In 1655 Huygens discovered a satellite around Saturn. This turned out to be the planet’s largest moon. In 1656 he 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 extremely thin. Every fourteen to fifteen years Earth moved into the same plane as the ring. If someone tried to observe Saturn from Earth at this time, Huygens said, he or she 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 plane. 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 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 1671-72. The second pair he discovered just before the ring crossing of 1685.

Over the next three centuries many more Saturn moons were discovered. In December 2004 astronomers at the Mauna Kea observatory in Hawaii found twelve previously unknown moons around Saturn. The latest discovery occurred in July 2007, when NASA reported that the Cassini spacecraft orbiting Saturn had captured an image of a new moon. According to NASA, in “Planetary Satellite Discovery Circumstances,” this discovery is the sixtieth moon found around Saturn.

Cassini and Saturn’s Rings

During the 1600s 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 the French astronomer Jean Chapelain (1595–1674). However, it was not generally accepted until the eighteenth century.

Modern astronomers believe the rings are composed of chunks of ice. These chunks range in size from tiny particles to as large as automobiles. (See Figure 8.2.) Saturn’s ring system is actually many ringlets of different sizes nestled within each other with gaps between ring systems. Scientists use letters to designate distinct ring systems around the planet. (See Table 8.1.)

URANUS

Uranus is the seventh planet from the Sun and the third largest planet in the solar system. It 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 sys-

tem, 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.

Herschel Discovers Uranus and Two of Its Moons

The astronomer William Herschel (1738-1822) was born in Germany, but lived and worked in Britain. In 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 (1738– 1820) 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: Titania and Oberon.

More Moons

During the mid-1800s two more moons were discovered around Uranus by the amateur British astronomer William Lassell (1799-1880). It was another century before the next moon, Miranda, was found by the American astronomer Gerrit P. Kuiper (1905-1973) in 1948. During the 1980s and 1990s more than a dozen new moons were added to the list. In 2003 three additional moons were discovered by the Hubble Space Telescope and astronomers at the Mauna Kea observatory in Hawaii: Margaret, Mab, and Cupid.

In “Planetary Satellite Discovery Circumstances,” NASA states that Uranus has twenty-seven known satellites. 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).

NEPTUNE

Neptune is the eighth planet from the Sun. The planet is far from Earth and extremely difficult to observe. It has a distinctive bluish hue when viewed through a telescope. It orbits around the Sun once in 165 Earth years.

NASA indicates in “Planetary Satellite Discovery Circumstances” that Neptune has thirteen 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 the planet for several decades. They were puzzled because its orbit did not follow the expected path. 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 Uranus’s orbit.

During the 1840s John Couch Adams (1819-1892) of Britain and Urbain Le Verrier (1811-1877) of France used mathematics to plot the location of this mystery planet. Adams presented his theory to George Airy (1801-1892), the Astronomer Royal of England. For some reason, Airy failed to pursue the matter and look for the unknown planet. Le Verrier submitted his theory to Johann Galle (1812-1910), the director of the Berlin Observatory. 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, Airy 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 he 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 1846 Lassell spotted the moon. It was named Triton, after the son of Poseidon, the sea god. The name was suggested by the French astronomer Camille Flammarion (1842-1925).

More Moons of Neptune

In 1949 Kuiper found another moon around Neptune. In 1989 images from the spacecraft Voyager 2 revealed six previously unknown moons. The most recent discoveries occurred in 2002 and 2003, when five new moons were added to the list, bringing the total to thirteen.

DWARF PLANETS

Dwarf planets are a new category of celestial bodies. The designation was created in 2006 by IAU resolution. Even though Pluto is the best-known dwarf planet, it was not the first one discovered. This distinction goes to Ceres, a small world named after a Roman goddess. Ceres was discovered in 1801 by the Italian astronomer Giuseppe Piazzi (1746–1826). He found it in the massive asteroid belt lying between Mars and Jupiter. Another dwarf planet is Eris. It was discovered in July 2005 by astronomers at the California Institute of Technology in Pasadena, California. Like Pluto, Eris is a trans-Neptunian object, meaning it lies beyond Neptune. Eris is named after a Greek goddess.

Tombaugh Discovers Pluto

Clyde Tombaugh (1906–1997) is credited with discovering the dwarf planet Pluto. Tombaugh made the discovery on February 18, 1930, while working at the Lowell Observatory in Flagstaff, Arizona. This famous observatory was founded in the 1890s by Percival Lowell (1855–1916). 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. 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 was 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 eleven-year-old British girl named Venetia Burney.

Pluto’s Moons

Pluto’s primary moon is Charon. It is named after a character in Greek mythology who ferried the souls of the dead across the river Styx to the underworld. On June 22, 1978, Charon was discovered by James W. Christy (1938–) at the U.S. Naval Observatory in Washington, D.C. 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 when compared to Pluto and the fixed stars. When the discovery was made public, Christy suggested the name that was assigned to the moon. In November 2005 images from the Hubble Space Telescope revealed that Pluto has two additional moons orbiting far from the planet. In 2006 the IAU gave the new moons the names of Nix (after Nyx, the mother of Charon) and Hydra (a mythical serpent associated with Pluto in Greek mythology).

Facts about Pluto

It takes Pluto 248 Earth years to circle the Sun. It has a highly elliptical orbit, in that sometimes it is closer to the Sun than Neptune. This last occurred between 1979 and 1999.

Pluto is believed to be a dark and icy world with a surface of frozen nitrogen, methane, and carbon dioxide. It has been observed and photographed only from great distances. No spacecraft have ever been near the dwarf planet. In January 2006 NASA launched the robotic probe New Horizons that should reach Pluto in 2015. It will provide the first-ever detailed images of the distant dwarf planet.

THE FAR PLANETS IN SCIENCE FICTION

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 A Journey in Other Worlds: A Romance of the Future (1894) by the American capitalist and inventor John Jacob Astor (1864–1912). In this story Jupiter is similar to a prehistoric Earth. Skeleton Men of Jupiter was an unfinished story by the American writer Edgar Rice Burroughs (1875–1950). It appeared in print during the 1940s. The story referred to a Jupiter-like world called Sasoom, and it was inhabited by creatures that looked like human skeletons. In Arthur C. Clarke’s (1917–2008) novel 2001 A Space Odyssey (1968), the exploration of the solar system reaches Saturn, whereas in the Stanley Kubrick (1928–1999) 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 seafaring creatures in science-fiction stories.

PIONEER

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 Moffett Field, California, for the agency’s Office of Space Science. The two spacecraft involved were Pioneer 10 and Pioneer 11. The total cost of their mission was approximately $350 million.

On each spacecraft was mounted a six-inch by nine-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 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 Canaveral Air Station 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 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 2008, Pioneer 10 was more than eight billion miles from Earth and was heading toward the star Aldebaran (the eye in the constellation Taurus), which is eighty-two light-years away. It will take the spacecraft over two million years to reach the star.

Pioneer 11

On April 6, 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 twenty-one thousand 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 thirteen thousand 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 four 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. Even though sending plutonium into space is controversial, NASA has used this 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.

VOYAGER

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 8.3.

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 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 flyby were one thousand times stronger than what is lethal to humans.
• Saturn’s rings consist of thousands of strands (ringlets). Saturn’s ringlets are not as uniform and separate as expected—some are kinked or braided together, and 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-two previously unknown moons (three around Jupiter, three around Saturn, ten around Uranus, and six around Neptune). The discovery of water ice on the surface of Europa was particularly exciting, because it raises the possibility that there is 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 nearly one million miles per day. Voyager 2 is a little slower than its sister ship.

Figure 8.4 shows the locations of the spacecraft in December 2003. Voyager 1 crossed into the heliosheath in December 2004. Voyager 2 did likewise in August 2007. Solar wind emanates from the Sun and forms a long “wind sock” that moves with the Sun as it journeys through space. The heliosheath is the outer layer of the heliosphere. The heliopause is the boundary between the heliosphere and interstellar space. It lies more than thirteen billion miles from Earth.

In August 2007 scientists celebrated the thirtieth anniversaries of the launches of the Voyager spacecraft, both of which were still returning data to Earth and described by NASA as “healthy.” At that time Voyager 1 was 9.7 billion miles and Voyager 2 was 7.8 billion miles from the Sun.

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 twelve-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 carry printed messages from President Jimmy Carter (1924–) and Kurt Waldheim (1918–2007), the secretary general of the United Nations.

GALILEO

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 8.5 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 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 table tennis 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 ball gets close enough to one of the blades 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 many flybys of the moons Europa, Ganymede, and Callisto and used 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 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 Io is constantly being reshaped by heavy volcanic activity.
• There is evidence of liquid water oceans beneath the icy surface of Europa and possibly Callisto.
• 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.

CASSINI

In 1997 NASA collaborated with the European Space Agency (ESA) and the Agenzia Spaziale Italiana (Italian Space Agency) to launch the Cassini mission to Saturn. (See Figure 8.6.) It was designed for a four-year orbit of the planet and to release a probe to land on Titan, Saturn’s largest moon. 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.

On October 15, 1997, the spacecraft was launched atop a Titan IV rocket from the Kennedy Space Center. Over the next three years it received two gravity assists from Venus and one each from Earth and Jupiter. Cassini arrived at Saturn in July 2004, becoming the first spacecraft ever to orbit the planet.

The Cassini orbiter is equipped with twelve scientific instruments. It also carried the Huygens probe with six instruments of its own. (See Figure 8.7.) The probe was released on December 25, 2004, and began its three-week journey to the surface of Titan. It penetrated the thick cloud cover that hides the moon and touched down on January 14, 2005. The probe sampled Titan’s atmosphere and provided the first photographs ever of its surface. It was active for nearly two and half hours during its descent and another hour and twelve minutes after landing before its battery power ceased. The orbiter continued to circle Saturn and conducted flybys of Titan and the smaller moons Enceladus, Hyperion, Dione, Rhea, Iape-tus, and Phoebe.

In “Cassini-Huygens Top 10 Science Highlights” (July 15, 2005, http://saturn.jpl.nasa.gov/news/features/feature20050715.cfm), NASA lists ten important discoveries made by the Cassini-Huygens mission:

• Titan’s surprise surface and organic atmosphere—the surface does not include global oceans as scientists expected, but is Earthlike in some ways. There is evidence of volcanoes, erosion, craters, dunes, and dry and wet lake beds. Titan’s atmosphere contains organic chemicals, such as benzene and methane. Scientists believe the moon experiences methane showers from clouds sweeping overhead.
• Saturn’s complex rings—the rings were found to have “strawlike clumps” several miles long and rotating ring particles. Cassini discovered an oxygen atmosphere that exists just above the rings.
• First detailed images of Phoebe—Cassini found this tiny moon battered and scarred by many large crater strikes. There was evidence of water ice and silicate and organic materials on the surface.
• Saturn’s colorful and violent atmosphere—scientists were surprised to find that the northern hemisphere of the planet appeared deep blue, rather than hazy yellow like the rest of the world. Also, violent lighting storms were detected in enormous thunderstorms nearly the size of Earth.
• Enceladus may have an atmosphere—magnetic field data suggest this small icy moon has an atmosphere around it.
• Saturn’s inner radiation belt—Cassini discovered a previously unknown radiation belt that circles the entire planet in between the cloud tops and the edge of the D ring.
• Dynamic ring-moon relationships—images revealed unexpected interactions between Saturn’s rings and moons, such as particle stealing.
• Saturn’s rotational speed—comparisons of Cassini measurements to those made by the Voyager spacecraft during the early 1980s suggest that Saturn’s internal rotation rate is slowing down.
• Massive mountains on Iapetus—scientists learned that there is a massive mountain range on the dark side of Iapetus. Some of the mountains exceed twelve miles in height. For comparison, Mount Everest on Earth is approximately 5.5 miles high.
• Cracks in Dione—Cassini images reveal that the moon’s terrain is creased with giant fractures.

Later in 2005 Cassini captured new images during flybys of the moons Mimas, Tethys, Hyperion, and Dione. In September 2007 the spacecraft flew within one thousand miles of Iapetus. Scientists call it “the two-faced moon,” because its surface appears snowy white in some places and dark black in others. Cassini’s images reveal that the moon is heavily cratered and has a mountain ridge along its equator. A similar flyby of Titan the following month provided new radar images of many hydrocarbon lakes and seas in the moon’s polar regions.

In October 2007 NASA celebrated the ten-year anniversary of Cassini’ s launch. That same month Miodrag Sremcevic et al. reported in “A Belt of Moonlets in Saturn’s A Ring” (Nature, vol. 44, October 25, 2007) that images taken by Cassini indicate the presence of a moonlet belt in Saturn’s A ring. The moonlets, which are described as the size of large boulders, are believed to have originated from a moon that once orbited the planet and was demolished by asteroid impacts.

As of March 2008, the orbiter continued its journey around Saturn conducting detailed studies of the planet, its rings, and its moons.

FUTURE MISSIONS TO THE FAR PLANETS

NASA’s next planned mission to a far planet is the Jupiter Polar Orbiter (Juno). Juno will assume a polar orbit and study the planet’s geology, atmosphere, and climate. It is scheduled to launch in 2010 and arrive at Jupiter in 2016. Other Jupiter missions in the conceptual stage for the late 2010s through early 2030s include the Europa Geophysical Explorer, a Jupiter flyby, and the Europa Astrobiology Lander. NASA also has plans for three Saturn missions during this same time period: a Saturn flyby and orbiters around Saturn’s moons Encela-dus and Titan.

PLUTO AND THE KUIPER BELT

As mentioned earlier, the Kuiper Belt consists of many icy worlds in a vast region that lies beyond Neptune in the solar system. Astronomers refer to celestial bodies in this region as Kuiper Belt Objects (KBOs). Since the first discovery of a KBO in 1992, scientists have determined that there are many thousands of these objects. Pluto is now considered a KBO, as are the recently discovered worlds Eris, Quaoar, Orcus, and Var-una. These objects lie far from Earth, and little is known about them.

New Horizons

New Horizons was launched on January 19, 2006, aboard an Atlas V rocket. More than a year later the spacecraft passed by Jupiter. New Horizons is scheduled to reach Pluto in late 2016 or early 2017. Following this encounter it will move into the Kuiper Belt and explore there through 2022.

The spacecraft includes seven scientific instruments designed to assess the geology and atmosphere of Pluto and its primary moon Charon and map their surface compositions. Charon is of particular interest to scientists, because it is believed to be covered by water ice. Following these encounters, New Horizons will perform flybys of objects in the Kuiper Belt.

New Horizons was conceived in 2001 and is the first mission to be conducted under NASA’s New Frontiers Program. The spacecraft is operated for NASA by the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.