Jupiter, named after the king of the Roman Gods, reigns supreme among the nine planets of our solar system, rivaling the Sun in its grandeur. Giant Jupiter contains two-thirds of the planetary mass of the solar system. In composition it resembles a small star. Its interior pressure may reach 100 million times the surface pressure on Earth. Jupiter's magnetic field is immense, even in proportion to the size of the planet, stretching millions of miles into the solar system. If the magnetic field were visible, it would rival the apparent size of our Moon. Electrical activity in Jupiter is so strong that it pours billions of watts into Earth's own magnetic field every day. No planet has greater influence on its neighbors.

Jupiter is the innermost of the 4 giant planets (the others are Saturn, Uranus, and Neptune), and clearly the most dynamic. Jupiter has its own 'mini solar system' of 49 moons. Scientists are most interested in the Galilean satellites - the four largest moons discovered by Galileo Galilei in 1610. Europa, may have an ocean under its frozen surface. Calisto's crater-pocked landscape may be the oldest in the solar system. Ganymede is the solar system's largest moon. It is bigger than Pluto and Mercury. And little Io has more volcanoes than anywhere else in the solar system.

Its atmosphere bristles with lightning and swirls with huge storm systems including the Great Red Spot, a storm that has persisted for at least 100---and perhaps as long as 300---years. With its dynamism, huge energy output, and entourage of satellites, Jupiter is in many ways like a small sun, and the Jovian system resembles a miniature solar system. Although Jupiter is a stellar composition----most of its mass is hydrogen and helium---it does not burn like the Sun. Models of star formation suggest that Jupiter's mass is only about one-eightieth of the mass needed for ignition, which occurs due to heating from internal gravitational collapse. Jupiter's smaller size leaves its center too cool to ignite, sustaining instead internal masses of liquefied gas.

Rings are important dynamical laboratories to look at the processes that probably went on billions of years ago when the Solar System was forming from a flattened disk of dust and gas. Furthermore, similar faint rings probably are associated with many small moons of the Solar System's other giant planets.

In the late 1970s, NASA's two Voyager spacecraft first revealed the structure of Jupiter's rings: a flattened main ring and an inner, cloud-like ring, called the halo, both composed of small, dark particles. One Voyager image seemed to indicate a third, faint outer ring.

New Galileo data reveal that this third ring, known as the gossamer ring because of its transparency, consists of two rings. One is embedded within the other, and both are composed of microscopic debris from two small moons, Amalthea and Thebe.

Scientists believe that dust is kicked off the small moons when they are struck by interplanetary meteoroids, or fragments of comets and asteroids, at speeds greatly magnified by Jupiter's huge gravitational field, like the cloud of chalk dust that rises when two erasers are banged together. The small moons are particularly vulnerable targets because of their relative closeness to the giant planet.

In these impacts, the meteoroid is going so fast it buries itself deep in the moon, then vaporizes and explodes, causing debris to be thrown off at such high velocity that it escapes the satellite's gravitational field. If the moon is too big, dust particles will not have enough velocity to escape the moon's gravitational field. With a diameter of just eight kilometers (five miles) and an orbit that lies just at the periphery of the main ring, tiny Adrastea is most perfectly suited for the job.

As dust particles are blasted off the moons, they enter orbits much like those of their source satellites, both in their distance and in their slight tilt relative to Jupiter's equatorial plane. A tilted orbit wobbles around a planet's equator, much like a hula hoop twirling around a person's waist. This close to Jupiter, orbits wobble back and forth in only a few months.

Jupiter's diameter is approximately 143,000 kilometers (86,000 miles). The ring system begins about 92,000 kilometers (55,000 miles) from Jupiter's center and extends to about 250,000 kilometers (150,000 miles) from the planet.

Jupiter holds clues to many of the mysteries of the early solar system. About 4.5 billion years ago, when the solar system formed out of a swirling mass of gases and dust called the solar nebula, Jupiter's core probably began as a solid mass of ice and rock about 15 times the bulk of Earth. The ice content of Jupiter's mass was high because it formed in the colder outer region of the solar system, where the nebula contained a lot of ice particles, principally water and methane. Probably because icy masses can adhere and compress into a single large body faster than plain rock can, the outer planets formed their cores before the rocky inner planets did. The gravity of the large icy cores of Jupiter and Saturn attracted most of the light hydrogen and helium gas in the nebula, and it is these gasses that we find dominating their atmospheres today. Jupiter, the innermost of the icy giants, "grew" the largest atmosphere. In fact, the face of Jupiter that we see is really just the top of its atmosphere. What goes on inside is even more intriguing.

Jupiter's massive atmosphere creates tremendous pressures as you move closer to the center of the planet. The substances inside the atmosphere are subject to extreme conditions, leading to exotic chemistry. For example, scientists have reason to believe that the inner layers of hydrogen in Jupiter's atmosphere, under the pressure of the atmosphere above, may have formed into a planet-encircling layer of what is called liquid metallic hydrogen. Not exactly an ocean, not exactly atmosphere, this layer of hydrogen would have properties that stretch our understanding of chemistry. Instead of the simple, free-moving and loosely bonded behavior of gaseous hydrogen, liquid metallic hydrogen is a strange matrix capable of conducting huge electrical currents. The persistent radio noise and improbably strong magnetic field of Jupiter could both emanate from this layer of metallic liquid. Some scientists theorize that beneath this layer there is no solid mass at the center of Jupiter, but that the unique temperature and pressure conditions sustain a core whose density is more like liquid or slush.

Farther from the planet's core, in what we can more certainly call the atmosphere, we see gases behaving in a more familiar manner, moving in general planetary circulations driven primarily by the rotation of the planet. Jupiter is believed to have three cloud layers in its atmosphere. At the top are clouds of ammonia ice; beneath that ammonium-hydrogen sulfide crystals; and in the lowest layer, water ice and perhaps liquid water. Jupiter is noteworthy for its turbulent cloudtops, and its long-standing storm, the Great Red Spot. The origins of these colorful features are uncertain, but scientists believe that they are caused by plumes of warmer gases that rise up from deep in the planet's interior. The plumes' colors are probably caused by their chemical content. Although the amount of carbon, for example, in the Jovian atmosphere is very small, carbon readily combines with hydrogen and trace amounts of oxygen to form a variety of gases such as carbon monoxide, methane, and other organic compounds. The orange and brown colors in Jupiter's clouds may be attributable to the presence of organic compounds, or sulfur and phosphorus.

Jupiter is one of the planets visible to the naked eye, and its path through the night sky has been traced for thousands of years. In 1610, the Italian astronomer Galileo Galilei discovered four large moons -- Io, Europa, Ganymede, and Callisto (known as the Galilean satellites) -- orbiting Jupiter. This was one of the earliest astronomical discoveries made with a telescope. It fueled the controversial argument of the time that the Sun and not the Earth was the center of the solar system. In the centuries since then, as telescopes improved, Jupiter because known primarily for its size and the Great Red Spot, which was imagined to be an island in a Jovian sea. As Earth-based astronomy continued to improve, we came to realize that the few fuzzy "surface" features we could see were constantly changing in position, size, and color, suggesting that they were atmospheric phenomena. With the advent of radio astronomy, we discovered that Jupiter is a source of strong radio-frequency noise, suggesting electrical activity. Out fascination with Jupiter increased.

In March 1972, NASA launched the Pioneer 10 spacecraft to observe the asteroid belt and Jupiter. Arriving at Jupiter in December 1973, Pioneer 10 revealed Jupiter's intense radiation output, its tremendous magnetic field, and the probability of a liquid interior. One year later, Pioneer 10's sister spacecraft, Pioneer 11, flew by Jupiter on its way to Saturn, and provided even more detailed imagery and measurements, including our first close-up look at the giant planet's polar regions. Then, in August and September 1977, NASA launched the two Voyager spacecraft to the outer solar system. The Voyagers' 1979 encounters with Jupiter provided us with startling, beautiful imagery of the king of the planets, revealing thousands of features never before seen. Swirling multicolored turbulence surrounded the Great Red Spot. Rising plumes and spinning eddies formed and dissipated, suggesting a strong source of heat bubbling up from within the planet.

The Voyager imagery, with its exposure of so many small details, told us that Jovian dynamics were much more complex than previously imagined. Yet many of the features resemble effects we know of in out own and other planetary atmospheres, magnified by the enormity and extremity of the Jovian environment. In studying Jupiter we can learn more about atmospheric effects and interactions that are subtle on Earth, such as magnetosphere-atmosphere interactions.

Subsequent missions to the giant planet will help us improve models of atmospheric dynamics, and will help us understand the chemistry and behavior of Earth's own relatively thin, but very precious, atmosphere.

Voyagers 1 and 2

still operational after more than 15 years in space and are traveling out of the Solar System. The two Voyagers are expected to last until at least the year 2015 when their radioisotope thermoelectric generators (RTG) power supplies are expected for fail. Their trajectories give negative evidence about possible planets beyond Pluto. Their next major scientific discovery should be the location of the heliopause. Low-frequency radio emissions believed to originate at the heliopause have been detected by both Voyagers.

Both Voyagers are using their ultraviolet spectrometers to map the heliosphere and study the incoming interstellar wind. The cosmic ray detectors are seeing the energy spectra of interstellar cosmic rays in the outer heliosphere

Voyager 1 has passed the Pioneer 10 spacecraft and is now the most distant human-made object in space.

Galileo

Jupiter orbiter and atmosphere probe, now in Jupiter orbit. It will make extensive surveys of the Jovian moons and the probe has descended into Jupiter's atmosphere to provide our first direct evidence of the interior of a gas giant.

Galileo has already returned the first resolved images of two asteroids, 951 Gaspra and 243 Ida, while in transit to Jupiter. It has also returned pictures of the impact of Comet SL9 onto Jupiter from its unique vantage point.

Efforts to unfurl the stuck High Gain Antenna (HGA) have essentially been abandoned. With its Low Gain Antenna Galileo transmits data at about 10 bits per second. JPL has developed a backup plan using enhancements of the receiving antennas in the Deep Space Network and data compression (JPEG-like for images, lossless compression for data from the other instruments) on the spacecraft. This should allow Galileo to achieve approximately 70% of its original science objectives with the much lower speed Low Gain Antenna. Long term Jovian weather monitoring, which is imagery intensive, will suffer the most.

After orbiting Jupiter 34 times and surviving four times the amount of radiation it was design to withstand, the resilient Galileo spacecraft is finally at the very end of its 14-year mission. To avoid even the most remote possibility of colliding with a pristine moon in the jovian system, the out-of-fuel spacecraft will dive into Jupiter on Sunday, Sept. 21, 2003.

Since its launch in 1989, the sturdy spacecraft traveled more than 4.6 billion kilometers (almost 2.8 billion miles), about the equivalent of seven times the distance between Earth and Jupiter. Despite communication problems and a temperamental tape recorder, Galileo returned 30 gigabytes of data, including 14,000 pictures.

This wealth of information drastically expanded our understanding of the solar system's biggest planet and its moons. The mission was possible because it drew its power from two long-lasting radioisotope thermoelectric generators provided by the Department of Energy.

Asteroids Unveiled

The exciting list of discoveries started even before Galileo was able to get a close glimpse of Jupiter. As it crossed the asteroid belt in October 1991, Galileo snapped images of Gaspra, returning the first ever close-up image of an asteroid. Less then a year later, the spacecraft got up close and personal with yet another asteroid, Ida. Images from Ida revealed the asteroid has its own little "moon," Dactyl, the first known moon of an asteroid.

Location, Location

In 1994 the spacecraft was in the right place at the right time and made the only direct observation of a comet impacting a planet. It took images of fragments of comet Shoemaker-Levy 9 crashing into Jupiter. Images of the impact, which was not visible from Earth, helped scientists better understand this type of event.

At Jupiter

Galileo began its tour of the jovian system in December 1995. Carefully designed orbits allowed the spacecraft to observe Jupiter's atmosphere, revealing numerous large thunderstorms many times larger than those on Earth, with lightning strikes up to 1,000 times more powerful than terrestrial lightning. Data collected by the descent probe made the first in-place studies of the planet's clouds and winds, and it furthered scientists' understanding of how Jupiter evolved. The probe also made measurements designed to assess the degree of evolution of Jupiter compared to the Sun.

As the first spacecraft in long-term residence in jovian orbit, Galileo also successfully studied the global structure and dynamics of Jupiter's magnetic field. Galileo also determined that Jupiter's ring system is formed by dust kicked up as interplanetary meteoroids smash into the planet's four small inner moons. Data also showed that Jupiter's outermost ring is actually made up of two rings, one embedded within another.

Moons' Wonders

Galileo extensively investigated the geologic diversity of Jupiter's four largest moons: Ganymede, Callisto, Io and Europa. Stunning images revealed the contrasting and changing surfaces of these moons.

Io has extensive volcanic activity, which is continually modifying the surface. The heat and the frequency of eruption can be 100 times more than that of Earth, something reminiscent of Earth's early days. The similarities make Io an ideal laboratory for the study of what Earth was like more than 3 billion years ago.

The moon Europa, Galileo unveiled, could be hiding a salty ocean up to 100 kilometers (62 miles) deep underneath its frozen surface. Images also reveal ice "rafts" the size of cities that have broken and drifted apart to create a scalloped and broken surface. There are also indications of volcanic ice flows, with liquid water flowing across the surface. These discoveries are particularly intriguing since liquid water is a key ingredient in the process that may lead to the formation of life.

The biggest discovery surrounding Ganymede was the presence of a magnetic field, the first moon of any planet known to have one. Images of this moon featured a faulted and fractured surface that demonstrated high tectonic activity. Like Europa and Io, Ganymede has a metallic core. Galileo magnetic data also provided evidence that Ganymede might have a liquid-saltwater layer as well.

Galileo determined that, while Callisto doesn't have a metallic core, its surface shows evidence of extensive erosion. Data collected raise the question of whether Callisto's surface may also hide an ocean.

Last Dance

Galileo's own discovery of a likely ocean hidden under Europa's surface raises the possibility of life there and concern about protecting it. For that reason, in its final victory lap the Galileo spacecraft will dive into the atmosphere of the gaseous planet and disintegrate. Predictably, some of the spacecraft findings raised intriguing questions that will have to be answered by future mission. But Galileo Galilei, the first modern astronomer, would be immensely proud of the discoveries made by the spacecraft that carries his name.