Jupiter is the fifth planet from the Sun and by far the largest.  Jupiter is more than twice as massive as all the other planets combined (318 times Earth).  It orbits 778,330,000 kilometers from the Sun, has a diameter of 142,984 kilometers, and a mass of 1.900x10 to the 27th kilograms.  Jupiter was the King of the Gods, the ruler of Olympus and the patron of the Roman state.  Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and Venus; at some times Mars is also brighter).  It has been known since prehistoric times.  Galileo's discovery, in 1610, of Jupiter's four large moons Io, Europa, Ganymede and Callisto (now known as the Galilean moons) was the first discovery of a center of motion not apparently centered on the Earth.  It was a major point in favor of Copernicus's heliocentric theory of the motions of the planets; Galileo's outspoken support of the Copernican theory got him arrested by the Inquisition.  He was forced to recant his beliefs and was imprisoned for the rest of his life.  Jupiter was first visited by Pioneer 10 in 1973 and later by Pioneer 11, Voyager 1, Voyager 2 and Ulysses.  The spacecraft Galileo is currently in orbit around Jupiter and will be sending back data for at least the next two years.

Jupiter is about 90% hydrogen and 10% helium with traces of methane, water, ammonia and "rock".  This is very close to the composition of the primordial Solar Nebula from which the entire solar system was formed.  Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium.  Our knowledge of the interior of Jupiter (and the other gas planets) is highly indirect and likely to remain so for some time.  (The data from Galileo's atmospheric probe goes down only about 150 km below the cloud tops.)  Jupiter probably has a core of rocky material amounting to something like 10 to 15 Earth-masses.  Above the core lies the main bulk of the planet in the form of liquid metallic hydrogen.  This exotic form of the most common of elements is possible only at pressures exceeding 4 million bars, as is the case in the interior of Jupiter (and Saturn).  Liquid metallic hydrogen consists of ionized protons and electrons (like the interior of the Sun but at a far lower temperature).  At the temperature and pressure of Jupiter's interior hydrogen is a liquid, not a gas.  It is an electrical conductor and the source of Jupiter's magnetic field.  This layer probably also contains some helium and traces of various "ices".  The outermost layer is composed primarily of ordinary molecular hydrogen and helium which is liquid in the interior and gaseous further out.  The atmosphere we see is just the very top of this deep layer.  Water, carbon dioxide, methane and other simple molecules are also present in tiny amounts.

Three distinct layers of clouds are believed to exist consisting of ammonia ice, ammonium hydrosulfide and a mixture of ice and water.  However, the preliminary results from the Galileo probe show only faint indications of clouds (one instrument seems to have detected the topmost layer while another may have seen the second).  But the probe's entry point (above) was unusual -- Earth-based telescopic observations and more recent observations by the Galileo orbiter suggest that the probe entry site may well have been one of the warmest and least cloudy areas on Jupiter at that time.  Data from the Galileo atmospheric probe also indicate that there is much less water than expected.  The expectation was that Jupiter's atmosphere would contain about twice the amount of oxygen (combined with the abundant hydrogen to make water) as the Sun.  But it now appears that the actual concentration is much less than the Sun's.  Also surprising was the high temperature and density of the uppermost parts of the atmosphere.  Jupiter and the other gas planets have high velocity winds which are confined in wide bands of latitude.  The winds blow in opposite directions in adjacent bands.  Slight chemical and temperature differences between these bands are responsible for the colored bands that dominate the planet's appearance.  The light colored bands are called zones; the dark ones belts.  The bands have been known for some time on Jupiter, but the complex vortices in the boundary regions between the bands were first seen by Voyager.  The data from the Galileo probe indicate that the winds are even faster than expected (more than 400 mph) and extend down into as far as the probe was able to observe; they may extend down thousands of kilometers into the interior. Jupiter's atmosphere was also found to be quite turbulent. This indicates that Jupiter's winds are driven in large part by its internal heat rather than from solar input as on Earth.

The vivid colors seen in Jupiter's clouds are probably the result of subtle chemical reactions of the trace elements in Jupiter's atmosphere, perhaps involving sulfur whose compounds take on a wide variety of colors, but the details are unknown.  The colors correlate with the cloud's altitude: blue lowest, followed by browns and whites, with reds highest.  Sometimes we see the lower layers through holes in the upper ones.

The Great Red Spot (GRS) has been seen by Earthly observers for more than 300 years (its discovery is usually attributed to Cassini, or Robert Hooke in the 17th century).  The GRS is an oval about 12,000 by 25,000 kilometers, big enough to hold two Earths.  Other smaller but similar spots have been known for decades.  Infrared observations and the direction of its rotation indicate that the GRS is a high-pressure region whose cloud tops are significantly higher and colder than the surrounding regions. Similar structures have been seen on Saturn and Neptune. It is not known how such structures can persist for so long.  Jupiter is just about as large in diameter as a gas planet can be.  If more material were to be added, it would be compressed by gravity such that the overall radius would increase only slightly.  A star can be larger only because of its internal (nuclear) heat source.  (But Jupiter would have to be at least 80 times more massive to become a star.)

Jupiter has rings like Saturn's, but much fainter and smaller.  They were totally unexpected and were only discovered when two of the Voyager 1 scientists insisted that after traveling 1 billion kilometers it was at least worth a quick look to see if any rings might be present.  Everyone else thought that the chance of finding anything was nil, but there they were.  They have since been imaged in the infra-red from ground-based telescopes and by Galileo.  Unlike Saturn's, Jupiter's rings are dark.  They're probably composed of very small grains of rocky material.  Unlike Saturn's rings, they seem to contain no ice.  Particles in Jupiter's rings probably don't stay there for long (due to atmospheric and magnetic drag). Galileo found clear evidence that the rings are continuously resupplied by dust formed by micrometeor impacts on the four inner moons, which are very energetic because of Jupiter's large gravitational field.  The inner halo ring is broadened by interactions with Jupiter's magnetic field.

In July 1994, Comet Shoemaker-Levy 9 collided with Jupiter with spectacular results.  The effects were clearly visible even with amateur telescopes. The debris from the collision was visible for nearly a year afterward with the Hubble Space Telescope.  When it is in the nighttime sky, Jupiter is often the brightest "star" in the sky (it is second only to Venus, which is seldom visible in a dark sky).  The four Galilean moons are easily visible with binoculars; a few bands and the Great Red Spot can be seen with a small astronomical telescope.