Jupiter is the largest planet in the solar system, and is an oblate spheroid about 142,984 km (88,846 miles) in diameter at the equator, and around 6½% less at the poles. It is more than 11 times the diameter of the Earth, and its volume is equivalent to 1,321 Earths, although it weighs only 318 times as much having a much lower density. It is the fifth planet out from the sun, which it orbits at an average distance of about 778,570,000 km (483,780,000 miles) in a slightly elliptical orbit, taking a little less than 12 Earth years (4,331.6 days) to complete. Although extremely large, Jupiter takes only 9 hours and 56 minutes to spin once on its axis, which is what causes the difference in equatorial and polar diameters. As Jupiter is classified as a gas giant, there is very little solid component to its structure.
Its atmosphere comprises about 75 percent hydrogen and 24 percent helium, by mass. The remaining 1% or so comprises carbon monoxide, water, ammonia, methane and other trace elements. Interior composition is a little different comprising about 71 percent hydrogen and 24 percent helium, and 5% other elements, again by mass. Jupiter may have a rocky core comprising up to 15% of its mass, but this is not certain. It probably had a rocky core earlier in its life which would have reduced in size over time, and may have disappeared entirely. Research continues on this subject.
Jupiter's outer temperature, near the top of the cloud layer, is about -145 degrees C. However, near the center, the temperature could be as high as 24,000 degrees C, which is hotter than the surface of the sun. Jupiter also radiates about twice as much heat as it receives from the sun due to the compression of the material at the center. It has the strongest magnetic field of any planet, and the Magnetic moment is about 20,000 times stronger than the Earth's. Jupiter's magnetosphere extends almost six million km (over 3½ million miles) towards the Sun, and almost a billion km (600 million miles) in the opposite direction, reaching Saturn's orbit.
The most spectacular visible feature of Jupiter is the Great Red Spot. At its widest, it is nearly 25,000 miles across. It is believed to be similar to a hurricane on Earth, but on a much larger scale. The color is probably derived from sulfur and phosphorus impurities in the ammonia crystals which comprise the bulk of the clouds in the upper atmosphere.
In July 1994, 21 pieces of the comet Shoemaker-Levy 9 fell into Jupiter's atmosphere. The impact explosions scattered debris over areas larger than the diameter of the Earth, and the scars on Jupiter's surface were visible for several months after the collisions. In 2009, another unknown object, probably about 1 km across, hit Jupiter and left a scar the size of the Pacific Ocean.
Jupiter has 63 confirmed moons, as at August 2010, of which 16 are known to be more than 6 miles across. The four largest satellites, known as the Galilean satellites following Galileo's discovery of them in 1610, are Ganymede, Europa, Io, and Callisto.
A number of spacecraft have visited Jupiter either as defined Jupiter missions, or as part of other missions. In 1973, Pioneer 10 was the first spacecraft to reach Jupiter, and in December 1974, Pioneer 11 came within 43,000 km (27,000 miles) of Jupiter on its way to Saturn. It was followed in 1979 by the two Voyager missions. Voyager made a number of discoveries including the existence of Jupiter's ring system, a number of new moons, and that there was current volcanic activity on Io. The Ulysses spacecraft flew close by Jupiter twice as part of its mission to study the Sun, firstly in February 1992 and again, more distantly, in 2003/4. The Galileo probe, launched on October 18, 1989 arrived at Jupiter on December 7, 1995 where it spent eight years observing Jupiter, its moons and rings, including sending a probe into the atmosphere. En route, it passed close to the asteroids Gaspra and Ida, discovering the latter's moon Dactyl. In September 2003, the mission having been completed, it was flown into Jupiter and destroyed. The most recent observations came from the New Horizons spacecraft, which passed Jupiter in February 2007 on its way to Pluto. In August 2011, the Juno spacecraft launches and is due to arrive in Jupiter orbit in July 2016 for a one year mission.