Composition and Atmosphere Edit
As a gas giant, Jupiter has nothing that could be considered a "surface" – its atmosphere simply transitions smoothly from gas to liquid to solid as it gets deeper. It's composed mostly of hydrogen and helium, with only small amounts of other elements. Numerous storms exist on its surface at any one time, notably the Great Red Spot, an anticyclonic storm the size of the Earth which has been active for at least three hundred years.
Jupiter's first four moons were discovered by Galileo Galilei in 1610, and today are known as the "Galilean" moons in his honor – of its total of 67 moons, the Galilean moons are the largest and most massive, making up 99.9997% of the total mass in orbit around Jupiter, and all except Europa are larger than the Earth's moon. The remaining 63 moons are very small by comparison, barely more than asteroids. The innermost Galilean moon, Io, is currently the only location in the solar system, other than the Earth, to exhibit active volcanism. Second is Europa, the smallest of the Galilean moons, covered in a shell of thick ice and theorized to possess an ocean of liquid water underneath this ice sheet. Third and fourth are Ganymede and Callisto, respectively. Both are composed mostly of water ice, and may also hold subsurface oceans like Europa.
Like all gas giants in the solar system, Jupiter has rings. However, they are tenuous and dark, being mostly composed of material ejected from Io's volcanoes, and the debris from meteoroid impacts of various moons.
Jupiter occupies a typical circular orbit, with a semi-major axis of 5.2 AU, an inclination of 1.3°, and a low eccentricity value of 0.04.
Jupiter wasn't explored directly until 1973, when Pioneer 10 conducted a flyby, returning the first images from beyond the orbit of Mars. Pioneer 10 was followed a few months later by Pioneer 11 (which later went on to explore Saturn). In 1979, the twin Voyager probes arrived. In 1992, the Ulysses probe used Jupiter's strong gravity to alter its trajectory, allowing it to pass over the poles of the Sun a few years later. In 1995, the Galileo spacecraft became the first to orbit Jupiter, and operated continuously until 2003, when it was intentionally crashed into the atmosphere to avoid any chance of the inactive spacecraft impacting (and contaminating) Europa. The Cassini spacecraft, en-route to Saturn, conducted a flyby in 2000, returning the highest resolution imagery of Jupiter to date. Finally, in 2007, the New Horizons spacecraft en-route to Pluto conducted a flyby of Jupiter, testing its instruments to help prepare for the Pluto encounter in 2015.