Jupiter is the most massive planet and the fifth out from the Sun and is aptly named after the king of the Roman gods (Zeus is his Greek equivalent). It is fascinating to study with small instruments, indeed binoculars show the four Galilean moons as well as the disc of Jupiter itself. Jupiter outshines all the stars except Sirius when it's at maximum distance from Earth. At its nearest, it is the fourth brightest object after the Sun, Moon and Venus.

Jupiter rotates faster than any other planet and rotates slightly faster at the poles, with a period varying between 9 hours 50 minutes and 9 hours 55 minutes. The Great Red Spot in the southern hemisphere drifts against its surroundings, an oval with maximum radius 12,500 km. Jupiter is a gas giant, with the cloud tops at around 120K in temperature.

The composition of Jupiter is 75% hydrogen and 25% helium by mass, with traces of ammonia, water and methane. Near the core, hydrogen ionizes under the extreme pressure, but the temperature is not high enough for nuclear fusion as in the Sun. This region causes the magnetic field. The core itself is most likely rocky material, though science fiction has suggested a novel diamond core. The core temperature is estimated at 20,000K due to gravitational compression rather than the Sun's energy.

The first exploration of Jupiter was made by Pioneer 10 in 1973, which was a flyby mission. Pioneer11 arrived in 1974. Voyager 1 and Voyager 2 arrived in 1979, designed to withstand Jupiter's magnetic field, which had almost knocked out the Pioneer probes. Ulysses used Jupiter for a gravity assist in 1992 on its way to the Sun. The recent Galileo arrived at Jupiter in 1995 after witnessing the comet Shoemaker-Levy crash into the planet. After orbiting 35 times, Galileo crashed into Jupiter itself in 2003.

The Galilean satellites are by far the largest and were discovered by Galileo in 1610. They are from nearest to farthest; Io, Europa, Ganymede and Callisto. Ganymede is the largest satellite in the Solar System, with a molten iron core, silicate mantle and ice crust. Callisto is second largest and probably about half rock, half ice.

Europa is fascinating, mostly silicate rock with an ice crust. There is a possibility of oceans under the ice. The surface is extremely smooth, but has dark bands crossing all over, up to 20 km in width.

Io is the third largest and is a little bigger than the Moon. Io is largely molten silicate rock with an iron core. It's surface changes all the time because of frequent volcanic activity, which makes maps useless. Io's orbit passes through the strong magnetic field of Jupiter, causing ions to be ripped from the moon.