• The largest planet of the solar system, 318 times as massive as the Earth and eleven times as large, with an equatorial radius of 72 000km. Jupiter has a mean distance of 778 million km from the Sun, which it takes 12 years to orbit. Jupiter is accompanied by a thin ring system and a huge array of satellites. Jupiter has long been a favourite object for terrestrial observers of the planets, who mapped its cloud belts and the spots and features on them, especially the Great Red Spot, as well as tracking its larger satellites, especially the Galilean group. Jupiter is also active in radio wavelengths with emissions like those from terrestrial lightning. The Voyager 1 and 2 probes generated a wealth of detailed information about Jupiter and its satellites, and allowed some of the mysteries of its cloud system to be resolved. The dominant colour of Jupiter, yellow, seems to be due mainly to the presence of sulphur. Jupiter’s overall meteorology appears stable, on the evidence of centuries of telescopic observation. Jupiter’s rings were discovered during the 1979 flyby by Voyager 1. Less startling than the Saturnian ring system, Jupiter’s differs from Saturn’s by having no defined inner edge – instead it fades away towards the cloud tops of the planet proper. But it has the same pattern of gaps influenced by outer satellites as the Saturnian ring system. Jupiter is massively the most influential object in the solar system after the Sun itself, accounting for 71 per cent of the mass of the solar system outside the Sun. Its gravitation greatly influences the orbits of asteroids and of comets entering the inner solar system. In 1994 Jupiter’s cloud system was rocked by the impact of comet Shoemaker Levy 9, spectacularly recorded by the Hubble Space Telescope and observers on Earth. It is possible that such impacts are also the explanation of short-lived spots in the Jupiter cloud system seen by earlier observers.