Jupiter

Jupiter is the biggest of the planets in the Solar System and is the fifth planet out from the Sun. It was first discovered by the ancients.

Jupiter has four larger moons together with as many as 50 smaller ones. The four biggest satellites are named Callisto, Europa, Ganymede and Io. All were discovered by Galileo Galilei in 1610 and are known collectively as the Galilean Satellites. Galileo initially believed them to be stars, as the discovery of Ganymede was the first time that a moon had been seen orbiting a planet other than Earth. Ganymede is Jupiter’s largest moon.

The name Jupiter came from the name of the greatest god in Roman mythology, probably due to its massive size. In Greek mythology, Jupiter is known as Zeus. Ganymede is named after a young Greek boy who was taken to Olympus by Zeus. It is the only Galilean moon to be named after a male.

As one of the four great gas giants, Jupiter belongs to a group of planets which are composed in the main of hydrogen and helium. There are three other gas giants, namely Neptune, Saturn and Uranus. Although in composition Jupiter is made up of large quantities of the gases hydrogen and helium, rock is believed to be at its core. Rings around Jupiter were not found until as recently as 1979. The Voyager 1 spacecraft identified three rings: the main ring, the inner ‘halo’ ring and the ‘gossamer’ ring. The main ring is flat, the halo ring is mostly cloud and the gossamer ring is composed of minute particles of debris.

The Great Red Spot is a storm on the surface of Jupiter and it can be seen through telescopes on Earth. It is constantly turning in an anticlockwise direction and takes about six days to complete a rotation. It is a permanent feature of the planet. In 2000, another spot that is much smaller in size was found over Jupiter’s southern hemisphere. Originally white in colour, it is gradually turning red, earning itself the nickname Red Spot Junior.

Jupiter orbits the Sun once every 11.86 years. It rotates faster than any other planet in the Solar System, completing a full rotation in about ten hours. Jupiter lies on a tilted axis but it is a tilt which is less pronounced than that of Earth’s. It does not therefore have seasons.

Among the first spacecraft to visit Jupiter were the Pioneer and Voyager missions. In 1995, the Galileo spacecraft orbited around the planet. In 1997, New Horizons was en route to Pluto when it passed close to Jupiter and its moons.