Jupiter is the fifth closest planet to the Sun and is the first of what are called the outer planets (being outside the asteroid belt). It is by far the largest planet in the solar system having two and a half times as much mass as all the other planets put together and one thousandth the mass of the Sun. This is so large that the Sun and Jupiter actually orbit each other about a point just outside of the Suns surface.
Jupiter orbits the Sun once very 12 years (at about 780 million km) and is comprised of gas (75% hydrogen and 24% helium) and is presumed to have a rocky core surrounded by a sea of liquid metallic hydrogen which forms a ball 110,000km in diameter. Jupiter’s total diameter is 142,984 km.
In the upper atmosphere is a cloud layer 50km thick. The clouds are comprised of ammonia crystals and other compounds which are arranged into bands moving at different speeds at different latitudes. The Great Red Spot is a large stable storm vortex laying between two layers.
Considering its size, Jupiter rotates very quickly at one rotation at just under once every 10 hours. This means that at the equator there is quite a large centrifugal force which means the planet has a pronounced bulge - its diameter around the equator is 9000km greater than the diameter measured at the poles.
Moons : Io, Europa, Ganymede and Callisto
Jupiter has many satellites (more than 60) but most of these are quite small (less than 10km diameter). The four largest moons (named above) which were discovered by Galileo Galilei in 1610 are named after the lovers of Zeus. These moons are generally larger than the Earth’s moon with diameters ranging from 3100km to 5200km. Three of the moons are locked together in an orbital resonance in which for every orbit Ganymede takes, Europa takes exactly two orbits and Io exactly four orbits.
The closest to Jupiter, Io, has over 400 volcano's and is incredibly geologically active. This is thought to be due to Jupiter’s strong gravitational field constantly squeezing the moon as it orbits which warms the moons interior.
The next of the Galilean moons is Europa. Its surface is very smooth and comprised of water ice, possibly floating on a sea of liquid water. It's thought to have a rocky centre and has a thin oxygen atmosphere. Because of the presence of water it is thought to be a good candidate to find life outside of the Earth.
Ganymede is the largest satellite in the solar system and is larger than the planet Mercury. It is also covered in ice but is less geologically active with its surface marked by craters and ridges.
Callisto, the last of the Galilean moons is comprised of even quantities of rock and ice and a thin atmosphere of carbon dioxide and oxygen. It is possible it has liquid water buried 100km below its surface.
Jupiter and Man
Jupiter is named after the Roman king of the gods also known as Jove who was based on the Greek god Zeus.
Jupiter was first visited by the Pioneer 10 spacecraft in 1973 closely followed by Pioneer 11 in 1974. These spacecraft obtained the first close-up images of Jupiter and its red spot and moons and also measured Jupiter’s massive magnetic field. They are still traveling out of the solar system, but have lost communications with Earth.
The next visitors were Voyager 1 and 2 in 1979 and discovered, among other things, the faint Jovian ring system, several new natural satellites, volcanic activity on Io.
The Ulysses spacecraft which was designed to study the Sun used Jupiter’s gravitational field (1992) to swing it out of the plane of the ecliptic to allow it to orbit over the Suns poles.
Galileo became the first spacecraft to orbit Jupiter in 1995, orbiting the planet for 7 years before being deliberately crashed into the planet in order to ensure that it did not crash into, and contaminate, Europa. During its mission it collected a huge amount of data on the entire Jovian system and even witnessed the Shoemaker-Levy 9 comet impact in Jupiter’s southern hemisphere.
The Cassini probe flew past in 2000 and imaged Jupiter’s atmosphere revealing many unknown features.
The New Horizons probe flew past in 2007 on its way to Pluto and studied the Jovian moons, magnetic field and ring system.
NASA currently has a mission underway to study Jupiter in detail from a polar orbit. Named Juno, the spacecraft launched in August 2011, arrived in July 2016 and will orbit the planet until 2018 when it will be de-orbited into Jupiter. It passes very close to Jupiter on each orbit and therefore has to survive Jupiter's intense radiation belts.
The next planned mission to the Jovian system will be the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2022.
Cassini Images of Jupiter
Voyager Images of Jupiter