Jupiter is the biggest planet in the Solar System – twice as heavy as all the other planets put together.
Jupiter has no surface for a spacecraft to land on because it is made mostly from helium gas and hydrogen. The massive pull of Jupiter’s gravity squeezes the hydrogen so hard that it is liquid.
Towards Jupiter’s core, immense pressure turns the hydrogen to solid metal.
The Ancient Greeks originally named the planet Zeus, after the king of their gods. Jupiter was the Romans’ name for Zeus.
Jupiter spins right round in less than ten hours, which means that the planet’s surface is moving at nearly 50,000 km/h.
Jupiter’s speedy spin makes its middle bulge out. It also churns up the planet’s metal core until it generates a hugely powerful magnetic field, ten times as strong as the Earth’s.
Jupiter has a Great Red Spot – a huge swirl of red clouds measuring more than 40,000 km across. The scientist Robert Hooke first noticed the spot in 1644.
Jupiter’s four biggest moons were first spotted by Galileo in the 17th century. Their names are Europa, Callisto and Ganymede.
Jupiter also has 17 smaller moons – Metis, Adastrea, Amalthea, Thebe, Leda, Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae, Sinope as well as five recent discoveries.
Jupiter is so massive that the pressure at its heart makes it glow very faintly with invisible infrared rays. Indeed, it glows as brightly as four million billion 100-watt light bulbs. But it is not quite big enough for nuclear reactions to start, and make it become a star.
The Galileo space probe reached Jupiter and its moons in the year 1995.
The Galilean moons are the four biggest of Jupiter’s moons. They were discovered by Galileo, centuries before astronomers identified the other, smaller ones.
Ganymede is the biggest of the Galilean moons – at 5268 km across, it is larger than the planet Mercury.
Ganymede looks hard but under its shell of solid ice is 900 km of slushy, half-melted ice and water.
Callisto is the second biggest, at 4806 km across.
Callisto is scarred with craters from bombardments early in the Solar System’s life.
Io is the third biggest, at 3642 km across.
Io’s surface is a mass of volcanoes, caused by it being stretched and squeezed by Jupiter’s massive gravity.
The smallest of the Galilean moons is Europa, at 3138 km across.
Europa is covered in ice and looks like a shiny, honey-coloured billiard ball from a distance.
A crater called Valhalla on Callisto is so big it makes the moon look like a giant eyeball.