As we know, the Sun is a star with eight planets orbiting it. But, Jupiter also seems to have its very own solar system with at least 63 very different moons orbiting it. Most of these moons are very small and are probably asteroids caught by Jupiter's strong gravitational pull to remain in orbit around the planet. This is similar to what is believed happened to Mars' moons, Phobos and Deimos. However, the four largest moons, Io, Europa, Ganymede and Callisto are like little worlds, each different from each other. They are not the rocky, dead worlds like Earth's Moon and planets like Mercury or dwarf planet Pluto, as many people believed. In fact, they were perhaps the biggest shock during the Voyager mission launched in 1977 to explore the four Gas Giants (Jupiter, Saturn, Uranus and Neptune).

These four moons (Io, Europa, Ganymede and Callisto) were discovered by Italian astronomer Galileo Galilei about four hundred years ago in 1610 when he noticed four points of light around Jupiter, believing them to be stars but later realising that they were moons. Above is a picture drawn by Galileo of the positions of three of the large moons when be believed them to be stars. (Galileo discovered the fourth moon at a later date). They are known as the Galilean satellites and are about to be visited again by the Galileo probe launched in 1996. Io, the closest large moon to Jupiter orbits the planet at a distance of 421,600 kilometres. Europa is 670,900 kilometres away, Ganymede is 1,070,000 kilometres away and Callisto is 1,880,000. After these moons, the next nearest moon is Leda, only 10 kilometres wide but 10,000,000 kilometres away from Jupiter, almost ten times further away than Callisto!

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