Callisto. Coloured Voyager 1 spacecraft image of the surface of Callisto, a moon of the planet Jupiter. Callisto is the faintest of the four Galilean moons, which were discovered by Galileo in 1610. It has a diameter of 4800 kilometres, and orbits Jupiter around 1.9 million kilometres from the planet. Callisto's surface is covered in impact craters, and it is thought to have little or no geological activity. This image was taken by Voyager 1 on 6th March, 1979, from a range of 202,000 kilometres.