Saturn has a large number of moons, 18 of which have been formally named. The 7th of these, Mimas, is a heavily cratered world composed largely of water ice. The surface of Mimas is scarred by a huge impact crater known as Herschel (after the astronomer who discovered Mimas). The crater, which is not visible in this picture, is 130 kilometres acros, almost one third the diameter of the moon. Fractures on the opposite side of Mimas from Herschel suggest the impact was almost large enough to completely destroy it. NASA's two Voyager spacecraft were launched in 1977 to explore the planets in the outer solar system. Voyager 1 flew past Saturn at a height of 124,000 kilometres in November 1980, having earlier visited Jupiter and several of its moons.
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