Saturn has a large number of moons, 18 of which have been formally named. Dione, the 12th of these, is composed of water ice and a proportion of denser material, probably silicate rock. It has two distinctly different hemispheres. Dione's trailing hemisphere features bright radiating patterns, thought to be features caused by some ancient resurfacing proces, posibly caused by vulcanicity. On the leading hemisphere, seen here, they have been obscured by impact craters. 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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