Most of the 21 moons of Uranus are small, with diameters of les than 150 kilometres, but there are 5 larger moons, of which Miranda is the smallest and closest to the planet. The surface of Miranda exhibits a strange mixture of features, with heavily cratered regions mixed up with areas of grooves, cliffs and valleys. It was originally thought that Miranda's odd landscape arose from the moon having been repeatedly shattered by impacts, then reasembling with some of the surface covered by material previously from the interior. More recent theories favour upwelling of partly melted ice being responsible for creating the features. NASA's two Voyager spacecraft were launched in 1977 to explore the planets in the outer solar system. Voyager 2 pased Uranus in 1986.
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