Callisto, photographed by Voyager 1, showing a large impact basin, 2600 kilometres acros, on the moon's surface. Known as 'Valhalla', this is the largest impact crater yet discovered in the Solar System. The concentric rings indicate that the impacting body might have come close to breaking through Callisto's crust. Callisto is the second largest of Jupiter's four main moons, which were discovered by Galileo, and is approximately the same size as the planet Mercury. The outermost of the four, Callisto takes 16.7 Earth days to orbit Jupiter. Two Voyager spacecraft were launched in 1977 to explore the planets in the outer solar system. Voyager 1 made its closest approached to Jupiter of 278,000 kilometres in March 1979 before flying on to Saturn.
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