Callisto, photographed by Voyager 2. 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. It is the most heavily cratered planetary satellite known and the bright areas are ejecta thrown out by relatively recent impact craters. NASA's two Voyager spacecraft were launched in 1977 to explore the planets in the outer solar system. Voyager 2 reached Jupiter in 1979 before flying on to reach Saturn in 1981, Uranus (1986) and Neptune (1989).
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