Callisto is the third largest moon in the solar system, with a diameter of 4820 km and an area larger than Asia. It is heavily cratered and has no major geographic features. It has a tenuous atmosphere and a subsurface ocean. It is considered a likely site for a future base due to its stable surface geology and location in a relatively radiation-free region around Jupiter. It orbits Jupiter at a distance of 1,880,000 km.
Callisto is a huge Jovian moon, the third largest satellite in the solar system, after Ganymede and Titan. With a diameter of 4820 km, Callisto is about 40% larger than the Moon and only one-third the size of the Earth. Callisto has an area of 7.30×107 km², larger than that of Asia.
Callisto is one of the Galilean satellites, discovered by Galileo Galilei in 1610 with one of the first telescopes. Callisto’s diameter is 99% that of Mercury, but it has a much lower mass, due to the enormous amount of ice it contains. Callisto is roughly half rock and half ice.
Callisto’s surface is extremely old and heavily cratered. The crater on its surface has almost reached saturation, that is, each new crater must obliterate an old one. Its surface looks like mud thrown by raindrops, but frozen into stone and ice.
As one of the most heavily cratered bodies in the solar system along with Mercury, Callisto has no major geographic features other than those associated with impacts. There are no major mountains, ridges or Callistoan lines. Some of its craters, notably the great Valhalla, form cracked cocentric ring structures.
Callisto has a tenuous atmosphere of carbon dioxide and molecular oxygen and a thin subsurface ocean 100-150km below. However, speculation about extraterrestrial life in the oceans of Jupiter’s moons has largely centered on Europa. Callisto is widely regarded as the most likely site of a future base, due to its location in a relatively radiation-free region around Jupiter and its stable surface geology. In contrast, Io is in one of the most radiation-saturated areas around Jupiter, its lowlands covered in sulfur, while volcanoes are constantly erupting all around it.
Callisto orbits Jupiter at a distance of 1,880,000 km, making it the fourth Galilean moon in terms of distance. In comparison, the Moon orbits the Earth at a distance of about 400,000 km, significantly closer.
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