Phobos
Phobos is the larger of Mars's two small moons, a dark, cratered body about 22 kilometres across.
Phobos is the inner and larger of Mars's two moons, discovered in 1877 by the American astronomer Asaph Hall. Its name means fear in Greek, after a son of the war god Ares. It is irregular in shape, about 27 by 22 by 18 kilometres, and orbits only about 6,000 kilometres above the Martian surface, circling three times a day.
Phobos is very dark and covered in craters and strange grooves. Its largest crater, Stickney, is about 9 kilometres across. Its orbit is slowly shrinking, and in perhaps 30 to 50 million years it will either crash into Mars or be torn apart into a ring.
Scientists debate whether Phobos is a captured asteroid or a body formed from debris after a giant impact on Mars. Japan's MMX mission plans to land and collect samples. Because impacts on Mars have showered Phobos with Martian debris for billions of years, the samples might even include pieces of Mars.
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Level: Explorer Ages 10 and up