Deimos is the smallest and outermost of the two satellites on Mars and the smallest named satellites, named for Deimos, a character in Greek mythology. It was discovered by Asaph Hall in 1877. In synchronous rotation with the planet, it travels an almost circular orbit, very close to the Martian equatorial plane, in 30.3 hours, a period slightly higher than the rotation of the red planet. It is irregular in shape. It has an average diameter of 12.4 km, an estimated mass of 1.4762 × 1015 kg and an average density of 1471 kg/m³. Its composition, supposedly similar to that of D3-type asteroids and extinct cometary nuclei, poses serious difficulties to scholars who try to explain its origin.
