The smaller and outermost of Mars' two satellites. It measures 12.6 km in size, and orbits Mars every 30.2 hours at a distance of about 23,500 km. It was discovered by the American astronomer Asaph Hall in 1877.
In Gk. mythology, Deimos, meaning "fear, terror," is one of the sons of Ares (Mars) and Aphrodite (Venus).