Deimos is the smallest and most external of the two satellites of Mars (the other is Fobos). It was discovered on 12 August 1877 by the astronomer Hall. It has an irregular shape, has an average diameter of 12.4 km and its composition, considered similar to that of the "D" type asteroids and extinct cometary nuclei, poses serious difficulties for scientists who try to explain its origin. Its surface is covered with a thick layer of reddish-colored of regolith and has lighter streaks.
In Greek mythology Deimos was one of the sons of Ares, god of war, and Aphrodite, goddess of beauty and accompanied his father in battle with his brother Fobos and the goddess Enio.