Deimos is Mars's smaller, outer moon, only 15 x 12 x 11 km in size. Deimos whirls around Mars in about 30 hours, at an average distance of 14,570 mi (23,459 km). Deimos was discovered by Asaph Hall in 1877, along with Mars's other moon, Phobos. Both moons are rotationally locked with Mars, always showing the same face to the planet; and both orbit Mars very close to the plane of its equator.
Since Phobos and Deimos both appear starlike in telescopes, we knew little of their appearance before the space age. Now that spacecraft have begun visiting Mars, we have images of Deimos' surface showing features as small as a few meters across. Deimos is a small, rocky body with a very dark surface. It has an albedo of only 0.07, which is similar to that of the C-type asteroids. This has fueled speculation that Phobos (and Deimos) both actually are captured asteroids.