Phobos

Phobos is the larger of Mars's two moons, at 27 x 22 x 18 km in size, and its orbit is the closer of the pair. It orbits only 6,000 km (3,700 miles) above the Martian surface, and goes around Mars three times per Martian day. Phobos's rotation is locked with Mars, always showing the same face to the planet; and it orbits Mars very close to the plane of the Martian equator.

Both Martian moons were discovered by Asaph Hall in 1877, and named after the Greek mythological companions of Mars, Fear (Phobos) and Terror (Deimos).

Until spacecraft began visiting Mars, we knew little of the appearance of either moon, since both appear starlike in telescopes. Now we have images of their surfaces showing features as small as a few meters across. Phobos is dominated by a large crater named Stickney, which is almost half the diameter of the moon itself.

Phobos and Deimos are both small rocky bodies, resembling asteroids. This has fueled speculation that they actually are captured asteroids. Because of Phobos' low orbit, it will eventually be destroyed by tidal forces. In about 11 million years, Phobos will either impact the surface of Mars or (more likely) break up into a short-lived planetary ring system.