Satellite of Mars
Diameter 22.2 km
Discovery date 18 August 1877
Discovered by Asaph Hall
Phobos (or Mars I) is one of Mars' moons along with Deimos. It is irregularly shaped and is therefore believed to be an asteroid which as been captured into Mars' gravity from the asteroid belt. Phobos is significantly larger than Deimos, and is closer to Mars.


Phobos is a heavily cratered potato-shaped moon with a prominent crater near one end. The crater is named Stickney. One orbit of Phobos around Mars takes 11.1 hours which is nearly three times shorter than Mars' other moon, Deimos, which takes 30.4 hours to orbit Mars once.