Mars has two small, irregular moons: Phobos and Deimos, discovered by Asaph Hall in 1877. Phobos orbits just 6,000 km above Mars — closer than any other moon to its planet — completing a circuit in under 7.7 hours and slowly spiralling inward toward eventual tidal destruction. Deimos, at 23,460 km, drifts more sedately with a 30-hour period. Both are thought to be captured D-type asteroids from the outer asteroid belt, though an alternative hypothesis proposes they formed from debris ejected by a giant impact on Mars. Phobos's Stickney crater and Deimos's smooth, cratered surface tell very different stories of bombardment and regolith evolution.