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Phobos (/ˈfoʊbəs/ FOH-bəs; Greek: Φόβος; systematic designation: Mars I) is the larger and closer of the two natural satellites of Mars. With a mean radius of 11.1 km (6.9 mi), Phobos is 7.24 times as massive as the second moon Deimos. It is named after the Greek god Phobos (which means "fear"), a son of Ares (Mars) and Aphrodite (Venus) . Both moons were discovered in 1877.

A small, irregularly shaped object, Phobos orbits about 9,400 km (5,800 mi) from the center of Mars, or about 6,000 km (3,700 mi) from the Martian surface, closer to its primary than any other known planetary moon. Phobos is one of the least reflective bodies in the Solar System, and features a large impact crater, Stickney crater. It orbits so close to the planet that it moves around Mars faster than Mars rotates. As a result, from the surface of Mars it appears to rise in the west, move across the sky in 4 h 15 min or less, and set in the east twice each Martian day. Due to its short orbital period and tidal interactions, Phobos's orbital radius is decreasing and it will eventually break up into a planetary ring.


Discovery

Phobos was discovered by astronomer Asaph Hall on August 18, 1877, at the United States Naval Observatory in Washington, D.C., at about 09:14 Greenwich Mean Time (contemporary sources, using the pre-1925 astronomical convention that began the day at noon, give the time of discovery as August 17 at 16:06 Washington mean time). Hall also discovered Deimos, Mars's other moon, on August 12, 1877 at about 07:48 UTC. The names, originally spelled Phobus and Deimus respectively, were suggested by Henry Madan (1838–1901), Science Master of Eton, based on Book XV of the Iliad, in which the god Ares summons Dread (Deimos) and Fear (Phobos).