Phobos (moon)

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

A small, irregularly shaped object, Phobos orbits about 9,377 km (5,827 mi) from the center of Mars, 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 itself rotates. As a result, from the surface of Mars it appears to rise in the west, move rapidly across the sky (in 4 h 15 min or less) and set in the east. Phobos's orbital radius is decreasing, and it will eventually either impact the surface of Mars or break up into a planetary ring.

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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).[8][9][10] Hall also discovered Deimos, Mars's other moon. 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).[11][12]

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