Phobos (foʊbəs, from Ancient Greek: Φόβος, lit. ‘fear’) is the innermost and larger of the two natural satellites of Mars, the other being Deimos. Phobos orbits closer to Mars than any other planet or satellite in the Solar System. It has an average radius of 11 km (7 mi) and a mass approximately 1/15 that of Mars. It is irregular in shape, with major axis lengths of 27 × 22 × 18 km (17 × 14 × 11 mi), and is one of the least reflective bodies in the Solar System having a albedo similar to charcoal. It rotates synchronously with Mars, meaning that one day on Phobos lasts about as long as one day on Mars—24 hours 39 minutes 35 seconds—and it keeps roughly the same face pointing toward its parent planet at all times due to tidal locking; however, because Phobos’s orbit lies outside that of Deimos, they rise in opposite directions with respect to the Martian horizon.
The origin of Phobos’s name is uncertain; it may be derived from Ancient Greek φόβος (phóbos), meaning “fear”, after Prometheus’ brother who brought doom upon mankind, or from Φώβος (Phōbos), son of Ares (Mars). The name “Phoebe” was given by William Herschel on 23 March 1787 during one his nightly telescopic observations while performing a star count. In Greek mythology, Phobos was often associated with fear, and Deimos with terror.