Phobos (IPA: /ˈfoʊbəs/) is one of the two small moons orbiting Mars, the other being Deimos. Both were discovered in 1877 by Asaph Hall. With a mean radius of 6.9 mi (11.1 km), Phobos is about 7 times as massive as Deimos. It is named after the Greek god Phobos which means "fear". Both Phobos and Deimos are likely to be captured asteroids, and both are small, battered, and irregularly shaped. Phobos orbits only at about 5,827 mi (9,377 km) from the center of Mars, making it the closest natural satellite to its parent planet in the solar system.