Phobos (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. 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…
(read more: Wikipedia)
image: Color image of Phobos, imaged by the Mars Reconnaissance Orbiter in 2008, NASA/JPL-Caltech/University of Arizona