Phobos is the larger and closer of the two natural satellites of Mars. With a mean radius of 11.27 km,it is 1.79787 times more massive than Mars’ 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 9376 km 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. It is getting closer at rate of about 1 meter every 100 years, so it is predicted that in about 50 million years it will break up into a planetary ring or collide with the planet. The temperatures range from about −4°C (25°F) to −112°C (−170°F), on the sunlit and shadowed sides respectively.