The inner → satellite of → Mars orbiting less than 6,000 km above the surface of Mars, closer to its → primary than any other → moon in the → solar system. Phobos is irregularly shaped, 27 x 22 x 18 km in size and orbits Mars in 0.319 days. Phobos' orbit is decaying at a rate of about 2 centimeters per year; it is therefore expected to break up and crash onto Mars within the next 50 million years. See also: → Roche limit, → orbit decay.
In Gk. mythology, Phobos is one of the sons of Ares (Mars) and Aphrodite (Venus). The name means "fear, panic, flight."