Mars has two very small elliptical-shaped moons, Deimos and Phobos—the Greek
names for the companions of the god Mars: Deimos (terror) and Phobos (fear). They
were discovered in Aug. 1877 by the American astronomer Asaph Hall (1829–1907)
of the U.S. Naval Observatory in Washington, DC.
The inner satellite, Phobos, is 16.78 mi (27 km) long, and it revolves around the
planet in 7.6 hours. The short orbital period of Phobos means that the satellite
travels around Mars three times in a Martian day. The outer moon, Deimos, is 9.32
mi (15 km) long, and it circles the planet in 30.35 hours.
Phobos orbits Mars at a distance of only 5,627 mi (9,378 km) and is closer to its
planet than any other moon in the solar system. Observation of Phobos has revealed
that the moon's orbit is actually decreasing downward; in about 40 million years it will
crash into the planet's surface or break up into a ring.