Mars is the fourth planet from the sun. Mars has two Moons, Phobos and Deimos. They were both discovered in 1877, by Asaph Hall at the Naval Observatory in Washington DC.
Phobos is the larger one of Mars’ moons, and it orbits closer to the surface of Mars than Deimos does. Phobos is 22 km in diameter, and orbits Mars three times every 24 hours. Phobos is slowly moving closer to Mars, and scientists believe that Phobos will eventually collide with Mars (in about 20-50 million years) or break up into pieces because of Mars’ gravitational pull. Phobos has an irregular shape, many craters, and dark terrain.
Left Image: Phobos from Mars Express. Image credit: G. Neukum (FU Berlin) et al., Mars Express, DLR, ESA, NASA (public domain). Right Image: Deimos: A Small Martian Moon. Image credit: Viking Project, JPL, NASA (public domain)
Deimos is the smaller of Mars’ two moons, and is one of the smallest moons in the solar system. Deimos is 12 km in diameter, and is more than twice as far away from Mars as Phobos is. Because it is significantly further away, Deimos takes over 30 hours to orbit Mars once. Deimos, like Phobos, has an irregular shape.
Astronomers think that both Phobos and Deimos were asteroids that got pulled into orbit around Mars by Mars’ gravity. Unlike Phobos, Deimos is much smoother and only has a few craters. In addition, because it is much further away, instead of colliding with Mars or breaking up into pieces, Deimos is likely to slowly move away and fling into space.
Phobos and Deimos were both named after Greek gods. These gods were thought to embody fear and terror: the word “phobos” in Greek means fear, and this is where the English word “phobia” comes from. In Greek mythology, Phobos and Deimos were the twin sons of Ares and Aphrodite. Ares is the Greek name for the Roman god of war, Mars.
Many scientists think that Phobos and Deimos need to be explored more than they have been so far. NASA’s Jet Propulsion Laboratory and Ames Research Center have both proposed missions to Phobos and Deimos. The SETI Institute (Search for Extraterrestrial Intelligence) also thinks that Phobos and Deimos are mysterious objects that are important stepping stones for the exploration of Mars in the future. The Massachusetts Institute of Technology (MIT), Stanford University, and NASA have proposed a partnership to explore Phobos in the near future… well, that’s if you consider 2030 the near future!
NASA Solar System Exploration - Phobos: Overview