The red planet is a small world with two tiny moons. Both were discovered by Asaph Hall in 1877 and named after Phobos ("Fear") and Deimos ("Terror" or "Panic"), the mythical attendants of Mars, the Roman god of war.
Unlike Earth's moon, Phobos and Deimos are irregular, potato-shaped satellites that might be asteroids captured by Mars's gravity.
Phobos and Deimos are much closer to Mars than the moon is to Earth. Phobos orbits Mars at about 9,380 kilometers (5,830 miles), on average, and Deimos is about 23,450 kilometers (14,570 miles) from the planet. In contrast, Earth's moon is roughly 384,400 kilometers (238,900 miles) out.
Both Martian moons—particularly Phobos—are likely doomed to crash onto Mars's surface in about 50 million years, very soon by cosmic standards.
Much sooner than that, humans might use Phobos as a base for Mars missions, because of its stock of raw building material and even rocket propellant and life support, not to mention its great view of the rusty world below.
Best of all, Phobos is far enough from the influence of Mars's gravity that colonists wouldn't need very big rockets to leave Phobos for other worlds.
Its only shortcoming is its small size.
Phobos is a mere 23 kilometers (14 miles) across at its widest, but because it's so close to Mars, from the planet's surface it looks about one-third as big as the moon does from Earth.
Both moons are easy to see from Mars, but both are much darker than the Earth's moon because the materials on their surfaces don't reflect as much light.
On Earth, a full moon is so bright in the sky that it obscures the view of many stars and other celestial objects. Mars's moons don't affect its night sky nearly as much.
Phobos's tight orbit lets it zip around Mars about once every seven hours of Earth time. This is much faster than Mars's rotation, so it isn't in the Martian sky for long—it rises and sets in about four hours and is hidden for about seven. It actually travels quickly enough across the sky that human eyes could see it move.
Also thanks to its high speed, Phobos frequently eclipses the sun from Mars's surface—more than 1,000 eclipses can happen in a Martian year, each lasting less than 20 seconds. All are partial eclipses because, as seen from the surface of Mars, Phobos covers only about one-quarter of the sun's face.
Despite its name, slower-moving Deimos inspires little terror. It takes about 30 hours to orbit Mars once, far too slow for human eyes to make out its motion.