Mars's two moons are potato-shaped rocky lumps that are unique in the inner solar system. Asaph Hall discovered the two satellites in 1877.

Facts about Mars's Satellites

Mars, the Red Planet, was named for the God of War; therefore, it is fitting that its two moons are named Phobos, which means fear, and Deimos, which means panic. Phobos is the larger of the two moons at 27 x 22 x 18 kilometers. Phobos is the closer satellite to Mars and orbits the planet at seven and a half hours. Deimos's measurements are 15 x 12 x 11 kilometers. Deimos is farther from Mars and orbits in just over thirty hours. Mars's day lasts for about 24 and a half hours; therefore Phobos orbits the planet about three times in a day while Deimos takes a bit longer than one day to complete an orbit.

The surface of Phobos contains one large crater named Stickney and parallel valleys that have been gouged out of the dust leading away from the crater. Other craters on Phobos are named after astronomers and characters in Jonathan Swift's Gulliver's Travels. The surface of Deimos is less distinct due to a thicker layer of dust covering impact scars. Two of the craters have been named Swift and Voltaire.