Moons of Mars (Phobos and Deimos)

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The planet Mars has two moons - Phobos and Deimos. They are irregular in shape and significantly smaller than the Earth's Moon.

Phobos has a diameter of 22.2 km (13.8 mi) and a mass of 1.08×1016 kg, while these measures for Deimos are 12.6 km (7.8 mi) and 2.0×1015 kg. Phobos orbits closer to Mars, with a semi-major axis of 9,377 km (5,827 mi) and an orbital period of 7.66 hours; Deimos's semi-major axis is 23,460 km (14,580 mi) with an orbital period of 30.35 hours.

Both Phobos and Deimos were discovered by Asaph Hall in August 1877 and are named after the Greek mythological twin characters Phobos and Deimos who accompanied their father, the god of war (know to Romans as Mars), into battle.