Moons of Mars

The two moons of Mars are Phobos and Deimos. Both moons were discovered in 1877 by Asaph Hall and are named after the characters Phobos (panic/fear) and Deimos (terror/dread) who, in Greek mythology, accompanied their father Ares, god of war, into battle. Ares was known as Mars to the Romans. It is possible that Mars may have moons smaller than 50–100 meters and a dust ring between Phobos and Deimos, but none have been discovered.

Of the two moons, Phobos orbits closer to Mars with a semi-major axis of 9,377 km (5,827 mi). Deimos orbits Mars with a semi-major axis of 23,460 km (14,580 mi).

Perhaps inspired by Johannes Kepler (and quoting Kepler's third law of planetary motion), Jonathan Swift's satire Gulliver's Travels (1726) refers to two moons in Part 3, Chapter 3 (the "Voyage to Laputa"), in which Laputa's astronomers are described as having discovered two satellites of Mars orbiting at distances of 3 and 5 Martian diameters with periods of 10 and 21.5 hours. The actual orbital distances of Phobos and Deimos are 1.4 and 3.5 Martian diameters, and their respective orbital periods are 7.6 and 30.3 hours. In the 20th century, V. G. Perminov, a spacecraft designer of early Soviet Mars and Venus spacecraft, speculated Swift found and deciphered records that Martians left on Earth. However, the view of most astronomers is that Swift was simply employing a common argument of the time, that as the inner planets Venus and Mercury had no satellites, Earth had one and Jupiter had four (known at the time), that Mars by analogy must have two. Furthermore, as they had not yet been discovered, it was reasoned that they must be small and close to Mars. This would lead Swift to making a roughly accurate estimate of their orbital distances and revolution periods. In addition Swift could have been helped in his calculations by his friend, the mathematician John Arbuthnot.

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