The two moons of Mars are Phobos and Deimos. They are irregular in shape. Both were discovered by American astronomer Asaph Hall in August 1877 and are named after the Greek mythological twin characters Phobos (fear and panic) and Deimos (terror and dread) who accompanied their father Ares into battle. Ares, god of war, was known to the Romans as Mars. Compared to the Earth's Moon, the moons Phobos and Deimos are small. Phobos has a diameter of 22.2 km (13.8 mi) and a mass of 1.08 kg, while Deimos measures 12.6 km (7.8 mi) across, with a mass of 2.0 kg. Phobos orbits closer to Mars, with a semi-major axis of and an orbital period of 7.66 hours; while Deimos orbits farther with a semi-major axis of and an orbital period of 30.35 hours. History Early speculation Speculation about the existence of the moons of Mars had begun when the moons of Jupiter were discovered. When Galileo Galilei, as a hidden report about his having observed two bumps on the sides of Saturn (later discovered to be its rings), used the anagram smaismrmilmepoetaleumibunenugttauiras for Altissimum planetam tergeminum observavi ("I have observed the most distant planet to have a triple form"), Johannes Kepler had misinterpreted it to mean Salve umbistineum geminatum Martia proles (Hello, furious twins, sons of Mars). Perhaps inspired by 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. Phobos and Deimos (both found in 1877, more than a century after Swift's novel) have actual orbital distances of 1.4 and 3.5 Martian diameters, and their respective orbital periods are 7.66 and 30.35 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