The farther moon of Mars.

Numbers (from

  • Size: 9 x 7 x 7 miles
  • Orbit Radius: 14,545 mi.
  • Revolution: 30.3 hours

The word Deimos is from a Greek word 'δειμος' meaning fear or terror, just like Phobos, Mars's other moon. Phobos and Deimos, according to the writeup at Phobos, were the horses that pulled Mars/Ares' chariot.

(That just seems a little too convenient for moons that were only discovered in 1877, but that's only in hindsight. If Asaph Hall had discovered three moons on Mars instead of two, they'd likely have had different names entirely...)

Deimos is the outer and smaller of Mars's two moons. Discovered in 1877 along with its inner counterpart Phobos by Asaph Hall, it was likewise named after a son of Ares and is Greek for "panic." Similar to Phobos in many other ways as well, it resembles the C-type asteroids found in the asteroid belt in both its composition and its irregular, cratered shape. It appears to be covered with a thick coat of regolith produced by asteroid impacts.

At 23,459 km from the planet's center, Deimos' completes an orbit every 1.26244 (Earth) days. Due to its irregular shape, its "diameter" varies from 15 to 11 km, with a mean of 12.6 km. Its mass is about 1.8e15 kg with a density of 1.7 gm/cm3.

Deimos' surface is actually less rough than that of Phobos, despite its lesser mass (and corresponding gravity) that should increase the chance of debris from asteroid impacts flying off into space. Its most prominent features are craters named Swift and Voltaire, each after the well-known writer of the same name. Swift (the writer) originally predicted twin Martian satellites in Gulliver's Travels, a century and a half before Hall's discovery. Gritchka tells me Voltaire mentioned Mars' moons in Micromégas.

Deimos is a tiny moon of Mars, perched right on the edge of high Mars orbit.

It is believed to be a captured C-Type asteroid; although nobody has any idea how it got to be there, or Phobos either for that matter. Any higher and it would have been destabilised by the tidal effects of the Suns gravity and left Mars entirely.

Other facts about Deimos is that it is very 'close' to the earth on the way back, less so on the way out. (By close I mean that the amount of propellent to return to the earth-moon system is small, as the return delta-v is only about 2.6km/s; and to travel there is about 6km/s; for comparison it takes about 9.5 km/s to get to low earth orbit from the earth; even a very tiny rocket can achieve 2.6km/s with ease. In terms of travel time, it's about 9 months in either direction; similar to some early sea travel.

Deimos's surface gravity is very, very weak. A man could jump off of the planet and go into an independent orbit around Mars; it's only about 20 km/h.

Finally, Deimos is believed by some scientists to contain ice. If so, mining this ice and using it for fuel could well open up space all the way from LEO to Mars and on to the main belt asteroids; rockets that are only currently able to reach LEO would be able to refuel there; perhaps even Jupiter and Saturn may be in reach. Certainly if the fuel is there returning it to LEO can be done cost effectively.

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