Mars: A Century of Exploration

The Moons: Phobos and Deimos

Mars has two moons, Phobos and Deimos, which were discovered in 1877. Both are thought to be C-type asteroids (which resemble the class of "carbonaceous chondrite" asteroids) which were "captured" by Mars.

Phobos is in an orbit 9,380 km from Mars and has a period of less than 8 hours (making the Martian month shorter than the Martian day!).
It is about 26 km long.
Deimos is in an orbit 23,460 km from Mars and has a period of about 30 hours. It is about 10 km long. Our first close-up look of these moons came with the Mariner 9 probe. This is a Viking 2 orbiter image of Deimos from a distance of 500 km. The surface appears smooth, but many craters are partially hidden by regolith (that is, by the soil and broken-up material).


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