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Deimos

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MOON

Deimos

The smaller, smoother moon of Mars — a likely captured asteroid drifting slowly outward.

Orbits Mars Outer moon of Mars
  • Smaller of the two Martian moons (15 u00d7 12.2 u00d7 11 km).
  • Orbits far enough from Mars that its orbit is slowly expanding (unlike Phobos).
  • Likely a captured C-type or D-type asteroid.
  • Best photographed by the UAE's Hope orbiter in 2023 u2014 the first detailed view of its trailing hemisphere.

Physical Properties

6 km
1.4762e15 kg
1.471 g/cm³
0.003 m/s²
0.00556 km/s
30.28 h
0.068

Orbit

Mars
23,463 km
0.00033
0.93°
1.263 d
1.3513 km/s

Sources & Further Reading

Numerical values (radius, mass, orbital elements, temperatures) are drawn from NASA NSSDC Planetary Fact Sheets, JPL Horizons, and the JPL Small-Body Database. Last refreshed: 2026-04-18 18:19:23.

Deimos is the smaller and more distant of the two moons of Mars. Like Phobos, it was discovered by Asaph Hall in August 1877. It orbits Mars every 30.3 hours at an altitude of about 23,460 km.

Deimos’s surface is far smoother in appearance than Phobos’s because a thick regolith partly fills its craters. Only two named craters exceed 2 km in diameter: Swift and Voltaire. In 2023 the UAE’s Hope orbiter captured the first detailed images of Deimos’s far side, revealing details of its trailing hemisphere never before seen.

Unlike Phobos, Deimos is outside Mars’s synchronous orbit, meaning tidal forces push it slowly outward — it will eventually escape Mars entirely. Its density and spectral properties are consistent with a C- or D-type carbonaceous asteroid, suggesting it may be a captured main-belt object rather than a body that formed in Mars’s orbit.