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Deimos viking2 big.jpg
Deimos by Viking Orbiter
Date of discovery Date of discovery::August 12, 1877
Name of discoverer Discoverer::Asaph Hall
Name origin Name origin::Greek ''deimos'' panic; attendant of Greek god of war
Orbital characteristics
Celestial class Member of::Moon
Primary Primary::Mars
Order from primary Order::2
Periareion Periapsis::23,455 km[1]
Apareion Apoapsis::23,465 km[1]
Semi-major axis Semi-major axis::23,460 km[2]
Orbital eccentricity Orbital eccentricity::0.0002[2]
Sidereal month Sidereal period::1.262 da[2][3]
Avg. orbital speed Orbital speed::1.36 km/s[4]
Inclination Inclination::1.8° to the ecliptic[2][4]
Rotational characteristics
Sidereal day Sidereal day::1.026 da[3]
Rotation speed Rotation speed::1.35 km/h
Axial tilt Axial tilt::0°
Physical characteristics
Mass 1.4 * 1015 kg[1]
Mean density Planet density::1,471 kg/m³[5]
Mean radius Mean radius::6.2 km[5]
Surface gravity Surface gravity::2.5e-3 m/s²[1]
Escape speed Escape speed::0.0056 km/s[1]
Surface area Lunar surface area::480 km²[1]
Mean temperature Mean temperature::233 K
Composition Composition::Rock and ice mix
Color Color::#999999
Albedo Albedo::0.068[5]

Deimos (from the Greek δειμος or deimos panic) is the outer, and the smaller, of the two moons of Mars.


Deimos was discovered by the astronomer Asaph Hall on August 12, 1877, at the United States Naval Observatory in Washington, DC.[6][7][8] The astronomer V. Knorre named the satellite Deimos (and also provided the name Phobos for the other satellite that Hall discovered six days later), per a suggestion by Henry G. Madan of Eton, based on the names given in The Iliad for the two servants of Ares, the Greek god of war, named Fear (Phobos) and Panic (Deimos).[9]

Orbital and physical characteristics

Deimos orbits Mars at a distance slightly further away than the distance of a synchronous orbit. For that reason, Deimos rises in the east and sets in the west of the Martian sky, about 2.7 days after its rising.[10]

Deimos is not round, but is shaped like a potato, with dimensions 15 x 12.2 x 11 km. Its largest surface feature is a 2.3 km diameter crater.[10] Deimos is heavily cratered but has a smooth-appearing surface. Its surface gravity is very weak, perhaps too weak to retain the ejecta from a crater impact. This ejecta is likely retained around Mars in a ring and redeposited as regolith on the surface of Deimos as it passes.[11]

Deimos' orbit is so little inclined with respect to the ecliptic that it makes daily transits across the Sun.[10]


The favored theory among conventional astronomers is that Deimos and its companion moon Phobos are captured C-type asteroids.[3][10] However, that theory is not universally accepted.[3]


The NASA Viking 1 and Visiting mission::Viking 2 orbiters both have taken close-up photographs of Deimos on the way to deliver their respective landing craft to the Martian surface. Since then, several missions have made flybys of Deimos, including the Visiting mission::Mars Reconnaissance Orbiter. The two Visiting mission::Mars Excursion Rovers have taken photographs of Deimos as seen from the surface of Mars.


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  1. 1.0 1.1 1.2 1.3 1.4 1.5 Calculated
  2. 2.0 2.1 2.2 2.3 "Planetary Satellite Mean Orbital Parameters," Solar System Dynamics, JPL, NASA. Accessed February 11, 2008.
  3. 3.0 3.1 3.2 3.3 "Fact Sheet for Deimos." Solar System Exploration, NASA. Accessed February 12, 2008.
  4. 4.0 4.1 Hamilton, Calvin J. "Mars' Moon Deimos." SolarViews, 2001. Accessed February 12, 2008.
  5. 5.0 5.1 5.2 "Planetary Satellite Physical Parameters." Solar System Dynamics, JPL, NASA. Accessed February 11, 2008.
  6. Authors unknown. "Notes: The Satellites of Mars." The Observatory, 1:181-185, 1877. Accessed February 11, 2008, from the SAO/NASA Astrophysics Data System, Harvard University.
  7. Hall, A. "Observations of the Satellites of Mars." Astronomische Nachrichten, 91(2161):11-14, 1877. Accessed February 11, 2008.
  8. Morley, TA. A catalogue of ground-based astrometric observations of the Martian satellites, 1877-1982, Astronomy and Astrophysics Supplement Series (ISSN 0365-0138), 77(2):209-226, February 1989. Accessed February 11, 2008.
  9. Knorre, V. "Entdeckung zweier Planeten." Astronomische Nachrichten, 92(2187):47-48, March 14, 1878. Accessed February 11, 2008.
  10. 10.0 10.1 10.2 10.3 "Entry for Deimos." The Planetary Society. Accessed February 12, 2008.
  11. "Mars: Extreme Planet: Deimos." Mars Exploration Program, NASA. Accessed February 12, 2008.

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