Deimos by Viking Orbiter
|Date of discovery||August 12, 1877|
|Name of discoverer||Asaph Hall|
|Name origin||Greek deimos panic; attendant of Greek god of war|
|Order from primary||2|
|Semi-major axis||23,460 km|
|Sidereal month||1.262 da|
|Avg. orbital speed||1,36 km/s|
|Inclination||1.8° to the ecliptic|
|Sidereal day||1.026 da|
|Rotational speed||1.35 km/h|
|Mass||1.4 * 1015 kg (2.5 * 10-8% earth)|
|Mean radius||6.2 km|
|Surface gravity||2.5 * 10-3 m/s² (2.6 * 10-4 g)|
|Escape speed||0.0056 km/s|
|Surface area||480 km²|
|Mean temperature||233 K|
|Composition||Rock and ice mix|
Deimos was discovered by the astronomer Asaph Hall on August 12, 1877, at the United States Naval Observatory in Washington, DC. 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).
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.
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. Deimos is heavily cratered but has a smooth-appearing surface. Its surface gravity is very weak, perhaps too week 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.
Deimos' orbit is so little inclined with respect to the ecliptic that it makes daily transits across the Sun. These transits are not, strictly speaking, eclipses because Deimos does no more than cast a shadow over the Sun as it passes between the Sun and Mars.
The NASA probes Viking 1 and Viking 2 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 Mars Reconnaissance Orbiter. The two Mars Excusion Rovers have taken photographs of Deimos as seen from the surface of Mars.
Animated image of Deimos making one of its daily transits of the Sun
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- 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.
- Hall, A. "Observations of the Satellites of Mars." Astronomische Nachrichten, 91(2161):11-14, 1877. Accessed February 11, 2008.
- 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.
- Knorre, V. "Entdeckung zweier Planeten." Astronomische Nachrichten, 92(2187):47-48, March 14, 1878. Accessed February 11, 2008.
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- "Mars: Extreme Planet: Deimos." Mars Exploration Program, NASA. Accessed February 12, 2008.