Deimos

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Deimos
Deimos by Viking Orbiter
Date of discovery	August 12, 187712 August 1877 3 Elul 5637 H 3 Elul 6035 AM
Name of discoverer	Asaph Hall
Name origin	Greek deimos panic; attendant of Greek god of war
Orbital characteristics
Celestial class	Moon
Primary	Mars
Order from primary	2
Periareion	23,455 km1.56787e-4 AU 14,574.261 mi[1]
Apoareion	23,465 km1.56854e-4 AU 14,580.475 mi[1]
Semi-major axis	23,460 km1.5682e-4 AU 14,577.368 mi[2]
Orbital eccentricity	0.0002[2]
Sidereal month	1.262 da0.00346 a[2][3]
Avg. orbital speed	1,36 km/s[4]
Inclination	1.8°0.0314 rad 2 grad to the ecliptic[2][4]
Rotational characteristics
Sidereal day	1.026 da24.624 h[3]
Rotation speed	1.35 km/h3.75e-4 km/s 2.33014e-4 mi/s 0.839 mph
Axial tilt	0°0 rad 0 grad
Physical characteristics
Mass	1.4 * 1015 kg1.9052e-8 M☾ 2.34271e-10 M⊕[1]
Mean density	1,471 kg/m³1.471 g/ml 91.832 lb/ft³[5]
Mean radius	6.2 km3.853 mi[5]
Surface gravity	2.5e-3 m/s²0.0082 ft/s² 2.54929e-4 g[1]
Escape speed	0.0056 km/s20.16 km/h 0.00348 mi/s 12.527 mph[1]
Surface area	480 km²185.329 mi² 1.26586e-5 A☾ 9.41044e-7 A⊕[1]
Mean temperature	233 K-40.15 °C -40.27 °F 419.4 °R
Composition	Rock and ice mix
Color	#999999
Albedo	0.068[5]

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

Contents 1 Discovery 2 Orbital and physical characteristics 3 Origin 4 Exploration 5 Gallery 6 References

Discovery

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 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.^[11]

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

Origin

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]

Exploration

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.

Gallery

Successive views of Phobos and Deimos photographed from "Husband Hill" by a Mars Excursion Rover

Animated image of Deimos making one of its daily transits of the Sun

References

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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