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{{Planet
| name=Deimos
| image=Deimos viking2 big.jpg
| caption=Deimos by [[Viking Orbiter]]
| date=August 12, 1877
| discname=[[Asaph Hall]]
| origname=Greek ''deimos'' panic; attendant of Greek god of war
| primary=Mars
| order=2
| periapsis=23,455 km<ref name=calc>Calculated</ref>
| apoapsis=23,465 km<ref name=calc/>
| semimajor=23,460 km<ref name=ssdelem>"[http://ssd.jpl.nasa.gov/?sat_elem Planetary Satellite Mean Orbital Parameters]," Solar System Dynamics, [[JPL]], [[NASA]]. Accessed February 11, 2008.</ref>
| eccentricity=0.0002<ref name=ssdelem/>
| sidereal=1.262 da<ref name=ssdelem/><ref name=nasa1>"[http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mar_Deimos&Display=Facts&System=Metric Fact Sheet for Deimos]." ''Solar System Exploration'', [[NASA]]. Accessed February 12, 2008.</ref>
| orbitspeed=1,36 km/s<ref name=solarviews>Hamilton, Calvin J. "[http://solarviews.com/eng/deimos.htm Mars' Moon Deimos]." ''SolarViews'', 2001. Accessed February 12, 2008.</ref>
| inclination=1.8°<ref name=ssdelem/><ref name=solarviews/>
| reference=the ecliptic
| siderealday=1.026 da<ref name=nasa1/>
| rotatespeed=1.35 km/h
| axialtilt=0°
| mass=1.4 * 10<sup>15</sup> kg (2.5 * 10<sup>-8</sup>% earth)<ref name=calc/>
| density=1,471 kg/m³<ref name=ssdphys>"[http://ssd.jpl.nasa.gov/?sat_phys_par Planetary Satellite Physical Parameters]." Solar System Dynamics, [[JPL]], [[NASA]]. Accessed February 11, 2008.</ref>
| meanradius=6.2 km<ref name=ssdphys/>
| surfacegrav=2.5 * 10<sup>-3</sup> m/s² (2.6 * 10<sup>-4</sup> ''g'')<ref name=calc/>
| escapespeed=0.0056 km/s<ref name=calc/>
| surfacearea= 480 km²<ref name=calc/>
| meantemp = 233 K
| composition=Rock and ice mix
| albedo=0.068<ref name=ssdphys/>
}}
'''Deimos''' (from the Greek δειμος or ''deimos'' panic) is the outer, and the smaller, of the two [[satellite]]s of [[Mars]].
== Discovery ==
Deimos was discovered by the astronomer [[Asaph Hall]] on August 12, 1877, at the [[United States Naval Observatory]] in [[Washington, DC]].<ref>Authors unknown. "[http://adsabs.harvard.edu//full/seri/Obs../0001//0000181.000.html Notes: The Satellites of Mars]." ''The Observatory'', 1:181-185, 1877. Accessed February 11, 2008, from the SAO/[[NASA]] Astrophysics Data System, [[Harvard University]].</ref><ref>Hall, A. "[http://adsabs.harvard.edu//full/seri/AN.../0091//0000013.000.html Observations of the Satellites of Mars]." ''Astronomische Nachrichten'', 91(2161):11-14, 1877. Accessed February 11, 2008.</ref><ref>Morley, TA. ''[http://adsabs.harvard.edu//full/seri/A+AS./0077//0000220.000.html 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.</ref> 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).<ref>Knorre, V. "[http://adsabs.harvard.edu//full/seri/AN.../0092//0000031.000.html Entdeckung zweier Planeten]." ''Astronomische Nachrichten'', 92(2187):47-48, March 14, 1878. Accessed February 11, 2008.</ref>
== 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.<ref name=planetsoc>"[http://www.planetary.org/explore/topics/mars/deimos.html Entry for Deimos]." The Planetary Society. Accessed February 12, 2008.</ref>
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.<ref name=planetsoc/> 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.<ref name=nasa2>"[http://mars.jpl.nasa.gov/facts/deimos.html Mars: Extreme Planet: Deimos]." Mars Exploration Program, [[NASA]]. Accessed February 12, 2008.</ref>
Deimos' orbit is so little inclined with respect to the [[ecliptic]] that it makes daily transits across the [[Sun]].<ref name=planetsoc/> 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 [[astronomy|astronomers]] is that Deimos and its companion moon [[Phobos]] are captured [[C-type asteroids]].<ref name=nasa1/><ref name=planetsoc/> However, that theory is not universally accepted.<ref name=nasa1/>
== 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 ==
<gallery>
Image:Deimos Phobos Husband Hill.jpg|Successive views of Phobos and Deimos photographed from "Husband Hill" by a Mars Excursion Rover
Image:Deimos transit.gif|Animated image of Deimos making one of its daily transits of the [[Sun]]
</gallery>
== References ==
<references/>
{{Solarsystem}}
| name=Deimos
| image=Deimos viking2 big.jpg
| caption=Deimos by [[Viking Orbiter]]
| date=August 12, 1877
| discname=[[Asaph Hall]]
| origname=Greek ''deimos'' panic; attendant of Greek god of war
| primary=Mars
| order=2
| periapsis=23,455 km<ref name=calc>Calculated</ref>
| apoapsis=23,465 km<ref name=calc/>
| semimajor=23,460 km<ref name=ssdelem>"[http://ssd.jpl.nasa.gov/?sat_elem Planetary Satellite Mean Orbital Parameters]," Solar System Dynamics, [[JPL]], [[NASA]]. Accessed February 11, 2008.</ref>
| eccentricity=0.0002<ref name=ssdelem/>
| sidereal=1.262 da<ref name=ssdelem/><ref name=nasa1>"[http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mar_Deimos&Display=Facts&System=Metric Fact Sheet for Deimos]." ''Solar System Exploration'', [[NASA]]. Accessed February 12, 2008.</ref>
| orbitspeed=1,36 km/s<ref name=solarviews>Hamilton, Calvin J. "[http://solarviews.com/eng/deimos.htm Mars' Moon Deimos]." ''SolarViews'', 2001. Accessed February 12, 2008.</ref>
| inclination=1.8°<ref name=ssdelem/><ref name=solarviews/>
| reference=the ecliptic
| siderealday=1.026 da<ref name=nasa1/>
| rotatespeed=1.35 km/h
| axialtilt=0°
| mass=1.4 * 10<sup>15</sup> kg (2.5 * 10<sup>-8</sup>% earth)<ref name=calc/>
| density=1,471 kg/m³<ref name=ssdphys>"[http://ssd.jpl.nasa.gov/?sat_phys_par Planetary Satellite Physical Parameters]." Solar System Dynamics, [[JPL]], [[NASA]]. Accessed February 11, 2008.</ref>
| meanradius=6.2 km<ref name=ssdphys/>
| surfacegrav=2.5 * 10<sup>-3</sup> m/s² (2.6 * 10<sup>-4</sup> ''g'')<ref name=calc/>
| escapespeed=0.0056 km/s<ref name=calc/>
| surfacearea= 480 km²<ref name=calc/>
| meantemp = 233 K
| composition=Rock and ice mix
| albedo=0.068<ref name=ssdphys/>
}}
'''Deimos''' (from the Greek δειμος or ''deimos'' panic) is the outer, and the smaller, of the two [[satellite]]s of [[Mars]].
== Discovery ==
Deimos was discovered by the astronomer [[Asaph Hall]] on August 12, 1877, at the [[United States Naval Observatory]] in [[Washington, DC]].<ref>Authors unknown. "[http://adsabs.harvard.edu//full/seri/Obs../0001//0000181.000.html Notes: The Satellites of Mars]." ''The Observatory'', 1:181-185, 1877. Accessed February 11, 2008, from the SAO/[[NASA]] Astrophysics Data System, [[Harvard University]].</ref><ref>Hall, A. "[http://adsabs.harvard.edu//full/seri/AN.../0091//0000013.000.html Observations of the Satellites of Mars]." ''Astronomische Nachrichten'', 91(2161):11-14, 1877. Accessed February 11, 2008.</ref><ref>Morley, TA. ''[http://adsabs.harvard.edu//full/seri/A+AS./0077//0000220.000.html 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.</ref> 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).<ref>Knorre, V. "[http://adsabs.harvard.edu//full/seri/AN.../0092//0000031.000.html Entdeckung zweier Planeten]." ''Astronomische Nachrichten'', 92(2187):47-48, March 14, 1878. Accessed February 11, 2008.</ref>
== 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.<ref name=planetsoc>"[http://www.planetary.org/explore/topics/mars/deimos.html Entry for Deimos]." The Planetary Society. Accessed February 12, 2008.</ref>
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.<ref name=planetsoc/> 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.<ref name=nasa2>"[http://mars.jpl.nasa.gov/facts/deimos.html Mars: Extreme Planet: Deimos]." Mars Exploration Program, [[NASA]]. Accessed February 12, 2008.</ref>
Deimos' orbit is so little inclined with respect to the [[ecliptic]] that it makes daily transits across the [[Sun]].<ref name=planetsoc/> 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 [[astronomy|astronomers]] is that Deimos and its companion moon [[Phobos]] are captured [[C-type asteroids]].<ref name=nasa1/><ref name=planetsoc/> However, that theory is not universally accepted.<ref name=nasa1/>
== 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 ==
<gallery>
Image:Deimos Phobos Husband Hill.jpg|Successive views of Phobos and Deimos photographed from "Husband Hill" by a Mars Excursion Rover
Image:Deimos transit.gif|Animated image of Deimos making one of its daily transits of the [[Sun]]
</gallery>
== References ==
<references/>
{{Solarsystem}}