Deimos (moon)

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Deimos

An image of Deimos taken by the Viking 1 orbiter.
Discovery
Discovered by	Asaph Hall
Discovered on	August 12 1877
Orbital characteristics
Mean radius	23,460 km
Eccentricity	0.0002
Revolution period	1.262 d
Avg. Orbital Speed	0.22 km/s
Inclination	0.93° (to Mars' equator) 1.793° (to the local Laplace plane) 27.58° (to the ecliptic)
Satellite of	Mars
Physical characteristics
Mean diameter	12.6 km (15.0×12×10.4)
Mass	kg
Mean density	2.2 g/cm3
Surface gravity	0.0039 m/s² (3.9 mm/s²)
Surface Gravity (Earth = 1)	0.00040 (400 µg)
Escape velocity	0.0069 km/s (6.9 m/s)
Rotation period	synchronous
Albedo	0.07
Surface temp.	≈233 K
Atmospheric pressure	no atmosphere

Discovery

Deimos was discovered on August 12, 1877 at about 07:48 UTC (given in contemporary sources as "August 11 14:40" Washington mean time using the old astronomical convention of beginning a day at noon, so 12 hours must be added to get the actual local mean time) [link].

Characteristics

Deimos transits the Sun, as seen by Mars Rover Opportunity

Deimos is probably an asteroid that was perturbed by Jupiter into an orbit that allowed it to be captured by Mars, though this hypothesis is still in some dispute. Like most bodies of its size, Deimos is highly nonspherical with dimensions of 15×12×10 km.

Deimos is composed of carbon-rich rock, much like C-type (carbonaceous chondrite) asteroids, and ice. It is cratered, but the surface is noticeably smoother than that of Phobos, caused by the partial filling of craters with regolith. The two largest craters, Swift and Voltaire, measure about 3 kilometres across.

As seen from Deimos, Mars would be 1000 times larger and 400 times brighter than the full Moon as seen from Earth, taking up a full 1/11 of the width of a celestial hemisphere.

As seen from Mars, Deimos has an angular diameter of no more than 2.5' and would therefore appear starlike to the naked eye. At its brightest ("full moon") it would be about as bright as Venus is from Earth; at the first or third quarter phase it would be about as bright as Vega. When Deimos passes in front of the Sun its angular diameter is only about 2.5 times the angular diameter for Venus during a transit of Venus from Earth. With a small telescope, a Martian observer could see Deimos' phases, which take 1.2648 days to run their course (Deimos' synodic period).

Orbits of Phobos and Deimos (to scale)

Unlike Phobos, which orbits so fast that it actually rises in the west and sets in the east, Deimos rises in the east and sets in the west. However, the orbital period of Deimos of about 30.4 hours exceeds the Martian solar day ("sol") of about 24.7 hours by such a small amount that it takes 2.7 days between rising and setting for an equatorial observer.

Because Deimos' orbit is relatively close to Mars and has only a very small inclination to Mars' equator, it cannot be seen from Martian latitudes greater than 82.7°.

Deimos in fiction

• In part 3 chapter 3 (the "Voyage to Laputa") of Jonathan Swift's famous satire Gulliver's Travels, a fictional work written in 1726, the astronomers of Laputa are described as having discovered two satellites of Mars orbiting at distances of 3 and 5 Martian diameters, respectively. The actual orbital distances of Phobos and Deimos are 1.4 and 3.5 Martian diameters, respectively. This is regarded as a coincidence; no telescope in Swift's day would have been even remotely powerful enough to discover these satellites.
• In the comic Sailor Moon, Deimos is the name of one of two crows belonging to Sailor Mars.

• Kim Stanley Robinson's Green Mars (1993) includes a detailed description of a manned landing on Deimos.
• The second episode of the Computer and video game Doom takes place in a UAC base on Deimos. Before the beginning of the game, Deimos disappears from martian orbit, and is reached by the unnamed Marine after he steps into a large teleporter on Phobos. Eventually, it is discovered that Deimos is floating above Hell itself.
• The UESC Marathon of the Marathon computer game series is a hollowed-out Deimos.

References

• Robert S. Richardson, "If You Were on Mars", [Astronomical Society of the Pacific Leaflets, 4 (1943) 214] (1943ASPL....4..214R) (Leaflet No. 178, December 1943)
• Contemporary accounts of the discovery of Phobos and Deimos:
• * [The Observatory, 1 (1877) 181]
• * [Astronomische Nachrichten, 91 (1878) 11/12]
• * [Monthly Notices of the Royal Astronomical Society, 38 (1878) 205]
• * [Astronomische Nachrichten, 92 (1878) 47/48]

See also

• Phobos, the other moon of Mars
• List of features on Phobos and Deimos
• Transit of Deimos from Mars

External links

• [Deimos rotation movie]
• [Animation of Deimos]

[http://encycl.opentopia.com/ edit ] Mars (satellites)

[[zh-min-nan:Deimos (oē-chheⁿ)]]

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