|October 19, 1978. The large crater (mostly in darkness) on the upper left of the image is Stickney.]]|
|Discovered by||Asaph Hall|
|Discovered on||August 18 1877|
|Revolution period||0.319 d (7 h 39.2 min)|
|Is a satellite of||Mars|
|Mean diameter||22.2 km (27×22×19)|
|Mean density||1.9 g/cm³|
|Surface gravity||m/s² (5 mm/sò)|
| Surface Gravity |
(Earth = 1)
|0.000,52 (520 õg)|
|Escape Velocity||0.011 km/s (11 m/s)|
|Atmospheric pressure||no atmosphere|
Phobos is the larger and innermost of Mars' two moonss, named after Phobos, son of Ares (Mars) from Greek Mythology. Phobos is closer to its primary than any other moon in the solar system, less than 6000 km above the surface of Mars. It is also one of the smaller moons in the solar system.
Phobos and Deimos were both discovered by American astronomer Asaph Hall. The names were suggested by Henry Madan (1838–1901), Science Master of Eton, from Book XV of the Illiad, where Ares summons Fear and Flight.
Phobos was discovered on August 18, 1877 at about 09:14 UTC (given in contemporary sources as "August 17 16:06" 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). 
Phobos orbits Mars below the synchronous orbit radius, meaning that it moves around Mars faster than Mars itself rotates. Thus it rises in the west, moves very rapidly across the sky (in 4 h 15 min or less) and sets in the east, usually twice a day (every 11 h 6 min). It is so close to the surface (in a low-inclination equatorial orbit) that it cannot be seen above the horizon from latitudes greater than 70.4°.
This low orbit means that Phobos will eventually be destroyed: tidal forces are lowering its orbit (currently at the rate of about 1.8 metres per century), and in about 50 million years it will either impact the surface of Mars or (more likely) break up into a planetary ring. Phobos is already below the Roche limit, but has not broken up because it is held together by more than just gravitational forces (tensile strength). The best estimate is that it will break up when its orbit radius drops to about 5000 km.
Phobos is a dark body that appears to be composed of C-type surface materials. It is similar to the C-type (blackish carbonaceous chondrite) asteroids that exist in the outer asteroid belt. Phobos's density is too low to be pure rock, however. It is probably composed of a mixture of rock and ice. The Soviet spacecraft Phobos 2 detected a faint but steady outgassing from Phobos. Unfortunately Phobos 2 died before it could determine the nature of the material, but it is most likely water. Recent images from Mars Global Surveyor indicates that Phobos is covered with a layer of fine dust about a metre thick, similar to the regolith on the Earth's Moon.
Phobos is heavily cratered. The most prominent feature on Phobos is the large crater named Stickney, the maiden name of Asaph Hall's wife Chloe Angeline Stickney Hall. Like Mimas's crater Herschel on a smaller scale, the impact that created Stickney must have almost shattered Phobos. The grooves and streaks on the surface were probably also caused by the Stickney impact. Phobos is highly nonspherical, with dimensions of 27 × 21.6 × 18.8 km.
Phobos is widely believed to be a captured asteroid. There is some speculation that it originated in the outer solar system rather than in the main asteroid belt. It is not known how the capture could have taken place.
As seen from Phobos, Mars would be 6400 times larger and 2500 times brighter than the full Moon as seen from Earth, taking up a full 1/4 of the width of a celestial hemisphere.
Sun, as seen by Mars Rover Opportunity]]
As seen from Mars's equator, Phobos is one-third the angular diameter of the full Moon as seen from Earth. Observers at higher Martian latitudes (less than the 70.4° latitude of invisibility) would see a smaller angular diameter because they are farther away from Phobos. Phobos' apparent size actually varies by up to 45% as it passes overhead, due to its proximity to Mars' surface. For an equatorial observer, for example, Phobos would be about 0,14ð upon rising and swell to 0,20ð by the time it reaches the zenith. By comparison, the Sun has an apparent size of about 0,35ð in the martian sky.
June 1, 2003 (NASA)]]
|Table of contents|
2 See also
3 External links
Contemporary accounts of the discovery of Phobos and Deimos: