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Phobos ( or {Link without Title} ) (' two Moon s (the other being Deimos ). It is named after the Greek God Phobos (which means "fear"), a son of Ares (Mars). Phobos is one of the smallest moons in the solar system, and orbits about 6000 Km (3728 Mi ) above the surface of Mars, closer to a major planet than any other moon. DISCOVERY See Also: Discovery of the Martian moons Phobos was discovered by American astronomer , Mars' other moon. The names, originally spelled ''Phobus'' and ''Deimus'' respectively, were suggested by Henry Madan (1838–1901), Science Master of Eton , from Book XV of the Iliad , where Ares summons Dread ( Deimos ) and Fear ( Phobos ).34 PHYSICAL CHARACTERISTICS on October 19 , 1978 . The large crater (mostly in darkness) on the upper left of the image is the Stickney .]] Phobos is a dark body that appears to be composed of Carbonaceous surface materials.5 It is similar to the C-type Asteroid s.6 Phobos' density is too low to be pure rock, however, and it is known to have significant Porosity .78Busch, M. W.; ''et al.''; 2007; ''Arecibo Radar Observations of Phobos and Deimos'', Icarus, Vol. 196, pp. 581-584 These results led to the suggestion that Phobos might contain a substantial reservoir of ice, but spectral observations have ruled out this hypothesis.9 Faint dust rings produced by Phobos and Deimos have long been predicted but attempts to observe these rings have, to date, failed. 10. Recent images from '' Mars Global Surveyor '' indicate that Phobos is covered with a layer of fine-grained Regolith at least 100 metres thick; it is believed to have been created by impacts from other bodies, but it is not known how the material stuck to an object with almost no gravity.11 Phobos is highly nonspherical, with dimensions of 27 × 21.6 × 18.8 km. Because of its shape alone, the gravity on its surface varies by about 210%; the tidal forces raised by Mars more than double this variation (to about 450%) because they compensate for a little more than half of Phobos' gravity at its sub- and anti-Mars poles. Phobos is heavily Crater ed.12 The most prominent surface feature is Stickney Crater , named after Asaph Hall's wife, Angeline Stickney Hall , Stickney being her maiden name. Like Mimas 's crater Herschel on a smaller scale, the impact that created Stickney must have almost shattered Phobos.13 Many grooves and streaks also cover the oddly shaped surface. The grooves are typically less than 30 m deep, 100 to 200 m wide, and up to 20 km in length, and were originally assumed to have been the result of the same impact that created Stickney. Analysis of results from the '' Mars Express '' spacecraft, however, revealed that the grooves are in fact independent of Stickney, and are deposits of material thrown into space by impacts on the Martian surface.14 The unique Kaidun Meteorite is claimed to be a piece of Phobos, but this has been difficult to verify since little is known about the detailed composition of the moon.15 ORBITAL CHARACTERISTICS Phobos's unusually close orbit around its parent planet produces some unusual effects. As seen from Phobos, Mars would appear 6400 times larger and 2500 times brighter than the full Moon appears from Earth, taking up a ¼ of the width of a celestial hemisphere. Phobos orbits Mars below the Synchronous Orbit radius, meaning that it moves around Mars faster than Mars itself rotates. Therefore it rises in the west, moves comparatively rapidly across the sky (in 4 h 15 min or less) and sets in the east, approximately twice a day (every 11 h 6 min). Since it is close to the surface and in an Equator ial orbit, it cannot be seen above the horizon from Latitude s greater than 70.4°. As seen from Mars' equator, Phobos would be one-third the angular diameter of the full would have an apparent size of about 0.35° in the Martian sky. Phobos' phases, inasmuch as they could be observed from Mars, take 0.3191 days to run their course (Phobos' Synodic period), a mere 13 seconds longer than Phobos' Sidereal Period . Solar transits See Also: Transit of Phobos from Mars , as seen by Mars Rover '' Opportunity '']] To an observer on the Martian surface, Phobos would regularly Transit across the Sun. It is not large enough to cover the Sun's disk, and so cannot cause a Total Eclipse . Several of these transits have been photographed by the Mars Rover '' Opportunity ''. During the transits, Phobos's shadow is cast on the surface of Mars, and has been photographed by several spacecraft. Death Phobos' low orbit means that it will eventually be destroyed: Tidal Force s are lowering its orbit, currently at the rate of about 1.8 metres per century, and in 30-80 million years it will either impact the surface of Mars or (more likely) break up into a Planetary Ring . Given Phobos' irregular shape and assuming that it is a Pile Of Rubble (specifically a Mohr-Coulomb Body ), it has been calculated that Phobos is stable with respect to tidal forces, but it is estimated that Phobos will pass the Roche Limit for a Rubble Pile when its orbital radius drops to about 7100 km, and will probably break up soon afterwards.Holsapple, K. A.; (2001); ''Equilibrium Configurations of Solid Cohesionless Bodies'' , Icarus, Vol. 154, pp. 432–448 ORIGIN Phobos and Deimos both have much in common with carbonaceous (C-type) Asteroid s, with Spectra , Albedo s and Densities very similar to those seen in C-type asteroids. This has led to speculation that both moons could have been captured into Martian orbit from the main Asteroid Belt .16 However, both moons have very circular orbits which lie almost exactly in Mars' Equatorial Plane . Captured moons would be expected to have eccentric orbits in random inclinations. Some evidence suggests that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large Planetesimal Craddock, R. A.; (1994); ''The Origin of Phobos and Deimos'', Abstracts of the 25th Annual Lunar and Planetary Science Conference, held in Houston, TX, 14-18 March 1994, p. 293. "HOLLOW PHOBOS" CLAIMS In the 1950 and 1960s, Phobos's unusual orbit and low density led to speculations that it might be a hollow artificial object. Around 1958 , Russian astrophysicist Iosif Samuilovich Shklovsky , studying the Secular acceleration of Phobos' orbital motion, suggested a "thin sheet metal" structure for Phobos, a suggestion which led to speculations that Phobos was of artificial origin.Shklovsky, I. S.; ''The Universe, Life, and Mind'', Academy of Sciences USSR, Moscow, 1962 Shklovsky based his analysis on estimates of the upper Martian atmosphere's density, and deduced that for the weak braking effect to be able to account for the secular acceleration, Phobos had to be very light —one calculation yielded a hollow iron sphere 16 km across but less than 6 cm thick.17 In a February 1960 letter to the journal ''Astronautics'', Singer, S. F. ; ''Astronautics'', February 1960 Siegfried Frederick Singer , then science advisor to President Eisenhower , came out in support of Shklovsky's theory, going as far as to state that " {Link without Title} purpose would probably be to sweep up radiation in Mars' atmosphere, so that Martians could safely operate around their planet". A few years later, in 1963 , Raymond H. Wilson Jr. , Chief of Applied Mathematics at NASA , allegedly announced to the Institute Of Aerospace Sciences that "Phobos might be a colossal base orbiting Mars", and that NASA itself was considering the possibility. Subsequently, however, the existence of the acceleration that had caused the claims was subjected to doubt,18 and the problem vanished by 1969 .19 The earlier studies had used an overestimated value of 5 cm/yr for the rate of altitude loss, which was later revised to 1.8 cm/yr. The secular acceleration is now attributed to tidal effects, which had not been considered in the earlier studies. The density of Phobos is now measured to be 1.9 g/cm³, which is inconsistent with a hollow shell. In addition, images obtained by the Viking Probes in the 1970s clearly showed a natural object, not an artificial one. Similar "hollow Moon" and " Hollow Earth " claims have been made. EXPLORATION Past missions Phobos has been photographed in close-up by several spacecraft whose primary mission has been to photograph Mars . The first was '' Mariner 9 '' in 1971 , followed by '' Viking 1 '' in 1977 , '' Mars Global Surveyor '' in 1998 and 2003 , and '' Mars Express '' in 2004 . The only dedicated Phobos probes have been the Soviet '' Phobos 1 '' and '' Phobos 2 ''; the first was lost en route to Mars , and the second returned some data and images before failing prior to its detailed examination of the moon. Future missions The Russian Space Agency is planning to launch a sample return mission to Phobos in 2009 , called '' Phobos-Grunt ''. Chinese surveying equipment will be included.20 , 2007 ) NAMED GEOLOGICAL FEATURES Geological features on Phobos are named after , named after the astronomer Johannes Kepler . Several craters have been named. {Link without Title} REFERENCES SEE ALSO
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