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Phobos (systematic designation: ) is the larger and innermost of the two natural satellites of Mars, the other being Deimos. Both moons were discovered in 1877 by American astronomer Asaph Hall. Phobos is a small, irregularly shaped object with a mean radius of ,〔 and is seven times more massive than Deimos, Mars's outer moon. Phobos is named after the Greek god Phobos, a son of Ares (Mars) and Aphrodite (Venus) which was the personification of Horror. The name "Phobos" is pronounced , or like the Greek . Phobos orbits from the Martian surface, closer to its primary than any other known planetary moon. It is so close that it orbits Mars faster than Mars rotates, and completes an orbit in just 7 hours and 39 minutes. As a result, from the surface of Mars it appears to rise in the west, move across the sky in 4 hours 15 min or less, and set in the east, twice each Martian day. Phobos is one of the least reflective bodies in the Solar System, and features a large impact crater, Stickney. The temperatures range from about −4 °C (25 °F) to −112 °C (−170 °F), on the sunlit and shadowed sides respectively.〔 Images and models indicate that Phobos may be a rubble pile held together by a thin crust, and that it is being torn apart by tidal interactions. Phobos is drawing closer to Mars by 2 meters every one hundred years, and it is predicted that in 30 to 50 million years it will collide with the planet or break up into a planetary ring.〔(【引用サイトリンク】title=NASA – Phobos )〕 ==Discovery== Phobos was discovered by astronomer Asaph Hall on 18 August 1877, at the United States Naval Observatory in Washington, D.C., at about 09:14 Greenwich Mean Time (contemporary sources, using the pre-1925 astronomical convention that began the day at noon, give the time of discovery as 17 August at 16:06 Washington mean time).〔 (Table II, p. 220: first observation of Phobos on 18 August 1877.38498)〕 Hall also discovered Deimos, Mars's other moon, on 12 August 1877 at about 07:48 UTC. The names, originally spelled ''Phobus'' and ''Deimus'' respectively, were suggested by Henry Madan (1838–1901), Science Master of Eton, based on Greek mythology, in which Phobos is a companion to the god Ares. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Phobos (moon)」の詳細全文を読む スポンサード リンク
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