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243 Ida () is an asteroid in the Koronis family of the asteroid belt. It was discovered on 29 September 1884 by Austrian astronomer Johann Palisa and named after a nymph from Greek mythology. Later telescopic observations categorized Ida as an S-type asteroid, the most numerous type in the inner asteroid belt. On 28 August 1993, Ida was visited by the spacecraft ''Galileo'', bound for Jupiter. It was the second asteroid to be visited by a spacecraft and the first found to have a natural satellite. Like all main-belt asteroids, Ida's orbit lies between the planets Mars and Jupiter. Its orbital period is 4.84 years, and its rotation period is 4.63 hours. Ida has an average diameter of . It is irregularly shaped and elongated, and apparently composed of two large objects connected together. Its surface is one of the most heavily cratered in the Solar System, featuring a wide variety of crater sizes and ages. Ida's moon, Dactyl, was discovered by mission member Ann Harch in images returned from ''Galileo''. It was named after the Dactyls, creatures which inhabited Mount Ida in Greek mythology. Dactyl, being only in diameter, is about one-twentieth the size of Ida. Its orbit around Ida could not be determined with much accuracy. However, the constraints of possible orbits allowed a rough determination of Ida's density, which revealed that it is depleted of metallic minerals. Dactyl and Ida share many characteristics, suggesting a common origin. The images returned from ''Galileo'', and the subsequent measurement of Ida's mass, provided new insights into the geology of S-type asteroids. Before the ''Galileo'' flyby, many different theories had been proposed to explain their mineral composition. Determining their composition permits a correlation between meteorites falling to the Earth and their origin in the asteroid belt. Data returned from the flyby pointed to S-type asteroids as the source for the ordinary chondrite meteorites, the most common type found on the Earth's surface. == Discovery and observations == Ida was discovered on 29 September 1884 by Austrian astronomer Johann Palisa at the Vienna Observatory. It was his 45th asteroid discovery. Ida was named by Moriz von Kuffner, a Viennese brewer and amateur astronomer. In Greek mythology, Ida was a nymph of Crete who raised the god Zeus. Ida was recognized as a member of the Koronis family by Kiyotsugu Hirayama, who proposed in 1918 that the group comprised the remnants of a destroyed precursor body.〔 Ida's reflection spectrum was measured on 16 September 1980 by astronomers David J. Tholen and Edward F. Tedesco as part of the eight-color asteroid survey (ECAS). Its spectrum matched those of the asteroids in the S-type classification.〔 〕 Many observations of Ida were made in early 1993 by the US Naval Observatory in Flagstaff and the Oak Ridge Observatory. These improved the measurement of Ida's orbit around the Sun and reduced the uncertainty of its position during the ''Galileo'' flyby from . 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「243 Ida」の詳細全文を読む スポンサード リンク
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