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The Jupiter trojans, commonly called Trojan asteroids or just Trojans, are a large group of asteroids that share the orbit of the planet Jupiter around the Sun. Relative to Jupiter, each trojan librates around one of Jupiter's two stable Lagrangian points, ', lying 60° ahead of the planet in its orbit, and ', 60° behind. Jupiter trojans are distributed in two elongated, curved regions around these Lagrangian points with an average semi-major axis of about 5.2 AU.〔 The first Jupiter trojan discovered, 588 Achilles, was spotted in 1906 by German astronomer Max Wolf.〔 A total of 6,178 Jupiter trojans have been found .〔(【引用サイトリンク】url=http://www.minorplanetcenter.org/iau/lists/Trojans.html )〕 By convention they are each named after a mythological figure from the Trojan War, hence the name "Trojan". The total number of Jupiter trojans larger than 1 km in diameter is believed to be about , approximately equal to the number of asteroids larger than 1 km in the asteroid belt.〔 Like main-belt asteroids, Jupiter trojans form families.〔 Jupiter trojans are dark bodies with reddish, featureless spectra. No firm evidence of the presence of water, or any other specific compound on their surface has been obtained, but it is thought that they are coated in tholins, organic polymers formed by the Sun's radiation. The Jupiter trojans' densities (as measured by studying binaries or rotational lightcurves) vary from 0.8 to 2.5 g·cm−3.〔 Jupiter trojans are thought to have been captured into their orbits during the early stages of the Solar System's formation or slightly later, during the migration of giant planets.〔 The term "trojan" has come to be used more generally to refer to other small Solar System bodies with similar relationships to larger bodies: for example, there are both Mars trojans and Neptune trojans, and Saturn has trojan moons. NASA has announced the discovery of an Earth trojan.〔(NASA's WISE Mission Finds First Trojan Asteroid Sharing Earth's Orbit 27 July 2011 )〕 The term "Trojan asteroid" is normally understood to specifically mean the Jupiter trojans because the first Trojans were discovered near Jupiter's orbit and Jupiter currently has by far the most known Trojans.〔 == Observational history == In 1772, Italian-born mathematician Joseph-Louis Lagrange, in studying the restricted three-body problem, predicted that a small body sharing an orbit with a planet but lying 60° ahead or behind it will be trapped near these points.〔 The trapped body will librate slowly around the point of equilibrium in a tadpole or horseshoe orbit.〔 These leading and trailing points are called the and Lagrange points.〔 However, no asteroids trapped in Lagrange points were observed until more than a century after Lagrange's hypothesis. Those associated with Jupiter were the first to be discovered.〔 E. E. Barnard made the first recorded observation of a trojan, (identified as A904 RD at the time), in 1904, but neither he nor others appreciated its significance at the time. Barnard believed he saw the recently discovered Saturnian satellite Phoebe, which was only two arc-minutes away in the sky at the time, or possibly an asteroid. The object's identity was not realized until its orbit was calculated in 1999.〔 The first recognized discovery of a trojan occurred in February 1906, when astronomer Max Wolf of Heidelberg-Königstuhl State Observatory discovered an asteroid at the Lagrangian point of the Sun–Jupiter system, later named 588 Achilles.〔 In 1906–1907 two more Jupiter trojans were found by fellow German astronomer August Kopff (624 Hektor and 617 Patroclus).〔 Hektor, like Achilles, belonged to the swarm ("ahead" of the planet in its orbit), whereas Patroclus was the first asteroid known to reside at the Lagrangian point ("behind" the planet).〔 By 1938, 11 Jupiter trojans had been detected.〔 This number increased to 14 only in 1961.〔 As instruments improved, the rate of discovery grew rapidly: by January 2000, a total of 257 had been discovered;〔 by May 2003, the number had grown to 1,600.〔 there are 3,898 known Jupiter trojans at and 2,049 at ,〔(【引用サイトリンク】accessdate =25 February 2014 )〕 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「Jupiter trojan」の詳細全文を読む スポンサード リンク
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