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thumb A minor-planet group is a population of minor planets that have a share broadly similar orbits. Members are generally unrelated to each other, unlike in an asteroid family, which often results from the break-up of a single asteroid. It is customary to name a group of asteroids after the first member of that group to be discovered, which is often the largest. ==Groups out to the orbit of Earth== There are relatively few asteroids that orbit close to the Sun. Several of these groups are hypothetical at this point in time, with no members having yet been discovered; as such, the names they have been given are provisional. *Vulcanoid asteroids are hypothetical asteroids that orbit entirely within the orbit of Mercury (have an aphelion of less than 0.3874 AU). A few searches for vulcanoids have been conducted but none have been discovered so far. *Apoheles are asteroids whose aphelion is less than 0.983 AU, meaning they orbit entirely within Earth's orbit. Other proposed names for this group are inner-Earth objects or interior Earth objects〔(NEA Orbital Elements )〕 (IEOs) and Atira asteroids, the first confirmed Apohele. As of March 2008 there are only five known Apoheles with an arc of observations greater than 20 days: (163693) Atira, , , and ; while there are other four possible candidates, but with a too short arc of observations: , , and . *Mercury-crosser asteroids having a perihelion smaller than Mercury's 0.3075 AU. *Venus-crosser asteroids having a perihelion smaller than Venus's 0.7184 AU. This group includes the above Mercury-crossers (if their aphelion is greater than Venus's perihelion. All known Mercury crossers satisfy this condition). *Earth-crosser asteroids having a perihelion smaller than Earth's 0.9833 AU. This group includes the above Mercury- and Venus-crossers, apart from the Apoheles. They are also divided into the * *Aten asteroids having a semi-major axis less than 1 AU, named after 2062 Aten. * *Apollo asteroids having a semi-major axis greater than 1 AU, named after 1862 Apollo. *Arjuna asteroids are somewhat vaguely defined as having orbits similar to Earth's; i.e. with an average orbital radius of around 1 AU and with low eccentricity and inclination. Due to the vagueness of this definition some asteroids belonging to the Apohele, Amor, Apollo or Aten groups can also be classified as Arjunas. The term was introduced by Spacewatch and does not refer to an existing asteroid; examples of Arjunas include 1991 VG. *Earth trojans are asteroids located in the Earth–Sun Lagrangian points and . Their location in the sky as observed from Earth's surface would be fixed at about 60 degrees east and west of the Sun, and as people tend to search for asteroids at much greater elongations few searches have been done in these locations. The only known Earth trojan is . *Near-Earth asteroids is a catch-all term for asteroids whose orbit closely approaches that of Earth. It includes almost all of the above groups, as well as the Amor asteroids. 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「List of minor-planet groups」の詳細全文を読む スポンサード リンク
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