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Extinct comets are comets that have expelled most of their volatile ice and have little left to form a tail or coma. The volatile material contained in the comet nucleus evaporates away, and all that remains is inert rock or rubble that can resemble an asteroid. Comets may go through a transition phase as they come close to extinction. A comet may be ''dormant'' rather than extinct, if its volatile component is sealed beneath an inactive surface layer. ==Nature of extinct comets== Extinct comets are those that have expelled most of their volatile ice and have little left to form a tail or coma. Over time, most of the volatile material contained in a comet nucleus evaporates away, and the comet becomes a small, dark, inert lump of rock or rubble〔("If comets melt, why do they seem to last for long periods of time?" ), ''Scientific American'', November 16, 1998〕 that can resemble an asteroid.〔("What is the difference between asteroids and comets?" ), Rosetta FAQ, ESA〕 Other related types of comet include ''transition comets'', that are close to becoming extinct, such as were looked for in the Hubble search for transition comets.〔Space Telescope Science Institute, '(Amateur Astronomers Will Use NASA's Hubble Space Telescope )', Media Release STScI-1992-23, 10 September 1992, retrieved 18 March 2009.〕 Comets such as C/2001 OG108 (LONEOS) may represent the transition between typical Halley-family or long-period comets and extinct comets. Damocloid asteroids have been studied as possible extinct cometary candidates due to the similarity of their orbital parameters with those of Halley-family comets.〔 ''Dormant comets'' are those within which volatiles may be sealed, but which have inactive surfaces. For example, 14827 Hypnos may be the nucleus of an extinct comet that is covered by a crust several centimeters thick that prevents any remaining volatiles from outgassing.〔 〕 The term ''dormant comet'' is also used to describe comets that may become active but are not actively outgassing. For example 60558 Echeclus has displayed a cometary coma and now also has the cometary designation 174P/Echeclus. After passing perihelion in early 2008, centaur 52872 Okyrhoe significantly brightened. 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「extinct comet」の詳細全文を読む スポンサード リンク
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