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Damocloid
Damocloids are minor planets such as 5335 Damocles and 1996 PW that have Halley-family or long-period highly eccentric orbits typical of periodic comets such as Halley's Comet, but without showing a cometary coma or tail. David Jewitt defines a damocloid as an object with a Tisserand's parameter relative to Jupiter TJ ≤ 2.〔 This can also loosely be defined as (q < 5.2 AU, a > 8.0 AU, and e > 0.75) or i > 90 deg, but this definition that does not focus on Jupiter excludes objects such as , , and .
Using the Tisserand parameter (T-Jupiter ≤ 2) there are currently 96 damocloid candidates.〔(【引用サイトリンク】publisher=JPL Solar System Dynamics ) Search parameters used: Limited by object type/group: Asteroids and T-Jupiter <= 2 〕 Of these objects, 74 have orbital observation arcs greater than 30 days providing reasonably decent orbits.〔〔 ((2013 list ) / (2011 list ) / (2010 list ))〕 Their average radius is eight kilometers assuming an albedo of 0.04. The albedos of four damocloids have been measured, and they are among the darkest objects known in the Solar System. Damocloids are reddish in color, but not as red as many Kuiper-belt objects or centaurs. Other damocloids include: , , , , and 20461 Dioretsa.
Retrograde objects such as Halley's Comet and damocloid can have relative velocities to Earth of .〔
==Origin==
Damocloids are thought to be nuclei of Halley-type comets that have lost all their volatile materials due to outgassing and become dormant. Such comets probably originate from the Oort cloud. This hypothesis is strengthened by the fact that a number of objects thought to be damocloids (and assigned minor-planet provisional designations) subsequently showed a coma and were confirmed to be comets: C/2001 OG108 (LONEOS), C/2002 CE10 (LINEAR), C/2002 VQ94 (LINEAR), C/2004 HV60 (Spacewatch) and possibly others. Another strong indication of cometary origin is the fact that some damocloids have retrograde orbits, unlike any other minor planets. (Objects with an inclination beyond 90 degrees up to 180 degrees are in a retrograde orbit and orbit in the opposite direction of other objects.)
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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