.jpg)
Anextinct comet is acomet that has expelled most of itsvolatile ice and has little left to form atail andcoma. In adormant comet, rather than being depleted, any remaining volatile components have been sealed beneath an inactive surface layer.
Due to the near lack of a coma and tail, an extinct or dormant comet may resemble anasteroid rather than a comet and blur the distinction between these two classes ofsmall Solar System bodies. When volatile materials such as nitrogen, water, carbon dioxide, ammonia, hydrogen and methane in thecomet nucleus have evaporated away, all that remains is an inert rock orrubble pile. A comet may go through a transition phase as it comes close to extinction.
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[1] that can resemble an asteroid.[2]
Other related types of comet includetransition comets, that are close to becoming extinct, such as were looked for in theHubble search for transition comets.[4] Comets such asC/2001 OG108 (LONEOS) may represent the transition between extinct comets and typicalHalley-type comets (periods of 20–200 years) orlong period comets (periods longer than 200 years).[5] Minor planets of the group ofdamocloids have been studied as possible extinct cometary candidates due to the similarity of their orbital parameters with those of Halley-type comets.[5]
Dormant comets are those within which volatiles may be sealed, but which have inactive surfaces. For example,14827 Hypnos may be thenucleus of an extinct comet that is covered by a crust several centimeters thick that prevents any remainingvolatiles fromoutgassing.[6]
The termdormant comet is also used to describe comets that may become active but are not actively outgassing. For example,60558 Echeclus has previously displayed a cometary coma and thus also has been given the cometary designation 174P/Echeclus. After passing perihelion in early 2008, centaur52872 Okyrhoe significantly brightened.[7]
When discovered, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "small Solar System body" was coined by theIAU in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due tosublimation of near-surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surfacevolatile ices and develop the appearance of asteroids. A further distinction is that comets typically have more eccentric orbits than most asteroids; most "asteroids" with notably eccentric orbits are probably dormant or extinct comets. Also, they are theorized to be common objects amongst the celestial bodies orbiting close to the Sun.[8]
Roughly six percent of thenear-Earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.[6][9][10]
Suspected or hypothesized extinct comets include:
.png)
