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An extinct comet is a
comet A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...
that has expelled most of its volatile ice and has little left to form a
tail The tail is the elongated section at the rear end of a bilaterian animal's body; in general, the term refers to a distinct, flexible appendage extending backwards from the midline of the torso. In vertebrate animals that evolution, evolved to los ...
and
coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to Nociception, respond normally to Pain, painful stimuli, light, or sound, lacks a normal Circadian rhythm, sleep-wake cycle and does not initiate ...
. In a dormant 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 an
asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
rather than a comet and blur the distinction between these two classes of
small Solar System bodies A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first IAU definition of planet, defined in 2006 by the International Astronomical Union (IAU) as fo ...
. When volatile materials such as nitrogen, water, carbon dioxide, ammonia, hydrogen and methane in the
comet nucleus The nucleus is the solid, central part of a comet, formerly termed a ''dirty snowball'' or an ''icy dirtball''. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublime and produce an atmosphe ...
have evaporated away, all that remains is an inert rock or
rubble pile In astronomy, a rubble pile is a celestial body that consists of numerous pieces of debris that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the various chunks that make the ...
. A comet may go through a transition phase as it comes close to extinction.


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 that can resemble an asteroid. 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 Hubble search for transition comets (Transition Comets—UV Search for OH Emissions in Asteroids) was a study involving amateur astronomers and the use of the Hubble Space Telescope, one of only six studies involving amateur astronomers approved by ...
. Comets such as C/2001 OG108 (LONEOS) may represent the transition between extinct comets and typical Halley-type comets (periods of 20–200 years) or long period comets (periods longer than 200 years). Minor planets of the group of damocloids have been studied as possible extinct cometary candidates due to the similarity of their orbital parameters with those of Halley-type comets.


Dormant comets

Dormant comets are those within which volatiles may be sealed, but which have inactive surfaces. For example, 14827 Hypnos may be the
nucleus Nucleus (: nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucleu ...
of an extinct comet that is covered by a crust several centimeters thick that prevents any remaining
volatiles Volatility or volatile may refer to: Chemistry * Volatility (chemistry), a measuring tendency of a substance or liquid to vaporize easily ** Volatile organic compounds, organic or carbon compounds that can evaporate at normal temperature and pre ...
from
outgassing Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation (whic ...
. The term ''dormant 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, centaur 52872 Okyrhoe significantly brightened.


Distinction between comets and asteroids

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 A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first defined in 2006 by the International Astronomical Union (IAU) as follows: "All other objects ...
" was coined by the
IAU The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...
in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation 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 surface volatile 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. Roughly six percent of the
near-Earth asteroid A near-Earth object (NEO) is any small Solar System body orbiting the Sun whose closest approach to the Sun (perihelion) is less than 1.3 times the Earth–Sun distance (astronomical unit, AU). This definition applies to the object's orbit aro ...
s are thought to be extinct nuclei of comets which no longer experience outgassing.


Extinct comets

Suspected or hypothesized extinct comets include: * * 14827 Hypnos * 2101 Adonis * 2015 TB145 *
3200 Phaethon 3200 Phaethon (; previously sometimes spelled Phæton), provisionally designated , is an active Apollo asteroid with an orbit that brings it closer to the Sun than any other named asteroid (though there are numerous unnamed asteroids with sm ...
* 3552 Don Quixote * P/2007 R5 (SOHO 1) * 1996 PW possibly an extinct long-period comet


See also

*
Centaur (minor planet) In planetary astronomy, a centaur is a small Solar System body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets. Centaurs generally have unstable orbits because of this; almost all the ...
* Damocloid asteroid *
List of minor planets and comets visited by spacecraft The following tables list all minor planets and comets that have been visited by robotic spacecraft. List of minor planets visited by spacecraft A total of 19 minor planets (asteroids, dwarf planets, and Kuiper belt objects) have been visi ...
* Lost comet


References


External links


"Low Albedos Among Extinct Comet Candidates", 2001


(NewScientist 5 March 2010) {{Portal bar, Astronomy, Stars, Spaceflight, Outer space, Solar System Comets Asteroids