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Extinct comet
Extinct comet
Extinct comet
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Comet nucleus of 9P/Tempel as imaged by the NASA Deep Impact space probe. This is not yet extinct but gives some idea of a cometary nucleus

An extinct comet is a comet that has expelled most of its volatile ice and has little left to form a tail and coma. 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 rather than a comet and blur the distinction between these two classes of small Solar System bodies. When volatile materials such as nitrogen, water, carbon dioxide, ammonia, hydrogen and methane in the comet nucleus have evaporated away, all that remains is an inert rock or rubble pile. A comet may go through a transition phase as it comes close to extinction.

Nature of extinct comets

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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]

Disintegration of asteroid P/2013 R3 observed by the Hubble Space Telescope between late-October 2013 and early-January 2014.[3]

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.[4] 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).[5] 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.[5]

Dormant comets

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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.[6]

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.[7]

Distinction between comets and asteroids

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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 the IAU 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.[8]

Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.[6][9][10]

Extinct comets

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Don Quixote (apmag 15) near perihelion in 2009.
The eccentric (e=0.66) comet-like orbit of Hypnos.

Suspected or hypothesized extinct comets include:

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Extinct comet

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from Grokipedia
An extinct comet is a that has depleted most or all of its volatile ices through repeated perihelion passages near the Sun, rendering it incapable of producing a or and causing it to resemble a rocky in appearance and behavior. These objects originate primarily from the Jupiter-family comets (JFCs), which are short-period comets influenced by Jupiter's gravity, or occasionally from long-period comets, and their physical evolution involves the sublimation of ices like , , and , often leaving behind a refractory crust of and rock. The formation of extinct comets typically occurs after hundreds to thousands of orbital cycles, with JFCs having an estimated physical lifetime of about 12,000 years or 1,600 perihelion passages before becoming dormant or fully extinct. Key characteristics include low albedos around 0.04, primitive carbonaceous spectral types (C, D, or F), elongated shapes, and densities ranging from 0.2 to 1.3 g/cm³, distinguishing them from typical asteroids despite their asteroidal orbits. They contribute significantly to the (NEO) population, with estimates suggesting that 6 ± 4% of NEOs are extinct JFCs, including about 140–200 objects larger than 1 km in diameter, and up to 0.3–3% of moderately bright NEOs being dormant comets. Notable examples include 3552 Don Quixote, identified as an extinct JFC with a faint cometary tail detected in infrared observations, and 107P/Wilson-Harrington, which showed cometary activity in the past but now appears asteroidal. In a 2015 study using the Spitzer Space Telescope, around 23 NEOs were confirmed as dormant short-period comets, highlighting their Kuiper Belt origins and potential as impactors, though their low activity makes detection challenging; more recent studies (2020–2025) have identified "dark comets" as a class of potential extinct or dormant comets exhibiting nongravitational acceleration without visible activity, with 14 known as of 2024 and expectations for more detections in 2025. Overall, extinct comets bridge the gap between volatile-rich comets and inert asteroids, providing insights into solar system evolution and the dynamical history of small bodies.

Definition and Characteristics

Definition

An extinct comet is defined as a comet nucleus that has lost nearly all of its volatile ices, such as water (H₂O), carbon monoxide (CO), and carbon dioxide (CO₂), through repeated sublimation during close solar approaches, thereby preventing the formation of a or . This loss of volatiles results in complete inactivity, with no detectable cometary activity observed even at perihelion, as any remaining ices are either depleted or insulated by a thick, nonvolatile crust formed from lag deposits. The term "extinct comet" emerged in astronomical literature during the late to characterize these bodies, which lose their distinctive cometary features and resemble asteroids after volatile exhaustion, with early candidates proposed based on dynamical evidence in the 1970s. Although they exhibit asteroidal characteristics in visible observations, extinct comets preserve their primordial cometary origins, setting them apart from true asteroids through orbital parameters indicative of icy body heritage; this contrasts with dormant comets, a transitional phase where volatiles persist but activity is temporarily suppressed.

Physical Properties

Extinct comets typically exhibit sizes ranging from 0.1 to 10 km in diameter, akin to small asteroids, with effective radii derived from observations and thermal modeling often falling between 0.1 and 8 km. Recent studies (as of 2024) have identified "dark comets" as additional candidates for extinct comets among near-Earth objects, exhibiting no visible activity but evidence of past cometary behavior through non-gravitational accelerations. Their surfaces display low geometric albedos, typically on the order of a few percent (p_v ≈ 0.02–0.05), rendering them darker than most . This low reflectivity, measured via visible and near-infrared photometry, arises from the accumulation of dark, materials over time, with median values around 0.04 for candidates in near-Earth and main-belt populations. Approximately 64% of such objects show comet-like albedos below 0.075, distinguishing them from brighter, rocky asteroid types. In visible and near-infrared spectra, extinct comets present very red to moderately red colors, indicative of organic-rich surfaces resulting from the devolatilization of primordial ices. These spectral slopes align closely with those of D-, P-, or C-type asteroids, often appearing even redder due to the presence of complex organic condensates and darkened carbon-rich residues. Featureless spectra in the 0.4–2.5 μm range further emphasize their primitive, processed exteriors without prominent absorption bands from fresh ices. Surface features of extinct comets consist of cratered, rocky exteriors overlaid by possible dust mantles, with no observable icy exposures. These characteristics, inferred from variations and imaging of analogs like main-belt candidates, suggest rough, regolith-covered terrains shaped by impacts and mantled by non-volatile debris from prior activity. Density estimates for extinct comets are typically around 0.5 ± 0.1 g/cm³, lower than typical rocky asteroids owing to their porous, primordial compositions retained from icy origins. This , calculated from measurements of similar nuclei and gravitational analyses, reflects high (up to 70–80%) and weak internal structure, consistent with rubble-pile models.

Formation and Evolution

Process of Becoming Extinct

Comets become extinct primarily through the progressive loss of volatile ices from their nuclei, driven by solar heating during repeated perihelion passages. The dominant mechanism is sublimation, where water ice and other volatiles transition directly from solid to gas under the influence of , particularly when the comet approaches within about 3 AU of the Sun. This process releases gas and entrained dust, forming the characteristic and , but it also erodes the nucleus surface over time. For short-period comets, which dominate the observed population of extinct candidates, this erosion occurs frequently due to their relatively close and repeated solar encounters. Thermal evolution plays a crucial role in this depletion, as solar radiation penetrates the porous nucleus, heating subsurface layers and triggering . Near-surface ices sublimate first, creating voids and channels that allow heat to propagate deeper, though is inefficient in low-density cometary material, with characteristic timescales exceeding 10^5 years for nuclei larger than 1 km. Deeper layers are exposed as overlying material is removed, leading to a gradual inward migration of the sublimation front. This non-uniform heating favors activity on sunward-facing regions, concentrating mass loss and altering the nucleus's over multiple orbits. As sublimation proceeds, a dust mantle forms from the accumulation of grains—primarily silicates and organics—that are not volatile and remain after ices are lost. These particles, ejected in jets during outbursts, settle back onto the surface, forming an insulating crust that reduces to underlying ices and suppresses further . Mantle thicknesses of just 10 cm can effectively halt activity by blocking solar radiation and trapping sublimated gases beneath the surface. In short-period comets with perihelia greater than 2 AU, stable mantles develop rapidly, often within the first few hundred orbits, transitioning the nucleus from active to dormant. The overall lifetime against for short-period comets is estimated at around 1,000 to 2,000 perihelion passages, or approximately 10,000 to 12,000 years, after which the volatile content is sufficiently depleted to cease observable activity. This duration aligns with dynamical models of Jupiter-family comets, where frequent solar approaches accelerate the process compared to long-period comets. Larger nuclei (radii >5 km) may persist longer due to greater initial volatile reserves, but smaller ones become inactive sooner. Cumulative mass loss is quantified through the sublimation rate, which follows the Hertz-Knudsen formula adapted for cometary conditions: the mass loss rate QQ is approximately Q=αAPsat2πmkTQ = \alpha A \frac{P_\text{sat}}{\sqrt{2\pi m k T}}
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