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The nucleus is the solid, central part of a
comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ...
, once termed a ''dirty snowball'' or an ''icy dirtball''. A cometary nucleus is composed of rock,
dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ...
, and frozen gases. When heated by the Sun, the gases sublime and produce an
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...
surrounding the nucleus known as the
coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...
. The force exerted on the coma by the Sun's
radiation pressure Radiation pressure is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength that is a ...
and
solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...
cause an enormous tail to form, which points away from the Sun. A typical comet nucleus has an
albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...
of 0.04. This is blacker than coal, and may be caused by a covering of dust. Results from the ''Rosetta'' and ''Philae'' spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of
planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System ...
s. Further, the ALICE spectrograph on ''Rosetta'' determined that
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
s (within above the comet nucleus) produced from
photoionization Photoionization is the physical process in which an ion is formed from the interaction of a photon with an atom or molecule. Cross section Not every interaction between a photon and an atom, or molecule, will result in photoionization. The prob ...
of
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...
s by
solar radiation Solar irradiance is the power per unit area ( surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ...
, and not
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
s from the Sun as thought earlier, are responsible for the degradation of water and
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
molecules released from the comet nucleus into its
coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...
. On 30 July 2015, scientists reported that the ''
Philae ; ar, فيلة; cop, ⲡⲓⲗⲁⲕ , alternate_name = , image = File:File, Asuán, Egipto, 2022-04-01, DD 93.jpg , alt = , caption = The temple of Isis from Philae at its current location on Agilkia Island in Lake Nasse ...
''
spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, ...
, that landed on
comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ...
67P/Churyumov-Gerasimenko in November 2014, detected at least 16
organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
s, of which four (including
acetamide Acetamide (systematic name: ethanamide) is an organic compound with the formula CH3CONH2. It is the simplest amide derived from acetic acid. It finds some use as a plasticizer and as an industrial solvent. The related compound ''N'',''N''-dime ...
,
acetone Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour. Acetone is miscibl ...
,
methyl isocyanate Methyl isocyanate (MIC) is an organic compound with the molecular formula CH3NCO. Synonyms are isocyanatomethane and methyl carbylamine. Methyl isocyanate is an intermediate chemical in the production of carbamate pesticides (such as carbaryl, ...
and
propionaldehyde Propionaldehyde or propanal is the organic compound with the formula CH3CH2CHO. It is the 3-carbon aldehyde. It is a colourless, flammable liquid with a slightly fruity odour. It is produced on a large scale industrially. Production Propionalde ...
) were detected for the first time on a comet.


Paradigm

Comet nuclei, at ~1 km to at times tens of kilometers, could not be resolved by telescopes. Even current giant telescopes would give just a few pixels on target, assuming nuclei were not obscured by comae when near Earth. An understanding of the nucleus, versus the phenomenon of the coma, had to be deduced, from multiple lines of evidence.


"Flying sandbank"

The "flying sandbank" model, first proposed in the late-1800s, posits a comet as a swarm of bodies, not a discrete object at all. Activity is the loss of both volatiles, and population members. This model was championed in midcentury by
Raymond Lyttleton Raymond Arthur Lyttleton FRS (7 May 1911 – 16 May 1995) was a British mathematician and theoretical astronomer. He was born in Warley Woods near Birmingham and educated at King Edward VI Five Ways school in Birmingham, going from there to C ...
, along with an origin. As the Sun passed through interstellar nebulosity, material would clump in wake eddies. Some would be lost, but some would remain in heliocentric orbits. The weak capture explained long, eccentric, inclined comet orbits. Ices ''per se'' were lacking; volatiles were stored by adsorption on grains.


"Dirty snowball"

Beginning in the 1950s,
Fred Lawrence Whipple Fred Lawrence Whipple (November 5, 1906 – August 30, 2004) was an American astronomer, who worked at the Harvard College Observatory for more than 70 years. Amongst his achievements were asteroid and comet discoveries, the " dirty snowball" h ...
published his "icy conglomerate" model. This was soon popularized as "dirty snowball." Comet orbits had been determined quite precisely, yet comets were at times recovered "off-schedule," by as much as days. Early comets could be explained by a "resisting medium"- such as "the aether", or the cumulative action of
meteoroids A meteoroid () is a small rocky or metallic body in outer space. Meteoroids are defined as objects significantly smaller than asteroids, ranging in size from grains to objects up to a meter wide. Objects smaller than this are classified as mic ...
against the front of the comet(s). But comets could return both early and late. Whipple argued that a gentle thrust from asymmetric emissions (now "nongravitational forces") better explained comet timing. This required that the emitter have cohesive strength- a single, solid nucleus with some proportion of volatiles. Lyttleton continued publishing flying-sandbank works as late as 1972. The death knell for the flying sandbank was Halley's Comet. Vega 2 and
Giotto Giotto di Bondone (; – January 8, 1337), known mononymously as Giotto ( , ) and Latinised as Giottus, was an Italian painter and architect from Florence during the Late Middle Ages. He worked during the Gothic/ Proto-Renaissance period. G ...
images showed a single body, emitting through a small number of jets.


"Icy dirtball"

It has been a long time since comet nuclei could be imagined as frozen snowballs. Whipple had already postulated a separate crust and interior. Before Halley's 1986 apparition, it appeared that an exposed ice surface would have some finite lifetime, even behind a coma. Halley's nucleus was predicted to be dark, not bright, due to preferential destruction/escape of gases, and retention of refractories. The term ''dust mantling'' has been in common use since more than 35 years. "the term ''dust mantling'' has been in common use since more than 35 years" The Halley results exceeded even these- comets are not merely dark, but among the darkest objects in the Solar System Furthermore, prior dust estimates were severe undercounts. Both finer grains and larger pebbles appeared in spacecraft detectors, but not ground telescopes. The volatile fraction also included organics, not merely water and other gases. Dust-ice ratios appeared much closer than thought. Extremely low densities (0.1 to 0.5 g cm-3) were derived. The nucleus was still assumed to be majority-ice, perhaps overwhelmingly so.


Modern theory

Three rendezvous missions aside, Halley was one example. Its unfavorable trajectory also caused brief flybys at extreme speed, at one time. More frequent missions broadened the sample of targets, using more advanced instruments. By chance, events such as the breakups of Shoemaker-Levy 9 and Schwassmann-Wachmann 3 contributed to our understanding. Densities were confirmed as quite low, ~0.6 g cm3. Comets were highly porous, and fragile on micro- and macro-scales. Refractory-to-ice ratios are much higher, at least 3:1, possibly ~5:1, ~6:1, or more. This is a full reversal from the dirty snowball model. The Rosetta science team has coined the term "mineral organices," for minerals and organics with a minor fraction of ices. Comets and
active asteroid Active asteroids are small Solar System bodies that have asteroid-like orbits but show comet-like visual characteristics. That is, they show comae, tails, or other visual evidence of mass-loss (like a comet), but their orbit remains within Jupi ...
s in the outer asteroid belt demonstrate that there can be a fine line separating the two categories of objects.


Origin

Comets, or their precursors, formed in the outer Solar System, possibly millions of years before planet formation. How and when comets formed is debated, with distinct implications for Solar System formation, dynamics, and geology. Three-dimensional computer simulations indicate the major structural features observed on cometary nuclei can be explained by pairwise low velocity accretion of weak cometesimals. The currently favored creation mechanism is that of the
nebular hypothesis The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System (as well as other planetary systems). It suggests the Solar System is formed from gas and dust orbiting t ...
, which states that comets are probably a remnant of the original planetesimal "building blocks" from which the planets grew. Astronomers think that comets originate in the
Oort cloud The Oort cloud (), sometimes called the Öpik–Oort cloud, first described in 1950 by the Dutch astronomer Jan Oort, is a theoretical concept of a cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from ...
, the scattered disk, and the outer
Main Belt The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
.


Size

Most cometary nuclei are thought to be no more than about 16 kilometers (10 miles) across. The largest comets that have come inside the orbit of
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...
are 95P/Chiron (≈200 km), C/2002 VQ94 (LINEAR) (≈100 km), Comet of 1729 (≈100 km), Hale–Bopp (≈60 km), 29P (≈60 km), 109P/Swift–Tuttle (≈26 km), and
28P/Neujmin 28P/Neujmin, also known as Neujmin 1, is a large periodic comet in the Solar System. With a perihelion distance (closest approach to the Sun) of 1.5 AU, this comet does not make close approaches to the Earth. The comet nucleus is estimated to ...
(≈21 km). The potato-shaped nucleus of
Halley's comet Halley's Comet or Comet Halley, officially designated 1P/Halley, is a short-period comet visible from Earth every 75–79 years. Halley is the only known short-period comet that is regularly visible to the naked eye from Earth, and thus the on ...
(15 × 8 × 8 km) contains equal amounts of ice and dust. During a flyby in September 2001, the
Deep Space 1 ''Deep Space 1'' (DS1) was a NASA technology demonstration spacecraft which flew by an asteroid and a comet. It was part of the New Millennium Program, dedicated to testing advanced technologies. Launched on 24 October 1998, the ''Deep Space ...
spacecraft observed the nucleus of Comet
Borrelly Alphonse Louis Nicolas Borrelly (December 8, 1842 – February 28, 1926) was a French astronomer. He joined the Marseille Observatory in 1864. In the course of his career, he discovered a number of asteroids and comets, including the periodic c ...
and found it to be about half the size (8×4×4 km) of the nucleus of Halley's Comet. Borrelly's nucleus was also potato-shaped and had a dark black surface. Like Halley's Comet, Comet Borrelly only released gas from small areas where holes in the crust exposed the ice to sunlight. The nucleus of comet Hale–Bopp was estimated to be 60 ± 20 km in diameter. Hale-Bopp appeared bright to the unaided eye because its unusually large nucleus gave off a great deal of dust and gas. The nucleus of P/2007 R5 is probably only 100–200 meters in diameter. The largest
centaurs A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse. Centaurs are thought of in many Greek myths as bein ...
(unstable, planet crossing, icy asteroids) are estimated to be 250 km to 300 km in diameter. Three of the largest would include
10199 Chariklo 10199 Chariklo is the largest confirmed centaur (small body of the outer Solar System). It orbits the Sun between Saturn and Uranus, grazing the orbit of Uranus. On 26 March 2014, astronomers announced the discovery of two rings (nickname ...
(258 km),
2060 Chiron 2060 Chiron is a small Solar System body in the outer Solar System, orbiting the Sun between Saturn and Uranus. Discovered in 1977 by Charles Kowal, it was the first-identified member of a new class of objects now known as centaurs—bodies o ...
(230 km), and (≈220 km). Known comets have been estimated to have an average density of 0.6 g/cm3. Below is a list of comets that have had estimated sizes, densities, and masses.


Composition

It was once thought that water-ice was the predominant constituent of the nucleus. In the dirty snowball model, dust is ejected when the ice retreats. Based on this, about 80% of the
Halley's Comet Halley's Comet or Comet Halley, officially designated 1P/Halley, is a short-period comet visible from Earth every 75–79 years. Halley is the only known short-period comet that is regularly visible to the naked eye from Earth, and thus the on ...
nucleus would be water ice, and frozen carbon monoxide ( CO) makes up another 15%. Much of the remainder is frozen carbon dioxide, methane, and ammonia. Scientists think that other comets are chemically similar to Halley's Comet. The nucleus of Halley's Comet is also an extremely dark black. Scientists think that the surface of the comet, and perhaps most other comets, is covered with a black crust of dust and rock that covers most of the ice. These comets release gas only when holes in this crust rotate toward the Sun, exposing the interior ice to the warming sunlight. This assumption was shown to be naive, starting at Halley. Coma composition does not represent nucleus composition, as activity selects for volatiles, and against refractories, including heavy organic fractions. Our understanding has evolved more toward mostly rock; recent estimates show that water is perhaps only 20-30% of the mass in typical nuclei. Instead, comets are predominantly organic materials and minerals. The composition of
water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous p ...
from Churyumov–Gerasimenko comet, as determined by the ''Rosetta'' mission, is substantially different from that found on Earth. The ratio of
deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one ...
to
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
in the water from the comet was determined to be three times that found for terrestrial water. This makes it unlikely that water on Earth came from comets such as Churyumov–Gerasimenko.


Structure

On 67P/Churyumov–Gerasimenko comet, some of the resulting water vapour may escape from the nucleus, but 80% of it recondenses in layers beneath the surface. This observation implies that the thin ice-rich layers exposed close to the surface may be a consequence of cometary activity and evolution, and that global layering does not necessarily occur early in the comet's formation history. Measurements carried out by the ''Philae'' lander on 67P/Churyumov–Gerasimenko comet, indicate that the dust layer could be as much as thick. Beneath that is hard ice, or a mixture of ice and dust.
Porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measur ...
appears to increase toward the center of the comet. While most scientists thought that all the evidence indicated that the structure of nuclei of comets is processed
rubble pile In astronomy, a rubble pile is a celestial body that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the ...
s of smaller ice planetesimals of a previous generation, the ''Rosetta'' mission dispelled the idea that comets are "rubble piles" of disparate material. The ''Rosetta'' mission indicated that comets may be "rubble piles" of disparate material. Data are not conclusive concerning the collisional environment during the formation and right afterwards.


Splitting

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart. Splitting comets include
3D/Biela Biela's Comet or Comet Biela (official designation: 3D/Biela) was a periodic Jupiter-family comet first recorded in 1772 by Montaigne and Messier and finally identified as periodic in 1826 by Wilhelm von Biela. It was subsequently observed to ...
in 1846, Shoemaker–Levy 9 in 1992, and 73P/Schwassmann–Wachmann from 1995 to 2006. Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC. Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact. Comets
42P/Neujmin 42P/Neujmin, also known as Neujmin 3, is a periodic comet 2 km in diameter. This comet and 53P/Van Biesbroeck are fragments of a parent comet that split in March 1845.53P/Van Biesbroeck appear to be fragments of a parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical.


Albedo

Cometary nuclei are among the darkest objects known to exist in the Solar System. The
Giotto Giotto di Bondone (; – January 8, 1337), known mononymously as Giotto ( , ) and Latinised as Giottus, was an Italian painter and architect from Florence during the Late Middle Ages. He worked during the Gothic/ Proto-Renaissance period. G ...
probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and
Deep Space 1 ''Deep Space 1'' (DS1) was a NASA technology demonstration spacecraft which flew by an asteroid and a comet. It was part of the New Millennium Program, dedicated to testing advanced technologies. Launched on 24 October 1998, the ''Deep Space ...
discovered that Comet Borrelly's surface reflects only 2.5–3.0% of the light that falls on it; by comparison, fresh asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their
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 (which ...
. Roughly six percent of the
near-Earth asteroid A near-Earth object (NEO) is any small Solar System body whose orbit brings it into proximity with Earth. By convention, a Solar System body is a NEO if its closest approach to the Sun ( perihelion) is less than 1.3 astronomical units (AU) ...
s are thought to be extinct nuclei of comets (see Extinct comets) which no longer experience outgassing. Two near-Earth asteroids with albedos this low include
14827 Hypnos 14827 Hypnos ('' prov. designation:'' ) is a highly eccentric, sub-kilometer-sized carbonaceous asteroid that is thought to be an extinct comet. It is classified as near-Earth object and potentially hazardous asteroid of the Apollo group. The a ...
and
3552 Don Quixote 3552 Don Quixote, provisionally designated , is an exceptionally eccentric asteroid, classified as a near-Earth object of the Amor group, Mars-crosser and Jupiter-crosser, as well as a centaur and extinct comet. Discovery and naming The as ...
.


Discovery and exploration

The first relatively close mission to a comet nucleus was space probe
Giotto Giotto di Bondone (; – January 8, 1337), known mononymously as Giotto ( , ) and Latinised as Giottus, was an Italian painter and architect from Florence during the Late Middle Ages. He worked during the Gothic/ Proto-Renaissance period. G ...
. This was the first time a nucleus was imaged at such proximity, coming as near as 596 km. The data was a revelation, showing for the first time the jets, the low-albedo surface, and organic compounds.Organic compounds (usually referred to as organics) does not imply life, it is just a class of chemicals: see
Organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J ...
.
During its flyby, Giotto was hit at least 12,000 times by particles, including a 1-gram fragment that caused a temporary loss of communication with Darmstadt. Halley was calculated to be ejecting three tonnes of material per second from seven jets, causing it to wobble over long time periods. Comet Grigg–Skjellerup's nucleus was visited after Halley, with Giotto approaching 100–200 km. Results from the ''Rosetta'' and ''Philae'' spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of
planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System ...
s. Further, the ALICE spectrograph on ''Rosetta'' determined that
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
s (within above the comet nucleus) produced from
photoionization Photoionization is the physical process in which an ion is formed from the interaction of a photon with an atom or molecule. Cross section Not every interaction between a photon and an atom, or molecule, will result in photoionization. The prob ...
of
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...
s by
solar radiation Solar irradiance is the power per unit area ( surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ...
, and not
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
s from the Sun as thought earlier, are responsible for the degradation of water and
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
molecules released from the comet nucleus into its
coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...
. Comets already visited are: *
Halley's Comet Halley's Comet or Comet Halley, officially designated 1P/Halley, is a short-period comet visible from Earth every 75–79 years. Halley is the only known short-period comet that is regularly visible to the naked eye from Earth, and thus the on ...
* 26P/Grigg-Skjellerup *
Tempel 1 Tempel 1 (official designation: 9P/Tempel) is a periodic Jupiter-family comet discovered by Wilhelm Tempel in 1867. It completes an orbit of the Sun every 5.5 years. Tempel 1 was the target of the ''Deep Impact'' space mission, which photogra ...
(also hit with impactor) * 19P/Borrelly *
81P/Wild Comet 81P/Wild, also known as Wild 2 (pronounced "vilt two") ( ), is a comet named after Swiss astronomer Paul Wild, who discovered it on January 6, 1978, using a 40-cm Schmidt telescope at Zimmerwald, Switzerland. For most of its 4.5 billion ...
* 103P/Hartley *
C/2013 A1 (Siding Spring) C/2013 A1 (Siding Spring) is an Oort cloud comet discovered on 3 January 2013 by Robert H. McNaught at Siding Spring Observatory using the Uppsala Southern Schmidt Telescope. At the time of discovery it was 7.2  AU from the Sun and loca ...
-unplanned encounter with Mars spacecraft * 67P/Churyumov–Gerasimenko (also landed on)


See also

*
Coma (cometary) The coma is the nebulous envelope around the nucleus of a comet, formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate. This gives a comet a "fuzzy" appearance when viewed in tel ...
*
Hypatia (stone) Hypatia is a small stone found in Egypt 1996, which may be the first known specimen of a comet nucleus on Earth, although defying physically-accepted models for hypervelocity processing of organic material. Discovery and name Hypatia was disc ...
* List of comets visited by spacecraft


References


External links


Nucleus of Halley's Comet
(15×8×8 km)

(5.5×4.0×3.3 km)


67/P by Rosetta
http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2014/08/comet_on_3_august_20142/14708015-1-eng-GB/Comet_on_3_August_2014.png 2]
ESA
{{DEFAULTSORT:Comet Nucleus Comets