HOME

TheInfoList



OR:

A comet is an icy,
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, ...
that, when passing close to the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, warms and begins to release gases, a process that is called
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 a ...
. This produces a visible atmosphere or
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 ...
, and sometimes also a
tail The tail is the section at the rear end of certain kinds of animals’ bodies; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, r ...
. These phenomena are due to the effects of
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 (W/m ...
and the
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 sola ...
acting upon the nucleus of the comet.
Comet nuclei The nucleus is the solid, central part of a comet, once 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 atmosphere sur ...
range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times Earth's diameter, while the tail may stretch beyond one
astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits t ...
. If sufficiently bright, a comet may be seen from Earth without the aid of a telescope and may subtend an arc of 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures and religions. Comets usually have highly
eccentric Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-center, in geometry * Eccentricity (graph theory) of a v ...
elliptical orbits, and they have a wide range of
orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...
s, ranging from several years to potentially several millions of years.
Short-period 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 are ...
s originate in the
Kuiper belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
or its associated
scattered disc The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc objec ...
, which lie beyond the orbit of
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
.
Long-period 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 are ...
s are thought to 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 2 ...
, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun from the Oort cloud by
gravitational perturbation In astronomy, perturbation is the complex motion of a massive body subjected to forces other than the gravitational attraction of a single other massive body. The other forces can include a third (fourth, fifth, etc.) body, resistance, as from ...
s caused by passing stars and the
galactic tide A galactic tide is a tidal force experienced by objects subject to the gravitational field of a galaxy such as the Milky Way. Particular areas of interest concerning galactic tides include galactic collisions, the disruption of dwarf or satellit ...
.
Hyperbolic comet This is a list of parabolic and hyperbolic comets in the Solar System. Many of these comets may come from the Oort cloud, or perhaps even have interstellar origin. The Oort Cloud is not gravitationally attracted enough to the Sun to form into a ...
s may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition. Comets are distinguished from
asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...
s by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma). However,
extinct comets 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 la ...
that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System. The discovery of
main-belt comet 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 Jup ...
s and active
centaur 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 being ...
minor planets has blurred the distinction between asteroids and comets. In the early 21st century, the discovery of some minor bodies with long-period comet orbits, but characteristics of inner solar system asteroids, were called
Manx comet A Manx comet is a rocky, minor, celestial body that has a long-period comet orbit. Unlike most bodies on a long-period comet orbit which typically sport long, bright tails, the Manx comet is tailless, more typical of an inner Solar System asteroid. ...
s. They are still classified as comets, such as C/2014 S3 (PANSTARRS). Twenty-seven Manx comets were found from 2013 to 2017. there are 4584 known comets. However, this represents a very small fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (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 2 ...
) is about one trillion. Roughly one comet per year is visible to the
naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying, light-collecting optical instrument, such as a telescope or microscope, or eye protection. Vision corrected to normal ...
, though many of those are faint and unspectacular. Particularly bright examples are called "
great comet A great comet is a comet that becomes exceptionally bright. There is no official definition; often the term is attached to comets such as Halley's Comet, which during certain appearances are bright enough to be noticed by casual observers who ar ...
s". Comets have been visited by unmanned probes such as the European Space Agency's ''
Rosetta Rosetta or Rashid (; ar, رشيد ' ; french: Rosette  ; cop, ϯⲣⲁϣⲓⲧ ''ti-Rashit'', Ancient Greek: Βολβιτίνη ''Bolbitinē'') is a port city of the Nile Delta, east of Alexandria, in Egypt's Beheira governorate. The Ro ...
'', which became the first to land a robotic spacecraft on a comet, and NASA's '' Deep Impact'', which blasted a crater on Comet
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 photograph ...
to study its interior.


Etymology

The word ''comet'' derives from the
Old English Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the early Middle Ages. It was brought to Great Britain by Anglo-Saxon settlement of Britain, Anglo ...
from the
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
or . That, in turn, is a
romanization Romanization or romanisation, in linguistics, is the conversion of text from a different writing system to the Roman (Latin) script, or a system for doing so. Methods of romanization include transliteration, for representing written text, and ...
of the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
'wearing long hair', and the ''
Oxford English Dictionary The ''Oxford English Dictionary'' (''OED'') is the first and foundational historical dictionary of the English language, published by Oxford University Press (OUP). It traces the historical development of the English language, providing a com ...
'' notes that the term () already meant 'long-haired star, comet' in Greek. was derived from () 'to wear the hair long', which was itself derived from () 'the hair of the head' and was used to mean 'the tail of a comet'. The
astronomical symbol Astronomical symbols are abstract pictorial symbols used to represent astronomical objects, theoretical constructs and observational events in European astronomy. The earliest forms of these symbols appear in Greek papyrus texts of late antiq ...
for comets (represented in
Unicode Unicode, formally The Unicode Standard,The formal version reference is is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expre ...
) is , consisting of a small disc with three hairlike extensions.


Physical characteristics


Nucleus

The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of
rock Rock most often refers to: * Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids * Rock music, a genre of popular music Rock or Rocks may also refer to: Places United Kingdom * Rock, Caerphilly, a location in Wales ...
,
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 homes ...
,
water ice Water ice could refer to: *Ice formed by water (as opposed to other substances) *The alternate term for various similar frozen fruit-flavoured desserts: **Italian ice primarily in Philadelphia and the Delaware Valley **Sorbet Sorbet (), also c ...
, and frozen
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 transpar ...
,
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
,
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
, and
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
. As such, they are popularly described as "dirty snowballs" after
Fred 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 ...
's model. Comets with a higher dust content have been called "icy dirtballs". The term "icy dirtballs" arose after observation of
Comet 9P/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 photograph ...
collision with an "impactor" probe sent by NASA Deep Impact mission in July 2005. Research conducted in 2014 suggests that comets are like " deep fried ice cream", in that their surfaces are formed of dense crystalline ice mixed with
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. The ...
s, while the interior ice is colder and less dense. The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include
methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...
,
hydrogen cyanide Hydrogen cyanide, sometimes called prussic acid, is a chemical compound with the formula HCN and structure . It is a colorless, extremely poisonous, and flammable liquid that boils slightly above room temperature, at . HCN is produced on an ...
,
formaldehyde Formaldehyde ( , ) (systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section F ...
,
ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...
,
ethane Ethane ( , ) is an organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is isolated on an industrial scale from natural gas and as a petr ...
, and perhaps more complex molecules such as long-chain
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
s and
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s. In 2009, it was confirmed that the amino acid
glycine Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid (carbamic acid is unstable), with the chemical formula NH2‐ CH2‐ COOH. Glycine is one of the proteinogeni ...
had been found in the comet dust recovered by NASA's Stardust mission. In August 2011, a report, based on
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...
studies of
meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...
s found on Earth, was published suggesting DNA and
RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
components (
adenine Adenine () ( symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its derivati ...
,
guanine Guanine () ( symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine (uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is called ...
, and related organic molecules) may have been formed on
asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...
s and comets. The outer surfaces of cometary nuclei have a very low
albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, 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 ...
, making them among the least reflective objects found 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. Giot ...
space probe A space probe is an artificial satellite that travels through space to collect scientific data. A space probe may orbit Earth; approach the Moon; travel through interplanetary space; flyby, orbit, or land or fly on other planetary bodies; or ent ...
found that the 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 o ...
(1P/Halley) reflects about four percent 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 1 ...
discovered that Comet Borrelly's surface reflects less than 3.0%; by comparison,
asphalt Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term a ...
reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like
tar Tar is a dark brown or black viscous liquid of hydrocarbons and free carbon, obtained from a wide variety of organic materials through destructive distillation. Tar can be produced from coal, wood, petroleum, or peat. "a dark brown or black bit ...
or
crude oil Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crude ...
. The low reflectivity of cometary surfaces causes them to absorb the heat that drives 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 a ...
processes. Comet nuclei with radii of up to have been observed, but ascertaining their exact size is difficult. The nucleus of 322P/SOHO is probably only in diameter. A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than across. Known comets have been estimated to have an average density of . Because of their low mass, comet nuclei do not become spherical under their own
gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
and therefore have irregular shapes. 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 the extinct nuclei of comets that no longer experience outgassing, including
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 ...
. Results from the ''Rosetta'' and ''Philae'' spacecraft show that the nucleus of
67P/Churyumov–Gerasimenko 67P/Churyumov–Gerasimenko (abbreviated as 67P or 67P/C–G) is a Jupiter-family comet, originally from the Kuiper belt, with a current orbital period of 6.45 years, a rotation period of approximately 12.4 hours and a maximum velocity of . Chu ...
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 a ...
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 no kn ...
s (within above the
comet nucleus The nucleus is the solid, central part of a comet, once termed a ''dirty snowball'' or an ''icy dirtball''. A cometary nucleus is composed of Rock (geology), rock, dust, and frozen gases. When heated by the Sun, the gases Sublimation (physics), ...
) 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 molecules 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 (W/m ...
, 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 always ...
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 transpar ...
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 bioch ...
s released from the comet nucleus into its coma. Instruments on the ''Philae'' lander found at least sixteen organic compounds at the comet's surface, four of which (
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''-dimeth ...
,
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 miscib ...
,
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, c ...
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 ...
) have been detected for the first time on a comet.


Coma

The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma". 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 sola ...
cause an enormous "tail" to form pointing away from the Sun. The coma is generally made of water and dust, with water making up to 90% of the
volatiles Volatiles are the group of chemical elements and chemical compounds that can be readily vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as refractory substances. On planet Earth, the term ' ...
that outflow from the nucleus when the comet is within 3 to 4
astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits t ...
s (450,000,000 to 600,000,000 km; 280,000,000 to 370,000,000 mi) of the Sun. The parent molecule is destroyed primarily through
photodissociation Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...
and to a much smaller extent
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 ...
, with the solar wind playing a minor role in the destruction of water compared to
photochemistry Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400  nm), visible light (400–7 ...
. Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by
light 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 ...
. Although the solid nucleus of comets is generally less than across, the coma may be thousands or millions of kilometers across, sometimes becoming larger than the Sun. For example, about a month after an outburst in October 2007, comet
17P/Holmes Comet Holmes (official designation: 17P/Holmes) is a periodic comet in the Solar System, discovered by the British amateur astronomer Edwin Holmes on November 6, 1892. Although normally a very faint object, Holmes became notable during its Octo ...
briefly had a tenuous dust atmosphere larger than the Sun. The
Great Comet of 1811 The Great Comet of 1811, formally designated C/1811 F1, is a comet that was visible to the naked eye for around 260 days, the longest recorded period of visibility until the appearance of Comet Hale–Bopp in 1997. In October 1811, at its bright ...
also had a coma roughly the diameter of the Sun. Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...
around from the Sun. At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail. Ion tails have been observed to extend one astronomical unit (150 million km) or more. Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflects sunlight directly while the gases glow from
ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
isation. Most comets are too faint to be visible without the aid of a
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
, but a few each decade become bright enough to be visible to the naked eye. Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to
Comet Holmes Comet Holmes (official designation: 17P/Holmes) is a periodic comet in the Solar System, discovered by the British amateur astronomer Edwin Holmes on November 6, 1892. Although normally a very faint object, Holmes became notable during its Octo ...
. In 1996, comets were found to emit
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10  picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
s. This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange". This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion by the emission of X-rays and
far ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
photons.


Bow shock

Bow shock In astrophysics, a bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby flowing ambient plasma such as the solar wind. For Earth and other magnetized planets, it is the boundary at which the speed of ...
s form as a result of the interaction between the solar wind and the cometary ionosphere, which is created by the ionization of gases in the coma. As the comet approaches the Sun, increasing outgassing rates cause the coma to expand, and the sunlight ionizes gases in the coma. When the solar wind passes through this ion coma, the bow shock appears. The first observations were made in the 1980s and 1990s as several spacecraft flew by comets
21P/Giacobini–Zinner Comet Giacobini–Zinner (officially designated 21P/Giacobini–Zinner) is a periodic comet in the Solar System. It was discovered by Michel Giacobini, who observed it in the constellation of Aquarius on December 20, 1900. It was recovered two o ...
, 1P/Halley, and
26P/Grigg–Skjellerup Comet Grigg–Skjellerup (formally designated 26P/Grigg–Skjellerup) is a periodic comet. It was visited by the Giotto probe in July 1992. The spacecraft came as close as 200 km, but could not take pictures because some instruments were dam ...
. It was then found that the bow shocks at comets are wider and more gradual than the sharp planetary bow shocks seen at, for example, Earth. These observations were all made near
perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...
when the bow shocks already were fully developed. The ''Rosetta'' spacecraft observed the bow shock at comet
67P/Churyumov–Gerasimenko 67P/Churyumov–Gerasimenko (abbreviated as 67P or 67P/C–G) is a Jupiter-family comet, originally from the Kuiper belt, with a current orbital period of 6.45 years, a rotation period of approximately 12.4 hours and a maximum velocity of . Chu ...
at an early stage of bow shock development when the outgassing increased during the comet's journey toward the Sun. This young bow shock was called the "infant bow shock". The infant bow shock is asymmetric and, relative to the distance to the nucleus, wider than fully developed bow shocks.


Tails

In the outer
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the
Kuiper belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
have been reported from observations by the
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
but these detections have been questioned. As a comet approaches the inner Solar System,
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 (W/m ...
causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail. At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. On occasions—such as when Earth passes through a comet's orbital plane, the
antitail An antitail is a spike projecting from a comet's coma which seems to go towards the Sun, and thus geometrically opposite to the other tails: the ''ion tail'' and the ''dust tail''. However, this phenomenon is an optical illusion that is seen f ...
, pointing in the opposite direction to the ion and dust tails, may be seen. The observation of antitails contributed significantly to the discovery of solar wind. The ion tail is formed as a result of the ionization by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an "induced
magnetosphere In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior dynam ...
" around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a
bow shock In astrophysics, a bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby flowing ambient plasma such as the solar wind. For Earth and other magnetized planets, it is the boundary at which the speed of ...
is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail. If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail,
magnetic reconnection Magnetic reconnection is a physical process occurring in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration. Magnetic reconnectio ...
occurs. This leads to a "tail disconnection event". This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of
Encke's Comet Comet Encke , or Encke's Comet (official designation: 2P/Encke), is a periodic comet that completes an orbit of the Sun once every 3.3 years. (This is the shortest period of a reasonably bright comet; the faint main-belt comet 311P/PanSTARRS has ...
was completely severed while the comet passed through a
coronal mass ejection A coronal mass ejection (CME) is a significant release of plasma and accompanying magnetic field from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted ...
. This event was observed by the STEREO space probe. In 2013,
ESA , owners = , headquarters = Paris, Île-de-France, France , coordinates = , spaceport = Guiana Space Centre , seal = File:ESA emblem seal.png , seal_size = 130px , image = Views in the Main Control Room (1205 ...
scientists reported that the
ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
of the planet
Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...
streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions."


Jets

Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser. These streams of gas and dust can cause the nucleus to spin, and even split apart. In 2010 it was revealed
dry ice Dry ice is the solid form of carbon dioxide. It is commonly used for temporary refrigeration as CO2 does not have a liquid state at normal atmospheric pressure and sublimates directly from the solid state to the gas state. It is used primarily a ...
(frozen carbon dioxide) can power jets of material flowing out of a comet nucleus. Infrared imaging of Hartley 2 shows such jets exiting and carrying with it dust grains into the coma.


Orbital characteristics

Most comets are
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 fol ...
with elongated
elliptical orbit In astrodynamics or celestial mechanics, an elliptic orbit or elliptical orbit is a Kepler orbit with an eccentricity of less than 1; this includes the special case of a circular orbit, with eccentricity equal to 0. In a stricter sense, it ...
s that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder. Comets are often classified according to the length of their
orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...
s: The longer the period the more elongated the ellipse.


Short period

Periodic comets Periodicity or periodic may refer to: Mathematics * Bott periodicity theorem, addresses Bott periodicity: a modulo-8 recurrence relation in the homotopy groups of classical groups * Periodic function, a function whose output contains values tha ...
or short-period comets are generally defined as those having orbital periods of less than 200 years. They usually orbit more-or-less in the
ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic again ...
plane in the same direction as the planets. Their orbits typically take them out to the region of the outer planets (
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...
and beyond) at
aphelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...
; for example, the aphelion of Halley's Comet is a little beyond the orbit of
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
. Comets whose aphelia are near a major planet's orbit are called its "family". Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits. At the shorter orbital period extreme, Encke's Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods less than 20 years and low inclinations (up to 30 degrees) to the ecliptic are called traditional Jupiter-family comets (JFCs). Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs). , 94 HTCs have been observed, compared with 744 identified JFCs. Recently discovered
main-belt comets 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 ...
form a distinct class, orbiting in more circular orbits within the
asteroid 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 ...
. Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations. Short-period comets have a tendency for their aphelia to coincide with a
giant planet The giant planets constitute a diverse type of planet much larger than Earth. They are usually primarily composed of low-boiling-point materials (volatiles), rather than rock or other solid matter, but massive solid planets can also exist. Ther ...
's semi-major axis, with the JFCs being the largest group. It is clear that comets coming in from 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 2 ...
often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods. Based on their orbital characteristics, short-period comets are thought to originate from the
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 being ...
and the Kuiper belt/
scattered disc The scattered disc (or scattered disk) is a distant circumstellar disc in the Solar System that is sparsely populated by icy small solar system bodies, which are a subset of the broader family of trans-Neptunian objects. The scattered-disc objec ...
—a disk of objects in the trans-Neptunian region—whereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer
Jan Hendrik Oort Jan Hendrik Oort ( or ; 28 April 1900 – 5 November 1992) was a Dutch astronomer who made significant contributions to the understanding of the Milky Way and who was a pioneer in the field of radio astronomy. His ''New York Times'' obituary ...
who hypothesized its existence). Vast swarms of comet-like bodies are thought to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards toward the Sun to form a visible comet. Unlike the return of periodic comets, whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable. When flung into the orbit of the sun, and being continuously dragged towards it, tons of matter are stripped from the comets which greatly influence their lifetime; the more stripped, the shorter they live and vice versa.


Long period

Long-period comets have highly
eccentric Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-center, in geometry * Eccentricity (graph theory) of a v ...
orbits and periods ranging from 200 years to thousands or even millions of years. An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System. For example, Comet McNaught had a heliocentric osculating eccentricity of 1.000019 near its perihelion passage
epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided by ...
in January 2007 but is bound to the Sun with roughly a 92,600-year orbit because the
eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-center, in geometry * Eccentricity (graph theory) of a v ...
drops below 1 as it moves farther from the Sun. The future orbit of a long-period comet is properly obtained when the
osculating orbit In astronomy, and in particular in astrodynamics, the osculating orbit of an object in space at a given moment in time is the gravitational Kepler orbit (i.e. an elliptic or other conic one) that it would have around its central body if perturba ...
is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System. By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the Solar System due to close passes by major planets are no longer properly considered as having "periods". The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as C/1999 F1 and C/2017 T2 (PANSTARRS) can have aphelion distances of nearly with orbital periods estimated around 6 million years. Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly
hyperbolic Hyperbolic is an adjective describing something that resembles or pertains to a hyperbola (a curve), to hyperbole (an overstatement or exaggeration), or to hyperbolic geometry. The following phenomena are described as ''hyperbolic'' because they ...
trajectories when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change. Single-apparition comets have a hyperbolic or parabolic
osculating orbit In astronomy, and in particular in astrodynamics, the osculating orbit of an object in space at a given moment in time is the gravitational Kepler orbit (i.e. an elliptic or other conic one) that it would have around its central body if perturba ...
which allows them to permanently exit the Solar System after a single pass of the Sun. The Sun's
Hill sphere The Hill sphere of an astronomical body is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sp ...
has an
unstable In numerous fields of study, the component of instability within a system is generally characterized by some of the outputs or internal states growing without bounds. Not all systems that are not stable are unstable; systems can also be mar ...
maximum boundary of . Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion that using a heliocentric unperturbed two-body
best-fit Curve fitting is the process of constructing a curve, or mathematical function, that has the best fit to a series of data points, possibly subject to constraints. Curve fitting can involve either interpolation, where an exact fit to the data is ...
suggests they may escape the Solar System. , only two objects have been discovered with an
eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-center, in geometry * Eccentricity (graph theory) of a v ...
significantly greater than one: 1I/ʻOumuamua and
2I/Borisov 2I/Borisov, originally designated C/2019 Q4 (Borisov), is the first observed rogue comet and the second observed interstellar interloper after ʻOumuamua. It was discovered by the Crimean amateur astronomer and telescope maker Gennadiy Borisov ...
, indicating an origin outside the Solar System. While ʻOumuamua, with an eccentricity of about 1.2, showed no optical signs of cometary activity during its passage through the inner Solar System in October 2017, changes to its trajectory—which suggests
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 a ...
—indicate that it is probably a comet. On the other hand, 2I/Borisov, with an estimated eccentricity of about 3.36, has been observed to have the coma feature of comets, and is considered the first detected
interstellar comet An interstellar object is an astronomical object (such as an asteroid, a comet, or a rogue planet, but not a star) in interstellar space that is not gravitationally bound to a star. This term can also be applied to an object that is on an interst ...
. Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known solar comet with a reasonable observation arc. Comets not expected to return to the inner Solar System include C/1980 E1, C/2000 U5, C/2001 Q4 (NEAT),
C/2009 R1 C/2009 R1 (McNaught), one of more than fifty comets known as Comet McNaught, is a non-periodic comet discovered by British-Australian astronomer Robert H. McNaught on September 9, 2009, using the Uppsala Southern Schmidt Telescope at Siding Sp ...
, C/1956 R1, and C/2007 F1 (LONEOS). Some authorities use the term "periodic comet" to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets), whereas others use it to mean exclusively short-period comets. Similarly, although the literal meaning of "non-periodic comet" is the same as "single-apparition comet", some use it to mean all comets that are not "periodic" in the second sense (that is, to also include all comets with a period greater than 200 years). Early observations have revealed a few genuinely hyperbolic (i.e. non-periodic) trajectories, but no more than could be accounted for by perturbations from Jupiter. Comets from
interstellar space Outer space, commonly shortened to space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. Outer space is not completely empty—it is a near-perfect vacuum containing a low density of particles, predo ...
are moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). When such objects enter the Solar System, they have a positive
specific orbital energy In the gravitational two-body problem, the specific orbital energy \varepsilon (or vis-viva energy) of two orbiting bodies is the constant sum of their mutual potential energy (\varepsilon_p) and their total kinetic energy (\varepsilon_k), divided ...
resulting in a positive velocity at infinity (v_\!) and have notably hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter's orbit, give or take one and perhaps two orders of
magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...
.


Oort cloud and Hills cloud

The Oort cloud is thought to occupy a vast space starting from between to as far as from the Sun. This cloud encases the celestial bodies that start at the middle of our solar system—the sun, all the way to outer limits of the Kuiper Belt. The Oort cloud consists of viable materials necessary for the creation of celestial bodies. The planets we have today, exist only because of the planetesimals (chunks of leftover space that assisted in the creation of planets) that were condensed and formed by the gravity of the sun. The eccentric made from these trapped planetesimals is why the Oort Cloud even exists. Some estimates place the outer edge at between . The region can be subdivided into a spherical outer Oort cloud of , and a doughnut-shaped inner cloud, the Hills cloud, of . The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets that fall to inside the orbit of
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
. The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981. Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo; it is seen as a possible source of new comets that resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.


Exocomets

Exocomet An exocomet, or extrasolar comet, is a comet outside the Solar System, which includes rogue comets and comets that orbit stars other than the Sun. The first exocomets were detected in 1987 around Beta Pictoris, a very young A-type main-sequence ...
s beyond the Solar System have also been detected and may be common in the
Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...
. The first exocomet system detected was around
Beta Pictoris Beta Pictoris (abbreviated β Pictoris or β Pic) is the second brightest star in the constellation Pictor. It is located from the Solar System, and is 1.75 times as massive and 8.7 times as luminous as the Sun. The Beta Pictoris sy ...
, a very young
A-type main-sequence star An A-type main-sequence star (A V) or A dwarf star is a main-sequence (hydrogen-burning) star of spectral type A and luminosity class V (five). These stars have spectra defined by strong hydrogen Balmer absorption lines. They measure between 1 ...
, in 1987. A total of 11 such exocomet systems have been identified , using the
absorption spectrum Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating f ...
caused by the large clouds of gas emitted by comets when passing close to their star. For ten years the
Kepler space telescope The Kepler space telescope is a disused space telescope launched by NASA in 2009 to discover Earth-sized planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched into an Earth-trailing heliocentric orb ...
was responsible for searching for planets and other forms outside of the solar system. The first transiting exocomets were found in February 2018 by a group consisting of professional astronomers and citizen scientists in light curves recorded by the Kepler Space Telescope. After Kepler Space Telescope retired in October 2018, a new telescope called TESS Telescope has taken over Kepler's mission. Since the launch of TESS, astronomers have discovered the transits of comets around the star Beta Pictoris using a light curve from TESS. Since TESS has taken over, astronomers have since been able to better distinguish exocomets with the spectroscopic method. New planets are detected by the white light curve method which is viewed as a symmetrical dip in the charts readings when a planet overshadows its parent star. However, after further evaluation of these light curves, it has been discovered that the asymmetrical patterns of the dips presented are caused by the tail of a comet or of hundreds of comets.


Effects of comets


Connection to meteor showers

As a comet is heated during close passes to the Sun,
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 a ...
of its icy components also releases solid debris too large to be swept away by
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 the solar wind. If Earth's orbit sends it through that trail of debris, which is composed mostly of fine grains of rocky material, there is likely to be a
meteor shower A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky. These meteors are caused by streams of cosmic debris called meteoroids entering Earth's atmosphere at extre ...
as Earth passes through. Denser trails of debris produce quick but intense meteor showers and less dense trails create longer but less intense showers. Typically, the density of the debris trail is related to how long ago the parent comet released the material. The
Perseid meteor shower The Perseids are a prolific meteor shower associated with the comet Swift–Tuttle. The meteors are called the Perseids because the point from which they appear to hail (called the radiant) lies in the constellation Perseus. Etymology The name ...
, for example, occurs every year between 9 and 13 August, when Earth passes through the orbit of
Comet Swift–Tuttle Comet Swift–Tuttle (formally designated 109P/Swift–Tuttle) is a large periodic comet with a 1995 ( osculating) orbital period of 133 years that is in a 1:11 orbital resonance with Jupiter. It fits the classical definition of a Halley-type co ...
. Halley's Comet is the source of the Orionid shower in October.


Comets and impact on life

Many comets and asteroids collided with Earth in its early stages. Many scientists think that comets bombarding the young Earth about 4 billion years ago brought the vast quantities of water that now fill Earth's oceans, or at least a significant portion of it. Others have cast doubt on this idea. The detection of organic molecules, including
polycyclic aromatic hydrocarbon A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. ...
s, in significant quantities in comets has led to speculation that comets or
meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...
s may have brought the precursors of life—or even life itself—to Earth. In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s that make up
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s through
shock synthesis Shock synthesis is the process of complex organic chemical creation through high velocity impact on simple amino acids, theorized to take place when a comet strikes a planetary body, or through the shock-wave created by a thunder clap. Hyper-vel ...
. The speed at which the comets entered the atmosphere, combined with the magnitude of energy created after initial contact, allowed smaller molecules to condense into the larger macro-molecules that served as the foundation for life. In 2015, scientists found significant amounts of molecular oxygen in the outgassings of comet 67P, suggesting that the molecule may occur more often than had been thought, and thus less an indicator of life as has been supposed. It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to Earth's
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, some of which may have survived as
lunar ice Lunar water is water that is present on the Moon. Diffuse water molecules can persist at the Moon's sunlit surface, as discovered by NASA's SOFIA observatory in 2020. Gradually water vapor is decomposed by sunlight, leaving hydrogen and oxyg ...
. Comet and
meteoroid 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 micr ...
impacts are also thought to be responsible for the existence of
tektite Tektites (from grc, τηκτός , meaning 'molten') are gravel-sized bodies composed of black, green, brown or grey natural glass formed from terrestrial debris ejected during meteorite impacts. The term was coined by Austrian geologist Franz ...
s and
australite Australites are tektites found in Australia. They are mostly dark or black, and have shapes including discs and bowls that are not seen in other tektites. NASA used the shape of "flanged button" australites in designing re-entry modules for the Ap ...
s.


Fear of comets

Fear of comets as acts of God and signs of impending doom was highest in Europe from AD 1200 to 1650. The year after the
Great Comet of 1618 The year 1618 in science and technology involved some significant events. Astronomy * March 8 – May 15 – Johannes Kepler formulates the third law of planetary motion. * July 21 – Pluto (not known at this time) reaches an aphelion. It nex ...
, for example,
Gotthard Arthusius Gotthard Arthus or Gotardus Artusius Dantiscanus (1568–1628) was a schoolmaster, historian and translator in early seventeenth-century Frankfurt. Life Gotthard Arthus was born in Danzig on 11 January 1568. In 1589 he matriculated at the Universi ...
published a pamphlet stating that it was a sign that the
Day of Judgment The Last Judgment, Final Judgment, Day of Reckoning, Day of Judgment, Judgment Day, Doomsday, Day of Resurrection or The Day of the Lord (; ar, یوم القيامة, translit=Yawm al-Qiyāmah or ar, یوم الدین, translit=Yawm ad-Dīn, ...
was near. He listed ten pages of comet-related disasters, including "earthquakes, floods, changes in river courses, hail storms, hot and dry weather, poor harvests, epidemics, war and treason and high prices". By 1700 most scholars concluded that such events occurred whether a comet was seen or not. Using
Edmond Halley Edmond (or Edmund) Halley (; – ) was an English astronomer, mathematician and physicist. He was the second Astronomer Royal in Britain, succeeding John Flamsteed in 1720. From an observatory he constructed on Saint Helena in 1676–77, H ...
's records of comet sightings, however,
William Whiston William Whiston (9 December 166722 August 1752) was an English theologian, historian, natural philosopher, and mathematician, a leading figure in the popularisation of the ideas of Isaac Newton. He is now probably best known for helping to inst ...
in 1711 wrote that the
Great Comet of 1680 C/1680 V1, also called the Great Comet of 1680, Kirch's Comet, and Newton's Comet, was the first comet discovered by telescope. It was discovered by Gottfried Kirch and was one of the brightest comets of the seventeenth century. Overview The co ...
had a periodicity of 574 years and was responsible for the worldwide flood in the Book of Genesis, by pouring water on Earth. His announcement revived for another century fear of comets, now as direct threats to the world instead of signs of disasters. Spectroscopic analysis in 1910 found the toxic gas
cyanogen Cyanogen is the chemical compound with the formula ( C N)2. It is a colorless and highly toxic gas with a pungent odor. The molecule is a pseudohalogen. Cyanogen molecules consist of two CN groups – analogous to diatomic halogen molecule ...
in the tail of Halley's Comet, causing panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public.


Fate of comets


Departure (ejection) from Solar System

If a comet is traveling fast enough, it may leave the Solar System. Such comets follow the open path of a hyperbola, and as such, they are called hyperbolic comets. Solar comets are only known to be ejected by
interacting Interaction is action that occurs between two or more objects, with broad use in philosophy and the sciences. It may refer to: Science * Interaction hypothesis, a theory of second language acquisition * Interaction (statistics) * Interactions o ...
with another object in the Solar System, such as Jupiter. An example of this is Comet C/1980 E1, which was shifted from an orbit of 7.1 million years around the Sun, to a hyperbolic trajectory, after a 1980 close pass by the planet Jupiter. (Solution using the Solar System
Barycenter In astronomy, the barycenter (or barycentre; ) is the center of mass of two or more bodies that orbit one another and is the point about which the bodies orbit. A barycenter is a dynamical point, not a physical object. It is an important conc ...
and
barycentric coordinates In mathematics, an affine space is a geometric structure that generalizes some of the properties of Euclidean spaces in such a way that these are independent of the concepts of distance and measure of angles, keeping only the properties related ...
. Select Ephemeris Type:Elements and Center:@0)
Interstellar comets such as 1I/ʻOumuamua and
2I/Borisov 2I/Borisov, originally designated C/2019 Q4 (Borisov), is the first observed rogue comet and the second observed interstellar interloper after ʻOumuamua. It was discovered by the Crimean amateur astronomer and telescope maker Gennadiy Borisov ...
never orbited the Sun and therefore do not require a 3rd-body interaction to be ejected from the Solar System.


Volatiles exhausted

Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages. Eventually most of the volatile material contained in a comet nucleus evaporates, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid. Some asteroids in elliptical orbits are now identified as extinct comets. Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.


Breakup and collisions

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart. A significant cometary disruption was that of
Comet Shoemaker–Levy 9 Comet Shoemaker–Levy 9 ( formally designated D/1993 F2) broke apart in July 1992 and collided with Jupiter in July 1994, providing the first direct observation of an extraterrestrial collision of Solar System objects. This generated a l ...
, which was discovered in 1993. A close encounter in July 1992 had broken it into pieces, and over a period of six days in July 1994, these pieces fell into Jupiter's atmosphere—the first time astronomers had observed a collision between two objects in the Solar System. Other 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 and
73P/Schwassmann–Wachmann 73P/Schwassmann–Wachmann, also known as Schwassmann–Wachmann 3, is a periodic comet that has a 5.4 year orbital period and that has been actively disintegrating since 1995. It last came to perihelion (closest approach to the Sun) in March 20 ...
from 1995 to 2006. Greek historian
Ephorus Ephorus of Cyme (; grc-gre, Ἔφορος ὁ Κυμαῖος, ''Ephoros ho Kymaios''; c. 400330 BC) was an ancient Greek historian known for his universal history. Biography Information on his biography is limited. He was born in Cyme, A ...
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. Some comets have been observed to break up during their perihelion passage, including great comets
West West or Occident is one of the four cardinal directions or points of the compass. It is the opposite direction from east and is the direction in which the Sunset, Sun sets on the Earth. Etymology The word "west" is a Germanic languages, German ...
and Ikeya–Seki. Biela's Comet was one significant example when it broke into two pieces during its passage through the perihelion in 1846. These two comets were seen separately in 1852, but never again afterward. Instead, spectacular
meteor showers A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky. These meteors are caused by streams of cosmic debris called meteoroids entering Earth's atmosphere at extre ...
were seen in 1872 and 1885 when the comet should have been visible. A minor meteor shower, the
Andromedids The Andromedids meteor shower is associated with Biela's Comet, the showers occurring as Earth passes through old streams left by the comet's tail. The comet was observed to have broken up by 1846; further drift of the pieces by 1852 suggested th ...
, occurs annually in November, and it is caused when Earth crosses the orbit of Biela's Comet. Some comets meet a more spectacular end – either falling into the Sun or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in July 1994 when Comet Shoemaker–Levy 9 broke up into pieces and collided with Jupiter.


Nomenclature

The names given to comets have followed several different conventions over the past two centuries. Prior to the early 20th century, most comets were referred to by the year when they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680", the "
Great Comet of 1882 The Great Comet of 1882 formally designated C/1882 R1, 1882 II, and 1882b, was a comet which became very bright in September 1882. It was a member of the Kreutz Sungrazers, a family of comets which pass within of the Sun's photosphere at perih ...
", and the "
Great January Comet of 1910 The Great January Comet of 1910, formally designated C/1910 A1 and often referred to as the Daylight Comet,. was a comet which appeared in January 1910. It was already visible to the naked eye when it was first noticed, and many people independent ...
". After Edmond Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759 by calculating its orbit, that comet became known as Halley's Comet. Similarly, the second and third known periodic comets, Encke's Comet and Biela's Comet, were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their appearance. In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet can be named after its discoverers or an instrument or program that helped to find it. For example, in 2019, astronomer
Gennadiy Borisov Gennadiy Vladimirovich Borisov (russian: Генна́дий Влади́мирович Бори́сов; born in 1962 in Kramatorsk) is a Crimean amateur telescope making, telescope maker and amateur astronomy, amateur astronomer who discovered ...
observed a comet that appeared to have originated outside of the solar system; the comet was named
2I/Borisov 2I/Borisov, originally designated C/2019 Q4 (Borisov), is the first observed rogue comet and the second observed interstellar interloper after ʻOumuamua. It was discovered by the Crimean amateur astronomer and telescope maker Gennadiy Borisov ...
after him.


History of study


Early observations and thought

From ancient sources, such as Chinese
oracle bone Oracle bones () are pieces of ox scapula and turtle plastron, which were used for pyromancy – a form of divination – in ancient China, mainly during the late Shang dynasty. ''Scapulimancy'' is the correct term if ox scapulae were used for th ...
s, it is known that comets have been noticed by humans for millennia. Until the sixteenth century, comets were usually considered bad
omen An omen (also called ''portent'') is a phenomenon that is believed to foretell the future, often signifying the advent of change. It was commonly believed in ancient times, and still believed by some today, that omens bring divine messages fr ...
s of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants.
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of phil ...
(384–322 BC) was the first known scientist to use various theories and observational facts to employ a consistent, structured cosmological theory of comets. He believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the
zodiac The zodiac is a belt-shaped region of the sky that extends approximately 8° north or south (as measured in celestial latitude) of the ecliptic, the Sun path, apparent path of the Sun across the celestial sphere over the course of the year. ...
and vary in brightness over the course of a few days. Aristotle's cometary theory arose from his observations and cosmological theory that everything in the cosmos is arranged in a distinct configuration. Part of this configuration was a clear separation between the celestial and terrestrial, believing comets to be strictly associated with the latter. According to Aristotle, comets must be within the sphere of the moon and clearly separated from the heavens. Also in the 4th century BC,
Apollonius of Myndus Apollonius ( grc, Απολλώνιος) of Myndus lived at the time of Alexander the Great, that is, the 4th century BCE, and was particularly skilled in explaining horoscopes. He professed to have learned his art from the Chaldeans. His statemen ...
supported the idea that comets moved like the planets. Aristotelian theory on comets continued to be widely accepted throughout the
Middle Ages In the history of Europe, the Middle Ages or medieval period lasted approximately from the late 5th to the late 15th centuries, similar to the post-classical period of global history. It began with the fall of the Western Roman Empire a ...
, despite several discoveries from various individuals challenging aspects of it. In the 1st century AD,
Seneca the Younger Lucius Annaeus Seneca the Younger (; 65 AD), usually known mononymously as Seneca, was a Stoic philosopher of Ancient Rome, a statesman, dramatist, and, in one work, satirist, from the post-Augustan age of Latin literature. Seneca was born in ...
questioned Aristotle's logic concerning comets. Because of their regular movement and imperviousness to wind, they cannot be atmospheric, and are more permanent than suggested by their brief flashes across the sky. He pointed out that only the tails are transparent and thus cloudlike, and argued that there is no reason to confine their orbits to the zodiac. In criticizing Apollonius of Myndus, Seneca argues, "A comet cuts through the upper regions of the universe and then finally becomes visible when it reaches the lowest point of its orbit." While Seneca did not author a substantial theory of his own, his arguments would spark much debate among Aristotle's critics in the 16th and 17th centuries. Also in the 1st century,
Pliny the Elder Gaius Plinius Secundus (AD 23/2479), called Pliny the Elder (), was a Roman author, naturalist and natural philosopher, and naval and army commander of the early Roman Empire, and a friend of the emperor Vespasian. He wrote the encyclopedic '' ...
believed that comets were connected with political unrest and death. Pliny observed comets as "human like", often describing their tails with "long hair" or "long beard". His system for classifying comets according to their color and shape was used for centuries. In
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
, by the 6th century astronomers believed that comets were celestial bodies that re-appeared periodically. This was the view expressed in the 6th century by the astronomers
Varāhamihira Varāhamihira ( 505 – 587), also called Varāha or Mihira, was an ancient Indian astrologer, astronomer, and polymath who lived in Ujjain (Madhya Pradesh, India). He was born at Kapitba in a Brahmin family, in the Avanti region, roughly co ...
and
Bhadrabahu Ācārya Bhadrabāhu (c. 367 - c. 298 BC) was, according to the ''Digambara'' sect of Jainism, the last '' Shruta Kevalin'' (all knowing by hearsay, that is indirectly) in Jainism . He was the last ''acharya'' of the undivided Jain ''sangha''. ...
, and the 10th-century astronomer
Bhaṭṭotpala Utpala or (') is the name of a 10th-century Indian commentator of Vārāha Mihira's ''Brihat Samhitā''. ''Brihat Samhitā'' is a Samhitā text of (Indian astrology and astronomy) . Samhitā is one of three branches of (Samhitā has many other ...
listed the names and estimated periods of certain comets, but it is not known how these figures were calculated or how accurate they were. In 1301, the Italian painter
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. Giot ...
was the first person to accurately and anatomically portray a comet. In his work ''
Adoration of the Magi The Adoration of the Magi or Adoration of the Kings is the name traditionally given to the subject in the Nativity of Jesus in art in which the three Magi, represented as kings, especially in the West, having found Jesus by following a star, ...
,'' Giotto's depiction of Halley's Comet in the place of the
Star of Bethlehem The Star of Bethlehem, or Christmas Star, appears in the nativity story of the Gospel of Matthew chapter 2 where "wise men from the East" (Magi) are inspired by the star to travel to Jerusalem. There, they meet King Herod of Judea, and ask him: ...
would go unmatched in accuracy until the 19th century and be bested only with the invention of photography. Astrological interpretations of comets proceeded to take precedence clear into the 15th century, despite the presence of modern scientific astronomy beginning to take root. Comets continued to forewarn of disaster, as seen in the ''
Luzerner Schilling The ''Luzerner Schilling'' (or ''Luzernerchronik'', Lucerne chronicle) is an illuminated manuscript of 1513, containing the chronicle of the history of the Swiss Confederation written by Diebold Schilling the Younger of Lucerne. The chronicle is ...
'' chronicles and in the warnings of
Pope Callixtus III Pope Callixtus III ( it, Callisto III, va, Calixt III, es, Calixto III; 31 December 1378 – 6 August 1458), born Alfonso de Borgia ( va, Alfons de Borja), was head of the Catholic Church and ruler of the Papal States from 8 April 1455 to his ...
. In 1578, German
Lutheran Lutheranism is one of the largest branches of Protestantism, identifying primarily with the theology of Martin Luther, the 16th-century German monk and reformer whose efforts to reform the theology and practice of the Catholic Church launched th ...
bishop Andreas Celichius defined comets as "the thick smoke of human sins ... kindled by the hot and fiery anger of the Supreme Heavenly Judge". The next year, Andreas Dudith stated that "If comets were caused by the sins of mortals, they would never be absent from the sky."


Scientific approach

Crude attempts at a
parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...
measurement of Halley's Comet were made in 1456, but were erroneous.
Regiomontanus Johannes Müller von Königsberg (6 June 1436 – 6 July 1476), better known as Regiomontanus (), was a mathematician, astrologer and astronomer of the German Renaissance, active in Vienna, Buda and Nuremberg. His contributions were instrumental ...
was the first to attempt to calculate
diurnal parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...
by observing the great comet of 1472. His predictions were not very accurate, but they were conducted in the hopes of estimating the distance of a comet from the Earth. In the 16th century,
Tycho Brahe Tycho Brahe ( ; born Tyge Ottesen Brahe; generally called Tycho (14 December 154624 October 1601) was a Danish astronomer, known for his comprehensive astronomical observations, generally considered to be the most accurate of his time. He was k ...
and
Michael Maestlin Michael Maestlin (also Mästlin, Möstlin, or Moestlin) (30 September 1550 – 26 October 1631) was a German astronomer and mathematician, known for being the mentor of Johannes Kepler. He was a student of Philipp Apian and was known as the tea ...
demonstrated that comets must exist outside of Earth's atmosphere by measuring the parallax of the
Great Comet of 1577 The Great Comet of 1577 (official designation: C/1577 V1) is a non-periodic comet that passed close to Earth during the year 1577 AD. Having an official designation beginning with "C" classes it as a non-periodic comet, and so it is not expected t ...
. Within the precision of the measurements, this implied the comet must be at least four times more distant than from Earth to the Moon. Based on observations in 1664,
Giovanni Borelli Giovanni Alfonso Borelli (; 28 January 1608 – 31 December 1679) was a Renaissance Italian physiologist, physicist, and mathematician. He contributed to the modern principle of scientific investigation by continuing Galileo's practice of testi ...
recorded the longitudes and latitudes of comets that he observed, and suggested that cometary orbits may be parabolic.
Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642) was an Italian astronomer, physicist and engineer, sometimes described as a polymath. Commonly referred to as Galileo, his name was pronounced (, ). He was ...
, one of the most renowned astronomers to date, even attempted writings on comets in ''
The Assayer ''The Assayer'' ( it, Il Saggiatore) was a book published in Rome by Galileo Galilei in October 1623 and is generally considered to be one of the pioneering works of the scientific method, first broaching the idea that the book of nature is to be ...
''. He rejected Brahe's theories on the parallax of comets and claimed that they may be a mere optical illusion. Intrigued as early scientists were about the nature of comets, Galileo could not help but throw about his own theories despite little personal observation. Maestlin's student
Johannes Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws ...
responded to these unjust criticisms in his work ''Hyperaspistes.''
Jakob Bernoulli Jacob Bernoulli (also known as James or Jacques; – 16 August 1705) was one of the many prominent mathematicians in the Bernoulli family. He was an early proponent of Leibnizian calculus and sided with Gottfried Wilhelm Leibniz during the Leib ...
published another attempt to explain comets (Conamen Novi Systematis Cometarum) in 1682. Also occurring in the early modern period was the study of comets and their astrological significance in medical disciplines. Many healers of this time considered medicine and astronomy to be inter-disciplinary and employed their knowledge of comets and other astrological signs for diagnosing and treating patients.
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a "natural philosopher"), widely recognised as one of the grea ...
, in his ''
Principia Mathematica The ''Principia Mathematica'' (often abbreviated ''PM'') is a three-volume work on the foundations of mathematics written by mathematician–philosophers Alfred North Whitehead and Bertrand Russell and published in 1910, 1912, and 1913. ...
'' of 1687, proved that an object moving under the influence of
gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
by an inverse square law must trace out an orbit shaped like one of the
conic section In mathematics, a conic section, quadratic curve or conic is a curve obtained as the intersection of the surface of a cone with a plane. The three types of conic section are the hyperbola, the parabola, and the ellipse; the circle is a specia ...
s, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example. He describes comets as compact and durable solid bodies moving in oblique orbit and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the Sun. He suspected that comets were the origin of the life-supporting component of air. He also pointed out that comets usually appear near the Sun, and therefore most likely orbit it. On their luminosity, he stated, "The comets shine by the Sun's light, which they reflect," with their tails illuminated by "the Sun's light reflected by a smoke arising from
he coma He or HE may refer to: Language * He (pronoun), an English pronoun * He (kana), the romanization of the Japanese kana へ * He (letter), the fifth letter of many Semitic alphabets * He (Cyrillic), a letter of the Cyrillic script called ''He'' in ...
. In 1705, Edmond Halley (1656–1742) applied Newton's method to 23 cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar
orbital element Orbital elements are the parameters required to uniquely identify a specific orbit. In celestial mechanics these elements are considered in two-body systems using a Kepler orbit. There are many different ways to mathematically describe the same ...
s, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation caused by Jupiter and
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 ...
. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758–9. Halley's predicted return date was later refined by a team of three French mathematicians:
Alexis Clairaut Alexis Claude Clairaut (; 13 May 1713 – 17 May 1765) was a French mathematician, astronomer, and geophysicist. He was a prominent Newtonian whose work helped to establish the validity of the principles and results that Sir Isaac Newton had out ...
,
Joseph Lalande Joseph is a common male given name, derived from the Hebrew Yosef (יוֹסֵף). "Joseph" is used, along with "Josef", mostly in English, French and partially German languages. This spelling is also found as a variant in the languages of the mo ...
, and
Nicole-Reine Lepaute Nicole-Reine Lepaute () née Étable de la Brière, also erroneously known as Hortense Lepaute, (5 January 1723 – 6 December 1788) was a French astronomer and human computer. Lepaute along with Alexis Clairaut and Jérôme Lalande calculated t ...
, who predicted the date of the comet's 1759 perihelion to within one month's accuracy. When the comet returned as predicted, it became known as Halley's Comet. As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755,
Immanuel Kant Immanuel Kant (, , ; 22 April 1724 – 12 February 1804) was a German philosopher and one of the central Enlightenment thinkers. Born in Königsberg, Kant's comprehensive and systematic works in epistemology, metaphysics, ethics, and ...
hypothesized in his '' Universal Natural History'' that comets were condensed from "primitive matter" beyond the known planets, which is "feebly moved" by gravity, then orbit at arbitrary inclinations, and are partially vaporized by the Sun's heat as they near perihelion. In 1836, the German mathematician
Friedrich Wilhelm Bessel Friedrich Wilhelm Bessel (; 22 July 1784 – 17 March 1846) was a German astronomer, mathematician, physicist, and geodesist. He was the first astronomer who determined reliable values for the distance from the sun to another star by the method ...
, after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the
jet force Jet force is the exhaust from some machine, esp. aircraft, propelling the object itself in the opposite direction as per Newton's laws of motion, Newton's Third Law. An understanding of jet force is intrinsic to the launching of drones, satellites, ...
s of evaporating material could be great enough to significantly alter a comet's orbit, and he argued that the non-gravitational movements of Encke's Comet resulted from this phenomenon. In the 19th century, the Astronomical Observatory of Padova was an epicenter in the observational study of comets. Led by
Giovanni Santini Giovanni Sante Gaspero Santini (b. Caprese in Tuscany, 30 June 1786; d. Noventa Padovana, 26 June 1877) was an Italian astronomer and mathematician. He received his first instruction from his parental uncle, the Abbot Giovanni Battista Santi ...
(1787–1877) and followed by Giuseppe Lorenzoni (1843–1914), this observatory was devoted to classical astronomy, mainly to the new comets and planets orbit calculation, with the goal of compiling a catalog of almost ten thousand stars. Situated in the Northern portion of Italy, observations from this observatory were key in establishing important geodetic, geographic, and astronomical calculations, such as the difference of longitude between Milan and Padua as well as Padua to Fiume. In addition to these geographic observations, correspondence within the observatory, particularly between Santini and another astronomer Giuseppe Toaldo, about the importance of comet and planetary orbital observations. In 1950,
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 ...
proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock. This "dirty snowball" model soon became accepted and appeared to be supported by the observations of an armada of
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, p ...
(including the
European Space Agency , owners = , headquarters = Paris, Île-de-France, France , coordinates = , spaceport = Guiana Space Centre , seal = File:ESA emblem seal.png , seal_size = 130px , image = Views in the Main Control Room (1205 ...
's ''
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. Giot ...
'' probe and the Soviet Union's ''
Vega 1 Vega 1 (along with its twin Vega 2) was a Soviet space probe, part of the Vega program. The spacecraft was a development of the earlier ''Venera'' craft. They were designed by Babakin Space Centre and constructed as 5VK by Lavochkin at Khimki. ...
'' and ''
Vega 2 Vega 2 (along with Vega 1) was a Soviet space probe part of the Vega program to explore Halley's comet and Venus. The spacecraft was a development of the earlier ''Venera'' craft. The name VeGa (ВеГа) combines the first two letters Russian wo ...
'') that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material. On 22 January 2014, ESA scientists reported the detection, for the first definitive time, 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 pha ...
on the
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to p ...
Ceres Ceres most commonly refers to: * Ceres (dwarf planet), the largest asteroid * Ceres (mythology), the Roman goddess of agriculture Ceres may also refer to: Places Brazil * Ceres, Goiás, Brazil * Ceres Microregion, in north-central Goiás ...
, the largest object in the asteroid belt. The detection was made by using the far-infrared abilities of the
Herschel Space Observatory The Herschel Space Observatory was a space observatory built and operated by the European Space Agency (ESA). It was active from 2009 to 2013, and was the largest infrared telescope ever launched until the launch of the James Webb Space Telesc ...
. The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids." On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, , and dust inside the comae of comets
C/2012 F6 (Lemmon) C/2012 F6 (Lemmon) is a long-period comet discovered in Leo on 23 March 2012, by A. R. Gibbs using the 1.5-m reflector at the Mt. Lemmon Survey, located at the summit of Mount Lemmon in the Santa Catalina Mountains north of Tucson, Arizona ...
and C/2012 S1 (ISON).


Spacecraft missions

*The
Halley Armada The Halley Armada is the name of a series of space probes, five of which were successful, sent to examine Halley's Comet during its 1986 sojourn through the inner Solar System, connected with apparition "1P/1982 U1". The armada included one probe ...
describes the collection of spacecraft missions that visited and/or made observations of Halley's Comet 1980s perihelion. The space shuttle ''Challenger'' was intended to do a study of Halley's Comet in 1986, but exploded shortly after being launched. *Deep Impact. Debate continues about how much ice is in a comet. In 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 1 ...
'' spacecraft obtained high-resolution images of the surface of Comet Borrelly. It was found that the surface of comet Borrelly is hot and dry, with a temperature of between , and extremely dark, suggesting that the ice has been removed by solar heating and maturation, or is hidden by the soot-like material that covers Borrelly. In July 2005, the '' Deep Impact'' probe blasted a crater on Comet
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 photograph ...
to study its interior. The mission yielded results suggesting that the majority of a comet's water ice is below the surface and that these reservoirs feed the jets of vaporized water that form the coma of Tempel 1. Renamed
EPOXI ''EPOXI'' was a compilation of NASA Discovery program missions led by the University of Maryland and principal investigator Michael A'Hearn, with co-operation from the Jet Propulsion Laboratory and Ball Aerospace. ''EPOXI'' uses the '' Deep Impa ...
, it made a flyby of Comet Hartley 2 on 4 November 2010. *Ulysses. In 2007, the
Ulysses probe ''Ulysses'' ( , ) was a robotic space probe whose primary mission was to orbit the Sun and study it at all latitudes. It was launched in 1990 and made three "fast latitude scans" of the Sun in 1994/1995, 2000/2001, and 2007/2008. In addition ...
unexpectedly passed through the tail of the comet
C/2006 P1 Comet McNaught, also known as the Great Comet of 2007 and given the designation C/2006 P1, is a non-periodic comet discovered on 7 August 2006 by British-Australian astronomer Robert H. McNaught using the Uppsala Southern Schmidt Telescope. ...
(McNaught) which was discovered in 2006. Ulysses was launched in 1990 and the intended mission was for Ulysses to orbit around the sun for further study at all latitudes. *Stardust. Data from the ''Stardust'' mission show that materials retrieved from the tail of Wild 2 were crystalline and could only have been "born in fire", at extremely high temperatures of over . Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk. As a result, comets also contain crystalline grains that formed in the early, hot inner Solar System. This is seen in comet spectra as well as in sample return missions. More recent still, the materials retrieved demonstrate that the "comet dust resembles asteroid materials". These new results have forced scientists to rethink the nature of comets and their distinction from asteroids. *Rosetta. The ''
Rosetta Rosetta or Rashid (; ar, رشيد ' ; french: Rosette  ; cop, ϯⲣⲁϣⲓⲧ ''ti-Rashit'', Ancient Greek: Βολβιτίνη ''Bolbitinē'') is a port city of the Nile Delta, east of Alexandria, in Egypt's Beheira governorate. The Ro ...
'' probe orbited Comet Churyumov–Gerasimenko. On 12 November 2014, its lander ''Philae'' successfully landed on the comet's surface, the first time a spacecraft has ever landed on such an object in history.


Classification


Great comets

Approximately once a decade, a comet becomes bright enough to be noticed by a casual observer, leading such comets to be designated as great comets. Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from Earth when at its brightest, it has a chance of becoming a great comet. However,
Comet Kohoutek Comet Kohoutek ( formally designated C/1973 E1 and formerly as 1973 XII and 1973f) is a comet that passed close to the Sun towards the end of 1973. Early predictions of the comet's peak brightness suggested that it had the potential to become o ...
in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so.
Comet West Comet West, formally designated C/1975 V1, 1976 VI, and 1975n, was a comet described as one of the brightest objects to pass through the inner Solar System in 1976. It is often described as a "great comet." History It was discovered photograp ...
, which appeared three years later, had much lower expectations but became an extremely impressive comet. The Great Comet of 1577 is a well-known example of a great comet. It passed near Earth as a
non-periodic comet The following is a list of comets with a very high eccentricity (generally 0.99 or higher) and a period of over 1,000 years that do not quite have a high enough velocity to escape the Solar System. Often, these comets, due to their extreme semimaj ...
and was seen by many, including well-known astronomers Tycho Brahe and Taqi ad-Din. Observations of this comet led to several significant findings regarding cometary science, especially for Brahe. The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession—
Comet Hyakutake Comet Hyakutake (, formally designated C/1996 B2) is a comet, discovered on 31 January 1996, that passed very close to Earth in March of that year. It was dubbed the Great Comet of 1996; its passage near the Earth was one of the closest com ...
in 1996, followed by Hale–Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. The first great comet of the 21st century was
C/2006 P1 Comet McNaught, also known as the Great Comet of 2007 and given the designation C/2006 P1, is a non-periodic comet discovered on 7 August 2006 by British-Australian astronomer Robert H. McNaught using the Uppsala Southern Schmidt Telescope. ...
(McNaught), which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.


Sungrazing comets

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometers. Although small sungrazers can be completely evaporated during such a close approach to the
Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, larger sungrazers can survive many perihelion passages. However, the strong
tidal force The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...
s they experience often lead to their fragmentation. About 90% of the sungrazers observed with
SOHO Soho is an area of the City of Westminster, part of the West End of London. Originally a fashionable district for the aristocracy, it has been one of the main entertainment districts in the capital since the 19th century. The area was develop ...
are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner Solar System. The remainder contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden, and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the
Quadrantids The Quadrantids (QUA) are a meteor shower that peaks in early January and whose radiant lies in the constellation Boötes. The zenithal hourly rate (ZHR) of this shower can be as high as that of two other reliably rich meteor showers, the Persei ...
and the
Arietids The Arietids are a strong meteor shower that lasts from May 22 to July 2 each year, and peaks on June 7. The Arietids, along with the Zeta Perseids, are the most intense daylight meteor showers of the year. The source of the shower is unknown, but ...
.


Unusual comets

Of the thousands of known comets, some exhibit unusual properties. Comet Encke (2P/Encke) orbits from outside the asteroid belt to just inside the orbit of the planet
Mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...
whereas the Comet
29P/Schwassmann–Wachmann Comet 29P/Schwassmann–Wachmann, also known as Schwassmann–Wachmann 1, was discovered on November 15, 1927, by Arnold Schwassmann and Arno Arthur Wachmann at the Hamburg Observatory in Bergedorf, Germany. It was discovered photographically, w ...
currently travels in a nearly circular orbit entirely between the orbits of Jupiter and Saturn.
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 orb ...
, whose unstable orbit is between Saturn and
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...
, was originally classified as an asteroid until a faint coma was noticed. Similarly, Comet Shoemaker–Levy 2 was originally designated asteroid .


Largest

The largest known periodic comet is 95P/Chiron at 200 km in diameter that comes to perihelion every 50 years just inside of Saturn's orbit at 8 AU. The largest known Oort cloud comet is suspected of being Comet Bernardinelli-Bernstein at ≈150 km that will not come to perihelion until January 2031 just outside of Saturn's orbit at 11 AU. The
Comet of 1729 The Comet of 1729, also known as C/1729 P1 or Comet Sarabat, was an assumed parabolic comet with an absolute magnitude of −3, the brightest ever observed for a comet;'' Comet Caesar'' (C/-43 K1) has, however, been calculated to have possibly had ...
is estimated to have been ≈100 km in diameter and came to perihelion inside of Jupiter's orbit at 4 AU.


Centaurs

Centaurs typically behave with characteristics of both asteroids and comets. Centaurs can be classified as comets such as
60558 Echeclus 60558 Echeclus is a centaur, approximately in diameter, located in the outer Solar System. It was discovered by Spacewatch in 2000 and initially classified as a minor planet with provisional designation (also written 2000 EC98). Research in 20 ...
, and 166P/NEAT. 166P/NEAT was discovered while it exhibited a coma, and so is classified as a comet despite its orbit, and
60558 Echeclus 60558 Echeclus is a centaur, approximately in diameter, located in the outer Solar System. It was discovered by Spacewatch in 2000 and initially classified as a minor planet with provisional designation (also written 2000 EC98). Research in 20 ...
was discovered without a coma but later became active, Y-J. Choi, P.R. Weissman, and D. Polishook ''(60558) 2000 EC_98'', IAU Circ., 8656 (Jan. 2006), 2. and was then classified as both a comet and an asteroid (174P/Echeclus). One plan for '' Cassini'' involved sending it to a centaur, but NASA decided to destroy it instead.


Observation

A comet may be discovered photographically using a wide-field
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
or visually with
binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...
. However, even without access to optical equipment, it is still possible for the
amateur astronomer Amateur astronomy is a hobby where participants enjoy observing or imaging celestial objects in the sky using the unaided eye, binoculars, or telescopes. Even though scientific research may not be their primary goal, some amateur astronomers m ...
to discover a sungrazing comet online by downloading images accumulated by some satellite observatories such as
SOHO Soho is an area of the City of Westminster, part of the West End of London. Originally a fashionable district for the aristocracy, it has been one of the main entertainment districts in the capital since the 19th century. The area was develop ...
. SOHO's 2000th comet was discovered by Polish amateur astronomer Michał Kusiak on 26 December 2010 and both discoverers of Hale–Bopp used amateur equipment (although Hale was not an amateur).


Lost

A number of periodic comets discovered in earlier decades or previous centuries are now
lost comet A lost comet is one which was not detected during its most recent perihelion passage. This generally happens when data is insufficient to reliably calculate the comet's location or if the solar elongation is unfavorable near perihelion passage. T ...
s. Their orbits were never known well enough to predict future appearances or the comets have disintegrated. However, occasionally a "new" comet is discovered, and calculation of its orbit shows it to be an old "lost" comet. An example is Comet
11P/Tempel–Swift–LINEAR 11P/Tempel–Swift–LINEAR is a periodic Jupiter-family comet in the Solar System. Ernst Wilhelm Leberecht Tempel (Marseille) originally discovered the comet on November 27, 1869, it was later observed by Lewis Swift ( Warner Observatory) on ...
, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by
LINEAR Linearity is the property of a mathematical relationship (''function'') that can be graphically represented as a straight line. Linearity is closely related to '' proportionality''. Examples in physics include rectilinear motion, the linear r ...
in 2001. There are at least 18 comets that fit this category.


In popular culture

The depiction of comets in
popular culture Popular culture (also called mass culture or pop culture) is generally recognized by members of a society as a set of practices, beliefs, artistic output (also known as, popular art or mass art) and objects that are dominant or prevalent in a ...
is firmly rooted in the long Western tradition of seeing comets as harbingers of doom and as omens of world-altering change. Halley's Comet alone has caused a slew of sensationalist publications of all sorts at each of its reappearances. It was especially noted that the birth and death of some notable persons coincided with separate appearances of the comet, such as with writers
Mark Twain Samuel Langhorne Clemens (November 30, 1835 – April 21, 1910), known by his pen name Mark Twain, was an American writer, humorist, entrepreneur, publisher, and lecturer. He was praised as the "greatest humorist the United States has p ...
(who correctly speculated that he'd "go out with the comet" in 1910) and
Eudora Welty Eudora Alice Welty (April 13, 1909 – July 23, 2001) was an American short story writer, novelist and photographer who wrote about the American South. Her novel ''The Optimist's Daughter'' won the Pulitzer Prize in 1973. Welty received numero ...
, to whose life
Mary Chapin Carpenter Mary may refer to: People * Mary (name), a feminine given name (includes a list of people with the name) Religious contexts * New Testament people named Mary, overview article linking to many of those below * Mary, mother of Jesus, also calle ...
dedicated the song "
Halley Came to Jackson ''Shooting Straight in the Dark'' is the third studio album by American country music artist Mary Chapin Carpenter. It was a #11 Country Album on the Billboard Country Albums chart. Four of its tracks became Billboard Hot Country Songs hits: "You ...
". In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that
cyanogen Cyanogen is the chemical compound with the formula ( C N)2. It is a colorless and highly toxic gas with a pungent odor. The molecule is a pseudohalogen. Cyanogen molecules consist of two CN groups – analogous to diatomic halogen molecule ...
in the tail might poison millions, whereas the appearance of Comet Hale–Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. In
science fiction Science fiction (sometimes shortened to Sci-Fi or SF) is a genre of speculative fiction which typically deals with imaginative and futuristic concepts such as advanced science and technology, space exploration, time travel, parallel unive ...
, the impact of comets has been depicted as a threat overcome by technology and heroism (as in the 1998 films '' Deep Impact'' and ''
Armageddon According to the Book of Revelation in the New Testament of the Christian Bible, Armageddon (, from grc, Ἁρμαγεδών ''Harmagedōn'', Late Latin: , from Hebrew: ''Har Məgīddō'') is the prophesied location of a gathering of armies ...
''), or as a trigger of global apocalypse (''
Lucifer's Hammer ''Lucifer's Hammer'' is a science fiction apocalyptic and post-apocalyptic science fiction, post-apocalypse-survival novel by American writers Larry Niven and Jerry Pournelle that was first published in 1977. It was nominated for the Hugo Award f ...
'', 1979) or zombies (''
Night of the Comet ''Night of the Comet'' is a 1984 American science fiction comedy horror film written and directed by Thom Eberhardt. It stars Catherine Mary Stewart, Robert Beltran, and Kelli Maroney as survivors of a comet that has turned most people into ei ...
'', 1984). In
Jules Verne Jules Gabriel Verne (;''Longman Pronunciation Dictionary''. ; 8 February 1828 – 24 March 1905) was a French novelist, poet, and playwright. His collaboration with the publisher Pierre-Jules Hetzel led to the creation of the ''Voyages extraor ...
's ''
Off on a Comet ''Off on a Comet'' (french: Hector Servadac) is an 1877 science fiction novel by French writer Jules Verne. Plot The story starts with a comet called Gallia, that touches the Earth in its flight and collects a few small chunks of it. The disas ...
'' a group of people are stranded on a comet orbiting the Sun, while a large crewed space expedition visits Halley's Comet in Sir Arthur C. Clarke's novel '' 2061: Odyssey Three''.


In Literature

The long-period comet first recorded by Pons in Florence on 15 July 1825 inspired
Lydia Sigourney Lydia Huntley Sigourney (September 1, 1791 – June 10, 1865), ''née'' Lydia Howard Huntley, was an American poet, author, and publisher during the early and mid 19th century. She was commonly known as the "Sweet Singer of Hartford." She had a ...
's humorous poem in which all the celestial bodies argue over the comet's appearance and purpose.


Gallery

File:Comet_C2020F3_NEOWISE_over_California_desert_landscape.png, Comet C/2020 F3 NEOWISE File:Comet P1 McNaught02 - 23-01-07-edited.jpg, Comet
C/2006 P1 Comet McNaught, also known as the Great Comet of 2007 and given the designation C/2006 P1, is a non-periodic comet discovered on 7 August 2006 by British-Australian astronomer Robert H. McNaught using the Uppsala Southern Schmidt Telescope. ...
(McNaught) taken from Victoria, Australia 2007 File:Great Comet of 1882.jpg, The
Great Comet of 1882 The Great Comet of 1882 formally designated C/1882 R1, 1882 II, and 1882b, was a comet which became very bright in September 1882. It was a member of the Kreutz Sungrazers, a family of comets which pass within of the Sun's photosphere at perih ...
is a member of the Kreutz group File:Great Comet 1861.jpg, Great Comet 1861 File:X-rays from Hyakutake.jpg,
Comet Hyakutake Comet Hyakutake (, formally designated C/1996 B2) is a comet, discovered on 31 January 1996, that passed very close to Earth in March of that year. It was dubbed the Great Comet of 1996; its passage near the Earth was one of the closest com ...
(
X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10  picometers to 10  nanometers, corresponding to frequencies in the range 30&nb ...
,
ROSAT ROSAT (short for Röntgensatellit; in German X-rays are called Röntgenstrahlen, in honour of Wilhelm Röntgen) was a German Aerospace Center-led satellite X-ray telescope, with instruments built by West Germany, the United Kingdom and the Uni ...
satellite) File:Asteroid P2013 P5 v2.jpg, "Active asteroid" 311P/PANSTARRS with several tails File:NASA-14090-Comet-C2013A1-SidingSpring-Hubble-20140311.jpg, Comet Siding Spring ( Hubble; 11 March 2014) File:Comets WISE.jpg, Mosaic of 20 comets discovered by the WISE space telescope File:PIA22419-Neowise-1stFourYearsDataFromDec2013-20180420.gif,
NEOWISE Wide-field Infrared Survey Explorer (WISE, observatory code C51, Explorer 92 and SMEX-6) is a NASA infrared astronomy space telescope in the Explorers Program. It was launched in December 2009, and placed in hibernation mode in February 2011, ...
– first four years of data starting in December 2013 File:Lovejoy-hi1a srem dec12 14.gif,
C/2011 W3 (Lovejoy) Comet Lovejoy, formally designated C/2011 W3 (Lovejoy), is a long-period comet and Kreutz sungrazer. It was discovered in November 2011 by Australian amateur astronomer Terry Lovejoy. The comet's perihelion took it through the Sun's corona on ...
heads towards the Sun File:ITS Impact.gif, View from the impactor in its last moments before hitting Comet
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 photograph ...
during the ''Deep Impact'' mission
;Videos File:NASA Developing Comet Harpoon for Sample Return.ogv, NASA is developing a comet harpoon for returning samples to Earth File:Encke tail rip off.ogg,
Comet Encke Comet Encke , or Encke's Comet (official designation: 2P/Encke), is a periodic comet that completes an orbit of the Sun once every 3.3 years. (This is the shortest period of a reasonably bright comet; the faint main-belt comet 311P/PanSTARRS has ...
loses its tail


See also

* ''
The Big Splash ''The Big Splash'' is a 1935 British comedy film directed by Leslie S. Hiscott and starring Frank Pettingell, Finlay Currie and Marguerite Allan. A millionaire hires a man to play his look-alike, double. It was made as a quota quickie at Beaconsf ...
'' *
Comet vintages Comet vintages are years during which an astronomical event, involving generally a "Great Comet", occurs prior to harvest. Throughout the history of wine, winemakers have attributed successful vintages and ideal weather conditions to the unexplaine ...
*
List of impact craters on Earth This list of impact craters on Earth contains a selection of the 190 confirmed craters given in the Earth Impact Database as of 2017. To keep the lists manageable, only the largest craters within a time period are included. Alphabetical lists f ...
*
List of possible impact structures on Earth This is a list of possible impact structures on Earth. More than 130 geophysical features on the surface of the Earth have been proposed as candidate sites for impact events by appearing several times in the literature and/or being endorsed by the ...
*
Lists of comets Non-periodic comets are seen only once. They are usually on near-parabolic orbits that will not return to the vicinity of the Sun for thousands of years, if ever. Periodic comets usually have elongated elliptical orbits, and usually return to th ...


References


Footnotes


Citations


Bibliography

*


Further reading

* *


External links

*
Comets
at NASA's Solar System Exploration
International Comet Quarterly
by Harvard University
Catalogue of the Solar System Small Bodies Orbital Evolution

Science Demos: Make a Comet
by the
National High Magnetic Field Laboratory The National High Magnetic Field Laboratory (MagLab) is a facility at Florida State University, the University of Florida, and Los Alamos National Laboratory in New Mexico, that performs magnetic field research in physics, biology, bioengineering ...

Comets: from myths to reality
exhibition on
Paris Observatory The Paris Observatory (french: Observatoire de Paris ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centers in the world. Its histor ...
digital library {{Authority control Astronomical objects Articles containing video clips Ice Extraterrestrial water Concepts in astronomy