View source for Comet

{{Short description|Natural object in space that releases gas}}
{{About|the astronomical object}}
{{Good article}}
{{Use American English|date=November 2019}}
{{Use dmy dates|date=November 2019}}
{{Infobox astronomical formation|image=File:Comet Hale-Bopp 1995O1.jpg|caption=[[Comet Hale–Bopp]] after passing [[perihelion]] in April 1997 |thing=Small solar system body|where=Star systems|density=0.6 g/cm<sup>3</sup> (average)|qid=Q3559|commonscat=Comets|size=Typically less than 10&nbsp;km wide (nucleus)<ref>{{Cite web |title=Ask an Astronomer |url=https://coolcosmos.ipac.caltech.edu/ask/182-What-is-the-size-of-a-comet-
 |access-date=2023-03-11 |website=Cool Cosmos}}</ref>}}
A '''comet''' is an icy, [[small Solar System body]] that warms and begins to release gases when passing close to the [[Sun]], a process called [[outgassing]]. This produces an extended, gravitationally unbound atmosphere or [[Coma (cometary)|coma]] surrounding the nucleus, and sometimes a [[Comet tail|tail]] of gas and dust gas blown out from the coma. These phenomena are due to the effects of [[solar radiation]] and the outstreaming [[solar wind]] plasma acting upon the nucleus of the comet. [[Comet nuclei]] 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]]. If sufficiently close and bright, a comet may be seen from Earth without the aid of a telescope and can [[Subtended angle|subtend]] an arc of up to 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures and religions.

Comets usually have highly [[Orbital eccentricity|eccentric]] elliptical orbits, and they have a wide range of [[orbital period]]s, ranging from several years to potentially several millions of years. [[#Short period|Short-period comet]]s originate in the [[Kuiper belt]] or its associated [[scattered disc]], which lie beyond the orbit of [[Neptune]]. [[#Long period|Long-period comet]]s are thought to originate in the [[Oort cloud]], a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star.<ref>{{cite book |title=Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe |publisher=Ecco/HarperCollins Publishers |location=New York |first=Lisa |last=Randall |pages=104–105 |date=2015 |isbn=978-0-06-232847-2}}</ref> Long-period comets are set in motion towards the Sun by [[gravitational perturbation]]s from [[List of nearest stars#Future and past|passing stars]] and the [[galactic tide]]. [[Hyperbolic comet]]s may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition.

[[Extinct comets]] that have passed close to the Sun many times have lost nearly all of their [[volatility (chemistry)|volatile]] ices and dust and may come to resemble small asteroids.<ref>{{cite web |title=What is the difference between asteroids and comets |url=http://www.esa.int/Our_Activities/Space_Science/Rosetta/Frequently_asked_questions |work=Rosetta's Frequently Asked Questions |publisher=European Space Agency |access-date=30 July 2013}}</ref> Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.<ref>{{cite web |title=What Are Asteroids And Comets |url=http://neo.jpl.nasa.gov/faq/#ast |archive-url=https://web.archive.org/web/20040628110338/http://neo.jpl.nasa.gov/faq/#ast |url-status=dead |archive-date=28 June 2004 |work=Near Earth Object Program FAQ |publisher=NASA |access-date=30 July 2013}}</ref><ref>{{cite journal |title=Comparison of Comet 81P/Wild 2 Dust with Interplanetary Dust from Comets |last1=Ishii |first1=H. A. |last2=Bradley |first2=J. P. |last3=Dai |first3=Z. R. |last4=Chi |first4=M. |last5=Kearsley |first5=A. T. |last6=Burchell |first6=M. J. |last7=Browning |first7=N. D. |last8=Molster |first8=F. |display-authors=1 |journal=Science |volume=319 |issue=5862 |pages=447–50 |date=2008 |doi=10.1126/science.1150683 |pmid=18218892 |bibcode=2008Sci...319..447I|s2cid=24339399 }}</ref> However, the discovery of [[main-belt comet]]s and active [[centaur (minor planet)|centaur]] minor planets has blurred the [[Asteroid#Terminology|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]]s. They are still classified as comets, such as C/2014 S3 (PANSTARRS).<ref>{{cite web |url=https://ssd.jpl.nasa.gov/sbdb.cgi?orb=1;sstr=2014+S3 |title=JPL Small-Body Database Browser C/2014 S3 (PANSTARRS)}}</ref> Twenty-seven Manx comets were found from 2013 to 2017.<ref>{{cite journal |title=Chasing Manxes: Long-Period Comets Without Tails |journal=AAA/Division for Planetary Sciences Meeting Abstracts |issue=49 |last1=Stephens |first1=Haynes |last2=Meech |first2=Karen Jean |last3=Kleyna |first3=Jan |last4=Keane |first4=Jacqueline |last5=Hainaut |first5=Olivier |last6=Yang |first6=Bin |last7=Wainscoat |first7=Richard J. |last8=Micheli |first8=Marco |last9=Bhatt |first9=Bhuwan |last10=Sahu |first10=Devendra |display-authors=1 |at=420.02 |date=October 2017 |volume=49 |bibcode=2017DPS....4942002S}}</ref>

{{As of|2021|11}}, there are 4,584 known comets.<ref>{{cite web |url=https://minorplanetcenter.net/|title=Comets Discovered |website=Minor Planet Center|access-date=27 April 2021}}</ref> 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]]) is about one trillion.<ref name="Erickson2003">{{cite book |url=https://books.google.com/books?id=lwbivW5YKoYC&pg=PA123 |title=Asteroids, Comets, and Meteorites: Cosmic Invaders of the Earth |series=The Living Earth |publisher=Infobase |location=New York |first=Jon |last=Erickson |page=123 |date=2003 |isbn=978-0-8160-4873-1}}</ref><ref name="Couper2014">{{cite book |url=https://books.google.com/books?id=YXkRBAAAQBAJ&pg=PA222 |title=The Planets: The Definitive Guide to Our Solar System |publisher=Dorling Kindersley |location=London |first1=Heather |last1=Couper |first2=Robert |last2=Dinwiddie |first3=John |last3=Farndon |first4=Nigel |last4=Henbest |first5=David W. |last5=Hughes |first6=Giles |last6=Sparrow |first7=Carole |last7=Stott |first8=Colin |last8=Stuart |display-authors=1 |page=222 |date=2014 |isbn=978-1-4654-3573-6}}</ref> Roughly one comet per year is visible to the [[naked eye]], though many of those are faint and unspectacular.<ref>{{cite journal |doi=10.1006/icar.1998.6048 |title=The Rate of Naked-Eye Comets from 101 BC to 1970 AD |date=1999 |last1=Licht |first1=A. |journal=Icarus |volume=137 |issue=2 |pages=355–356 |bibcode=1999Icar..137..355L}}</ref> Particularly bright examples are called "[[great comet]]s". Comets have been visited by uncrewed probes such as NASA's ''[[Deep Impact (spacecraft)|Deep Impact]]'', which blasted a crater on Comet [[Tempel 1]] to study its interior, and the European Space Agency's ''[[Rosetta (spacecraft)|Rosetta]]'', which became the first to land a robotic spacecraft on a comet.<ref name="Philae">{{cite web |url=http://www.esa.int/Our_Activities/Space_Science/Rosetta/Touchdown!_Rosetta_s_Philae_probe_lands_on_comet |title=Touchdown! Rosetta's Philae Probe Lands on Comet |publisher=European Space Agency |date=12 November 2014 |access-date=11 December 2017}}</ref>

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== Etymology ==
[[File:Anglo-Saxon Chronicle - cometa (British Library Cotton MS Tiberius A VI, folio 10v).jpg|thumb|A comet was mentioned in the ''[[Anglo-Saxon Chronicle]]'' that allegedly made an appearance in 729 AD.]]
The word ''comet'' derives from the [[Old English]] {{lang|ang|cometa}} from the [[Latin]] {{lang|la|comēta}} or {{lang|la|comētēs}}. That, in turn, is a [[Romanization of Greek|romanization]] of the [[Ancient Greek|Greek]] {{lang|grc|κομήτης}} 'wearing long hair', and the ''[[Oxford English Dictionary]]'' notes that the term ({{lang|grc|ἀστὴρ}}) {{lang|grc|κομήτης}} already meant 'long-haired star, comet' in Greek. {{lang|grc|Κομήτης}} was derived from {{lang|grc|κομᾶν}} ({{transliteration|grc|koman}}) 'to wear the hair long', which was itself derived from {{lang|grc|κόμη}} ({{transliteration|grc|komē}}) 'the hair of the head' and was used to mean 'the tail of a comet'.<ref>{{OED|comet}}</ref><ref>{{cite dictionary |url=http://etymonline.com/?term=comet |title=Comet (n.) |dictionary=Online Etymology Dictionary |last=Harper |first=Douglas |access-date=30 July 2013}}</ref>

The [[astronomical symbol]] for comets (represented in [[Unicode]]) is {{Unichar|2604|Comet}}, consisting of a small disc with three hairlike extensions.<ref>{{cite book |url=https://archive.org/stream/encyclopediaame01unkngoog#page/n202/mode/2up |title=The Encyclopedia Americana: A Library of Universal Knowledge |volume=26 |publisher=The Encyclopedia Americana Corp. |pages=162–163 |date=1920}}</ref>

== Physical characteristics ==
[[File:Structure of a comet.jpg|thumb|Structure of a comet]]

=== Nucleus ===
[[File:Comet Hartley 2.jpg|thumb|Nucleus of [[103P/Hartley]] as imaged during a [[Deep Impact (spacecraft)|spacecraft flyby]]. The nucleus is about 2&nbsp;km in length.]]
{{Main|Comet nucleus}}
The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of [[rock (geology)|rock]], [[Comet dust|dust]], [[ice|water ice]], and frozen [[carbon dioxide]], [[carbon monoxide]], [[methane]], and [[ammonia]].<ref>{{cite journal |bibcode=1998A&A...330..375G |title=Making a comet nucleus |last1=Greenberg |first1=J. Mayo |volume=330 |date=1998 |pages=375 |journal=[[Astronomy & Astrophysics]]}}</ref> As such, they are popularly described as "dirty snowballs" after [[Fred Whipple]]'s model.<ref>{{cite web |url=http://starryskies.com/solar_system/Comet/dirty_snowballs.html |title=Dirty Snowballs in Space |publisher=Starryskies |access-date=15 August 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130129035627/http://starryskies.com/solar_system/Comet/dirty_snowballs.html |archive-date=29 January 2013}}</ref> Comets with a higher dust content have been called "icy dirtballs".<ref>{{cite news |url=http://www.timeshighereducation.co.uk/news/evidence-from-esas-rosetta-spacecraft-suggests-that-comets-are-more-icy-dirtball-than-dirty-snowball/199168.article |title=Evidence from ESA's Rosetta Spacecraft Suggests that Comets are more "Icy Dirtball" than "Dirty Snowball" |date=21 October 2005 |work=Times Higher Education}}</ref> The term "icy dirtballs" arose after observation of [[Tempel 1|Comet 9P/Tempel 1]] collision with an "impactor" probe sent by NASA Deep Impact mission in July 2005. Research conducted in 2014 suggests that comets are like "[[Fried ice cream|deep fried ice cream]]", in that their surfaces are formed of dense crystalline ice mixed with [[organic compound]]s, while the interior ice is colder and less dense.<ref name="NASA-20150210" />

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. Nuclei contain a variety of organic compounds, which may include [[methanol]], [[hydrogen cyanide]], [[formaldehyde]], [[ethanol]], [[ethane]], and perhaps more complex molecules such as long-chain [[hydrocarbon]]s and [[amino acid]]s.<ref>{{cite web |last=Meech |first=M. |title=1997 Apparition of Comet Hale–Bopp: What We Can Learn from Bright Comets |url=http://www.psrd.hawaii.edu/Feb97/Bright.html |publisher=Planetary Science Research Discoveries |date=24 March 1997 |access-date=30 April 2013}}</ref><ref>{{cite web |title=Stardust Findings Suggest Comets More Complex Than Thought |url=http://stardust.jpl.nasa.gov/news/news110.html |publisher=NASA |date=14 December 2006 |access-date=31 July 2013}}</ref> In 2009, it was confirmed that the amino acid [[glycine]] had been found in the comet dust recovered by NASA's [[Stardust (spacecraft)|Stardust mission]].<ref>{{cite journal |doi=10.1111/j.1945-5100.2009.tb01224.x |title=Cometary glycine detected in samples returned by Stardust |date=2009 |last1=Elsila |first1=Jamie E. |last2=Glavin |first2=Daniel P. |last3=Dworkin |first3=Jason P. |display-authors=1 |journal=Meteoritics & Planetary Science |volume=44 |issue=9 |pages=1323 |bibcode=2009M&PS...44.1323E|doi-access=free }}</ref> In August 2011, a report, based on [[NASA]] studies of [[meteorite]]s found on Earth, was published suggesting [[DNA]] and [[RNA]] components ([[adenine]], [[guanine]], and related organic molecules) may have been formed on [[asteroid]]s and comets.<ref name="Callahan">{{cite journal |doi=10.1073/pnas.1106493108 |title=Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases |date=2011 |last1=Callahan |first1=M. P. |last2=Smith |first2=K. E. |last3=Cleaves |first3=H. J. |last4=Ruzicka |first4=J. |last5=Stern |first5=J. C. |last6=Glavin |first6=D. P. |last7=House |first7=C. H. |last8=Dworkin |first8=J. P. |display-authors=1 |journal=Proceedings of the National Academy of Sciences |volume=108 |issue=34 |pages=13995–8 |bibcode=2011PNAS..10813995C |pmid=21836052 |pmc=3161613|doi-access=free }}</ref><ref name="Steigerwald">{{cite web |last=Steigerwald |first=John |title=NASA Researchers: DNA Building Blocks Can Be Made in Space |url=http://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html |publisher=NASA |date=8 August 2011 |access-date=31 July 2013 |archive-date=26 April 2020 |archive-url=https://web.archive.org/web/20200426055700/https://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html |url-status=dead }}</ref>

The outer surfaces of cometary nuclei have a very low [[albedo]], making them among the least reflective objects found in the Solar System. The [[Giotto (spacecraft)|Giotto]] [[space probe]] found that the nucleus of [[Halley's Comet]] (1P/Halley) reflects about four percent of the light that falls on it,<ref name="dark">{{cite journal |title=The Activity and Size of the Nucleus of Comet Hale-Bopp (C/1995 O1) |journal=Science |last1=Weaver |first1=H. A. |last2=Feldman |first2=P. D. |last3=a'Hearn |first3=M. F. |last4=Arpigny |first4=C. |last5=Brandt |first5=J. C. |last6=Festou |first6=M. C. |last7=Haken |first7=M. |last8=McPhate |first8=J. B. |last9=Stern |first9=S. A. |last10=Tozzi |first10=G. P. |display-authors=1 |volume=275 |issue=5308 |pages=1900–1904 |date=1997 |pmid=9072959 |doi=10.1126/science.275.5308.1900 |bibcode=1997Sci...275.1900W|s2cid=25489175 }}</ref> and [[Deep Space 1]] discovered that [[19P/Borrelly|Comet Borrelly]]'s surface reflects less than 3.0%;<ref name="dark" /> by comparison, [[Bitumen|asphalt]] reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter [[Volatility (chemistry)|volatile]] [[Chemical compound|compounds]], leaving behind larger organic compounds that tend to be very dark, like [[tar]] or [[petroleum|crude oil]]. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their [[outgassing]] processes.<ref>{{cite book |url=https://books.google.com/books?id=PRqVqQKao9QC&pg=PA91 |page=91 |title=Habitability and Cosmic Catastrophes |isbn=978-3-540-76945-3 |last1=Hanslmeier |first1=Arnold |date=2008|publisher=Springer }}</ref>

Comet nuclei with radii of up to {{convert|30|km|mi|sp=us}} have been observed,<ref>{{cite journal |doi=10.1023/A:1021545031431 |title=The Nucleus of Comet Hale-Bopp (C/1995 O1): Size and Activity |date=2000 |last1=Fernández |first1=Yanga R. |journal=Earth, Moon, and Planets |volume=89 |issue=1 |pages=3–25 |bibcode=2002EM&P...89....3F|s2cid=189899565 }}</ref> but ascertaining their exact size is difficult.<ref>{{cite web |url=http://www2.ess.ucla.edu/~jewitt/nucleus.html | first=David | last=Jewitt |title=The Cometary Nucleus |publisher=Department of Earth and Space Sciences, UCLA |date=April 2003 |access-date=31 July 2013}}</ref> The nucleus of [[322P/SOHO]] is probably only {{convert|100|-|200|m|ft|sp=us}} in diameter.<ref name="soho1">{{cite web |title=SOHO's new catch: its first officially periodic comet |publisher=European Space Agency |url=http://www.esa.int/Our_Activities/Space_Science/SOHO_s_new_catch_its_first_officially_periodic_comet |access-date=16 August 2013}}</ref> 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 {{convert|100|m|ft|sp=us}} across.<ref>{{harvnb|Sagan|Druyan|1997|p=137}}</ref> Known comets have been estimated to have an average density of {{convert|0.6|g/cm3|oz/cuin|abbr=on}}.<ref name="Britt2006">{{cite journal |bibcode=2006LPI....37.2214B |title=Small Body Density and Porosity: New Data, New Insights |last1=Britt |first1=D. T. |last2=Consolmagno |first2=G. J. |last3=Merline |first3=W. J. |display-authors=1 |volume=37 |date=2006 |pages=2214 |journal=37th Annual Lunar and Planetary Science Conference |url=http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2214.pdf |access-date=25 August 2013 |archive-url=https://web.archive.org/web/20081217064607/http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2214.pdf |archive-date=17 December 2008 |url-status=dead}}</ref> Because of their low mass, comet nuclei do not [[gravitational collapse|become spherical]] under their own [[gravity]] and therefore have irregular shapes.<ref>{{cite web |url=https://history.nasa.gov/SP-467/ch7.htm |title=The Geology of Small Bodies |date=January 1984 |publisher=NASA |access-date=15 August 2013 |last1=Veverka |first1=J. }}</ref>

[[File:Comet wild 2.jpg|thumb|220px|Comet [[81P/Wild]] exhibits jets on light side and dark side, stark relief, and is dry.]]
Roughly six percent of the [[near-Earth asteroid]]s are thought to be the [[Extinct comet|extinct nuclei of comets]] that no longer experience outgassing,<ref name="dormant">{{cite journal |doi=10.1016/j.icarus.2006.02.016 |arxiv=astro-ph/0603106v2 |date=2006 |title=The size–frequency distribution of dormant Jupiter family comets |last1=Whitman |first1=K. |last2=Morbidelli |first2=A. |last3=Jedicke |first3=R. |display-authors=1 |journal=Icarus |volume=183 |issue=1 |pages=101–114 |bibcode=2006Icar..183..101W|s2cid=14026673 }}</ref> including [[14827 Hypnos]] and [[3552 Don Quixote]].

Results from the [[Rosetta (spacecraft)|''Rosetta'']] and [[Philae (spacecraft)|''Philae'']] spacecraft show that the nucleus of [[67P/Churyumov–Gerasimenko]] has no magnetic field, which suggests that magnetism may not have played a role in the early formation of [[planetesimal]]s.<ref name="esa20150414">{{cite news |url=http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_and_Philae_find_comet_not_magnetised |title=Rosetta and Philae Find Comet Not Magnetised |publisher=European Space Agency |first=Markus |last=Bauer |date=14 April 2015 |access-date=14 April 2015}}</ref><ref name="nature20150414">{{cite journal |title=Rosetta's comet has no magnetic field |journal=[[Nature (journal)|Nature]] |first=Quirin |last=Schiermeier |date=14 April 2015 |doi=10.1038/nature.2015.17327|s2cid=123964604 }}</ref> Further, the [[Rosetta (spacecraft)#Instruments|ALICE spectrograph]] on ''Rosetta'' determined that [[electron]]s (within {{convert|1|km|mi|abbr=on}} above the [[comet nucleus]]) produced from [[photoionization]] of water molecules by [[Sunlight|solar radiation]], and not [[photon]]s from the Sun as thought earlier, are responsible for the degradation of water and [[carbon dioxide]] [[molecule]]s released from the comet nucleus into its coma.<ref name="NASA-20150602">{{cite web |last1=Agle |first1=D. C. |last2=Brown |first2=Dwayne |last3=Fohn |first3=Joe |last4=Bauer |first4=Markus |display-authors=1 |title=NASA Instrument on Rosetta Makes Comet Atmosphere Discovery |url=http://www.jpl.nasa.gov/news/news.php?feature=4609 |date=2 June 2015 |publisher=[[NASA]] |access-date=2 June 2015}}</ref><ref name="AA-20150602">{{cite journal |last1=Feldman |first1=Paul D. |last2=A'Hearn |first2=Michael F. |last3=Bertaux |first3=Jean-Loup |last4=Feaga |first4=Lori M. |last5=Parker |first5=Joel Wm. |last6=Schindhelm |first6=Eric |last7=Steiffl |first7=Andrew J. |last8=Stern |first8=S. Alan |last9=Weaver |first9=Harold A. |last10=Sierks |first10=Holger |last11=Vincent |first11=Jean-Baptiste |display-authors=1 |title=Measurements of the near-nucleus coma of comet 67P/Churyumov-Gerasimenko with the Alice far-ultraviolet spectrograph on Rosetta |url=http://www.aanda.org/articles/aa/pdf/forth/aa25925-15.pdf |archive-url=https://web.archive.org/web/20150608071334/http://www.aanda.org/articles/aa/pdf/forth/aa25925-15.pdf |archive-date=2015-06-08 |url-status=live |date=2 June 2015 |journal=[[Astronomy & Astrophysics]] |doi=10.1051/0004-6361/201525925 |access-date=3 June 2015 |arxiv=1506.01203 |bibcode=2015A&A...583A...8F |volume=583 |pages=A8|s2cid=119104807 }}</ref> Instruments on the ''Philae'' lander found at least sixteen organic compounds at the comet's surface, four of which ([[acetamide]], [[acetone]], [[methyl isocyanate]] and [[propionaldehyde]]) have been detected for the first time on a comet.<ref name="wapo20150730">{{cite news |url=https://www.washingtonpost.com/world/philae-probe-finds-evidence-that-comets-can-be-cosmic-labs/2015/07/30/63a2fc0e-36e5-11e5-ab7b-6416d97c73c2_story.html |archive-url=https://web.archive.org/web/20181223235109/https://www.washingtonpost.com/world/philae-probe-finds-evidence-that-comets-can-be-cosmic-labs/2015/07/30/63a2fc0e-36e5-11e5-ab7b-6416d97c73c2_story.html |url-status=dead |archive-date=23 December 2018 |title=Philae probe finds evidence that comets can be cosmic labs |newspaper=The Washington Post |agency=Associated Press |first=Frank |last=Jordans |date=30 July 2015 |access-date=30 July 2015}}</ref><ref name="esa20150730">{{cite web |url=http://www.esa.int/Our_Activities/Space_Science/Rosetta/Science_on_the_surface_of_a_comet |title=Science on the Surface of a Comet |publisher=European Space Agency |date=30 July 2015 |access-date=30 July 2015}}</ref><ref name="SCI-20150731">{{cite journal |last1=Bibring |first1=J.-P. |last2=Taylor |first2=M.G.G.T. |last3=Alexander |first3=C. |last4=Auster |first4=U. |last5=Biele |first5=J. |last6=Finzi |first6=A. Ercoli |last7=Goesmann |first7=F. |last8=Klingehoefer |first8=G. |last9=Kofman |first9=W. |last10=Mottola |first10=S. |last11=Seidenstiker |first11=K.J. |last12=Spohn |first12=T. |last13=Wright |first13=I. |display-authors=1 |title=Philae's First Days on the Comet – Introduction to Special Issue |date=31 July 2015 |journal=[[Science (journal)|Science]] |volume=349 |number=6247 |page=493 |doi=10.1126/science.aac5116 |bibcode=2015Sci...349..493B |pmid=26228139|doi-access=free }}</ref>

{| class="wikitable center" style="text-align: center; width: 530px; margin: 0.1em auto;"
|+Properties of some comets
|-
! width="120" | Name
! width="120" | Dimensions<br />(km)
! width="120" | Density<br />([[Gram|g]]/cm<sup>3</sup>)
! width="120" | Mass<br />([[Kilogram|kg]])<ref name="mass">
Halley: Using the [[Volume#Formulas|volume of an ellipsoid]] of 15×8×8&nbsp;km * a [[rubble pile]] density of 0.6&nbsp;g/cm<sup>3</sup> yields a mass (m=d*v) of 3.02E+14&nbsp;kg.<br />
Tempel 1: Using a spherical diameter of 6.25&nbsp;km; [[Volume#Formulas|volume of a sphere]] * a density of 0.62&nbsp;g/cm<sup>3</sup> yields a mass of 7.9E+13&nbsp;kg.<br />
19P/Borrelly: Using the [[Volume#Formulas|volume of an ellipsoid]] of 8x4x4km * a density of 0.3&nbsp;g/cm<sup>3</sup> yields a mass of 2.0E+13&nbsp;kg.<br />
81P/Wild: Using the [[Volume#Formulas|volume of an ellipsoid]] of 5.5x4.0x3.3&nbsp;km * a density of 0.6&nbsp;g/cm<sup>3</sup> yields a mass of 2.28E+13&nbsp;kg.</ref>
!Refs
|-
| align="left" | [[Halley's Comet]]
| 15 × 8 × 8
| 0.6
| 3{{e|14}}
|<ref>{{cite web |url=https://astrosociety.org/file_download/inline/6f0b9235-b5eb-40e6-88a3-6dd45e3b6f92 |title=What Have We Learned About Halley's Comet? |date=1986 |publisher=Astronomical Society of the Pacific |access-date=4 October 2013}}</ref><ref>{{cite journal |title=Is the nucleus of Comet Halley a low density body? |journal=Nature |last1=Sagdeev |first1=R. Z. |last2=Elyasberg |first2=P. E. |last3=Moroz |first3=V. I. |display-authors=1 |volume=331 |issue=6153 |pages=240 |date=1988 |issn=0028-0836 |doi=10.1038/331240a0 |bibcode=1988Natur.331..240S|s2cid=4335780 }}</ref>
|-
| align="left" | [[Tempel 1]]
| 7.6 × 4.9
| 0.62
| 7.9{{e|13}}
|<ref name="Britt2006" /><ref>{{cite web |url=http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=9P |title=9P/Tempel 1 |publisher=JPL |access-date=16 August 2013}}</ref>
|-
| align="left" | [[19P/Borrelly]]
| 8 × 4 × 4
| 0.3
| 2.0{{e|13}}
|<ref name="Britt2006" />
|-
| align="left" | [[81P/Wild]]
| 5.5 × 4.0 × 3.3
| 0.6
| 2.3{{e|13}}
|<ref name="Britt2006" /><ref name="wild2">{{cite web |title=Comet 81P/Wild 2 |publisher=The Planetary Society |url=http://www.planetary.org/explore/topics/asteroids_and_comets/wild2.html |access-date=20 November 2007 |url-status=dead |archive-url=https://web.archive.org/web/20090106004009/http://planetary.org./explore/topics/asteroids_and_comets/wild2.html |archive-date=6 January 2009 }}</ref>
|-
| align="left" | [[67P/Churyumov–Gerasimenko]]
| 4.1 × 3.3 × 1.8
| 0.47
| 1.0{{e|13}}
|<ref>{{cite web |url=http://www.esa.int/spaceinimages/Images/2015/01/Comet_vital_statistics |title=Comet vital statistics |publisher=European Space Agency |date=22 January 2015 |access-date=24 January 2015}}</ref><ref>{{cite web |url=http://blogs.esa.int/rosetta/2014/08/21/determining-the-mass-of-comet-67pc-g/ |title=Determining the mass of comet 67P/C-G |publisher=European Space Agency |first=Emily |last=Baldwin |date=21 August 2014 |access-date=21 August 2014}}</ref>
|}

=== Coma ===
{{Main|Coma (cometary)}}

[[File:Hubble's Last Look at Comet ISON Before Perihelion.jpg|thumb|[[Hubble Space Telescope|Hubble]] image of [[Comet ISON]] shortly before [[perihelion]].<ref>{{cite web |url=http://www.spacetelescope.org/images/opo1347a/ |title=Hubble's Last Look at Comet ISON Before Perihelion |publisher=European Space Agency |date=19 November 2013 |access-date=20 November 2013}}</ref>]]

[[File:Comet borrelly.jpg|thumb|left|[[19P/Borrelly|Comet Borrelly]] exhibits jets, but has no surface ice.]]

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]] and [[solar wind]] cause an enormous "tail" to form pointing away from the Sun.<ref>{{cite book |url=https://books.google.com/books?id=4zjv84hHNPcC&pg=PA66 |title=A Complete Manual of Amateur Astronomy: Tools and Techniques for Astronomical Observations |last1=Clay Sherrod |first1=P.  |last2=Koed |first2=Thomas L. |name-list-style=amp |page=66 |date=2003 |publisher=Courier Corporation |isbn=978-0-486-15216-5}}</ref>

The coma is generally made of water and dust, with water making up to 90% of the [[Volatile (astrogeology)|volatiles]] that outflow from the nucleus when the comet is within 3 to 4 [[astronomical unit]]s (450,000,000 to 600,000,000&nbsp;km; 280,000,000 to 370,000,000&nbsp;mi) of the Sun.<ref name=Combi2004>{{cite book |url=http://www.lpi.usra.edu/books/CometsII/7023.pdf |archive-url=https://web.archive.org/web/20070315183630/http://www.lpi.usra.edu/books/CometsII/7023.pdf |archive-date=2007-03-15 |url-status=live |title=Gas dynamics and kinetics in the cometary coma: Theory and observations |journal=Comets II |last1=Combi |first1=Michael R. |last2=Harris |first2=Walter M. |last3=Smyth |first3=William H. |display-authors=1 |pages=523 |date=2004 |doi=10.2307/j.ctv1v7zdq5.34 |bibcode=2004come.book..523C}}</ref> The {{H2O}} parent molecule is destroyed primarily through [[photodissociation]] and to a much smaller extent [[photoionization]], with the solar wind playing a minor role in the destruction of water compared to [[photochemistry]].<ref name=Combi2004/> 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 [[radiation pressure|light pressure]].<ref>{{cite web |url=http://migall.fastmail.fm/astronomy/solar_system/small_bodies/hale_bop/jpl/define.htm |title=Comet Definitions |publisher=Michael Gallagher |last=Morris |first=Charles S. |access-date=31 August 2013}}</ref>

Although the solid nucleus of comets is generally less than {{convert|60|km|mi|sp=us}} across, the coma may be thousands or millions of kilometers across, sometimes becoming larger than the Sun.<ref>{{cite journal |doi=10.1023/A:1021512317744 |bibcode=2002EM&P...90...67L |date=2002 |first1=Rosine |last1=Lallement |last2=Bertaux |first2=Jean-Loup |last3=Szegö |first3=Karöly |last4=Nemeth |first4=Szilvia |display-authors=1 |journal=Earth, Moon, and Planets |volume=90 |pages=67–76 |title=The Shadow of Comet Hale–Bopp in Lyman-Alpha|issue=1 |s2cid=118200399 }}</ref> For example, about a month after an outburst in October 2007, comet [[17P/Holmes]] briefly had a tenuous dust atmosphere larger than the Sun.<ref name=atmosphere2>{{cite web |author-link=David C. Jewitt |last=Jewitt |first=David |url=http://www2.ess.ucla.edu/~jewitt/holmes.html |title=The Splintering of Comet 17P/Holmes During a Mega-Outburst |publisher=University of Hawaii |access-date=30 August 2013}}</ref> The [[Great Comet of 1811]] had a coma roughly the diameter of the Sun.<ref name="primer">{{cite web |title=The Comet Primer |work=Gary W. Kronk's Cometography |last=Kronk |first=Gary W. |url=http://cometography.com/educate/comintro.html |access-date=30 August 2013 |url-status=dead |archive-url=https://web.archive.org/web/20110517043903/http://cometography.com/educate/comintro.html |archive-date=17 May 2011 }}</ref> Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of [[Mars]] around {{convert|1.5|AU}} from the Sun.<ref name="primer"/> 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.<ref name="primer"/> Ion tails have been observed to extend one astronomical unit (150&nbsp;million km) or more.<ref name=atmosphere2/>

[[File:PIA20119-CometChristensen-C2006W3-CO2-WISE-20100420.jpg|thumb|C/2006 W3 (Christensen) emitting carbon gas (IR image)]]
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]]isation.<ref name="le">{{cite web |url=http://www.le.ac.uk/ph/faulkes/web/planets/r_pl_comets.html |title=Comets |publisher=University of Leicester |last1=Brinkworth |first1=Carolyn |last2=Thomas |first2=Claire |name-list-style=amp |access-date=31 July 2013}}</ref> Most comets are too faint to be visible without the aid of a [[telescope]], but a few each decade become bright enough to be visible to the naked eye.<ref>{{cite book |url=https://books.google.com/books?id=caYpAQAAMAAJ |page=75 |title=A field guide to the stars and planets |isbn=978-0-395-93432-6 |last=Pasachoff |first=Jay M |date=2000|publisher=Houghton Mifflin }}</ref> 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 [[17P/Holmes|Comet Holmes]].<ref name=atmosphere1>{{cite web |last=Jewitt |first=David |url=http://www2.ess.ucla.edu/~jewitt/holmes.html |title=Comet Holmes Bigger Than The Sun |publisher=Institute for Astronomy at the University of Hawaii |access-date=31 July 2013}}</ref>

In 1996, comets were found to emit [[X-ray]]s.<ref>{{cite journal |doi=10.1126/science.274.5285.205 |title=Discovery of X-ray and Extreme Ultraviolet Emission from Comet C/Hyakutake 1996 B2 |date=1996 |last1=Lisse |first1=C. M. |last2=Dennerl |first2=K. |last3=Englhauser |first3=J. |last4=Harden |first4=M. |last5=Marshall |first5=F. E. |last6=Mumma |first6=M. J. |last7=Petre |first7=R. |last8=Pye |first8=J. P. |last9=Ricketts |first9=M. J. |display-authors=1 |journal=Science |volume=274 |issue=5285 |pages=205 |last10=Schmitt |first10=J. |last11=Trumper |first11=J. |last12=West |first12=R. G. |bibcode=1996Sci...274..205L |s2cid=122700701 |url=https://zenodo.org/record/1231082}}</ref> This greatly surprised astronomers because X-ray emission is usually associated with very [[black-body radiation|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]] photons.<ref>{{cite journal |title=Charge Exchange-Induced X-Ray Emission from Comet C/1999 S4 (LINEAR) |journal=Science |last1=Lisse |first1=C. M. |last2=Christian |first2=D. J. |last3=Dennerl |first3=K. |last4=Meech |first4=K. J. |last5=Petre |first5=R. |last6=Weaver |first6=H. A. |last7=Wolk |first7=S. J. |display-authors=1 |volume=292 |issue=5520 |pages=1343–8 |date=2001 |doi=10.1126/science.292.5520.1343 |bibcode=2001Sci...292.1343L |pmid=11359004}}</ref>

===Bow shock===
[[Bow shock]]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]],<ref>{{cite journal |title=The Bow wave of Comet Giacobini-Zinner – ICE magnetic field observations |journal=Geophysical Research Letters |last1=Jones |first1=D. E. |last2=Smith |first2=E. J. |last3=Slavin |first3=J. A. |last4=Tsurutani |first4=B. T. |last5=Siscoe |first5=G. L. |last6=Mendis |first6=D. A. |display-authors=1 |volume=13 |issue=3 |pages=243–246 |date=March 1986 |bibcode=1986GeoRL..13..243J |doi=10.1029/GL013i003p00243}}</ref> 1P/Halley,<ref>{{cite journal |title=First in situ plasma and neutral gas measurements at comet Halley |journal=Nature |first1=K. I.|last1=Gringauz|first2= T. I. |last2=Gombosi |first3=A. P. |last3=Remizov |first4=I. |last4=Apáthy |first5=I. |last5=Szemerey |first6=M. I. |last6=Verigin |first7=L. I. |last7=Denchikova |first8=A. V. |last8=Dyachkov |first9=E. |last9=Keppler |first10=I. N. |last10=Klimenko |first11=A. K. |last11=Richter |first12=A. J. |last12=Somogyi |first13=K. |last13=Szegő |first14=S. |last14=Szendrő |first15=M. |last15=Tátrallyay |first16=A. |last16=Varga |first17= G. A. |last17=Vladimirova |display-authors=1 |volume=321 |pages=282–285 |date=15 May 1986 |bibcode=1986Natur.321..282G |doi=10.1038/321282a0|s2cid=117920356 }}</ref> and [[26P/Grigg–Skjellerup]].<ref>{{cite journal |title=First results from the Giotto magnetometer experiment during the P/Grigg-Skjellerup encounter |journal=[[Astronomy & Astrophysics]] |first1=F. M. |last1=Neubauer |first2=H. |last2=Marschall |first3=M. |last3=Pohl |first4=K.-H. |last4=Glassmeier |first5=G. |last5=Musmann |first6=F. |last6=Mariani |first7=M. H. |last7=Acuna |first8=L. F. |last8=Burlaga |first9=N. F. |last9=Ness |first10=M. K. |last10=Wallis |first11=H. U. |last11=Schmidt |first12=E. |last12=Ungstrup |display-authors=1 |volume=268 |issue=2 |pages=L5–L8 |date=February 1993 |bibcode=1993A&A...268L...5N}}</ref> 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]] when the bow shocks already were fully developed.

The [[Rosetta (spacecraft)|''Rosetta'']] spacecraft observed the bow shock at comet [[67P/Churyumov–Gerasimenko]] 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.<ref>{{cite journal |title=The infant bow shock: a new frontier at a weak activity comet |journal=[[Astronomy & Astrophysics]] |last1=Gunell |first1=H. |last2=Goetz |first2=C. |last3=Simon Wedlund |first3=C. |last4=Lindkvist |first4=J. |last5=Hamrin |first5=M. |last6=Nilsson |first6=H. |last7=LLera |first7=K. |last8=Eriksson |first8=A. |last9=Holmström |first9=M. |display-authors=1 |volume=619 |at=L2 |date=November 2018 |doi=10.1051/0004-6361/201834225 |bibcode=2018A&A...619L...2G |url=https://www.duo.uio.no/bitstream/10852/67125/1/aa34225-18.pdf |archive-url=https://web.archive.org/web/20190430061032/https://www.duo.uio.no/bitstream/10852/67125/1/aa34225-18.pdf |archive-date=2019-04-30 |url-status=live|doi-access=free }}</ref>

=== Tails ===
{{Main|Comet tail}}
[[File:Cometorbit01.svg|thumb|left|280px|Typical direction of tails during a comet's orbit near the Sun]]
In the outer [[Solar System]], 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]] have been reported from observations by the [[Hubble Space Telescope]]<ref name="Cochran1995">{{cite journal |bibcode=1995ApJ...455..342C |title=The Discovery of Halley-sized Kuiper Belt Objects Using the Hubble Space Telescope |last1=Cochran |first1=Anita L. |last2=Levison |first2=Harold F. |last3=Stern |first3=S. Alan |last4=Duncan |first4=Martin J. |display-authors=1 |volume=455 |date=1995 |pages=342 |journal=The Astrophysical Journal |doi=10.1086/176581 |arxiv=astro-ph/9509100|s2cid=118159645 }}</ref><ref name="Cochran1998">{{cite journal |doi=10.1086/311515 |title=The Calibration of the Hubble Space Telescope Kuiper Belt Object Search:Setting the Record Straight |date=1998 |last1=Cochran |first1=Anita L. |last2=Levison |first2=Harold F. |last3=Tamblyn |first3=Peter |last4=Stern |first4=S. Alan |last5=Duncan |first5=Martin J. |display-authors=1 |journal=The Astrophysical Journal |volume=503 |issue=1 |pages=L89 |arxiv=astro-ph/9806210 |bibcode=1998ApJ...503L..89C|s2cid=18215327 }}</ref> but these detections have been questioned.<ref name="Brown1997">{{cite journal |doi=10.1086/311009 |title=An Analysis of the Statistics of the \ITAL Hubble Space Telescope\/ITAL] Kuiper Belt Object Search |date=1997 |last1=Brown |first1=Michael E. |last2=Kulkarni |first2=Shrinivas R. |last3=Liggett |first3=Timothy J. |display-authors=1 |journal=The Astrophysical Journal |volume=490 |issue=1 |pages=L119–L122 |bibcode=1997ApJ...490L.119B|doi-access=free }}</ref><ref name="Jewitt1996">{{cite journal |bibcode=1996AJ....112.1225J |title=The Mauna Kea-Cerro-Tololo (MKCT) Kuiper Belt and Centaur Survey |last1=Jewitt |first1=David |last2=Luu |first2=Jane |last3=Chen |first3=Jun |display-authors=1 |volume=112 |date=1996 |pages=1225 |journal=The Astronomical Journal |doi=10.1086/118093}}</ref> As a comet approaches the inner Solar System, [[solar radiation]] 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.<ref name="le"/> 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.<ref>{{cite book |url=https://books.google.com/books?id=S4xDhVCxAQIC&pg=PA422 |page=422 |title=The Cambridge Guide to the Solar System |isbn=978-1-139-49417-5 |last=Lang |first=Kenneth R. |date=2011|publisher=Cambridge University Press }}</ref> On occasions—such as when Earth passes through a comet's orbital plane, the [[antitail]], pointing in the opposite direction to the ion and dust tails, may be seen.<ref>{{Cite APOD |title=PanSTARRS: The Anti Tail Comet |date=29 June 2013 |access-date=31 July 2013}}</ref>

[[File:Comet Parts.svg|thumb|upright|Diagram of a comet showing the [[Antitail|dust trail]], the dust tail, and the ion gas tail formed by [[solar wind]].]]
The observation of antitails contributed significantly to the discovery of solar wind.<ref>{{cite journal |doi=10.1007/BF00225271 |title=The plasma tails of comets and the interplanetary plasma |date=1963 |last1=Biermann |first1=L. |journal=Space Science Reviews |volume=1 |issue=3 |page=553 |bibcode=1963SSRv....1..553B|s2cid=120731934 }}</ref> 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]]" 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]] 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.<ref name="pp 864">{{cite book |title=An Introduction to Modern Astrophysics |publisher=Addison-Wesley |last1=Carroll |first1=B. W. |last2=Ostlie |first2=D. A. |name-list-style=amp |pages=864–874 |date=1996 |isbn=0-201-54730-9}}</ref>

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]] occurs. This leads to a "tail disconnection event".<ref name="pp 864" /> 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]] was completely severed while the comet passed through a [[coronal mass ejection]]. This event was observed by the [[STEREO|STEREO space probe]].<ref>{{cite journal |title=The Heliospheric Imagers Onboard the STEREO Mission |journal=Solar Physics |last1=Eyles |first1=C. J. |last2=Harrison |first2=R. A. |last3=Davis |first3=C. J. |last4=Waltham |first4=N. R. |last5=Shaughnessy |first5=B. M. |last6=Mapson-Menard |first6=H. C. A. |last7=Bewsher |first7=D. |last8=Crothers |first8=S. R. |last9=Davies |first9=J. A. |last10=Simnett |first10=G. M. |last11=Howard |first11=R. A. |last12=Moses |first12=J. D. |last13=Newmark |first13=J. S. |last14=Socker |first14=D. G. |last15=Halain |first15=J.-P. |last16=Defise |first16=J.-M. |last17=Mazy |first17=E. |last18=Rochus |first18=P. |display-authors=1 |volume=254 |issue=2 |pages=387 |date=2008 |doi=10.1007/s11207-008-9299-0 |bibcode=2009SoPh..254..387E |hdl=2268/15675 |s2cid=54977854 |url=https://orbi.uliege.be/bitstream/2268/15675/1/The%20Heliospheric%20Imagers%20Onboard%20the%20STEREO.pdf |archive-url=https://web.archive.org/web/20180722165455/https://orbi.uliege.be/bitstream/2268/15675/1/The%20Heliospheric%20Imagers%20Onboard%20the%20STEREO.pdf |archive-date=2018-07-22 |url-status=live}}</ref>

In 2013, [[ESA]] scientists reported that the [[ionosphere]] of the planet [[Venus]] streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions."<ref name="ESA-20130129">{{cite web |title=When A Planet Behaves Like A Comet |url=http://www.esa.int/Our_Activities/Space_Science/When_a_planet_behaves_like_a_comet |date=29 January 2013 |publisher=European Space Agency |access-date=30 August 2013}}</ref><ref name="Space-20130130">{{cite web |last=Kramer |first=Miriam |title=Venus Can Have 'Comet-Like' Atmosphere |url=http://www.space.com/19537-venus-comet-atmosphere.html |date=30 January 2013 |publisher=Space.com |access-date=30 August 2013}}</ref>

=== Jets ===
[[File:Hartley2jets2 epoxi big.jpg|thumb|170px|Gas and snow jets of [[103P/Hartley]]]]
Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser.<ref name=jets>{{cite web |url=http://hubblesite.org/hubble_discoveries/comet_ison/blogs/comets-and-jets |title=Comets and Jets |work=Hubblesite.org |date=12 November 2013}}</ref> These streams of gas and dust can cause the nucleus to spin, and even split apart.<ref name=jets/> In 2010 it was revealed that [[Sublimation (phase transition)|sublimation]] of [[dry ice]] (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus.<ref>{{cite news |url=http://www.astronomynow.com/news/n1011/11hartley/ |title=Dry ice fuels comet jets |work=Astronomy Now |first=Emily |last=Baldwin |date=11 November 2010 |archive-url=https://web.archive.org/web/20131217034053/http://www.astronomynow.com/news/n1011/11hartley/ |archive-date=17 December 2013}}</ref> Infrared imaging of Hartley&nbsp;2 shows such jets exiting and carrying with it dust grains into the coma.<ref>{{cite news |url=https://www.nytimes.com/2010/11/19/science/space/19comet.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2010/11/19/science/space/19comet.html |archive-date=2022-01-01 |url-access=limited |title=Comet Hartley 2 Is Spewing Ice, NASA Photos Show |work=The New York Times |last1=Chang |first1=Kenneth |date=18 November 2010}}{{cbignore}}</ref>

== Orbital characteristics ==

Most comets are [[small Solar System bodies]] with elongated [[elliptical orbit]]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.<ref>{{cite web |url=http://www.st-andrews.ac.uk/~bds2/ltsn/ljm/JAVA/COMETORB/COMET.HTM |title=The Orbit of a Comet |publisher=University of St Andrews |access-date=1 September 2013}}</ref> Comets are often classified according to the length of their [[orbital period]]s: The longer the period the more elongated the ellipse.

=== {{anchor|Short-period comet}} Short period ===
{{Main|List of numbered comets|List of Halley-type comets}}

[[List of periodic comets|Periodic comets]] or short-period comets are generally defined as those having orbital periods of less than 200 years.<ref>{{cite journal |title=The origin of short-period comets |journal=The Astrophysical Journal Letters |first1=Martin |last1=Duncan |first2=Thomas |last2=Quinn |first3=Scott |last3=Tremaine |display-authors=1 |volume=328 |pages=L69–L73 |date=May 1988 |doi=10.1086/185162 |bibcode=1988ApJ...328L..69D|doi-access=free }}</ref> They usually orbit more-or-less in the [[ecliptic]] plane in the same direction as the planets.<ref>{{cite book |url=https://books.google.com/books?id=Ox5hCOc9A2AC&pg=PA117 |page=117 |title=Our Cosmic Origins: From the Big Bang to the Emergence of Life and Intelligence |isbn=978-0-521-79480-0 |last=Delsemme |first=Armand H. |date=2001|publisher=Cambridge University Press }}</ref> Their orbits typically take them out to the region of the outer planets ([[Jupiter]] and beyond) at [[aphelion]]; for example, the aphelion of Halley's Comet is a little beyond the orbit of [[Neptune]]. Comets whose aphelia are near a major planet's orbit are called its "family".<ref name=Wilson1909>{{cite journal |last=Wilson |first=H. C. |title=The Comet Families of Saturn, Uranus and Neptune |journal=Popular Astronomy |volume=17 |pages=629–633 |date=1909 |bibcode=1909PA.....17..629W}}</ref> Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.<ref>{{cite web |url=http://www.uwgb.edu/dutchs/PLANETS/Comets.HTM |title=Comets |first=Steven |last=Dutch |publisher=Natural and Applied Sciences, University of Wisconsin |access-date=31 July 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130729122906/http://www.uwgb.edu/dutchs/PLANETS/Comets.HTM |archive-date=29 July 2013}}</ref>

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).<ref>{{cite web |url=http://www.dtm.ciw.edu/users/sheppard/satellites/jf.html |title=The Jupiter Family Comets |publisher=Department of Terrestrial Magnetism Carnegie Institution of Washington |access-date=11 August 2013}}</ref><ref name="britastro">{{cite web |url=http://www.britastro.org/projectalcock/Comets%20where%20are%20they.htm |title=Comets – where are they ? |date=6 November 2012 |publisher=British Astronomical Association |access-date=11 August 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130805211248/http://www.britastro.org/projectalcock/Comets%20where%20are%20they.htm |archive-date=5 August 2013}}</ref> 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).<ref name="Morbidelli2006">{{cite journal |doi=10.1007/s11214-008-9405-5 |title=Dynamical Origin of Comets and Their Reservoirs |date=2008 |last1=Duncan |first1=Martin J. |journal=Space Science Reviews |volume=138 |issue=1–4 |pages=109–126 |bibcode=2008SSRv..138..109D|s2cid=121848873 }}</ref><ref name=jewitt2002>{{Cite journal |doi=10.1086/338692 |title=From Kuiper Belt Object to Cometary Nucleus: The Missing Ultrared Matter |date=2002 |last1=Jewitt |first1=David C. |journal=The Astronomical Journal |volume=123 |issue=2 |pages=1039–1049 |bibcode=2002AJ....123.1039J|s2cid=122240711 |doi-access=free }}</ref> {{As of| January 2025}} there are 73 known Encke-type comets (six of which are classified as [[Near-earth objects]] (NEOs)), 106 HTCs (36 of which are NEOs), and 815 JFCs (153 of which are NEOs).<ref name="SSD-JPL">{{cite web |title=Small-Body Database Query |url=https://ssd.jpl.nasa.gov/tools/sbdb_query.html#!#results |website=Solar System Dynamics - Jet Propulsion Laboratory |publisher=NASA - California Institute of Technology |access-date=2025-01-28}}</ref>

Recently discovered [[main-belt comets]] form a distinct class, orbiting in more circular orbits within the [[asteroid belt]].<ref name="NYT-20221118">{{cite news |last=Andrews |first=Robin George |title=The Mysterious Comets That Hide in the Asteroid Belt - Comets normally fly in from the far reaches of space. Yet astronomers have found them seemingly misplaced in the asteroid belt. Why are they there? |url=https://www.nytimes.com/2022/11/18/science/comet-asteroid-belt-space.html |date=18 November 2022 |work=[[The New York Times]] |accessdate=18 November 2022 }}</ref><ref>{{cite web |last=Reddy |first=Francis |title=New comet class in Earth's backyard |url=http://www.astronomy.com/sitecore/content/Home/News-Observing/News/2006/04/New%20comet%20class%20in%20Earths%20backyard.aspx?sc_lang=en |work=Astronomy |date=3 April 2006 |access-date=31 July 2013 |archive-date=24 May 2014 |archive-url=https://web.archive.org/web/20140524023305/http://www.astronomy.com/sitecore/content/Home/News-Observing/News/2006/04/New%20comet%20class%20in%20Earths%20backyard.aspx?sc_lang=en |url-status=dead }}</ref>

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further [[Perturbation (astronomy)|gravitational perturbations]].<ref>{{cite web |url=https://www.e-education.psu.edu/astro801/content/l11_p9.html |title=Comets |publisher=The Pennsylvania State University |access-date=8 August 2013}}</ref> Short-period comets have a tendency for their aphelia to coincide with a [[giant planet]]'s semi-major axis, with the JFCs being the largest group.<ref name="britastro"/> It is clear that comets coming in from the [[Oort cloud]] 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.<ref>{{harvnb|Sagan|Druyan|1997|pp=102–104}}</ref><ref>{{cite book |url=https://books.google.com/books?id=w7E_uwj0Lc8C&pg=PA246 |page=246 |title=In Quest of the Solar System |isbn=978-0-7637-9477-4 |last=Koupelis |first=Theo |date=2010|publisher=Jones & Bartlett Publishers }}</ref>

Based on their orbital characteristics, short-period comets are thought to originate from the [[Centaur (minor planet)|centaurs]] and the Kuiper belt/[[scattered disc]]<ref>{{cite web |last=Davidsson |first=Björn J. R. |title=Comets – Relics from the birth of the Solar System |url=http://www.astro.uu.se/~bjorn/eng_comet.html |publisher=Uppsala University |date=2008 |access-date=30 July 2013 |archive-url=https://web.archive.org/web/20130119065421/http://www.astro.uu.se/~bjorn/eng_comet.html |archive-date=19 January 2013 |url-status=dead }}</ref> —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]] who hypothesized its existence).<ref>{{cite journal |bibcode=1950BAN....11...91O |title=The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin |last1=Oort |first1=J. H. |volume=11 |date=1950 |pages=91 |journal=Bulletin of the Astronomical Institutes of the Netherlands}}</ref> 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.<ref>{{cite book |url=https://books.google.com/books?id=PRqVqQKao9QC&pg=PA152 |page=152 |title=Habitability and Cosmic Catastrophes |isbn=978-3-540-76945-3 |last=Hanslmeier |first=Arnold |date=2008|publisher=Springer }}</ref> 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.<ref>{{Cite web|url=http://planetfacts.org/short-period-comet/|title=What is A Short Period Comet – Less than 200 Year Orbital Cycle|last=Rocheleau|first=Jake|date=2011-09-12|website=Planet Facts|language=en|access-date=2019-12-01}}</ref>

=== Long period ===
{{See also|List of long-period comets|List of near-parabolic comets|List of hyperbolic comets}}
[[File:Comet Kohoutek orbit p391.svg|thumb|Orbits of [[Comet Kohoutek]] (red) and Earth (blue), illustrating the high [[Orbital eccentricity|eccentricity]] of its orbit and its rapid motion when close to the Sun.]]

Long-period comets have highly [[Orbital eccentricity|eccentric]] orbits and periods ranging from 200 years to thousands or even millions of years.<ref name="SBP"/> An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System.<ref name=Elenin2011>{{cite web |url=http://spaceobs.org/en/2011/03/07/vliyanie-planet-gigantov-na-orbitu-komety-c2010-x1-elenin/ |title=Influence of giant planets on the orbit of comet C/2010 X1 |first=Leonid |last=Elenin |date=7 March 2011 |access-date=11 August 2013 |archive-date=19 March 2012 |archive-url=https://web.archive.org/web/20120319014011/http://spaceobs.org/en/2011/03/07/vliyanie-planet-gigantov-na-orbitu-komety-c2010-x1-elenin/ |url-status=dead }}</ref> For example, [[C/2006 P1|Comet McNaught]] had a heliocentric osculating eccentricity of 1.000019 near its perihelion passage [[Epoch (astronomy)|epoch]] in January 2007 but is bound to the Sun with roughly a 92,600-year orbit because the [[Orbital eccentricity|eccentricity]] 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]] is computed at an epoch after leaving the planetary region and is calculated with respect to the [[Barycentric coordinates (astronomy)|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 {{convert|70000|AU|pc ly|abbr=on}} with orbital periods estimated around 6&nbsp;million years.

Single-apparition or non-periodic comets are similar to long-period comets because they have [[parabolic trajectory|parabolic]] or slightly [[hyperbolic trajectory|hyperbolic]] trajectories<ref name="SBP">{{cite web |title=Small Bodies: Profile |url=http://pds.jpl.nasa.gov/planets/special/smbod.htm |publisher=NASA/JPL |date=29 October 2008 |access-date=11 August 2013}}</ref> 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]] which allows them to permanently exit the Solar System after a single pass of the Sun.<ref>{{cite book |url=https://books.google.com/books?id=3K9Fhu2q-8gC&pg=PA21 |title=Astronomy and Astrophysics |last1=Joardar |first1=S. |last2=Bhattacharya |first2=A. B. |last3=Bhattacharya |first3=R. |display-authors=1 |page=21 |date=2008 |publisher=Jones & Bartlett Learning |isbn=978-0-7637-7786-9}}</ref> The Sun's [[Hill sphere]] has an [[unstable]] maximum boundary of {{convert|230000|AU|pc ly|abbr=on}}.<ref name=Chebotarev1964>{{cite journal |bibcode=1964SvA.....7..618C |title=Gravitational Spheres of the Major Planets, Moon and Sun |last1=Chebotarev |first1=G. A. |volume=7 |date=1964 |pages=618 |journal=Soviet Astronomy}}</ref> Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion<ref name="e1">{{cite web |title=JPL Small-Body Database Search Engine: e > 1 |publisher=JPL |url=http://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=all;obj_kind=all;obj_numbered=all;OBJ_field=0;ORB_field=0;c1_group=ORB;c1_item=Bg;c1_op=%3E;c1_value=1;table_format=HTML;max_rows=100;format_option=comp;c_fields=AcBgBiBjBqChCk;.cgifields=format_option;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=com_orbit_class&query=1&c_sort=BgD |access-date=13 August 2013}}</ref> that using a heliocentric unperturbed [[Two-body problem|two-body]] [[curve fitting|best-fit]] suggests they may escape the Solar System.

{{As of|2022}}, only two objects have been discovered with an [[Eccentricity (mathematics)|eccentricity]] significantly greater than one: [[ʻOumuamua|1I/ʻOumuamua]] and [[2I/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]]—indicate that it is probably a comet.<ref name="Oumuamua">{{cite news |url=https://www.space.com/41015-interstellar-visitor-oumuamua-comet-after-all.html |title=Interstellar Visitor 'Oumuamua Is a Comet After All |work=Space.com |first=Chelsea |last=Gohd |date=27 June 2018 |access-date=27 September 2018}}</ref> 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 object|interstellar comet]].<ref>{{Cite news |url=https://www.sciencenews.org/article/astronomy-interstellar-comet-space |title=Astronomers have spotted a second interstellar object |work=Science News |last=Grossman |first=Lisa |date=12 September 2019 |access-date=16 September 2019}}</ref><ref name="cnn">{{cite news |url=https://www.cnn.com/2019/09/24/world/second-interstellar-visitor-confirmed-scn-trnd/ |title=2nd interstellar visitor to our solar system confirmed and named |publisher=CNN |last1=Strickland |first1=Ashley |date=27 September 2019}}</ref> Comet [[C/1980 E1]] had an orbital period of roughly 7.1&nbsp;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.<ref name="C/1980E1-jpl">{{cite web |url=http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=1980E1 |title=C/1980 E1 (Bowell) |work=[[JPL Small-Body Database]] |type=1986-12-02 last obs |access-date=13 August 2013}}</ref> 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/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),<ref>{{cite encyclopedia |title=Comet |url=https://www.britannica.com/eb/article-54344/comet |encyclopedia=[[Encyclopædia Britannica Online]] |access-date=13 August 2013}}</ref> whereas others use it to mean exclusively short-period comets.<ref name="SBP"/> 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 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]] 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]] resulting in a positive [[hyperbolic excess velocity|velocity at infinity]] (<math>v_{\infty}\!</math>) 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 [[Order of magnitude|magnitude]].<ref>{{cite journal |title=On the nondetection of extrasolar comets |journal=The Astrophysical Journal |last1=McGlynn |first1=Thomas A. |last2=Chapman |first2=Robert D. |name-list-style=amp |volume=346 |at=L105 |date=1989 |doi=10.1086/185590 |bibcode=1989ApJ...346L.105M|doi-access=free }}</ref>

{| class="wikitable" style="width: 600px; text-align: center; font-size: 1em; margin: 0.4em auto;"
|-
|+ [[Hyperbolic comet]] discoveries<ref name="e1-name">{{cite web |title=JPL Small-Body Database Search Engine: e > 1 (sorted by name) |publisher=JPL |url=https://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=all;obj_kind=all;obj_numbered=all;OBJ_field=0;ORB_field=0;c1_group=ORB;c1_item=Bg;c1_op=%3E;c1_value=1;table_format=HTML;max_rows=100;format_option=comp;c_fields=AcBgBiBjBqCiCkCn;.cgifields=format_option;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=com_orbit_class&query=1&c_sort=AcD |access-date=7 December 2020}}</ref>
|-
! Year !! 2007 !! 2008 !! 2009 !! 2010 !! 2011 !! 2012 !! 2013 !! 2014 !! 2015 !! 2016 !! 2017 !! 2018 !! 2019 !! 2020
|-
! Number
| 12 || 7 || 8 || 4 || 13 || 10 || 16 || 9 || 16 || 5 || 18 || 10 || 15 || 17
|}

=== Oort cloud and Hills cloud ===
[[File:Small objects in the Solar System ESA25188647.jpg|thumb|upright=1.5|The [[Oort cloud]] thought to surround the Solar System. Showed with Kuiper Belt and Asteroid Belt for comparison.]]
{{Main|Oort cloud|Hills cloud}}
The Oort cloud is thought to occupy a vast space starting from between {{convert|2000|and|5000|AU|ly|2|abbr=on}}<ref name=book>{{cite book |chapter=Comet Populations and Cometary Dynamics |title=Encyclopedia of the Solar System |publisher=Academic Press |first1=Harold F. |last1=Levison |first2=Luke |last2=Donnes |name-list-style=amp |editor1-first=Lucy-Ann Adams |editor1-last=McFadden |editor2-first=Torrence V. |editor2-last=Johnson |editor3-first=Paul Robert |editor3-last=Weissman |edition=2nd |pages=[https://archive.org/details/encyclopediaofso0000unse_u6d1/page/575 575–588] |date=2007 |isbn=978-0-12-088589-3 |chapter-url=https://archive.org/details/encyclopediaofso0000unse_u6d1/page/575 }}</ref> to as far as {{convert|50000|AU|ly|2|abbr=on}}<ref name=Morbidelli2006/> from the Sun. This cloud encases the celestial bodies that start at the middle of the 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 Solar System's planets 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.<ref>{{Cite web|url=https://solarsystem.nasa.gov/solar-system/oort-cloud/in-depth|title=In Depth {{!}} Oort Cloud|website=NASA Solar System Exploration|access-date=2019-12-01}}</ref> Some estimates place the outer edge at between {{convert|100000|and|200000|AU|ly|2|abbr=on}}.<ref name= book /> The region can be subdivided into a spherical outer Oort cloud of {{convert|20000|-|50000|AU|ly|2|abbr=on}}, and a doughnut-shaped inner cloud, the Hills cloud, of {{convert|2000|-|20000|AU|ly|2|abbr=on}}.<ref>{{cite book |first=Lisa |last=Randall |title=Dark matter and the dinosaurs: The astounding interconnectedness of the universe |date=2015 |publisher=Harper Collins Publishers |isbn=978-0-06-232847-2 |pages=115}}</ref> 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]].<ref name=Morbidelli2006/> The inner Oort cloud is also known as the Hills cloud, named after [[Jack G. Hills]], who proposed its existence in 1981.<ref name="hills1981" /> Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo;<ref name="hills1981">{{cite journal |first=Jack G. |last=Hills |date=1981 |title=Comet showers and the steady-state infall of comets from the Oort Cloud |journal=[[The Astronomical Journal]] |volume=86 |pages=1730–1740 |bibcode=1981AJ.....86.1730H |doi=10.1086/113058|doi-access=free }}</ref><ref name="levison2001">{{cite journal |title=The Origin of Halley-Type Comets: Probing the Inner Oort Cloud |journal=[[The Astronomical Journal]] |first1=Harold F. |last1=Levison |first2=Luke |last2=Dones |first3=Martin J. |last3=Duncan |display-authors=1 |volume=121 |issue=4 |pages=2253–2267 |date=2001 |bibcode=2001AJ....121.2253L |doi=10.1086/319943|doi-access=free }}</ref><ref name="Donahue1991">{{cite book |editor-first=Thomas M. | editor-last=Donahue |others=Trivers, Kathleen Kearney and Abramson, David M. |date=1991 |title=Planetary Sciences: American and Soviet Research, Proceedings from the U.S.–U.S.S.R. Workshop on Planetary Sciences |url=http://books.nap.edu/openbook.php?record_id=1790&page=R1 |publisher=National Academy Press |page=251 |isbn=0-309-04333-6 |access-date=18 March 2008|doi=10.17226/1790 }}</ref> 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.<ref name="Julio1997">{{cite journal |first=Julio A. |last=Fernéndez |date=1997 |title=The Formation of the Oort Cloud and the Primitive Galactic Environment |url=http://www.gps.caltech.edu/classes/ge133/reading/oort.pdf |journal=[[Icarus (journal)|Icarus]] |volume=219 |issue=1 |pages=106–119 |access-date=18 March 2008 |bibcode=1997Icar..129..106F |doi=10.1006/icar.1997.5754 |archive-date=24 July 2012 |archive-url=https://web.archive.org/web/20120724192955/http://www.gps.caltech.edu/classes/ge133/reading/oort.pdf |url-status=dead }}</ref>

=== Exocomets ===
{{Main|Exocomet}}
[[Exocomet]]s beyond the Solar System have been detected and may be common in the [[Milky Way]].<ref name="berk">{{cite web |title=Exocomets may be as common as exoplanets |url=http://newscenter.berkeley.edu/2013/01/07/exocomets-may-be-as-common-as-exoplanets/ |date=7 January 2013 |publisher=UC Berkeley |access-date=30 July 2013 |last=Sanders |first=Robert}}</ref> The first exocomet system detected was around [[Beta Pictoris]], a very young [[A-type main-sequence star]], in 1987.<ref name="Space-20130107">{{cite web |title='Exocomets' Common Across Milky Way Galaxy |url=http://www.space.com/19156-exocomets-alien-solar-systems.html |date=7 January 2013 |publisher=Space.com |access-date=8 January 2013 |url-status=dead |archive-url=https://web.archive.org/web/20140916085824/http://www.space.com/19156-exocomets-alien-solar-systems.html |archive-date=16 September 2014 }}</ref><ref name="Beust1990">{{cite journal |bibcode=1990A&A...236..202B |title=The Beta Pictoris circumstellar disk. X – Numerical simulations of infalling evaporating bodies |last1=Beust |first1=H. |last2=Lagrange-Henri |first2=A.M. |last3=Vidal-Madjar |first3=A. |last4=Ferlet |first4=R. |display-authors=1 |volume=236 |date=1990 |pages=202–216 |journal=[[Astronomy & Astrophysics]] |issn=0004-6361}}</ref> A total of 11 such exocomet systems have been identified {{as of|lc=y|2013}}, using the [[Absorption spectroscopy|absorption spectrum]] caused by the large clouds of gas emitted by comets when passing close to their star.<ref name="berk"/><ref name="Space-20130107" /> For ten years the [[Kepler space telescope]] 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 science|citizen scientists]] in light curves recorded by the Kepler Space Telescope.<ref>{{Cite web|url=https://www.newsweek.com/comets-detected-outside-our-solar-system-first-time-696446|title=Astronomers have detected comets outside our solar system for the first time ever|first=Meghan|last=Bartels|date=2017-10-30|website=Newsweek|language=en|access-date=2019-12-01}}</ref><ref>{{Cite journal|last1=Rappaport|first1=S.|last2=Vanderburg|first2=A.|last3=Jacobs|first3=T.|last4=LaCourse|first4=D.|last5=Jenkins|first5=J.|last6=Kraus|first6=A.|last7=Rizzuto|first7=A.|last8=Latham|first8=D. W.|last9=Bieryla|first9=A.|last10=Lazarevic|first10=M.|last11=Schmitt|first11=A.|date=2018-02-21|title=Likely transiting exocomets detected by Kepler|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=474|issue=2|pages=1453–1468|arxiv=1708.06069|doi=10.1093/mnras/stx2735|doi-access=free |pmid=29755143|pmc=5943639|bibcode=2018MNRAS.474.1453R|issn=0035-8711}}</ref> 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.<ref>{{Cite web|url=https://astronomy.com/news/2019/04/tess-spots-its-first-exocomet-around-one-of-the-skys-brightest-stars|title=TESS spots its first exocomet around one of the sky's brightest stars|first=Jake|last=Parks|date=April 3, 2019|website=Astronomy.com|access-date=2019-11-25}}</ref><ref>{{Cite journal|last1=Zieba|first1=S.|last2=Zwintz|first2=K.|last3=Kenworthy|first3=M. A.|last4=Kennedy|first4=G. M.|date=2019-05-01|title=Transiting exocomets detected in broadband light by TESS in the β Pictoris system|journal=Astronomy & Astrophysics|language=en|volume=625|pages=L13|arxiv=1903.11071|doi=10.1051/0004-6361/201935552|bibcode=2019A&A...625L..13Z|s2cid=85529617|issn=0004-6361}}</ref> 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.<ref>{{Cite web|url=https://www.sciencealert.com/nasa-s-new-planet-hunter-has-detected-its-first-exocomet-orbiting-an-alien-star|title=NASA's New Planet Hunter Has Detected an 'Exocomet' Orbiting an Alien Star|last=Starr|first=Michelle|website=ScienceAlert|date=2 April 2019 |language=en-gb|access-date=2019-12-01}}</ref>

== Effects of comets ==
[[File:PSM V18 D201 Shower of perseids sept 6 and 7.jpg|thumb|Diagram of [[Perseids|Perseid meteors]]]]

=== Connection to meteor showers ===
As a comet is heated during close passes to the Sun, [[outgassing]] of its icy components releases solid debris too large to be swept away by [[radiation pressure]] and the solar wind.<ref>{{harvnb|Sagan|Druyan|1997|p=235}}</ref> 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]] 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.<ref name="sciam19990920">{{cite web |url=https://www.scientificamerican.com/article/what-causes-a-meteor-show/ |title=What causes a meteor shower? |work=[[Scientific American]] |first=Gregory A. |last=Lyzenga |date=20 September 1999 |access-date=21 November 2019}}</ref><ref name="natgeo20190207">{{cite web |url=https://www.nationalgeographic.com/science/space/reference/meteor-showers/ |archive-url=https://web.archive.org/web/20190507121359/https://www.nationalgeographic.com/science/space/reference/meteor-showers/ |url-status=dead |archive-date=7 May 2019 |title=Meteor showers, explained |work=[[National Geographic]] |first=Victoria |last=Jaggard |date=7 February 2019 |access-date=21 November 2019}}</ref> The [[Perseids|Perseid meteor shower]], for example, occurs every year between 9 and 13 August, when Earth passes through the orbit of [[Comet Swift–Tuttle]]. Halley's Comet is the source of the [[Orionids|Orionid shower]] in October.<ref name=showers>{{cite web |url=http://meteorshowersonline.com/major_meteor_showers.html |title=Major Meteor Showers |publisher=Meteor Showers Online |access-date=31 July 2013 |archive-url=https://web.archive.org/web/20130724111133/http://meteorshowersonline.com/major_meteor_showers.html |archive-date=24 July 2013 |url-status=dead}}</ref><ref>{{cite web |url=https://www.weather.gov/fsd/meteor |title=Meteors and Meteor Showers |publisher=United States National Weather Service |access-date=21 November 2019}}</ref>

=== 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&nbsp;billion years ago brought the [[Origin of water on Earth|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.<ref>{{cite web |last=Muir |first=Hazel |title=Earth's water brewed at home, not in space |url=https://www.newscientist.com/article/dn12693 |work=New Scientist |date=25 September 2007 |access-date=30 August 2013}}</ref> The detection of organic molecules, including [[polycyclic aromatic hydrocarbon]]s,<ref name="NASA-20150210">{{cite web |last=Clavin |first=Whitney |title=Why Comets Are Like Deep Fried Ice Cream |url=http://www.jpl.nasa.gov/news/news.php?feature=4480 |date=10 February 2015 |publisher=[[NASA]] |access-date=10 February 2015}}</ref> in significant quantities in comets has led to speculation that comets or [[meteorite]]s may have brought the precursors of life—or even life itself—to Earth.<ref>{{cite book |url=https://books.google.com/books?id=Dfn0VoICrBYC&pg=PA315 |page=315 |title=Comets |isbn=978-1-4020-3495-4 |last=Fernández |first=Julio A. |date=2006|publisher=Springer }}</ref> In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the [[amino acid]]s that make up [[protein]]s through [[shock synthesis]].<ref>{{cite journal |doi=10.1038/ngeo1930 |title=Shock synthesis of amino acids from impacting cometary and icy planet surface analogues |date=2013 |last1=Martins |first1=Zita |last2=Price |first2=Mark C. |last3=Goldman |first3=Nir |last4=Sephton |first4=Mark A. |last5=Burchell |first5=Mark J. |display-authors=1 |journal=Nature Geoscience |bibcode=2013NatGe...6.1045M |volume=6 |issue=12 |pages=1045–1049}}</ref> 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.<ref>{{Cite web|url=https://www.astrobio.net/also-in-news/did-comet-impacts-jump-start-life-on-earth/|title=Did comet impacts jump-start life on Earth?|date=2019-10-18|website=Astrobiology Magazine|language=en-US|access-date=2019-12-01 |archive-url=https://web.archive.org/web/20210308143059/https://www.astrobio.net/also-in-news/did-comet-impacts-jump-start-life-on-earth/ |archive-date=2021-03-08 |url-status=usurped}}</ref> 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.<ref>Oregonian (29 Oct 2015), "Comet's oxygen shakes theories on solar system", p. A5</ref>

It is suspected that comet impacts have, over long timescales, delivered significant quantities of water to Earth's [[Moon]], some of which may have survived as [[lunar ice]].<ref>{{cite web |url=http://lunarscience.nasa.gov/articles/water-discovered-in-apollo-moon-rocks-likely-came-from-comets/ |title=Water Discovered in Apollo Moon Rocks Likely Came from Comets |access-date=7 September 2013 |publisher=NASA}}</ref> Comet and [[meteoroid]] impacts are thought to be responsible for the existence of [[tektite]]s and [[australite]]s.<ref>{{cite web |url=http://museumvictoria.com.au/discoverycentre/infosheets/australites/ |archive-url=https://web.archive.org/web/20080726071448/http://museumvictoria.com.au/DiscoveryCentre/Infosheets/Australites/ |url-status=dead |archive-date=26 July 2008 |title=Australites |publisher=Museum Victoria |access-date=7 September 2013}}</ref>

===Fear of comets===
Fear of comets as [[act of God|acts of God]] and signs of impending doom was highest in Europe from AD 1200 to 1650.{{r|ley196710}} The year after the [[Great Comet of 1618]], for example, [[Gotthard Arthusius]] published a pamphlet stating that it was a sign that the [[Day of Judgment]] was near.<ref>{{cite book |url=https://gallica.bnf.fr/ark:/12148/btv1b2600275q |title=Cometa orientalis: Kurtze vnd eygentliche Beschreibung deß newen Cometen, so im November deß abgelauffenen 1618 |via=Gallica.fr |publisher=Sigismund Latomus |location=[[Frankfurt on Main|Franckfurt-am-Mayn]] |first=Gothard |last=Arthusius |author-link=Gotthard Arthusius |date=1619}}</ref> 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".{{r|ley196710}}

By 1700 most scholars concluded that such events occurred whether a comet was seen or not. Using [[Edmond Halley]]'s records of comet sightings, however, [[William Whiston]] in 1711 wrote that the [[Great Comet of 1680]] had a periodicity of 574 years and was responsible for the [[Genesis flood narrative|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.<ref name="ley196710">{{Cite magazine |url=https://archive.org/stream/Galaxy_v26n01_1967-10_modified#page/n83/mode/2up |title=The Worst of All the Comets |department=For Your Information |magazine=[[Galaxy Science Fiction]] |last=Ley |first=Willy |volume=26 |issue=1 |pages=96–105 |date=October 1967}}</ref> Spectroscopic analysis in 1910 found the toxic gas [[cyanogen]] in the tail of Halley's Comet,<ref>{{Cite news |url=https://www.nytimes.com/1910/02/08/archives/comets-poisonous-tail-yerkes-observatory-finds-cyanogen-in-spectrum.html |title=Yerkes Observatory Finds Cyanogen in Spectrum of Halley's Comet |work=[[The New York Times]] |date=8 February 1910 |access-date=8 January 2018}}</ref> causing panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public.<ref>{{Cite news |url=https://www.universetoday.com/40778/interesting-facts-about-comets/ |title=Interesting Facts About Comets |work=Universe Today |first=Jerry |last=Coffey |date=20 September 2009 |access-date=8 January 2018}}</ref>

== 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 [[Perturbation (astronomy)|interacting]] with another object in the Solar System, such as Jupiter.<ref>{{cite journal |bibcode=1991JBAA..101..119H |title=On hyperbolic comets |last1=Hughes |first1=D. W. |volume=101 |date=1991 |pages=119 |journal=Journal of the British Astronomical Association}}</ref> An example of this is Comet [[C/1980 E1]], which was shifted from an orbit of 7.1&nbsp;million years around the Sun, to a hyperbolic trajectory, after a 1980 close pass by the planet Jupiter.<ref name=barycenter>{{cite web |author=[[JPL Horizons On-Line Ephemeris System|Horizons]] output |url=http://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=sb&sstr=C/1980+E1 |title=Barycentric Osculating Orbital Elements for Comet C/1980 E1 |access-date=9 March 2011}} (Solution using the Solar System [[Center of mass#Barycenter in astrophysics and astronomy|Barycenter]] and [[Barycentric coordinates (astronomy)|barycentric coordinates]]. Select Ephemeris Type:Elements and Center:@0)</ref> Interstellar comets such as [['Oumuamua|1I/ʻOumuamua]] and [[2I/Borisov]] never orbited the Sun and therefore do not require a 3rd-body interaction to be ejected from the Solar System.

=== Extinction ===
{{Main|Extinct comet}}

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.<ref name=dormant/> 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.<ref>{{cite web |last=Lyzenga |first=Greg |title=If comets melt, why do they seem to last for long periods of time |url=http://www.scientificamerican.com/article.cfm?id=if-comets-melt-why-do-the |work=[[Scientific American]] |date=16 November 1998 |access-date=13 August 2013}}</ref> Some asteroids in elliptical orbits are now identified as extinct comets.<ref>{{cite journal |url=http://www.boulder.swri.edu/~hal/PDF/asteroids3.pdf |archive-url=https://web.archive.org/web/20040202155133/http://www.boulder.swri.edu/~hal/PDF/asteroids3.pdf |archive-date=2004-02-02 |url-status=live |title=Evolution of Comets into Asteroids |journal=Asteroids III |last1=Bottke |first1=William F. Jr. |last2=Levison |first2=Harold F. |name-list-style=amp |page=669 |date=2002 |bibcode=2002aste.book..669W}}</ref><ref>{{cite journal |title=Are the IRAS-detected Apollo asteroids extinct comets? |journal=Monthly Notices of the Royal Astronomical Society |first=J. K. |last=Davies |volume=221 |pages=19P–23P |date=July 1986 |doi=10.1093/mnras/221.1.19P |bibcode=1986MNRAS.221P..19D|doi-access=free }}</ref><ref>{{cite book |chapter=The Comet-Asteroid Transition: Recent Telescopic Observations |title=Asteroids, Comets, Meteors 1993: Proceedings of the 160th Symposium of the International Astronomical Union, Held in Belgirate, Italy, June 14–18, 1993 |volume=160 |publisher=Springer |first=L. A. |last=McFadden |editor1-first=Andrea |editor1-last=Milani |editor2-first=Michel |editor2-last=Di Martino |editor3-first=A. |editor3-last=Cellino |page=95 |date=1994 |bibcode=1994IAUS..160...95M}}</ref><ref>{{cite journal |title=The enigmatic object 2201 Oljato: Is it an asteroid or an evolved comet? |journal=Journal of Geophysical Research |first1=L. A. |last1=McFadden |first2=A. L. |last2=Cochran |first3=E. S. |last3=Barker |first4=D. P. |last4=Cruikshank |first5=W. K. |last5=Hartmann |display-authors=1 |volume=98 |issue=E2 |pages=3031–3041 |date=February 1993 |doi=10.1029/92JE01895 |bibcode=1993JGR....98.3031M}}</ref> Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.<ref name=dormant/>

=== Breakup and collisions ===

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart.<ref>{{cite news |url=http://news.bbc.co.uk/1/hi/sci/tech/2153650.stm |title=Astronomers see comet break-up |date=26 July 2002 |work=BBC News |last=Whitehouse |first=David}}</ref> A significant cometary disruption was that of [[Comet Shoemaker–Levy 9]], 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.<ref>{{cite web |last=Kronk |first=Gary W. |title=D/1993 F2 Shoemaker–Levy 9 |url=http://cometography.com/pcomets/1993f2.html |work=Gary W. Kronk's Cometography |access-date=27 April 2009 |url-status=dead |archive-url=https://web.archive.org/web/20080509145539/http://cometography.com/pcomets/1993f2.html |archive-date=9 May 2008 }}</ref><ref>{{cite web |url=http://www2.jpl.nasa.gov/sl9/background.html |title=Comet Shoemaker–Levy Background |publisher=JPL |access-date=23 September 2013}}</ref> Other splitting comets include [[3D/Biela]] in 1846 and [[73P/Schwassmann–Wachmann]] from 1995 to 2006.<ref name=spitzer2006>{{cite web |date=10 May 2006 |title=Spitzer Telescope Sees Trail of Comet Crumbs |last=Whitney |first=Clavin |url=http://www.spitzer.caltech.edu/news/239-ssc2006-13-Spitzer-Telescope-Sees-Trail-of-Comet-Crumbs |access-date=16 August 2013}}</ref> Greek historian [[Ephorus]] reported that a comet split apart as far back as the winter of 372–373 BC.<ref name="great">{{cite web |title=Great Comets in History |first=Donald K. |last=Yeomans |publisher=JPL |url=http://ssd.jpl.nasa.gov/?great_comets |date=April 2007 |access-date=16 August 2013}}</ref> Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.<ref name=split>{{cite book |bibcode=2004come.book..301B |url=http://www.lpi.usra.edu/books/CometsII/7011.pdf |archive-url=https://web.archive.org/web/20090318153712/http://www.lpi.usra.edu/books/CometsII/7011.pdf |archive-date=2009-03-18 |url-status=live |title=Split comets |last1=Boehnhardt |first1=H. |date=2004 |pages=301 |journal=Comets II|doi=10.2307/j.ctv1v7zdq5.25 }}</ref>

Comets [[42P/Neujmin]] and [[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.<ref name="DPS35">{{cite journal |bibcode=2003DPS....35.4705P |title=Are Comets 42P/Neujmin 3 and 53P/Van Biesbroeck Parts of one Comet? |last1=Pittichova |first1=Jand |last2=Meech |first2=Karen J. |last3=Valsecchi |first3=Giovanni B. |last4=Pittich |first4=Eduard M. |display-authors=1 |volume=35 |date=2003 |pages=1011 |journal=Bulletin of the American Astronomical Society}}</ref> Another group of comets that is the result of fragmentation episodes is the Liller comet family made of C/1988 A1 (Liller), C/1996 Q1 (Tabur), C/2015 F3 (SWAN), C/2019 Y1 (ATLAS), and [[C/2023 V5 (Leonard)]].<ref name="Sekanina-2016">{{Cite journal
  |first1     =  Zdenek |last1 = Sekanina
  |first2     =  Rainer |last2 =  Kracht
  |date       = 1 May 2016
  |title      = Pairs and Groups of Genetically Related Long-period Comets and Proposed Identity of the Mysterious Lick Object of 1921
  |journal    = [[The Astrophysical Journal]]
  |volume     = 823
  |issue      = 1
  |pages      = 2 (26 pages)
  |bibcode    = 2016ApJ...823....2S 
  |doi        = 10.3847/0004-637X/823/1/2
  |arxiv      = 1510.06445 
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  |first1     = Carlos |last1 = de la Fuente Marcos
  |first2     = Raúl |last2 = de la Fuente Marcos
  |date       = 27 November 2023
  |title      = Second-generation Fragments of a Comet Split in the Making: The Liller Family Comets
  |journal    = Research Notes of the American Astronomical Society
  |volume     = 7
  |issue      = 11
  |pages      = 249 (3 pages)
  |bibcode    = 2023RNAAS...7..249D 
  |doi        = 10.3847/2515-5172/ad0f27
  |arxiv      = 
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Some comets have been observed to break up during their perihelion passage, including great comets [[Comet West|West]] and [[Comet Ikeya–Seki|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]] were seen in 1872 and 1885 when the comet should have been visible. A minor meteor shower, the [[Andromedids]], occurs annually in November, and it is caused when Earth crosses the orbit of Biela's Comet.<ref>{{cite web |url=http://meteorshowersonline.com/showers/andromedids.html |title=The Andromedids |publisher=Meteor Showers Online |access-date=27 April 2009 |archive-url=https://web.archive.org/web/20130122144223/http://meteorshowersonline.com/showers/andromedids.html |archive-date=22 January 2013 |url-status=dead}}</ref>

Some comets meet a more spectacular end – either falling into the Sun<ref>{{cite web |title=SOHO analyses a kamikaze comet |url=http://sci.esa.int/soho/26188-soho-analyses-a-kamikaze-comet/ |publisher=European Space Agency |date=23 February 2001 |access-date=30 August 2013}}</ref> or colliding with 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.<ref>{{cite web |url=http://nssdc.gsfc.nasa.gov/planetary/comet.html |title=Comet Shoemaker–Levy 9 Collision with Jupiter |access-date=30 August 2013 |publisher=National Space Science Data Center}}</ref>

{{multiple image
| direction         = horizontal
| align             = center
| total_width       = 800
| image1            = Jupiter showing SL9 impact sites.jpg
| image2            = Schwassman-Wachmann3-B-HST.gif
| image3            = C2015D1-SOHO.jpg
| image4            = 14060-Asteroid-P2013R3-Disintegration-20140306.jpg
| caption1          = Brown spots mark impact sites of [[Comet Shoemaker–Levy 9]] on Jupiter
| caption2          = The break up of [[73P/Schwassmann–Wachmann]] within three days (1995)
| caption3          = Ghost tail of C/2015 D1 (SOHO) after passage at the Sun
| caption4          = Disintegration of [[Main-belt comet#Activity|P/2013 R3]] (2014)<ref name="NASA-20140306">{{cite web |url=http://www.nasa.gov/press/2014/march/nasas-hubble-telescope-witnesses-asteroids-mysterious-disintegration-1 |title=Release 14-060: NASA's Hubble Telescope Witnesses Asteroid's Mysterious Disintegration |publisher=[[NASA]] |last1=Harrington |first1=J.D. |last2=Villard |first2=Ray |name-list-style=amp |date=6 March 2014 |access-date=6 March 2014}}</ref>
}}

{{clear}}

== Nomenclature ==
{{Main|Naming of comets}}
[[File:Halley's Comet, 1910.JPG|thumb|[[Halley's Comet]] in 1910]]

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]]", and the "[[Great January Comet of 1910]]".

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]].<ref>{{cite web |last=Ridpath |first=Ian |author-link=Ian Ridpath |title=Halley and his Comet |url=http://www.ianridpath.com/halley/halley4.htm |work=A brief history of Halley's Comet |date=3 July 2008 |access-date=14 August 2013}}</ref> Similarly, the second and third known periodic comets, [[Encke's Comet]]<ref name="KronkEncke">{{cite web |last=Kronk |first=Gary W. |title=2P/Encke |url=http://cometography.com/pcomets/002p.html |work=Gary W. Kronk's Cometography |access-date=14 August 2013}}</ref> and [[Biela's Comet]],<ref name="KronkBiela">{{cite web |last=Kronk |first=Gary W. |title=3D/Biela |url=http://cometography.com/pcomets/003d.html |work=Gary W. Kronk's Cometography |access-date=14 August 2013}}</ref> 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.<ref name="harvcom"/>

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.<ref name="harvcom">{{cite web |url=http://www.icq.eps.harvard.edu/cometnames.html |title=Comet Names and Designations; Comet Naming and Nomenclature; Names of Comets |publisher=Harvard University |access-date=7 September 2013}}</ref> For example, in 2019, astronomer [[Gennadiy Borisov]] observed a comet that appeared to have originated outside of the solar system; the comet was named [[2I/Borisov]] after him.<ref name=Christensen2019>{{cite press release|last1=Christensen |first1=Lars Lindberg|title=Naming of new interstellar visitor: 2I/Borisov|website=International Astronomical Union|url=https://www.iau.org/news/pressreleases/detail/iau1910/|access-date=24 September 2019}}</ref>

== History of study ==
{{Main|Observational history of comets}}

=== Early observations and thought ===
[[File:Bayeux Tapestry scene32 Halley comet.jpg|thumb|235px|[[Halley's Comet]] appeared in 1066, prior to the [[Battle of Hastings]], and is depicted in the [[Bayeux Tapestry]].]]
[[File:Tychonis Brahe De mundi aetherei recentioribus phaenomenis liber secundus.png|thumb|Page from a treatise by [[Tycho Brahe]] depicting his [[geocentric]] view of the [[Great Comet of 1577]]]]

From ancient sources, such as Chinese [[oracle bone]]s, it is known that comets have been noticed by humans for millennia.<ref>{{cite web |url=http://www.lib.cam.ac.uk/mulu/oracle.html |title=Chinese Oracle Bones |publisher=Cambridge University Library |access-date=14 August 2013 |archive-url=https://web.archive.org/web/20131005100532/http://www.lib.cam.ac.uk/mulu/oracle.html |archive-date=5 October 2013 |url-status=dead}}</ref> Until the sixteenth century, comets were usually considered bad [[omen]]s of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants.<ref>{{cite web|last=Ridpath|first=Ian|author-link=Ian Ridpath|date=8 July 2008|title=Comet lore|url=http://www.ianridpath.com/halley/halley1.htm|access-date=14 August 2013|work=A brief history of Halley's Comet}}</ref><ref>{{harvnb|Sagan|Druyan|1997|p=14}}</ref>

[[Aristotle]] (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]] 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.<ref>{{cite book|last=Heidarzadeh|first=Tofigh|url=https://books.google.com/books?id=Fo-GY4J1h4cC&pg=PA1|title=A History of Physical Theories of Comets, From Aristotle to Whipple|date=2008|publisher=[[Springer Science+Business Media]]|isbn=978-1-4020-8323-5|page=1|lccn=2008924856}}</ref> 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]] supported the idea that comets moved like the planets.{{Sfn|Sagan|Druyan|1997|p=48}} Aristotelian theory on comets continued to be widely accepted throughout the [[Middle Ages]], despite several discoveries from various individuals challenging aspects of it.<ref name=":0">{{cite journal|last1=Barker|first1=Peter|last2=Goldstein|first2=Bernard R.|name-list-style=amp|date=September 1988|title=The role of comets in the Copernican revolution|journal=Studies in History and Philosophy of Science Part A|volume=19|issue=3|pages=299–319|doi=10.1016/0039-3681(88)90002-7|bibcode=1988SHPSA..19..299B}}</ref>

In the 1st century AD, [[Seneca the Younger]] questioned Aristotle's logic concerning comets. Because of their regular movement and imperviousness to wind, they cannot be atmospheric,{{Sfn|Sagan|Druyan|1997|p=26}} and are more permanent than suggested by their brief flashes across the sky.{{efn|"I do not think that a comet is just a sudden fire, but that it is among the eternal works of nature." {{harv|Sagan|Druyan|1997|p=26}}}} 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.{{Sfn|Sagan|Druyan|1997|p=26}} 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."{{Sfn|Sagan|Druyan|1997|pp=26–27}} While Seneca did not author a substantial theory of his own,<ref>{{Cite book|last=Heidarzadeh|first=Tofigh|url=https://books.google.com/books?id=Fo-GY4J1h4cC&q=a+history+of+physical+theories+on+comets&pg=PR8|title=A History of Physical Theories of Comets, From Aristotle to Whipple|date=2008-05-23|publisher=Springer Science & Business Media|isbn=978-1-4020-8323-5|language=en}}</ref> his arguments would spark much debate among Aristotle's critics in the 16th and 17th centuries.<ref name=":0" />{{efn|Seneca is quoted as stating, "Why ... are we surprised that comets, such a rare spectacle in the universe, are not yet grasped by fixed laws and that their beginning and end are not known, when their return is at vast intervals? ... The time will come when diligent research over very long periods of time will bring to light things which now lie hidden."{{sfn|Sagan|Druyan|1997|pp=37–38}}}}

In the 1st century AD, [[Pliny the Elder]] believed that comets were connected with political unrest and death.<ref>{{harvnb|Sagan|Druyan|1997|pp=27–28}}</ref> Pliny observed comets as "human like", often describing their tails with "long hair" or "long beard".<ref>{{cite book|last=Hellman|first=C. Doris|url=https://books.google.com/books?id=1_MVAwAAQBAJ&pg=PA36|title=The Comet of 1577: Its Place in the History of Astronomy|publisher=AMS Press|year=1971|isbn=0-404-51510-X|series=Columbia University Studies in the Social Sciences No. 510|page=36|lccn=72-110569|orig-year=1944}}</ref> His system for classifying comets according to their color and shape was used for centuries.<ref name=":2" />

In [[Indian astronomy|India]], by the 6th century AD astronomers believed that comets were apparitions that re-appeared periodically. This was the view expressed in the 6th century by the astronomers [[Varāhamihira]] and [[Bhadrabahu III|Bhadrabahu]], and the 10th-century astronomer [[Bhaṭṭotpala]] listed the names and estimated periods of certain comets, but it is not known how these figures were calculated or how accurate they were.<ref>{{cite book|last1=Kelley|first1=David H.|url=https://books.google.com/books?id=ILBuYcGASxcC&pg=PA293|title=Exploring Ancient Skies: A Survey of Ancient and Cultural Astronomy|last2=Milone|first2=Eugene F.|date=2011|publisher=[[Springer Science+Business Media]]|isbn=978-1-4419-7624-6|edition=2nd|page=293|doi=10.1007/978-1-4419-7624-6|bibcode=2011eas..book.....K |oclc=710113366|name-list-style=amp}}</ref><ref>{{Cite journal |last=Sharma |first=S. D. |date=1987 |title=Periodic Nature of Cometary Motions as Known to Indian Astronomers Before Eleventh Century A.D |url=https://www.cambridge.org/core/journals/international-astronomical-union-colloquium/article/periodic-nature-of-cometary-motions-as-known-to-indian-astronomers-before-eleventh-century-ad/CB4A864D932B389D94E6842EAD3F980F |journal=International Astronomical Union Colloquium |language=en |volume=91 |pages=109–112 |doi=10.1017/S0252921100105925 |issn=0252-9211}}</ref>

There is a claim that an Arab scholar in 1258 noted several recurrent appearances of a comet (or a type of comet), and though it's not clear if he considered it to be a single periodic comet, it might have been a comet with a period of around 63 years.<ref>{{Cite journal |last=Khalil Konsul |first=Eng. |date=2019-08-01 |title=Comet Observations in Arab Heritage Books: Did the Arabs Discover the Periodicity of Comets Before Edmund Halley? |url=https://ui.adsabs.harvard.edu/abs/2019ASPC..520...83K |journal=ASP Conference Series |volume=520 |pages=83|bibcode=2019ASPC..520...83K }}</ref>

In 1301, the Italian painter [[Giotto]] was the first person to accurately and anatomically portray a comet. In his work ''[[Adoration of the Magi]],'' Giotto's depiction of Halley's Comet in the place of the [[Star of Bethlehem]] would go unmatched in accuracy until the 19th century and be bested only with the invention of photography.<ref name=":1">{{Cite journal|last=Olson|first=Roberta J.M.|author-link=Roberta Olson|date=1984|title=... And They Saw Stars: Renaissance Representations of Comets and Pretelescopic Astronomy|journal=Art Journal|volume=44|issue=3|pages=216–224|doi=10.2307/776821|jstor=776821}}</ref>

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]]'' chronicles and in the warnings of [[Pope Callixtus III]].<ref name=":1" /> In 1578, German [[Lutheran]] bishop Andreas Celichius defined comets as "the thick smoke of human sins&nbsp;... kindled by the hot and fiery anger of the [[God|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."{{sfn|Sagan|Druyan|1997|pp=32–33}}

=== Scientific approach ===
Crude attempts at a [[parallax]] measurement of Halley's Comet were made in 1456, but were erroneous.{{Sfn|Sagan|Druyan|1997|p=36}} [[Regiomontanus]] was the first to attempt to calculate [[diurnal parallax]] 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 Earth.<ref name=":2">{{Cite book |last1=Brandt |first1=John C. |url=https://books.google.com/books?id=CcSUeymd-14C&q=regiomontanus+on+comets&pg=PR7 |title=Introduction to Comets |last2=Chapman |first2=Robert D. |date=2004-03-11 |publisher=Cambridge University Press |isbn=978-0-521-00466-4 |pages=6–11 |language=en}}</ref>

In the 16th century, [[Tycho Brahe]] and [[Michael Maestlin]] demonstrated that comets must exist outside of Earth's atmosphere by measuring the parallax of the [[Great Comet of 1577]].<ref>{{Cite journal|last=Barker|first=Peter|date=2002-06-01|title=Constructing Copernicus|journal=Perspectives on Science|volume=10|issue=2|pages=208–227|doi=10.1162/106361402321147531|s2cid=57563317|issn=1063-6145}}</ref> Within the precision of the measurements, this implied the comet must be at least four times more distant than from Earth to the Moon.<ref name="ESO part I">{{cite web |title=A Brief History of Comets I (until 1950) |url=http://www.eso.org/public/events/astro-evt/hale-bopp/comet-history-1.html |publisher=[[European Southern Observatory]] |access-date=14 August 2013}}</ref><ref>{{harvnb|Sagan|Druyan|1997|p=37}}</ref> Based on observations in 1664, [[Giovanni Alfonso Borelli|Giovanni Borelli]] recorded the longitudes and latitudes of comets that he observed, and suggested that cometary orbits may be parabolic.<ref>{{cite journal |title=Giovanni Borelli and the Comets of 1664–65 |journal=Journal for the History of Astronomy |first=Luciano |last=Boschiero |volume=40 |issue=1 |pages=11–30 |date=February 2009 |doi=10.1177/002182860904000103|bibcode=2009JHA....40...11B |s2cid=118350308 }}</ref> Despite being a skilled astronomer, in his 1623 book ''[[The Assayer]]'', [[Galileo Galilei]] rejected Brahe's theories on the parallax of comets and claimed that they may be a mere optical illusion, despite little personal observation.<ref name=":2" /> In 1625, Maestlin's student [[Johannes Kepler]] upheld that Brahe's view of cometary parallax was correct.<ref name=":2" /> Additionally, mathematician [[Jacob Bernoulli]] published a treatise on comets in 1682.

During the [[early modern period]] comets were studied for 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.<ref>{{Cite journal|last=Lanuza Navarro|first=Tayra M. C.|date=2006|title=Medical astrology in Spain during the seventeenth century|journal=Cronos (Valencia, Spain)|volume=9|pages=59–84|issn=1139-711X|pmid=18543450}}</ref>

[[Isaac Newton]], in his ''[[Philosophiæ Naturalis Principia Mathematica|Principia Mathematica]]'' of 1687, proved that an object moving under the influence of [[gravity]] by an inverse square law must trace out an orbit shaped like one of the [[conic section]]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.<ref>{{cite book |last=Newton |first=Isaac |author-link=Isaac Newton |chapter=Lib. 3, Prop. 41 |title=Philosophiæ Naturalis Principia Mathematica |publisher=[[Royal Society of London]] |date=1687 |isbn=0-521-07647-1 |title-link=Philosophiæ Naturalis Principia Mathematica}}</ref>
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.<ref>{{harvnb|Sagan|Druyan|1997|pp=306–307}}</ref> He pointed out that comets usually appear near the Sun, and therefore most likely orbit it.{{Sfn|Sagan|Druyan|1997|p=26}} 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 [the coma]".{{Sfn|Sagan|Druyan|1997|p=26}}

[[File:Newton Comet1680.jpg|thumb|290px|The orbit of the comet of 1680, fitted to a [[parabola]], as shown in [[Isaac Newton|Newton]]'s ''[[Philosophiæ Naturalis Principia Mathematica|Principia]]'']]

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]]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]]. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758–59.<ref name="Halley">{{cite journal |doi=10.1098/rstl.1704.0064 |bibcode=1704RSPT...24.1882H |title=Astronomiae Cometicae Synopsis, Autore Edmundo Halleio apud Oxonienses. Geometriae Professore Saviliano, & Reg. Soc. S |date=1704 |last1=Halleio |first1=E. |journal=Philosophical Transactions of the Royal Society of London |volume=24 |issue=289–304 |pages=1882–1899 |s2cid=186209887 |url=http://rstl.royalsocietypublishing.org/content/24/289-304/1882.full.pdf |archive-url=https://web.archive.org/web/20170430210149/http://rstl.royalsocietypublishing.org/content/24/289-304/1882.full.pdf |archive-date=2017-04-30 |url-status=live|doi-access=free }}</ref> Halley's predicted return date was later refined by a team of three French mathematicians: [[Alexis Clairaut]], [[Joseph Lalande]], and [[Nicole-Reine Lepaute]], who predicted the date of the comet's 1759 perihelion to within one month's accuracy.<ref>On 1758 November 14, Alexis Clairaut announced to the Royal Academy of Sciences in Paris his prediction of the date at which Halley's comet would return:
* Clairaut (January 1759) [https://babel.hathitrust.org/cgi/pt?id=ucm.5324346563;view=1up;seq=44 "Mémoire sur la cométe de 1682,"] ''Le Journal des Sçavans'', pp. 38–45. On p. 44, Clairaut predicts that Halley's comet would return in mid April 1759. From [https://babel.hathitrust.org/cgi/pt?id=ucm.5324346563;view=1up;seq=50 p. 44] (translated from French): " <!-- ... ''il me paroît que la Cométe attendue doit passer à son périhélie vers le milieu du mois d'Avril prochain."'' ( --> ... it seems to me that the expected comet must pass its perihelion towards the middle of next April." On p. 40, Clairaut states that his prediction might be slightly incorrect due to the presence of unknown planets beyond Saturn: "<!-- ''Un corps qui passe dans des régions aussi éloignées, & qui échappe à nos yeux pendant des intervalles aussi longs, pourroit être soumis à des forces totalement inconnues; telles que l'action d'autres Cométes, ou même de quelque planéte toujours trop distante du Soleil pour être jamais apperçue."'' ( -->A body [i.e., Halley's comet] which passes into regions so remote, and which escapes our eyes during such long intervals, might be subjected to totally unknown forces; such as the action of other comets, or even of some planet always too far from the sun to ever be perceived."
On 1759 April 7, the French astronomer [[Joseph-Nicolas Delisle]] announced to the Royal Academy of Sciences in Paris that he and his assistant [[Charles Messier]] had observed the return of Halley's comet, as predicted:
* de l'Isle (June 1759) [https://babel.hathitrust.org/cgi/pt?id=ucm.5324346563;view=1up;seq=360 "Lettre de M. de l'Isle ... contenant la découverte du retour de la Comète de 1682, ... "] (Letter from Mr. de l'Isle ... containing the discovery of the return of the comet of 1682), ''Le Journal des Sçavans'', pp. 356–364.
De l'Isle subsequently admitted that the comet's return had first been seen by a German amateur astronomer and farmer, [[Johann Georg Palitzsch|Georg Palitzsch]]:
* de l'Isle (August 1759) [https://babel.hathitrust.org/cgi/pt?id=ucm.5324346563;view=1up;seq=527 "Seconde lettre de M. de l'Isle,"] ''Le Journal des Sçavans'', pp. 523–529. From p. 526 (translated from French): " <!-- ... ''j'ai reçu une Lettre d'Heidelberg le premier Avril au soir, dans laquelle l'on m'écrit que l'on a publié à Leipsick le 24 Janvier de cette année un Mémoire Allemand dans lequel il est dit que cette Comète a été vue en Saxe par un Paysan, nommé Palisch, le 25 & 26 Décembre de l'année dernière; j'ai bien de la peine à concevoir comment ce Paysan aura pû la découvrir, cette Comète, ... "'' ( --> ... I received a letter from Heidelberg on the first of April in the evening, in which it is written to me that there had been published at Leipzig on the 24th of January of this year a German memoir in which it is said that this comet had been seen in Saxony by a peasant, named Palisch, on the 25th and 26th of December of last year; I can hardly conceive how this peasant could have discovered it, this comet ... "
The story behind the rediscovery of Halley's comet was given by Joseph Lalande in: 
* Delalande, ''Tables astronomiques de M. Halley, ... Et l'Histoire de la Comete de 1759.'' [Astronomical tables of Mr. Halley, ... and the history of the comet of 1759.] (Paris, France: Durand, 1759), [https://babel.hathitrust.org/cgi/pt?id=mdp.39015006990892;view=1up;seq=105 pp. 91 ff.] Lalande acknowledged the contributions of Madame Lepaute to predicting the return of Halley's comet on p. 110. From [https://babel.hathitrust.org/cgi/pt?id=mdp.39015006990892;view=1up;seq=124 p. 110] (translated from French): "<!-- ''... mais il faut convenir que cette suite immense de détails m'eût semblé effrayante, si ''Madame LEPAUTE'', appliquée depuis long-temps & avec succès aux calculs Astronomiques, n'en eût partagé le travail."'' ( --> ... but it must be admitted that this immense series of details would have seemed frightening to me if ''Madame LEPAUTE'', [who has] long applied [herself] successfully to astronomical calculations, had not shared in the work."
See also: 
* Broughton, Peter (1985) "The first predicted return of comet Halley", ''Journal for the History of Astronomy'', '''16''' : 123–132. Available at: [http://adsabs.harvard.edu/full/1985JHA....16..123B Astrophysics Data System]
* Clairaut, [https://books.google.com/books?id=WQRbq4SA_zkC&pg=PP7''Théorie du mouvement des comètes,'' ...] [Theory of the movement of comets, ...] (Paris, France: Michel Lambert, 1760); see especially the preface.</ref>{{sfn|Sagan|Druyan|1997|p=93}} When the comet returned as predicted, it became known as Halley's Comet.<ref>{{cite book |url=https://books.google.com/books?id=DFgMAaU3vA8C&pg=PA35 |page=35 |title=The Greatest Comets in History: Broom Stars and Celestial Scimitars |isbn=978-0-387-09513-4 |last=Wong |first=Yau-Chuen |date=2008|publisher=Springer }}</ref>
{{Quote box |quote=From his huge vapouring train perhaps to shake<br />Reviving moisture on the numerous orbs,<br />Thro' which his long ellipsis winds; perhaps<br />To lend new fuel to declining suns,<br />To light up worlds, and feed th' ethereal fire. |source=[[James Thomson (poet, born 1700)|James Thomson]] ''[[The Seasons (Thomson poem)|The Seasons]]'' (1730; 1748)<ref>{{cite book |url=https://books.google.com/books?id=F--di9y22yUC&pg=PA67 |title=The Background of Thomson's Seasons |isbn=978-0-8166-5950-0 |page=67 |last=McKillop |first=Alan Dugald |date=1942|publisher=U of Minnesota Press }}</ref> |style=padding: 8px 12px;}}

As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, [[Immanuel Kant]] hypothesized in his ''[[Universal Natural History and Theory of the Heavens|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.{{Sfn|Sagan|Druyan|1997|pp=84–87}} In 1836, the German mathematician [[Friedrich Wilhelm Bessel]], after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the [[jet force]]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.<ref>{{harvnb|Sagan|Druyan|1997|p=126}}</ref>

In the 19th century, the Astronomical Observatory of Padova was an epicenter in the observational study of comets. Led by [[Giovanni Santini]] (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.<ref>{{Cite journal|last=Pigatto|first=Luisa|date=December 2009|title=The correspondence of Giovanni Santini and Giuseppe Lorenzoni, directors of the Astronomical Observatory of Padua in the 19th Century|journal=Annals of Geophysics|volume=52|pages=595–604}}</ref> Correspondence within the observatory, particularly between Santini and another astronomer Giuseppe Toaldo, mentioned the importance of comet and planetary orbital observations.<ref>Pigatto, L. (1988): Santini e gli strumenti della Specola, in Giovanni Santini astronomo, "Atti e Memorie dell'Accademia Patavina di Scienze, Lettere ed Arti", (Padova), XCIX (1986–1987), 187–198.</ref>

In 1950, [[Fred Lawrence Whipple]] proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock.<ref name="Whipple">{{cite journal |doi=10.1086/145272 |title=A comet model. I. The acceleration of Comet Encke |date=1950 |last1=Whipple |first1=F. L. |journal=The Astrophysical Journal |volume=111 |pages=375 |bibcode=1950ApJ...111..375W}}</ref> This "dirty snowball" model soon became accepted and appeared to be supported by the observations of an armada of [[spacecraft]] (including the [[European Space Agency]]'s ''[[Giotto mission|Giotto]]'' probe and the Soviet Union's ''[[Vega 1]]'' and ''[[Vega 2]]'') that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material.<ref>{{cite book |url=https://books.google.com/books?id=E4NfZ9FDcc8C&pg=PA156 |page=156 |title=Magic Universe:A Grand Tour of Modern Science |isbn=978-0-19-162235-9 |last1=Calder |first1=Nigel |date=13 October 2005|publisher=OUP Oxford }}</ref>

On 22 January 2014, [[European Space Agency|ESA scientists]] reported the detection, for the first definitive time, of [[water vapor]] on the [[dwarf planet]] [[Ceres (dwarf planet)|Ceres]], the largest object in the asteroid belt.<ref name="KüppersO'Rourke2014">{{cite journal |last1=Küppers |first1=Michael |last2=O'Rourke |first2=Laurence |last3=Bockelée-Morvan |first3=Dominique|author3-link=Dominique Bockelée-Morvan |last4=Zakharov |first4=Vladimir |last5=Lee |first5=Seungwon |last6=von Allmen |first6=Paul |last7=Carry |first7=Benoît |last8=Teyssier |first8=David |last9=Marston |first9=Anthony |last10=Müller |first10=Thomas |last11=Crovisier |first11=Jacques |last12=Barucci |first12=M. Antonietta |last13=Moreno |first13=Raphael |title=Localized sources of water vapour on the dwarf planet (1) Ceres |journal=Nature |volume=505 |issue=7484 |date=2014 |pages=525–527 |issn=0028-0836 |doi=10.1038/nature12918 |bibcode=2014Natur.505..525K |pmid=24451541|s2cid=4448395 }}</ref> The detection was made by using the [[Far-infrared astronomy|far-infrared abilities]] of the [[Herschel Space Observatory]].<ref name="NASA-20140122">{{cite web |last1=Harrington |first1=J.D. |title=Herschel Telescope Detects Water on Dwarf Planet – Release 14-021 |url=http://www.nasa.gov/press/2014/january/herschel-telescope-detects-water-on-dwarf-planet |date=22 January 2014 |publisher=[[NASA]] |access-date=22 January 2014}}</ref> 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."<ref name="NASA-20140122" /> On 11 August 2014, astronomers released studies, using the [[Atacama Large Millimeter Array|Atacama Large Millimeter/Submillimeter Array (ALMA)]] for the first time, that detailed the distribution of [[Hydrogen cyanide|HCN]], [[Hydrogen isocyanide|HNC]], [[Formaldehyde|{{chem2|H2CO}}]], and dust inside the [[Coma (cometary)|comae]] of comets [[C/2012 F6 (Lemmon)]] and [[Comet ISON|C/2012 S1 (ISON)]].<ref name="NASA-20140811">{{cite web |url=http://www.nasa.gov/press/2014/august/goddard/nasa-s-3-d-study-of-comets-reveals-chemical-factory-at-work |title=Release 14-038: NASA's 3-D Study of Comets Reveals Chemical Factory at Work |publisher=[[NASA]] |last1=Zubritsky |first1=Elizabeth |last2=Neal-Jones |first2=Nancy |name-list-style=amp |date=11 August 2014 |access-date=12 August 2014}}</ref><ref name="AJL-20140811">{{cite journal |last=Cordiner |first=M. A. |title=Mapping the Release of Volatiles in the Inner Comae of Comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON) Using the Atacama Large Millimeter/Submillimeter Array |date=11 August 2014 |journal=[[The Astrophysical Journal]] |volume=792 |number=1 |doi=10.1088/2041-8205/792/1/L2 |display-authors=etal |arxiv=1408.2458 |bibcode=2014ApJ...792L...2C |pages=L2|s2cid=26277035 }}</ref>

=== Spacecraft missions ===
{{See also|List of comets visited by spacecraft|List of missions to comets}}
*The [[Halley Armada]] 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]]'' spacecraft obtained high-resolution images of the surface of [[19P/Borrelly|Comet Borrelly]]. It was found that the surface of comet Borrelly is hot and dry, with a temperature of between {{convert|26|and|71|C|F}}, 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.<ref>{{cite web |title=NASA Spacecraft Finds Comet Has Hot, Dry Surface |url=http://www.jpl.nasa.gov/releases/2002/release_2002_80.html |publisher=JPL |date=5 April 2002 |access-date=22 August 2013 |archive-date=12 October 2012 |archive-url=https://web.archive.org/web/20121012140123/http://www.jpl.nasa.gov/releases/2002/release_2002_80.html |url-status=dead }}</ref> In July 2005, the ''[[Deep Impact (spacecraft)|Deep Impact]]'' probe blasted a crater on Comet [[Tempel 1]] 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.<ref>{{cite web |title=NASA's 'Deep Impact' Team Reports First Evidence of Cometary Ice |url=http://www.brown.edu/Administration/News_Bureau/2005-06/05-072.html |publisher=Brown University |date=2 February 2006 |access-date=22 August 2013}}</ref> Renamed [[EPOXI]], it made a flyby of [[103P/Hartley|Comet Hartley 2]] on 4 November 2010.
*'''Ulysses'''. In 2007, the [[Ulysses (spacecraft)|Ulysses probe]] unexpectedly passed through the tail of the comet [[C/2006 P1 (McNaught)|C/2006 P1]] (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 (spacecraft)|''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 {{convert|1000|C|F}}.<ref>{{cite news |last=Rincon |first=Paul |title=Comets 'are born of fire and ice' |url=http://news.bbc.co.uk/2/hi/science/nature/4801968.stm |work=BBC News |date=14 March 2006 |access-date=7 September 2013}}</ref><ref>{{cite web |last=Malik |first=T. |title=NASA's Stardust Comet Samples Contain Minerals Born in Fire |url=http://www.space.com/scienceastronomy/060313_stardust_update.html |publisher=Space.com |date=13 March 2006 |access-date=7 September 2013}}</ref> 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.<ref>{{cite journal |doi=10.1038/nature03088 |title=The building blocks of planets within the 'terrestrial' region of protoplanetary disks |date=2004 |last1=Van Boekel |first1=R. |last2=Min |first2=M. |last3=Leinert |first3=Ch. |last4=Waters |first4=L.B.F.M. |last5=Richichi |first5=A. |last6=Chesneau |first6=O. |last7=Dominik |first7=C. |last8=Jaffe |first8=W. |last9=Dutrey |first9=A. |last10=Graser |first10=U. |last11=Henning |first11=Th. |last12=De Jong |first12=J. |last13=Köhler |first13=R. |last14=De Koter |first14=A. |last15=Lopez |first15=B. |last16=Malbet |first16=F. |last17=Morel |first17=S. |last18=Paresce |first18=F. |last19=Perrin |first19=G. |last20=Preibisch |first20=Th. |last21=Przygodda |first21=F. |last22=Schöller |first22=M. |last23=Wittkowski |first23=M. |display-authors=1 |journal=Nature |volume=432 |issue=7016 |pages=479–82 |pmid=15565147 |bibcode=2004Natur.432..479V|s2cid=4362887 }}</ref> As a result, comets 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".<ref>{{cite web |title=Stardust comet dust resembles asteroid materials |url=https://publicaffairs.llnl.gov/news/news_releases/2008/NR-08-01-05.html |publisher=Lawrence Livermore National Laboratory |date=24 January 2008 |access-date=7 September 2013 |url-status=dead |archive-url=https://web.archive.org/web/20100528001619/https://publicaffairs.llnl.gov/news/news_releases/2008/NR-08-01-05.html |archive-date=28 May 2010 }}</ref> These new results have forced scientists to rethink the nature of comets and their distinction from asteroids.<ref>{{cite news |last=Dunham |first=Will |title=Dust samples prompt rethink about comets |url=https://www.reuters.com/article/us-comet-idUSN2537011620080126 |work=Reuters |date=25 January 2008 |access-date=7 September 2013}}</ref>
*'''Rosetta'''. The ''[[Rosetta (spacecraft)|Rosetta]]'' probe orbited [[67P/Churyumov–Gerasimenko|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.<ref>{{cite web |url=http://sci.esa.int/rosetta/34479-rosetta-ready-to-explore-a-comet-s-realm/ |title=Rosetta Ready To Explore A Comet's Realm |publisher=European Space Agency |date=12 January 2004 |access-date=7 September 2013}}</ref>

== Classification ==

=== Great comets ===
{{Main|Great comet}}{{See also|Great Comet of 1577}}
[[File:Von einem Schrecklichen vnd Wunderbarlichen Cometen so sich den Dienstag nach Martini dieses lauffenden M. D. Lxxvij. Jahrs am Himmel erzeiget hat (grayscale).png|thumb|300px|Woodcut of the [[Great Comet of 1577]]]]

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.<ref name="great"/> 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.<ref>{{cite book |url=https://books.google.com/books?id=vQwwAAAAMAAJ |page=274 |title=The World Almanac and Book of Facts 1996 |isbn=978-0-88687-780-4 |last=Famighetti |first=Robert |date=1995|publisher=Newspaper Enterprise Association }}</ref> 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]] in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so.<ref>{{cite web |url=http://www.universetoday.com/97561/new-sun-skirting-comet-could-provide-dazzling-display-in-2013/ |title=New 'Sun-Skirting' Comet Could Provide Dazzling Display in 2013 |work=Universe Today |access-date=7 September 2013 |last=Atkinson |first=Nancy|date=25 September 2012 }}</ref> [[Comet West]], which appeared three years later, had much lower expectations but became an extremely impressive comet.<ref>{{cite web |last=Kronk |first=Gary W. |title=C/1975 V1 (West) |url=http://cometography.com/lcomets/1975v1.html |work=Gary W. Kronk's Cometography |access-date=7 September 2013}}</ref>

The Great Comet of 1577 is a well-known example of a great comet. It passed near Earth as a [[List of near-parabolic comets|non-periodic comet]] and was seen by many, including well-known astronomers Tycho Brahe and [[Taqi ad-Din Muhammad ibn Ma'ruf|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]] 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]] (McNaught), which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.<ref>{{cite web |url=http://hubblesite.org/hubble_discoveries/comet_ison/blogs/great-moments-in-comet-history-comet-mcnaught |title=Great Moments in Comet History: Comet McNaught |publisher=Hubblesite |access-date=15 August 2013}}</ref>

=== Sungrazing comets ===
{{Main|Sungrazing comet}}

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometers.<ref>{{cite book |url=https://books.google.com/books?id=qU95h4yKia4C&pg=PA34 |page=34 |title=Hunting and Imaging Comets |isbn=978-1-4419-6905-7 |last=Mobberley |first=Martin |date=2010|publisher=Springer }}</ref> Although small sungrazers can be completely evaporated during such a close approach to the [[Sun]], larger sungrazers can survive many perihelion passages. However, the strong [[tidal force]]s they experience often lead to their fragmentation.<ref>{{cite journal |bibcode=1966IrAJ....7..141O |title=Sun-Grazing Comets and Tidal Disruption |last=Opik |first=E. J. |volume=7 |date=1966 |pages=141 |journal=Irish Astronomical Journal}}</ref>

About 90% of the sungrazers observed with [[Solar and Heliospheric Observatory|SOHO]] are members of the [[Kreutz sungrazer|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.<ref name="Bailey">{{cite journal |last1=Hahn |first1=M. E. |last2=Chambers |first2=J. E. |last3=Hahn |first3=G. |display-authors=1 |title=Origin of sungrazers: a frequent cometary end-state |journal=[[Astronomy & Astrophysics]] |volume=257 |issue=1 |pages=315–322 |date=1992 |bibcode=1992A&A...257..315B}}</ref> 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 [[96P/Machholz|Comet 96P/Machholz]], which is the parent of two [[meteor shower|meteor streams]], the [[Quadrantids]] and the [[Arietids]].<ref name="Ohtsuka">{{cite journal |last1=Yoshikawa |first1=K. |last2=Nakano |first2=S. |last3=Yoshikawa |first3=M. |display-authors=1 |title=On the Association among Periodic Comet 96P/Machholz, Arietids, the Marsden Comet Group, and the Kracht Comet Group |url=http://pasj.asj.or.jp/v55/n1/550127/55012319.pdf |archive-url=https://web.archive.org/web/20181005091950/http://pasj.asj.or.jp/v55/n1/550127/55012319.pdf |archive-date=2018-10-05 |url-status=live |journal=Publications of the Astronomical Society of Japan |volume=55 |issue=1 |pages=321–324 |date=2003 |doi=10.1093/pasj/55.1.321 |bibcode=2003PASJ...55..321O|doi-access=free }}</ref>

=== Unusual comets ===
{{See also|#Fate of comets|Swastika#Comet}}
[[File:Euler-Diagram bodies in the Solar System.jpg|thumb|300x300px|[[Euler diagram]] showing the types of bodies in the Solar System]]
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 (planet)|Mercury]] whereas the Comet [[29P/Schwassmann–Wachmann]] currently travels in a nearly circular orbit entirely between the orbits of Jupiter and Saturn.<ref>{{cite web |last=Kronk |first=Gary W. |title=29P/Schwassmann–Wachmann 1 |url=http://cometography.com/pcomets/029p.html |work=Gary W. Kronk's Cometography |access-date=22 September 2013}}</ref> [[2060 Chiron]], whose unstable orbit is between Saturn and [[Uranus]], was originally classified as an asteroid until a faint coma was noticed.<ref>{{cite web |last=Kronk |first=Gary W. |title=95P/Chiron |url=http://cometography.com/pcomets/095p.html |work=Gary W. Kronk's Cometography |access-date=27 April 2009}}</ref> Similarly, [[137P/Shoemaker–Levy|Comet Shoemaker–Levy 2]] was originally designated asteroid {{mp|1990 UL|3}}.<ref>{{cite web |last=Kronk |first=Gary W. |title=137P/Shoemaker–Levy 2 |url=http://cometography.com/pcomets/137p.html |work=Gary W. Kronk's Cometography |access-date=27 April 2009}}</ref>

=== Largest ===
The largest known periodic comet is [[95P/Chiron]] at 200&nbsp;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&nbsp;km that will not come to perihelion until January 2031 just outside of Saturn's orbit at 11 AU. The [[Comet of 1729]] is estimated to have been ≈100&nbsp;km in diameter and came to perihelion inside of Jupiter's orbit at 4 AU.

=== Centaurs ===
{{Main|Centaur (minor planet)}}

Centaurs typically behave with characteristics of both asteroids and comets.<ref name=Horner2004a>{{cite journal |last1=Horner |first1=J. |last2=Evans |first2=N.W. |last3=Bailey |first3=M. E. |display-authors=1 |title=Simulations of the Population of Centaurs I: The Bulk Statistics |year=2004 |arxiv=astro-ph/0407400 |doi=10.1111/j.1365-2966.2004.08240.x |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=354 |issue=3 |pages=798–810 |doi-access=free |bibcode=2004MNRAS.354..798H|s2cid=16002759 }}</ref> Centaurs can be classified as comets such as [[60558 Echeclus]], 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]] was discovered without a coma but later became active,<ref name="Choietal2006">
Y-J. Choi, P.R. Weissman, and D. Polishook ''(60558) 2000 EC_98'', IAU Circ., '''8656''' (Jan. 2006), 2.</ref> and was then classified as both a comet and an asteroid (174P/Echeclus). One plan for ''[[Cassini–Huygens|Cassini]]'' involved sending it to a centaur, but NASA decided to destroy it instead.<ref name="usra_0903">{{cite web |last1=Pappalardo |first1=Bob |last2=Spiker |first2=Linda |name-list-style=amp |url=http://www.lpi.usra.edu/opag/march09/presentations/pappalardo.pdf |title=Cassini Proposed Extended-Extended Mission (XXM) |publisher=Lunar and Planetary Institute |date=15 March 2009 |url-status=live |archive-date=18 July 2012 |archive-url=https://web.archive.org/web/20120718145449/http://www.lpi.usra.edu/opag/march09/presentations/pappalardo.pdf}}</ref>

== Observation ==

A comet may be discovered photographically using a wide-field [[telescope]] or visually with [[binoculars]]. However, even without access to optical equipment, it is still possible for the [[amateur astronomer]] to discover a sungrazing comet online by downloading images accumulated by some satellite observatories such as [[Solar and Heliospheric Observatory|SOHO]].<ref name=soho2>{{cite web |last=Farmer |first=Steve E. Jr. |title=Getting Started – SOHO Comet Hunting Techniques/Instructions |url=http://www.cometary.net/searching_for_soho_comets.htm |publisher=Red Barn Observatory |access-date=25 August 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130404234938/http://www.cometary.net/searching_for_soho_comets.htm |archive-date=4 April 2013}}</ref> SOHO's 2000th comet was discovered by Polish amateur astronomer Michał Kusiak on 26 December 2010<ref>{{cite web |url=http://sohowww.nascom.nasa.gov/hotshots/2010_12_28/ |title=SOHO |date=28 December 2010 |publisher=NASA |access-date=25 August 2013}}</ref> and both discoverers of Hale–Bopp used amateur equipment (although Hale was not an amateur).

=== Lost ===
{{Main|Lost comet}}

A number of periodic comets discovered in earlier decades or previous centuries are now [[lost comet]]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]], discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by [[LINEAR]] in 2001.<ref name="kronk">{{cite web |last=Kronk |first=Gary W. |title=11P/Tempel–Swift–LINEAR |url=http://cometography.com/pcomets/011p.html |work=Gary W. Kronk's Cometography |access-date=27 April 2009}}</ref> There are at least 18 comets that fit this category.<ref name="Meyer">{{cite web |url=http://www.comethunter.de/lost.html |title=Lost periodic comets |work=The Catalogue of Comet Discoveries |last=Meyer |first=M. |date=2013 |access-date=18 July 2015}}</ref>

== In popular culture ==
{{See also|Comets in fiction|Category:Fiction about impact events}}

The depiction of comets in [[popular culture]] is firmly rooted in the long Western tradition of seeing comets as harbingers of doom and as omens of world-altering change.<ref name="Van Riper 29">{{cite book |url=https://books.google.com/books?id=JADiKdzkJqsC&pg=PA27 |pages=27–29 |title=Science in Popular Culture: A Reference Guide |isbn=978-0-313-31822-1 |last1=Bowdoin Van Riper |first1=A |date=2002|publisher=Greenwood Publishing }}</ref> 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]] (who correctly speculated that he'd "go out with the comet" in 1910)<ref name="Van Riper 29" /> and [[Eudora Welty]], to whose life [[Mary Chapin Carpenter]] dedicated the song "[[Halley Came to Jackson]]".<ref name="Van Riper 29" />

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]] in the tail might poison millions,<ref>{{cite web |last=Ridpath |first=Ian |author-link=Ian Ridpath |title=Awaiting the Comet |work=A brief history of Halley's Comet |url=http://www.ianridpath.com/halley/halley11.htm |date=3 July 2008 |access-date=15 August 2013}}</ref> whereas the appearance of Comet Hale–Bopp in 1997 triggered the mass suicide of the [[Heaven's Gate (religious group)|Heaven's Gate]] cult.<ref>{{cite news |first=B. Drummond Jr. |last=Ayres |title=Families Learning of 39 Cultists Who Died Willingly |url=https://www.nytimes.com/1997/03/29/us/families-learning-of-39-cultists-who-died-willingly.html |quote=According to material the group posted on its Internet site, the timing of the suicides were probably related to the arrival of the Hale–Bopp comet, which members seemed to regard as a cosmic emissary beckoning them to another world |work=[[The New York Times]] |date=29 March 1997 |access-date=20 August 2013}}</ref>

In [[science fiction]], the [[impact event|impact of comets]] has been depicted as a threat overcome by technology and heroism (as in the 1998 films ''[[Deep Impact (film)|Deep Impact]]'' and ''[[Armageddon (1998 film)|Armageddon]]''), or as a trigger of global apocalypse (''[[Lucifer's Hammer]]'', 1979) or zombies (''[[Night of the Comet]]'', 1984).<ref name="Van Riper 29" /> In [[Jules Verne]]'s ''[[Off on a Comet]]'' 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]]''.<ref>{{cite news |url=https://www.latimes.com/archives/la-xpm-1987-12-06-bk-26609-story.html |title=The View From Halley's Comet – 2061: Odyssey Three by Arthur C. Clarke |work=Los Angeles Times |last=Brin |first=David |date=6 December 1987}}</ref>

==In literature==
The long-period comet first recorded by [[Jean-Louis Pons|Pons]] in [[Florence]] on 15 July 1825 inspired [[Lydia Sigourney]]'s humorous poem {{ws|[[s:Poems Sigourney 1827/The Comet of 1825|The Comet of 1825]]}} in which all the celestial bodies argue over the comet's appearance and purpose.

== Gallery ==

<gallery mode="packed" heights="175">
File:Comet_C2020F3_NEOWISE_over_California_desert_landscape.png|[[Comet Neowise|Comet C/2020 F3 NEOWISE]], July 2020
File:Comet P1 McNaught02 - 23-01-07-edited.jpg|Comet [[C/2006 P1]] (McNaught) taken from Victoria, Australia 2007
File:Great Comet of 1882.jpg|The [[Great Comet of 1882]] is a member of the [[Kreutz sungrazer|Kreutz group]]
File:Great Comet 1861.jpg|Austrian astronomer [[Edmund Weiss]] sketched the [[C/1861 J1|Great Comet of 1861]]
File:X-rays from Hyakutake.jpg|[[Comet Hyakutake]] ([[X-ray]], [[ROSAT]] satellite)
File:Asteroid P2013 P5 v2.jpg|"Active asteroid" [[311P/PANSTARRS]] with several tails<ref>{{cite web |url=https://hubblesite.org/contents/news-releases/2013/news-2013-52.html |title=NASA's Hubble Sees Asteroid Spout Six Comet-like Tails |publisher=NASA |website=Hubblesite.org |date=7 November 2013 |access-date=21 November 2019}}</ref>
File:NASA-14090-Comet-C2013A1-SidingSpring-Hubble-20140311.jpg|[[C/2013 A1|Comet Siding Spring]] ([[Hubble Space Telescope|Hubble]]; 11 March 2014)
File:Comets WISE.jpg|Mosaic of 20 comets discovered by the [[Wide-field Infrared Survey Explorer|WISE]] space telescope
File:PIA22419-Neowise-1stFourYearsDataFromDec2013-20180420.gif|[[NEOWISE]] – Comets appear in yellow in [[Neowise]]'s first four years of collecting data (December 2013 to December 2017)
File:Lovejoy-hi1a srem dec12 14.gif|The [[STEREO]] solar observatory filmed [[C/2011 W3|Comet Lovejoy]] moving against the [[solar wind]] as it approached the Sun in December 2011
File:ITS Impact.gif|View from ''[[Deep Impact (spacecraft)|Deep Impact]]''{{'}}s impactor in its last moments before hitting [[Tempel 1|Comet Tempel 1]], July 4, 2005
</gallery>

;Videos
<gallery mode="packed" heights="175">
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|The [[STEREO]] solar observatory filmed [[Comet Encke]] temporarily losing its tail, April 20, 2007
</gallery>
{{clear}}

==See also==
* ''[[The Big Splash (book)|The Big Splash]]''
* [[Comet vintages]]
* [[List of impact craters on Earth]]
* [[List of possible impact structures on Earth]]
* [[Lists of comets]]

== References ==
===Footnotes===
{{notelist}}

===Citations===
{{Reflist}}

=== Bibliography ===
* {{cite book |url=https://books.google.com/books?id=LhkoowKFaTsC |title=Comet |publisher=Random House |location=New York |first1=Carl |last1=Sagan |author-link1=Carl Sagan |first2=Ann |last2=Druyan |author-link2=Ann Druyan |name-list-style=amp |year=1997 |isbn=978-0-3078-0105-0 }}

== Further reading ==
* {{cite book |title=Comets, Popular Culture, and the Birth of Modern Cosmology |publisher=[[Princeton University Press]] |first=Sara J. |last=Schechner |date=1997 |isbn=978-0-691-01150-9}}
* {{cite book |title=Introduction to Comets |publisher=[[Cambridge University Press]] |first1=John C. |last1=Brandt |first2=Robert D. |last2=Chapman |name-list-style=amp |edition=2nd |date=2004 |isbn=978-0-521-80863-7}}

== External links ==
* [https://solarsystem.nasa.gov/small-bodies/comets/overview/ Comets] at NASA's Solar System Exploration
* [http://www.icq.eps.harvard.edu/ International Comet Quarterly] by Harvard University
* [http://smallbodies.ru/en/ Catalogue of the Solar System Small Bodies Orbital Evolution]
* [https://nationalmaglab.org/education/magnet-academy/watch-play/demos/make-a-comet Science Demos: Make a Comet] by the [[National High Magnetic Field Laboratory]]
* [https://bibnum.obspm.fr/exhibits/show/cometes Comets: from myths to reality], exhibition on [[Paris Observatory]] digital library

{{Comets|nonobject=yes}}
{{Meteorites}}
{{Solar System}}
{{Small Solar System bodies}}
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