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Comet
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{{Short description|Natural object in space that releases gas}} {{About|the astronomical object}} {{good article}} {{pp-semi-vandalism|small=yes}} {{pp-move|small=yes}} {{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 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> {{TOC limit|3}}
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