Editing Comet (section)

== 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>