Editing Comet (section)

=== {{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>