Editing Comet (section)

=== 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>

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|+ [[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>
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