Editing Halley's Comet (section)

==Orbit and origin==
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|image1=Halley's Comet animation.gif
|caption1=The orbital path of Halley, against the orbits of the planets (''animation'')
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|caption2=[[Orionids|Orionid meteor]] originating from Halley's Comet streaking the sky below the [[Milky Way]] and to the right of [[Venus]]
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Halley's orbital period has varied between 74 and 80 years since 240 BC.<ref name="yeo"/> Its orbit around the [[Sun]] is highly [[Ellipse|elliptical]], with an [[orbital eccentricity]] of 0.967 (with 0 being a circle and 1 being a [[parabolic trajectory]]). The perihelion, the point in the comet's orbit when it is nearest the Sun, is {{convert|0.59|au|e6km|abbr=unit|lk=on}}. This is between the orbits of [[Mercury (planet)|Mercury]] and [[Venus]]. Its [[aphelion]], or farthest distance from the Sun, is {{convert|35|au|e9km|abbr=unit}}, roughly the orbital distance of [[Pluto]]. Unlike the overwhelming majority of objects in the Solar System, Halley's orbit is [[Retrograde and direct motion|retrograde]]; it orbits the Sun in the opposite direction to the planets, or, clockwise from above the Sun's north pole.<ref name="Hromakina2021"/> The orbit is inclined by 18° to the [[ecliptic]], with much of it lying south of the ecliptic.<ref name="Russell1988"/> This is usually represented as 162°, to account for Halley's retrograde orbit.<ref name="NASA_chapter5"/><ref name=MPC/> The 1910 passage was at a [[relative velocity]] of {{convert|70.56|km/s|mph|abbr=on}}.<ref name="jpldata"/> Because its orbit comes close to Earth's in two places, Halley is associated with two [[meteor shower]]s: the [[Eta Aquariids]] in early May, and the [[Orionids]] in late October.<ref name="Streams"/>

Halley is classified as a ''periodic'' or ''short-period [[comet]]'': one with an orbit lasting 200 years or less.<ref name="Morbidelli2006"/> This contrasts it with long-period comets, whose orbits last for thousands of years. Periodic comets have an average inclination to the ecliptic of only ten degrees, and an orbital period of just 6.5 years, so Halley's orbit is atypical.<ref name="hughes1987"/> Most short-period comets (those with orbital periods shorter than 20 years and inclinations of 30 degrees or less) are called Jupiter-family comets.<ref name="Morbidelli2006"/> Those resembling 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.<ref name="Morbidelli2006"/><ref name="jewitt2002"/> {{As of|2024}}, 105 Halley-type comets have been observed, compared with 816 identified Jupiter-family comets.<ref name="jpl_JFC-list"/><ref name="yfernandez"/>

The orbits of the Halley-type comets suggest that they were originally long-period comets whose orbits were perturbed by the gravity of the giant planets and directed into the inner Solar System.<ref name="Morbidelli2006"/> If Halley was once a long-period comet, it is likely to have originated in the [[Oort cloud]],<ref name="jewitt2002"/> a sphere of cometary bodies around [[Oort Cloud#Structure and composition|20,000–50,000 au]] from the Sun. Conversely the Jupiter-family comets are generally believed to originate in the [[Kuiper belt]],<ref name="jewitt2002"/> a flat disc of icy debris between 30 au (Neptune's orbit) and 50 au from the Sun (in the [[scattered disc]]). Another point of origin for the Halley-type comets was proposed in 2008, when a [[trans-Neptunian object]] with a retrograde orbit similar to Halley's was discovered, {{mpl|2008 KV|42}}, whose orbit takes it from just outside that of Uranus to twice the distance of Pluto. It may be a member of a new population of small Solar System bodies that serves as the source of Halley-type comets.<ref name="Gladman2009"/>

Halley has probably been in its current orbit for 16,000–200,000 years, although it is not possible to numerically integrate its orbit for more than a few tens of apparitions, and close approaches before 837 AD can only be verified from recorded observations.<ref name="Olsson1987"/> The non-gravitational effects can be crucial;<ref name="Olsson1987"/> as Halley approaches the Sun, it expels jets of sublimating gas from its surface, which knock it very slightly off its orbital path. These orbital changes cause delays in its [[perihelion]] passage of four days on average.{{sfn|Yeomans|1991|pp=260–261}}

In 1989, [[Boris Chirikov]] and Vitold Vecheslavov performed an analysis of 46 apparitions of Halley's Comet taken from historical records and computer simulations, which showed that its dynamics were chaotic and unpredictable on long timescales.<ref name="Chirikov1989b"/> Halley's projected [[Mean sojourn time|dynamical lifetime]] is estimated to be about 10&nbsp;million years.<ref name="Chirikov1989"/> The dynamics of its orbit can be approximately described by a two-dimensional [[symplectomorphism|symplectic map]], known as the [[Kepler map]], a solution to the [[restricted three-body problem]] for highly eccentric orbits.<ref name="Chirikov1989"/><ref name="Lages2018"/> Based on records from the 1910 apparition, [[David Hughes (astronomer)|David Hughes]] calculated in 1985 that Halley's nucleus has been reduced in mass by 80 to 90% over the last 2,000 to 3,000 revolutions, and that it will most likely disappear completely after another 2,300 perihelion passages.<ref name="Hughes1985"/> More recent work suggests that Halley will evaporate, or split in two, within the next few tens of thousands of years, or will be ejected from the Solar System within a few hundred thousand years.<ref name="Williams2015"/><!-- added this because it summarises jewitt2002 which is very technical and hard to read--><ref name="jewitt2002"/>