Editing Comet Hale–Bopp (section)

== Orbital changes ==
[[File:Hale-Bopp orbit.svg|thumb|right|Hale–Bopp at perihelion on April 1, 1997]]
{{multiple image
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| header = Animation of Hale–Bopp orbit
| image1 = Animation of Hale-Bopp orbit.gif
| caption1 = Polar view
| image2 = Animation of Hale–Bopp orbit - equatorial view.gif
| caption2 = Equatorial view
| footer = {{legend2|magenta|  Hale–Bopp }}{{·}}{{legend2|Cyan|[[Mercury (planet)|Mercury]]}}{{·}}{{legend2| OrangeRed |[[Venus]] }}{{·}}{{legend2|Royalblue|Earth}} {{·}}{{legend2|Gold|[[Mars]] }}{{·}}{{legend2|Lime|[[Jupiter]] }} }}

The comet likely made its previous [[perihelion]] approximately 4,200 years ago,{{r|Yeomans_1997}} roughly the year 2215&nbsp;BC.{{r|Marsden_1997}} The estimated closest approach to Earth was 1.4&nbsp;au, and it may have been observed in [[ancient Egypt]] during the [[6th dynasty]] reign of the Pharaoh [[Pepi II]] (Reign: 2247 – c. 2216 BC). Pepi's [[pyramid]] at [[Saqqara]] contains a text referring to an "nhh-star" as a companion of the pharaoh in the heavens, where "{{lang|egy|nhh}}" is the [[hieroglyph]] for long hair.{{r|Weeks_1998}}

Hale–Bopp may have had a near collision with Jupiter in 2215&nbsp;BC, which probably caused a dramatic change in its orbit, and 2215&nbsp;BC may have been its first passage through the inner Solar System from the [[Oort cloud]].{{r|Marsden_1997}} The comet's current orbit is almost [[perpendicular]] to the plane of the [[ecliptic]], so further close approaches to planets will be rare. However, in April 1996 the comet passed within {{convert|0.77|AU|e6km|abbr=unit}} of [[Jupiter]], close enough for its orbit to be measurably affected by the planet's [[gravity]].{{r|Yeomans_1997}} The comet's orbit was shortened considerably to a period of roughly 2,399&nbsp;years,{{r|barycenter}} and it will next return to the inner [[Solar System]] around the year 4385.{{r|Hale-Bopp4385}} Its greatest distance from the Sun ([[aphelion]]) will be about 354&nbsp;AU,{{r|jpl1}} reduced from about 525&nbsp;AU.{{r|epoch1600|Marsden_1997|Williams_2005}}

The estimated probability of Hale–Bopp's striking Earth in future passages through the inner Solar System is remote, about 2.5×10<sup>−9</sup> per orbit.{{r|Weissman_2007}} However, given that the comet nucleus is around {{cvt|60|km|mi}} in diameter,{{r|jpl1}} the consequences of such an impact would be apocalyptic. Weissman conservatively estimates the diameter at {{cvt|35|km|mi}}; an estimated density of 0.6 g/cm<sup>3</sup> then gives a cometary mass of 1.3×10<sup>19</sup> g. At a probable impact velocity of 52.5&nbsp;km/s, impact energy can be calculated as 1.9×10<sup>32</sup> [[erg]]s, or 4.4×10<sup>9</sup> [[TNT equivalent|megatons]], about 44 times the estimated energy of the [[Alvarez hypothesis|K-T impact event]].{{r|Weissman_2007}}

Over many orbits, the cumulative effect of gravitational perturbations on comets with high orbital [[inclination]]s and small perihelion distances is generally to reduce the perihelion distance to very small values. Hale–Bopp has about a 15%&nbsp;chance of eventually becoming a [[sungrazing comet]] through this process.{{r|Bailey_1996}} If such is the case, it could undergo huge mass loss, or break up into smaller pieces like the [[Kreutz sungrazer]]s.  It would also be extremely bright, due to a combination of closeness to the Sun and nuclei size, potentially exceeding [[Halley's Comet]] in 837 AD.