{{short description|Periodic comet}}
{{short description| comet}}
{{Use dmy dates|date=January 2026}}
{{Use dmy dates|date=January 2026}}
{{Infobox comet
{{Infobox comet
| physical_ref = {{r|jpldata}}
| physical_ref = {{r|jpldata}}
| mean_radius = {{cvt|4.28|km|mi}}{{r|Snodgrass_2007}}
| mean_radius = {{cvt|4.28|km|mi}}{{r|Snodgrass_2007}}
| spectral_type = {{unbulleted|(B–V) {{=}} {{val|0.88|0.02}}{{r|Xu_2024}}|(V–R) {{=}} {{val|0.27|0.02}}{{r|Xu_2024}}}}
| M1 = 9.1
| M1 = 9.1
| M2 = 12.5
| M2 = 12.5
== Orbit ==
== Orbit ==
Comet 78P/Gehrels’ [[Apsis|aphelion]] at a distance of {{convert|5.4|AU|e6km|abbr=unit}} from the Sun meant that its orbit is frequently [[Perturbation (astronomy)|perturbed]] by Jupiter.{{r|jpldata}} On 15 September 2029, the comet will pass within {{convert|0.018|AU|e6km|abbr=unit}} of Jupiter and be strongly perturbed.{{r|jpldata}} By the year 2200, the comet will have a [[Centaur (minor planet)|centaur]]-like orbit with a perihelion (closest distance to the Sun) near Jupiter.{{r|Horizons2200}} This outward migration from a perihelion of {{convert|2.01|AU|e6km|abbr=unit}} to {{convert|5.057|AU|e6km|abbr=unit}} could cause the comet to go dormant.
Comet 78P/Gehrels’ [[Apsis|aphelion]] at a distance of {{convert|5.4|AU|e6km|abbr=unit}} from the Sun meant that its orbit is frequently [[Perturbation (astronomy)|perturbed]] by Jupiter.{{r|jpldata}} On 15 September 2029, the comet will pass within {{convert|0.018|AU|e6km|abbr=unit}} of Jupiter and be strongly perturbed.{{r|jpldata}} By the year 2200, the comet will have a [[Centaur (minor planet)|centaur]]-like orbit with a perihelion (closest distance to the Sun) near Jupiter.{{r|Horizons2200}} This outward migration from a perihelion of {{convert|2.01|AU|e6km|abbr=unit}} to {{convert|5.057|AU|e6km|abbr=unit}} could cause the comet to go dormant.
<div style=”float:right; margin:2px;”>
<div style=”float:right; margin:2px;”>
| s2cid= 55536237
| s2cid= 55536237
| doi= 10.1111/j.1365-2966.2008.12900.x
| doi= 10.1111/j.1365-2966.2008.12900.x
| doi-access= free }}
</ref>
<ref name=”Xu_2024″>{{cite journal
| author1= R. Xu
| author2= J. Shi
| author3= Y. Ma
| display-authors= etal
| title= Activity Analysis on 68P/Klemola and 78P/Gehrels 2 in 2018–2020 Perihelion Passage
| journal= The Astronomical Journal
| year= 2024
| volume= 167
| issue= 3
| pages= 111–124
| bibcode= 2024AJ….167..111X
| doi= 10.3847/1538-3881/ad1ff2
| doi-access= free }}
| doi-access= free }}
</ref>
</ref>
Jupiter-family comet
78P/Gehrels, also known as Gehrels 2, is a Jupiter-family comet with a current orbital period of 7.22 years. It is the second periodic comet discovered by American astronomer, Tom Gehrels.
Observational history
[edit]
It was discovered by Tom Gehrels on photographic plates exposed between 29 September and 5 October 1973 at the Palomar Observatory.[1] It had a brightness of apparent magnitude of 15. Brian G. Marsden computed the parabolic and elliptical orbits which suggested an orbital period of 8.76 years, later revising the data to give a perihelion date of 30 November 1963 and orbital period of 7.93 years.[8]
The comet’s predicted next appearance was observed by W. and A. Cochran at the McDonald Observatory, Texas on 8 June 1981.[9][10] It was observed again in 1989[11] and in 1997, when favourable conditions meant that brightness increased to magnitude 12.[8] It has subsequently been observed on every apparition since.[2]
The dynamical history of this comet indicated that it had resided within the planetary region of the Solar System for about a million years, but its migration to the inner Solar System is only relatively recent.[7] Comet 78P/Gehrels’ aphelion at a distance of 5.4 AU (810 million km) from the Sun meant that its orbit is frequently perturbed by Jupiter.[4] On 15 September 2029, the comet will pass within 0.018 AU (2.7 million km) of Jupiter and be strongly perturbed.[4] By the year 2200, the comet will have a centaur-like orbit with a perihelion (closest distance to the Sun) near Jupiter.[12] This outward migration from a perihelion of 2.01 AU (301 million km) to 5.057 AU (756.5 million km) could cause the comet to go dormant.
