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78P/Gehrels

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78P/Gehrels
Comet Gehrels 2 photographed from the Zwicky Transient Facility on 27 January 2019
Discovery[1]
Discovered byTom Gehrels
Discovery sitePalomar Observatory, Arizona, USA
Discovery date29 September 1973
Designations
P/1973 S1, P/1981 L1
  • Gehrels 2
  • 1973 XI, 1981 XVII
  • 1989 XVII
  • 1973n, 1981f, 1989n
Orbital characteristics[2][3]
Epoch9 June 2026
(JD 2461200.5)
Observation arc52.247 years
Number of
observations		8,640
Aphelion5.459 AU
7.091 AU (2038)[4]
Perihelion2.004 AU
4.121 AU (2032)[5]
Semi-major axis3.732 AU
Eccentricity0.4629
Orbital period7.209 years
Inclination6.2577°
210.49°
Argument of
periapsis		192.80°
Mean anomaly357.80°
Last perihelion25 June 2026[6]
2 April 2019[6][7]
Next perihelion9 January 2032[5]
TJupiter2.887
Earth MOID1.022 AU
Jupiter MOID0.024 AU
Physical characteristics[2]
Mean radius
4.21 km (2.62 mi)[8]
Mean density
550±80 kg/m3[9]
  • (B–V) = 0.88±0.02[10]
  • (V–R) = 0.27±0.02[10]
Comet total
magnitude (M1)		9.1
Comet nuclear
magnitude (M2)		12.5

78P/Gehrels, also known as Gehrels 2, is a Jupiter-family comet roughly 8 km in diameter with a current orbital period of 7.2 years. It is the second periodic comet discovered by American astronomer, Tom Gehrels.

At perihelion on 25 June 2026 the solar elongation was 33 degrees at an apparent magnitude of approximately 15,[11][6] The closest approach to Earth will be on 31 December 2026 at a distance of 1.56 AU (233 million km).[11] The comet will make a very close approach to Jupiter in September 2029[2] that will move the orbit outwards and the comet will next come to perihelion on 9 January 2032 at a distance of 4.1 AU from the Sun.[5]

Observational history

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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.[12]

The comet's predicted next appearance was observed by W. and A. Cochran at the McDonald Observatory, Texas on 8 June 1981.[13][14] It was observed again in 1989[15] and in 1997, when favourable conditions meant that brightness increased to magnitude 12.[12] It has subsequently been observed on every apparition since,[6] with the first polarimetric observations of the comet were conducted at the Girawali Observatory in 2012.[16]

Physical characteristics

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Initial CCD observations of 78P/Gehrels in 2003 indicated that it has a small nucleus of only around 1.54 km (0.96 mi).[17] Photometry obtained from the Isaac Newton Telescope in 2006 revealed it has an effective radius of about 4.21 km (2.62 mi) instead, however this might represent its upper limit.[8] The maximum dust production rate during its 2019 apparition was measured at Afρ = 1028.1±13.3 cm, making it one of the most active Jupiter-family comets ever known.[10][18]

Orbit

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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,[10] with its first encounter with Jupiter occurred only about 500 years ago.[19] 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.[2] On 15 September 2029, the comet will pass within 0.018 AU (2.7 million km) of Jupiter—slightly outside the orbit of Callisto—and be strongly perturbed.[2] By the year 2200, the comet will have a centaur-like orbit with a perihelion (closest distance to the Sun) near Jupiter.[20] 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.

The Outward Migration of 78P/Gehrels
Year (epoch)2009[2]20302200[20]
Semi-major axis3.736.029.37
Perihelion2.004.084.99
Aphelion5.467.9613.7
Comet 78P passing within 0.018 AU (2.7 million km) of Jupiter in 2029.[2]

References

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  1. 1 2 T. Gehrels; R. Adams; C. Vesely; et al. (31 October 1973). B. G. Marsden (ed.). "Comet Gehrels (1973n)". IAU Circular. 2589 (1). Bibcode:1973IAUC.2589....1G.
  2. 1 2 3 4 5 6 7 "78P/Gehrels 2 – JPL Small-Body Database Lookup". ssd.jpl.nasa.gov. Jet Propulsion Laboratory. Retrieved 11 May 2009.
  3. ↑ "78P/Gehrels Orbit". Minor Planet Center. Retrieved 29 June 2020.
  4. ↑ "Horizons Batch for 78P/Gehrels 2 (90000845) on 2038-Jul-16". JPL Horizons. Retrieved 25 July 2026. (JPL#K263/26/Soln.date: 2026-Jul-20)
  5. 1 2 3 "Horizons Batch for 78P/Gehrels 2 (90000845) on 2032-Jan-09" (Perihelion occurs when rdot flips from negative to positive). JPL Horizons. Retrieved 22 July 2026. (JPL#K263/26/Soln.date: 2026-Jul-20)
  6. 1 2 3 4 S. Yoshida (3 October 2006). "78P/Gehrels 2". www.aerith.net. Retrieved 24 February 2010.
  7. ↑ S. Nakano (10 June 2011). "78P/Gehrels 2 (NK 2102)". OAA Computing and Minor Planet Sections. Retrieved 18 February 2012.
  8. 1 2 C. S. Snodgrass; S. C. Lowry; A. Fitzsimmons (2007). "Optical observations of 23 distant Jupiter Family Comets, including 36P/Whipple at multiple phase angles" (PDF). Monthly Notices of the Royal Astronomical Society. 385 (2): 737–756. arXiv:0712.4204. Bibcode:2008MNRAS.385..737S. doi:10.1111/j.1365-2966.2008.12900.x. S2CID 55536237.
  9. ↑ M. L. Paradowski (2022). "A New Indirect Method of Determining Density of Cometary Nuclei" (PDF). Acta Astronomica. 72 (2): 141–159. Bibcode:2022AcA....72..141P. doi:10.32023/0001-5237/72.2.4. ISSN 0001-5237.
  10. 1 2 3 4 R. Xu; J. Shi; Y. Ma; et al. (2024). "Activity Analysis on 68P/Klemola and 78P/Gehrels 2 in 2018–2020 Perihelion Passage". The Astronomical Journal. 167 (3): 111–124. Bibcode:2024AJ....167..111X. doi:10.3847/1538-3881/ad1ff2.
  11. 1 2 G. van Buitenen. "78P/Gehrels". astro.vanbuitenen.nl. Retrieved 12 April 2026.
  12. 1 2 G. W. Kronk. "78P/Gehrels 2". Cometography.com. Retrieved 25 February 2015.
  13. ↑ A. Cochran (12 June 1981). B. G. Marsden (ed.). "Periodic Comet Gehrels 2 (1981f)". IAU Circular. 3612 (1). Bibcode:1981IAUC.3612....1C.
  14. ↑ A. Cochran; W. Cochran (8 July 1981). D. W. Green (ed.). "Periodic Comet Gehrels 2 (1981f)". IAU Circular. 3615 (4). Bibcode:1981IAUC.3615....4C.
  15. ↑ J. Gibson (3 July 1989). D. W. Green (ed.). "Periodic Comet Gehrels 2 (1989n)". IAU Circular. 4800 (1). Bibcode:1989IAUC.4800....1G.
  16. ↑ S. R. Choudhury; E. Hadamcik; A. K. Sen (2012). "A study of comet 78P/Gehrels by imaging polarimetry". Journal of Quantitative Spectroscopy and Radiative Transfer. 146: 444–451. Bibcode:2014JQSRT.146..444R. doi:10.1016/j.jqsrt.2014.02.010.
  17. ↑ S. C. Lowry; P. R. Weissman (2003). "CCD observations of distant comets from Palomar and Steward Observatories". Icarus. 164 (2): 492–503. Bibcode:2003Icar..164..492L. doi:10.1016/S0019-1035(03)00129-5.
  18. ↑ E. M. Epifani; P. Palumbo (2011). "The dust coma environment of the short period comets 32P/Comas Solá, 56P/Slaughter-Burnham, and 78P/Gehrels 2 from ground-based observations" (PDF). Astronomy & Astrophysics. 525: 62–76. Bibcode:2011A&A...525A..62M. doi:10.1051/0004-6361/201014989.
  19. ↑ F. J. Pozuelos; F. Moreno; F. Aceituno; et al. (2014). "Dust environment and dynamical history of a sample of short-period comets" (PDF). Astronomy & Astrophysics. 568: 3–16. arXiv:1406.6220. Bibcode:2014A&A...568A...3P. doi:10.1051/0004-6361/201423762.
  20. 1 2 "Barycentric Osculating Orbital Elements for Comet 78P/Gehrels 2 in epoch 1800 and 2200". JPL Horizons On-Line Ephemeris System. Jet Propulsion Laboratory. Retrieved 11 May 2009. (Solution using the Solar System's barycenter (Sun+Jupiter). Select Ephemeris Type:Elements and Center:@0)
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