 Comet Gehrels 2 photographed from theZwicky Transient Facility on 27 January 2019 | |
| Discovery[1] | |
|---|---|
| Discovered by | Tom Gehrels |
| Discovery site | Palomar Observatory, Arizona, USA |
| Discovery date | 29 September 1973 |
| Designations | |
| P/1973 S1, P/1981 L1 | |
| |
| Orbital characteristics[4][5] | |
| Epoch | 21 November 2025 (JD 2461000.5) |
| Observation arc | 52.247 years |
| Number of observations | 8,640 |
| Aphelion | 5.461 AU |
| Perihelion | 2.005 AU |
| Semi-major axis | 3.733 AU |
| Eccentricity | 0.46299 |
| Orbital period | 7.212 years |
| Inclination | 6.257° |
| 210.49° | |
| Argument of periapsis | 192.78° |
| Mean anomaly | 330.47° |
| Last perihelion | 2 April 2019[2][3] |
| Next perihelion | 25 June 2026 |
| TJupiter | 2.887 |
| EarthMOID | 1.022 AU |
| JupiterMOID | 0.024 AU |
| Physical characteristics[4] | |
Mean radius | 4.21 km (2.62 mi)[6] |
| Comet total magnitude (M1) | 9.1 |
| Comet nuclear magnitude (M2) | 12.5 |
78P/Gehrels, also known asGehrels 2, is aJupiter-family comet with a current orbital period of 7.22 years. It is the second periodic comet discovered by American astronomer,Tom Gehrels.
It was discovered byTom Gehrels on photographic plates exposed between 29 September and 5 October 1973 at thePalomar Observatory.[1] It had a brightness ofapparent 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 theMcDonald 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] with the first polarimetric observations of the comet were conducted at theGirawali Observatory in 2012.[12]
InitialCCD observations of 78P/Gehrels in 2003 indicated that it has a smallnucleus of only around 1.54 km (0.96 mi).[13] Photometry obtained from theIsaac 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.[6] 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.[7][14]
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] with its first encounter with Jupiter occurred only about 500 years ago.[15] Comet 78P/Gehrels'aphelion at a distance of 5.4 AU (810 million km) from the Sun meant that its orbit is frequentlyperturbed 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 acentaur-like orbit with a perihelion (closest distance to the Sun) near Jupiter.[16] 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.
| Year (epoch) | 2009[4] | 2030 | 2200[16] |
|---|---|---|---|
| Semi-major axis | 3.73 | 6.02 | 9.37 |
| Perihelion | 2.00 | 4.08 | 4.99 |
| Aphelion | 5.46 | 7.96 | 13.7 |
| Numbered comets | ||
|---|---|---|
| Previous 77P/Longmore | 78P/Gehrels | Next 79P/du Toit–Hartley |
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