78P/Gehrels
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
It was discovered by Tom Gehrels on photographic plates exposed between 29 September and 5 October 1973 at the Palomar Observatory. 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.The comet's predicted next appearance was observed by W. and A. Cochran at the McDonald Observatory, Texas on 8 June 1981. It was observed again in 1989 and in 1997, when favourable conditions meant that brightness increased to magnitude 12. It has subsequently been observed on every apparition since, with the first polarimetric observations of the comet were conducted at the Girawali Observatory in 2012.
Physical characteristics
Initial CCD observations of 78P/Gehrels in 2003 indicated that it has a small nucleus of only around. Photometry obtained from the Isaac Newton Telescope in 2006 revealed it has an effective radius of about instead, however this might represent its upper limit. The maximum dust production rate during its 2019 apparition was measured at Afρ, making it one of the most active Jupiter-family comets ever known.Orbit
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, with its first encounter with Jupiter occurred only about 500 years ago. Comet 78P/Gehrels' aphelion at a distance of from the Sun meant that its orbit is frequently perturbed by Jupiter. On 15 September 2029, the comet will pass within of Jupiter and be strongly perturbed. By the year 2200, the comet will have a centaur-like orbit with a perihelion near Jupiter. This outward migration from a perihelion of to could cause the comet to go dormant.| Year | 2009 | 2030 | 2200 |
| Semi-major axis | 3.73 | 6.02 | 9.37 |
| Perihelion | 2.00 | 4.08 | 4.99 |
| Aphelion | 5.46 | 7.96 | 13.7 |