78P/Gehrels

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]

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]

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.