|Discovered by||Tom Gehrels|
|Discovery date||September 29, 1973|
|1973 XI; 1981 XVII; 1989 XVII|
|Orbital characteristics A|
|Epoch||March 6, 2006|
|Semi-major axis||3.735 AU|
|Orbital period||7.22 yr|
|Last perihelion||April 2, 2019|
January 12, 2012
October 27, 2004
It was discovered by Tom Gehrels at the Lunar and Planetary Laboratory, Arizona, USA 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 in 1981 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 in 2004 when it reached magnitude 10, 2012, and 2018.
Comet 78P/Gehrels' aphelion (furthest distance from the Sun) of 5.4AU is in the zone of control of the giant planet Jupiter and the orbit of the comet is frequently perturbed by Jupiter. On September 15, 2029, the comet will pass within 0.018 AU (2.7 million kilometers) of Jupiter and be strongly perturbed. By the year 2200, the comet will have a centaur-like orbit with a perihelion (closest distance to the Sun) near Jupiter. This outward migration from a perihelion of 2AU to a perihelion of ~5AU could cause the comet to go dormant.