Astronomer Tom Gehrels discovered this comet accidently. Some plates obtained on routine survey for minor planets revealed a comet. The photos exposed on a 122–Schmidt Telescope at the Palomar Observatory were a diffused image of a comet. As astronomer himself was the first to discover it, the comet was named after him. Comet 78P/Gehrels was discovered on 30th September, 1973 and was officially announced on 31st October.
Observations made about this comet 87P/Gehrels 2 include:
- The comet appeared diffused.
- It exhibited a fan–shaped tail.
- An elliptical and parabolic orbit was provided by astronomer B. G Marsden.
At the time of its discovery in September 1973, the magnitude of comet 78P/Gehrels2 was estimated at around 15–16. A pre–discovery image obtained in an image of early November revealed a magnitude of 16. The comet was followed and images obtained on 7th March, 1975 revealed a magnitude of about 21–21.5.
The perihelion distance (nearest to the sun) of comet 78P/Gehrels 2 is noted at 2.009 AU and the aphelion distance (far from the sun) is at 5.462 AU. Astronomical predictions states that this comet is greatly perturbed by Jupiter. There are indications that a close encounter (of 0.018 AU) with Jupiter on 15th September, 2029 will increase its perihelion and push the comet into a dormant state.
The orbital period of comet 78P/Gehrels 2 is noted at about 7.22 years.
Comet 78P/Gehrels 2 was recovered by W. A Cochran of MacDonald Observatory, Texas, USA around 8th June, 1981.
Comet 78P/Gehrels 2 had a close encounter with Jupiter in 1971. It passed through 0.9 AU, which decreased its perihelion and led to its discovery.
The comet 78P/ Gehrels 2 had last appeared on 12th January, 2012 and is next expected to appear on 2nd April, 2019.