Copyright © 1997 by Herman Mikuz (Crni Vhr Observatory, Slovenia)
CCD image of periodic comet 78P/Gehrels 2 obtained on 1997 December 28.95UT with 36-cm, f/6.7 Schmidt-Cassegrain telescope and R filter. Exposure time was 5 minutes. (The Author has converted the image to black and white to better represent its appearance.)
Tom Gehrels (Lunar and Planetary Laboratory, Arizona, USA) discovered this comet while examining plates exposed with Palomar Observatory's 122-cm Schmidt Telescope during a minor planet survey. The comet's diffuse image was noted on plates exposed on 1973 September 29, September 30, October 4, and October 5. The brightness was estimated as 15-16, while a fan-shaped tail extended about two arc minutes.
The comet was officially announced on October 31, at which time B. G. Marsden (Central Bureau for Astronomical Telegrams) provided both a parabolic and elliptical orbit. The latter indicated an orbital period of 8.76 years. This orbit allowed numerous immediate observations, and the short-period nature of the orbit was confirmed on November 7 when Marsden issued a more precise orbit indicating a perihelion date of 1973 November 30 and an orbital period of 7.93 years. This orbit indicated the comet had passed 0.9 AU from Jupiter in 1971, and that prior to that encounter the orbital period had been 8.5 years.
A prediscovery image was identified in early November. This image was identified by E. Helin (California Institute of Technology, USA) on a plate exposed with the Palomar Observatory 46-cm Schmidt telescope on September 28. The brightness was estimated as magnitude 16.
The comet's maximum magnitude in 1973 never exceeded 15, with the comet steadily fading after November of that year. The comet had passed closest to Earth (1.3729 AU) on October 21. Astronomers continued to follow the comet until 1975 March 7 when photographed at magnitude 21 to 21.5.
During 1980 Marsden worked on a solution to recovery this comet during the expected 1981 return. His prediction enabled the comet to be found by W. and A. Cochran (McDonald Observatory, Texas, USA) on 1981 June 8. The brightness was then estimated as 19.5 and the position indicated Marsden's prediction was only 0.15 day off. The maximum brightness of the 1981 return never exceeded 16. The comet was closest to Earth (1.3738 AU) on October 26.
The comet next returned during 1989. It passed closest to Earth (1.3559 AU) on October 31 and passed closest to the sun (2.348 AU) on November 3. The maximum brightness was near 14.
The 1997 return of this comet was the most favorable since its discovery. It passed closest to the sun on 1997 August 7, and was closest to Earth (1.3119 AU) on December 14. Because of the decreasing distance from Earth after perihelion, the comet was expected to attain a maximum brightness of about 12.9 during October and early November, and such a magnitude was reached. However, the comet apparently became brighter during the period of December through February, with numerous visual magnitude estimates typically near 12, if not slightly brighter, during January. The coma diameter was then near 1.5 arc minutes and CCD images were then revealing a diffuse tail.
Copyright © 1998 by Stefano Sposetti
This image was taken by Stefano Sposetti on 1998 January 6.8. He used a 0.20-m f/6.3 Celestron telescope and a Hi-SIS22 CCD camera. This is a composite of 60 30-second exposures. North is up and east is left. (Thanks to Stefano Sposetti for permission to use this image.)