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90P/Gehrels 1

Past, Present, and Future Orbits by Kazuo Kinoshita


     T. Gehrels (Palomar Observatory, California, USA) discovered this comet on a photographic plate obtained with the 122-cm Schmidt Telescope on 1972 October 11.44. The magnitude was estimated as 19 and the comet was described as diffuse, with slight condensation, but no tail. The comet was again detected on a plate exposed with the same telescope on October 14.20. Although moonlight interfered during the remainder of the month, B. G. Marsden was able to calculate a set of search ephemerides which enabled Gehrels to again photograph the comet on November 10.24, when the magnitude was again estimated as 19.

Historical Highlights

APPARITION OF 1973: Discovery
[Perihelion Date=1973 January 24.67; Period=14.51 years]
     The first orbit was calculated by B. G. Marsden and revealed the comet was probably moving in a short-period orbit. Marsden provided a rough perihelion date of 1973 February and a period of 15 years. As further observations arrived, the orbit was refined to a perihelion date of January 24.74 and a period of 14.52 years. [With the comet being observed at future appartiions, the perihelion date was revised to January 24.67, while the period was 14.51 years].
     The comet remained a photographic object throughout this apparition and may have been at its brightest in December, when E. Roemer (Lunar and Planetary Laboratory, Catalina Station, Arizona, USA) determined the nuclear magnitude as about 18.2 on the 3rd, while Roemer and her colleague L. M. Vaughn (Steward Observatory, Kitt Peak, Arizona, USA) gave the nuclear magnitude as 18.7 on the 13th. The comet was last detected on 1973 September 23.48, when Roemer and G. Reskin photographed the comet with the 229-cm reflector at Steward Observatory. Roemer photographed the comet's predicted position on both October 21 and December 31, but no trace was found.

[Perihelion Date=1987 August 10.21; Period=15.06 years]
     S. Nakano used the positions from the discovery apparition to calculate a new orbit. He then integrated the motion to this apparition and predicted the comet would arrive at perihelion on 1987 August 14.81. J. V. Scotti (Steward Observatory, Kitt Peak, Arizona, USA) recovered the comet on 1987 August 29.43, using the 91-cm Spacewatch reflector. Another CCD image by Scotti on August 31 revealed a magnitude of 17.1. He added that the coma was moderately condensed, while a tail extended 82" in PA 262°. The only visual observation of this apparition was made by D. H. Levy (Lunar and Planetary Laboratory, Catalina station, Arizona, USA) on October 26, when he used the 154-cm reflector to estimate the magnitude as 15.0. The brightest CCD magnitude was obtained by Scotti and T. Gehrels on November 16, when the 91-cm Spacewatch reflector revealed a total magnitude of 16.7 and a nuclear magnitude of 19.3. The comet was last detected on 1988 March 13.20, when Gehrels and Scotti gave the total magnitude as 18.1 and the nuclear magnitude as 21.1.

[Perihelion Date=2002 June 22.97; Period=14.84 years]
     P. Rocher used the positions spanning 1972 to 1988 to calculate a new orbit. He then integrated the motion to this apparition and predicted the comet would arrive at perihelion on 2002 June 23.04. The comet was recovered by O. R. Hainaut, S. Holdstock, and A. C. Delsanti (European Southern Observatory, La Silla, Chile) on 2001 August 10.38, using the 154-cm Danish telescope. The total magnitude was determined as 20.8. The comet was probably at its brightest in November and December 2002, when several astronomers provided total magnitudes of 16 to 17 using CCD cameras. The comet was last detected on 2003 February 19.79, when A. Nakamura (Japan) found on it on a CCD image exposed with the 60-cm reflector. He judged the magnitude as 18.4.