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161P/Hartley-IRAS

Past, Present, and Future Orbits by Kazuo Kinoshita

M. Jäger and G. Rhemann image of 161P exposed on 2005 July 14
Copyright © 2005 by Michael Jäger and Gerald Rhemann (Austria)

This image was obtained on 2005 July 14 by M. Jäger and G. Rhemann.

Discovery

     M. Hartley (U. K. Schmidt Telescope Unit, Australia) found the trail of a comet on a photographic plate exposed on 1983 November 4.47. He estimated the magnitude as 15. Before a confirmation plate could be obtained, J. Davies and S. Green (University of Leicester, England) reported on November 11 that the Infrared Astronomy Satellite (IRAS) had imaged a comet on November 10.35. They estimated the magnitude as 15.5. K. S. Russell (U. K. Schmidt Telescope Unit) was asked by the Central Bureau for Astronomical Telegrams to try and confirm the comet, at which time Russell noted that Hartley's comet and the IRAS comet might be identical. Russell exposed a plate on November 14, through clouds and in bright moonlight, but nothing was found. Russell finally managed to confirm the comet and the suggested identity on November 23.44. He estimated the magnitude as 15.5.

Historical Highlights

  • The first parabolic orbit was published with the discovery announcement on 1983 November 25. B. G. Marsden (Central Bureau for Astronomical Telegrams) took the five available positions and determined the perihelion date as 1983 December 31.14. The orbit was described as "rough." On December 5, Marsden and D. W. E. Green noted that although this orbit was still adequately representing the positions, further observations were "required in order to investigate an indicated possible departure from parabolic motion." The elliptical orbit was confirmed on December 9, when Marsden gave the perihelion date as 1984 January 8.93 and the period as about 20.7 years. He said the period was very uncertain and that if it was 28 years the comet might have passed close to Jupiter in 1957. As it turned out the comet's elliptical orbit had a perihelion date of January 8.71 and a period of 21.45 years. The comet had passed close to Jupiter on 1921 August 21 at a distance of 0.44 AU.
  • There are indications that the comet rapidly brightened. B. Skiff (Lowell Observatory, Arizona, USA) reported a photographic magnitude of 12 on November 30, while D. Levy (Tucson, Arizona, USA) and J. Bortle (Stormville, New York, USA) estimated the magnitude as about 11.5 and 10.6, respectively, on December 1. In addition, a photographic observation by J. Gibson (Palomar Observatory, California, USA) on November 28 showed a hint of tail. It was predicted that the comet would slowly fade, but the comet had other ideas. After apparently holding at about magnitude 11 throughout most of January 1984, C. S. Morris (California, USA) reported the magnitude as 10.5 on January 29, 10.3 on February 5, and 9.9 on February 11. The comet peaked about two weeks later, when the magnitude was given as 7.8 by Bortle on February 23, 7.4 by Morris on the 24th, and 7.7 by Green on the 25th. Green noted the magnitude had already dropped to 8.6 on February 27 and this was confirmed in the nights that followed by several other observers. Bortle noted the comet was fainter than 9 on March 8 and fainter than 10 on March 27. Although the brightness changed very little during April, it attained a magnitude of 11 after the first week of May and was fainter than 12 by the end of the month. The comet was last detected on June 4.10 by R. E. McCrosky, G. Schwartz, and C.-Y. Shao (Oak Ridge Observatory).
  • The comet was recovered on 2004 November 3, when R. H. McNaught (Siding Spring Observatory) estimated the magnitude as 19.5. The comet passed perihelion on 2005 June 20. The comet passed closest to Earth on 2005 July 11 (1.48 AU). The comet was brighter than magnitude 11 by mid-July.
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