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123P/West-Hartley

Past, Present, and Future Orbits by Kazuo Kinoshita

Vega Observatory image of 123P exposed on 2004 January 24
Copyright © 2004 by Mihelcic Matej (Vega Observatory, Slovenia)

This image was obtained on 2004 January 24.18, by Mihelcic Matej using a 10-inch f/6.3 reflector and an MX5 CCD camera. It is a 240-second exposure.

Discovery

     Richard M. West (European Southern Observatory) discovered this comet on 1989 May 11 while examing plates exposed by G. Pizarro (La Silla) on 1989 March 14.32 for the ESO Quick Blue Survey extension. He described the comet as a condensation enveloped in a diffuse halo and noted a tail extending 0.5 arc minute toward PA 310°. He initially gave the daily motion as 210 arc seconds, but with only one available plate the direction of motion was ambiguous and was therefore given as PA 45° or PA 225°.
     Upon receiving the information, Brian G. Marsden (Central Bureau for Astronomical Telegrams) computed a series of ephemerides based on assumptions of both parabolic and elliptical motion. R. H. McNaught noted a plate exposed with the U. K. Schmidt on 1989 April 28 covered the area expected if the comet was heading toward PA 225°. His failure to locate the comet, however, seemed to eliminate the elliptical solution and one of the parabolic solutions.
     Malcolm Hartley discovered a comet on a J survey plate exposed with the U. K. Schmidt on 1989 May 28.47 by S. M. Hughes. McNaught subsequently noted the position was actually close to that expected if West's comet was following Marsden's elliptical solution and that if the two comets were the same, the comet would have been just outside the April 28 plate's field of view. Hartley's plate revealed the comet's total magnitude was 17.0 and that a tail extended four arc minutes toward PA 305°.
     Marsden subsequently took the precise positions measured by West and Hartley and successfully linked them. They indicated the comet had passed perihelion on 1988 October 1.6. The perihelion distance was given as 2.42 AU and the orbital period was 6.61 years. Marsden considered the orbit was "still very uncertain."

Historical Highlights

  • With the comet several months passed perihelion at the time of discovery and with it being rather faint, observations were not numerous. Investigations of photographs obtained at other observatories revealed the comet was present on a plate obtained at Palomar Observatory on May 7.30. Following the comet's discovery announcement, T. Seki (Geisei, Japan) photographed it on May 30 and Pizarro photographed it on May 31. The latter observer estimated the magnitude as 18. Observers at La Silla further detected the comet on June 1 and 8. The final observation of the comet was made at Mount John Observatory on June 30.46.
  • Following the 1989 June 1 observation, Marsden revised his earlier published elliptical orbit. The perihelion date was 1988 October 6.2, the perihelion distance was 2.133 AU, and the orbital period was 7.56 years. Later orbits using more than one apparition indicated only slight changes to this orbit.
  • S. Nakano investigated the positions obtained in 1989, determined a new orbit, and integrated it forward to the comet's next perihelion. He predicted this would come on 1996 May 12.04. T. Gehrels and J. V. Scotti (Lunar and Planetary Laboratory, Arizona, USA) recovered the comet on 1995 September 21.47 with the 0.9-m Spacewatch telescope at Kitt Peak. The total magnitude was estimated as between 18.8 and 19.1, the magnitude of the nuclear condensation was 21.1, the coma diameter was 11 arc seconds, and the tail extended 22 arc seconds toward PA 268°. The precise positions indicated Nakano's prediction required a correction of +0.91 day. The comet was followed by various observatories until 1996 August 5.48, at which time the comet was detected at Yatsuka. The total magnitude was then given as 14.2.
  • The comet last passed perihelion on 2003 December 9. Magnitude estimates generally exceeded 15 as 2004 began and peaked at between magnitude 14 and 14.5 during February.
  • Close approaches to planets: This comet made 2 close approaches to Jupiter during the 20th century. It makes no close approaches to within 1 AU of any planet during the 21st century. (From the orbital work of Kazuo Kinoshita)
    • 0.79 AU from Jupiter on 1939 December 25
      • decreased perihelion distance from 2.33 AU to 2.23 AU
      • decreased orbital period from 8.29 to 7.95 years
    • 0.68 AU from Jupiter on 1963 September 21
      • decreased perihelion distance from 2.23 AU to 2.18 AU
      • decreased orbital period from 7.94 to 7.71 years

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