G A R Y   W.   K R O N K ' S   C O M E T O G R A P H Y


Past, Present, and Future Orbits by Kazuo Kinoshita

G. Sostero and E. Guido image of 76P exposed on 2006 December 20
Copyright © 2006 by G. Sostero and E. Guido (Remanzacco Observatory, Italy)

This image was taken on 2006 December 20.17 by G. Sostero and E. Guido. It was taken with a 45-cm f/4.4 reflector and an 1001E IMG CCD camera. It is composed of nine unfiltered 120-second exposures.


     The events leading up to the discovery of this comet are among the strangest ever recorded. It would certainly be appropriate to refer to this comet as the object that couldn't be lost!
     During January 1975, Richard M. West (European Southern Observatory Sky Atlas Laboratory, Geneva) was examining plates exposed by Pizarro and Ballereau at the European Southern Observatory at La Silla (Chile). On a plate exposed on 1974 October 15.13, he found a trail, which he described as diffuse, with a magnitude of 12. The comet also exhibited a central condensation and a tail less than 1 degree long. Searches through other ESO plates revealed no trace of the comet and West was left with a single image taken three months earlier and showing ambiguous motion (either to the east-southeast or west-northwest). The comet was presumed lost.
     Chapter two of this story began near the end of February of 1975, when Lubos Kohoutek (Hamburg Observatory, Germany) found a cometary object on a plate exposed on February 9. As the motion was ambiguous, he began searching for this comet on February 27 in areas southwest and northeast of the February 9 position. Kohoutek found a comet to the southwest on a photograph exposed on February 27.78. He estimated the magnitude as 13 and described the comet as diffuse, with a condensation. On March 2, word came from Japan that Toshihiko Ikemura (Shinshiro) was not able to find Kohoutek's comet in the extrapolated position, but he had discovered a comet on March 1.52, at a position northeast of Kohoutek's February 27 position. He estimated the magnitude as 12, and his confirmation on March 2.45 revealed it heading in the opposite direction to Kohoutek's reported comet.
     B. G. Marsden began examining the February positions and immediately noted that when Ikemura's comet was extrapolated backwards it aligned almost perfectly with Kohoutek's February 27 comet. Thus, while attempting to confirm the February 9 comet, Kohoutek found a second one which was independently found by Ikemura. The February 9 object was latter photographed and became known as 75P/Kohoutek.
     Additional images of comet Kohoutek-Ikemura were obtained at several observatories during the next week. K. Hurukawa and T. Hirayama computed the first parabolic orbit, which was published on March 3. It indicated a perihelion date of 1975 March 23.45, a perihelion distance of 0.940 AU, and an inclination of 13.0°. After a few more days of observations had been received, Marsden began his orbital computations and soon noted that comet Kohoutek-Ikemura was identical to the comet West had found on the October plates. The comet became officially known as West-Kohoutek-Ikemura.

Historical Highlights

  • The comet's elliptical nature was first established by B. G. Marsden on 1975 March 7, following the link of the February and March observations to that of October. Marsden commented, "It is also clear that the comet passed only 0.01 AU from Jupiter in March 1972." Later computations by G. W. Kronk using a more precise orbit revealed the comet passed 0.012 AU from Jupiter late on 1972 March 22. Prior to the comet's entry into Jupiter's sphere of influence during the latter half of 1971, the comet moved in an elliptical orbit with a period of about 30 years and a perihelion distance of 4.78 AU.
  • The comet returned to perihelion in 1981. Computations by S. W. Milbourn during 1980 predicted the perihelion date would be April 12. H.-E. Schuster recovered the comet on 1980 November 12 and the measured position indicated a correction of -1.37 days needed to be applied to Milbourn's prediction. This was not an especially favorable apparition as the comet reached a maximum brightness of only 17.
  • The comet was also seen at its returns in 1987 and 1993. The latter return was rather favorable as observers reported maximum magnitudes slightly brighter than 13.
  • The comet made a very close approach to Mars on 2000 June 5, with Kenji Muraoka (Japan) giving the smallest distance between the comet and Mars as 0.04215 AU, which is equal to 3,914,000 miles or 6,303,000 kilometers. Although this comet has been bright enough for large amateur telescopes to see in the past, this close approach was not observable by even the largest telescopes as the comet was then situated less than 7° from the sun. On a sad note, the Author began receiving a large amount of e-mail from people who had stumbled across a web site that was predicting doom for our planet. A visit to the web site revealed the claim that astronomers were wrong about their calculations and that the comet was actually going to hit Mars, which would subsequently throw pieces of the red planet directly toward our planet! People must have a short memory, because the same web site had failed on all of its previous attempts to predict doom by comet. There was never a chance of collision, which meant there was no chance of pieces of Mars hitting us. This just represented a close approach of a comet and that planet.
  • The comet was recovered on 2006 August 30 by Peter Birtwistle (Great Shefford Observatory, England). He gave the nuclear magnitude as 18.8.
  • Additional Images

    G. Rhemann image of 76p exposed on 1993 December 6
    Copyright © 1993 by Gerald Rhemann

    This image was obtained on 1993 December 6.88 UT with the 225/255/435mm Schmidt camera. Exposure time was 5 minutes and the photographic emulsion was Kodak Pro Gold 400/120. The comet's total magnitude was then about 13. (The image has been cropped by the webmaster to save space and reversed to better represent the visual appearance of the comet.)

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