26 October 2019
Perihelion Distance (q)
Aphelion Distance (Q)
Click for NASA orbit diagram
Hills Observatory: 1 January 2013 to 26 May 2020
The events leading up to the discovery of this comet are among the strangest ever recorded.
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 15 October 1974, 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 one 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.
Near the end of February of 1975, Lubos Kohoutek (Hamburg Observatory, Germany) found a
cometary object on a plate exposed on 9 February. As the motion was ambiguous, he began
searching for this comet on 27 February in areas southwest and northeast of the 9 February
position. Kohoutek found a comet to the southwest on a photograph exposed on 27 February. He
estimated the magnitude as 13 and described the comet as diffuse, with a condensation. On 2
March, 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 1 March, at a position
northeast of Kohoutek's 27 February position. He estimated the magnitude as 12, and his
confirmation on 2 March revealed it heading in the opposite direction to Kohoutek's reported
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 27
February comet. Thus, while attempting to confirm the 9 February comet, Kohoutek found a
second one which was independently found by Ikemura. The 9 February 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 3 March. It indicated a perihelion date of 23 March 1975, 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-
The comet's elliptical nature was first established by B. G. Marsden on 7 March 1975, 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 22 March 1972. 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 12 April. H.-E. Schuster recovered the comet on 12 November 1980
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 5 June 2000, 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.
The comet was recovered on 2006 August 30 by Peter Birtwistle (Great Shefford Observatory,
England). He gave the nuclear magnitude as 18.8.
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