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26P/Grigg-Skjellerup

Past, Present, and Future Orbits by Kazuo Kinoshita

Discovery

     John Grigg (Thames, New Zealand) was engaged in his monthly sky survey when he discovered this comet in Virgo on 1902 July 23.27. Grigg was using a 3.5-inch refractor (power=25x) and said the comet looked like an extremely faint nebula, and was "about twice the diameter of Jupiter." Grigg was able to catch a glimpse of the comet under mostly cloudy skies on the 24th, but his solid confirmation did not come until the skies cleared on the 27th. After that observation he could finally send out notices. Again battling the weather, Grigg was able to again observe the comet on July 30 and August 2 and 3, the latter of which was his final observation. Grigg's notices failed to reach the authorities in time, so that he remained the only observer. Shortly after his final observation he computed a parabolic orbit. Up to that time he thought he might have caught the return of some other periodic comet, but his orbit revealed this was a hitherto unknown comet.
     James Francis Skjellerup (Cape Town, South Africa) discovered a comet on 1922 May 17. A notice was sent out and observations were soon made at numerous observatories around the world. The general concensus during the earliest observations was that the comet's total magnitude was around 11, while the coma diameter was between 4 and 5 arc minutes. By the end of the month astronomers had already found that the observations were indicating the comet was moving in a short-period orbit. Shortly after mid-June, R. T. Crawford and W. F. Meyer (Berkeley Astronomical Department, California, USA) pointed out the similarities between the orbits of Grigg's comet of 1902 and that found by Skjellerup. Because of the rough nature of Grigg's observations, Crawford and Meyer added that identity could not be completely confirmed until the comet returned in 1927. Identity was fully confirmed in that year.
     During 1986 Lubor Kresak (Astronomical Institute, Slovak Academy of Sciences) suggested this comet had actually been seen in 1808 by Jean Louis Pons. Pons had reported seeing a comet on February 6 and 9 of that year and said it was about one degree in diameter, suggesting it was near Earth. On the latter date he said it was in Ophiuchus and made a drawing showing its location near two globular clusters. Kresak said a backward integration of the orbit of Grigg-Skjellerup by N. A. Belyaev revealed this comet would have passed about 0.12 AU from Earth during February 1808.

Historical Highlights

  • The comet has been observed at every return since its 1922 rediscovery. The maximum brightness typically reaches 9 or 9.5 during the returns when it passes perihelion during April or May. This last happened in 1977 and 1982. It can also have rather unfavorable apparitions when the total magnitude fails to become brighter than 16.
  • Since 1725 the comet has experienced several encounters with Jupiter which have acted to steadily increase the comet's perihelion distance from 0.77 AU to the present 0.99 AU.
  • The current alignment of this comet's orbit allows it to produce a meteor shower around April 23. The Pi Puppids were discovered in 1972 following the prediction by H. B. Ridley that the comet could now produce a meteor shower. Activity is best seen by observers in the Southern Hemisphere where hourly rates have reached values as high as 42, but only in those years closest to the comet's perihelion passage. Activity in other years is basically nonexistent.
  • The 1997 return was not one of the more favorable returns. The August 30 perihelion date prevented the comet from coming closer than 1.76 AU from Earth. The result was a probable maximum magnitude near 13. Unfortunately the maximum brightness happened when the comet was less than 30 degrees from the sun at the end of August. Only a handful of observations were made in the months before and after perihelion. The 2002 return was even more unfavorable and no observations were reported.
  • Close approaches to planets: This comet made 12 close approaches to Earth and 4 close approaches to Jupiter during the 20th century. It makes 4 close approaches to Earth and one close approach to Jupiter during the first half of the 21st century. (From the orbital work of Kazuo Kinoshita)
    • 0.17 AU from Jupiter on 1905 January 22
      • increased perihelion distance from 0.75 AU to 0.90 AU
      • increased orbital period from 4.83 to 4.99 years
    • 0.51 AU from Earth on 1907 July 4
    • 0.47 AU from Earth on 1912 June 30
    • 0.27 AU from Earth on 1922 June 12
    • 0.20 AU from Earth on 1927 June 3
    • 0.24 AU from Earth on 1932 June 7
    • 0.86 AU from Jupiter on 1940 October 27
      • decreased perihelion distance from 0.91 AU to 0.86 AU
      • decreased orbital period from 5.02 to 4.90 years
    • 0.37 AU from Earth on 1942 June 9
    • 0.16 AU from Earth on 1947 April 12
    • 0.65 AU from Earth on 1952 March 9
    • 0.33 AU from Jupiter on 1964 March 17
      • increased perihelion distance from 0.86 AU to 1.00 AU
      • increased orbital period from 4.91 to 5.12 years
    • 0.82 AU from Earth on 1972 February 12
    • 0.18 AU from Earth on 1977 April 2
    • 0.33 AU from Earth on 1982 May 31
    • 0.86 AU from Earth on 1987 July 10
    • 0.51 AU from Jupiter on 1999 September 18
      • increased perihelion distance from 1.00 AU to 1.12 AU
      • increased orbital period from 5.11 to 5.31 years
    • 0.56 AU from Earth on 2008 March 23
    • 0.63 AU from Earth on 2029 March 13
    • 0.84 AU from Earth on 2034 June 21
    • 0.28 AU from Jupiter on 2047 March 25
      • increased perihelion distance from 1.09 AU to 1.34 AU
      • increased orbital period from 5.26 to 5.63 years
    • 0.87 AU from Earth on 2050 April 13

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