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31P/Schwassmann-Wachmann 2

Past, Present, and Future Orbits by Kazuo Kinoshita

H. Mikuz image of 31P exposed on 1993 December 12
Copyright © 1993 by Herman Mikuz (Crni Vrh Observatory, Slovenia)

This image was obtained on 1993 December 12.15 UT with a 20-cm, f/2 Baker-Shmidt telescope, V filter, and ST-6 CCD. Exposure time was 5 minutes. (The image was cropped and reversed by the webmaster to save space.)


     A. Schwassmann and A. A. Wachmann (Hamburg Observatory, Bergedorf, Germany) discovered this comet in southern Auriga on photographs exposed on 1929 January 17. It was described as magnitude 11.
     Shortly after the discovery was announced, prediscovery images were found on plates exposed at Yerkes Observatory on January 4, 7, and 12, at Harvard Observatory on December 19 and January 9, and at Tokyo Observatory on December 8 and 19.

Historical Highlights

  • The comet slowly faded after its discovery because the distance from Earth increased as it approached the sun. It was last seen on June 6 at magnitude 14.5. The comet was first recognised as moving in a short-period orbit on January 21 when George van Biesbroeck and Y. C. Chang computed a period of 6.83 years. After the comet was last seen the orbit was revised and a period of 6.43 years was determined.
  • The first predicted return was that of 1935 and astronomers realised the comet would be unfavorably placed. Nevertheless, early attempts were made to find the comet and on 1934 December 11 it was found by van Biesbroeck at magnitude 16.5. The indicated correction to the predicted perihelion date was -2.6 days.
  • The comet has been seen at every apparition, with the comet frequently reaching magnitude 13 and sometimes becoming brighter than 12, as in 1942 and 1981. In 1973 L. Kresak suggested the comet could probably be seen throughout its orbit and therefore be classed as an annual comet. An attempt was made to find the comet as it passed through aphelion during 1977 December, but these failed and the comet was not found until 1979 December 14. This marked the earliest the comet had ever been recovered, but further attempts were made to find the comet near aphelion. Although the comet has still not been seen at aphelion, astronomers came closer than ever before in 1991 when a Kitt Peak telescope found the comet on September 12, nearly a year after aphelion and 865 days before perihelion.
  • Close approaches to planets: The comet experienced two close approaches to Jupiter during the 20th century and makes one more approach during the 21st century. (From the orbital work of Kazuo Kinoshita)
    • 0.18 AU from Jupiter on 1926 March 22
      • decreased perihelion distance from 3.56 AU to 2.09 AU
      • decreased orbital period from 9.29 to 6.42 years
      • contributed to comet's discovery
    • 0.25 AU from Jupiter on 1997 March 18
      • increased perihelion distance from 2.07 AU to 3.41 AU
      • increased orbital period from 6.39 to 8.72 years
    • 0.84 AU from Jupiter on 2033 November 28
      • decreases perihelion distance from 3.41 AU to 2.90 AU
      • decreases orbital period from 8.70 to 7.76 years

    Additional Images

    G. Rhemann image of 31P on 1994 January 16
    Copyright © 1994 by Gerald Rhemann (Austria)

    Periodic comet 31P (lower left) moves near the star field of the open cluster M44 (upper right). This photograph was taken by Gerald Rhemann on 1994 January 16.03, using a 171/200/257mm Schmidt camera and hypered Technical Pan 2415. This is a 6-minute exposure and he estimated the comet's magnitude as 11.0.