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39P/Oterma

Past, Present, and Future Orbits by Kazuo Kinoshita

Discovery

     This comet was discovered by L. Oterma (Turku University Observatory, Finland) on a minor planet survey plate exposed with the 50-cm Schmidt-Väisälä camera on 1943 March 27.97, while it moving slowly within the constellation Virgo. Confirmation plates were obtained by Y. Väisälä (Turku University Observatory) on April 3.93 and Oterma on April 8.86. Oterma gave the magnitude as 15.1 on the latter date. Prediscovery images were later found on plates exposed by Väisälä on 1942 February 17.81 and February 18.77.

Historical Highlights

  • The comet faded very slowly following its discovery and had dropped to only magnitude 16 by the first days of July. It had also moved very slowly, which delayed the calculation of a reliable orbit. On July 14, L. E. Cunningham and R. N. Thomas published an elliptical orbit which revealed the comet had passed perihelion during 1942 October at a distance of 3.44 AU. The orbital period was given as 8.00 years. They also pointed out that the comet had passed close to Jupiter during 1938. The comet's orbital eccentricity was so small that the suggestion was made that the comet might be observable throughout its orbit.
  • The comet was recovered during 1944 January after it had sufficiently moved out of the sun's glare. It was described as small and round, with a magnitude of 16.5. Revised calculations revealed the comet had passed perihelion a few months earlier than had been determined late in 1943--more precisely, 1942 August 21. The comet's maximum brightness during 1944 was 16.
  • The comet continued to fade after 1944, but astronomers continued to follow it. The maximum magnitude reached 17 in 1945, 17.2 in 1946, 17.5 in 1947. Similarly, the coma diameter declined from 12 arc seconds in 1945 to only 3 arc seconds in 1947.
  • Apparition of 1950: The comet had passed aphelion during the latter half of 1946, and changes were beginning to be seen during 1948. First, the comet attained a maximum brightness of between 15.5 and 16. Second, increased activity was apparent as the coma diameter reached 6 arc seconds and a tail was frequently photographed. Similar numbers were also reported during 1949. The comet next passed perihelion during 1950. The maximum magnitude reached 14.5, while the coma attained a diameter of about 30 arc seconds. As the comet moved away from the sun it followed the same slow fading pattern noted during the 1940s.
  • The comet next passed perihelion during 1958, and this marked the last observed perihelion passage for quite some time. Astronomers had noted that the comet's orbit was not particularly stable because of close approaches to Jupiter. It seems Jupiter was not much of a factor prior to 1770, but in that year the comet passed 0.41 AU from Saturn which acted to decrease its perihelion distance from 6.2 AU to 5.7 AU. This put the comet well within Jupiter's sphere of influence. The comet managed to avoid Jupiter for 167 years, but in 1937 it passed only 0.16 AU away. This decreased the perihelion distance from 5.79 AU to 3.39 AU. This new perihelion distance allowed the comet to become significantly brighter than in the past and set the stage for the 1943 discovery. As the comet headed away from the sun following its 1958 perihelion, a new close encounter with Jupiter in 1963 increased the perihelion distance from 3.39 AU to 5.47 AU.
  • The comet was missed at its next expected return in 1983, with astronomers determining it was too faint to be detected with then existing telescopes. B. G. Marsden published a prediction for the 2002 apparition. The comet was recovered by Y. R. Fernandez (Mauna Kea, Hawaii, USA) on CCD images obtained with the 2.2-m reflector on 2001 August 13.43. It was described as stellar, with a magnitude of 22.0. The object was confirmed by K. J. Meech and J. Pittichova (Mauna Kea) on August 20 and 21. The comet was located just 2 arc minutes from Marsden's predicted position. With the orbit now precisely known, Meech located images on CCD frames obtained on 1999 May 9 and July 15, and these also confirmed very faint images reported by D. C. Jewitt, J. X. Luu, and C. A. Trujillo (Mauna Kea) on 1998 May 1 and 22. In the present orbit, the comet can not become much brighter than magnitude 22.
  • Close approaches to planets: The comet experienced two close approaches to Jupiter during the 20th century and makes two more approaches to Jupiter and one approach to Saturn during the 21st century.
    • 0.165 AU from Jupiter on 1937 October 27
      • decreased perihelion distance from 5.79 AU to 3.39 AU
      • decreased orbital period from 18.11 to 7.89 years
    • 0.095 AU from Jupiter on 1963 April 12
      • increased perihelion distance from 3.39 AU to 5.47 AU
      • increased orbital period from 7.88 to 19.43 years
    • 1.01 AU from Saturn on 2011 June 3 & 1.17 AU from Jupiter on 2023 July 12
      • increased perihelion distance from 5.47 AU to 5.71 AU
      • decreased orbital period from 19.49 to 19.43 years
    • 0.89 AU from Jupiter on 2025 January 15
      • increased perihelion distance from 5.71 AU to 5.91 AU
      • increased orbital period from 19.43 to 18.51 years

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