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38P/Stephan-Oterma

Past, Present, and Future Orbits by Kazuo Kinoshita

J.
Copyright 1980 by Johnny Bremseth (Norway)

This drawing was made by Johnny Bremseth on 1980 December 5.78. He used a 0.20-m f/10 Schmidt-Cassegrain reflector. The field width is 50 arc minutes. (Thanks to Terje Wilhelmsen for permission to use this image.)

Discovery

     J. E. Coggia (Marseilles, France) found what he thought was an uncataloged nebula on 1867 January 22.9. The sky clouded up almost immediately and remained completely cloudy until the night of January 24, when E. J. M. Stephan (Marseille, France) checked on the nebula through a brief break in the clouds and saw that it had moved. Stephan was able to confirm this was a comet on January 25.86. Stephan said the comet was rather brillant, round, with a very marked nucleus. The initial announcements did not mention Coggia's name and the comet was named after Stephan. E. W. L. Tempel (Marseille, France) independently discovered this comet on January 28.86 near Pi Arietis. Although he knew of Stephan's discovery, he noted a distinct difference in the description from his and figured he had found a different comet. Tempel described the comet as very faint and about 3 arc minutes across.
     Liisi Oterma (Turku, Finland) discovered this comet on 1942 November 6.00. It was described as magnitude 13. Oterma confirmed the discovery on November 6.84. It was described as magnitude 13, with a slow northward motion. A short time after the announcement, Fred L. Whipple (Harvard College Observatory, Massachusetts, USA) found a pre-discovery image on a patrol plate exposed on November 5.23. The magnitude was also estimated as 13.

Historical Highlights

  • Apparition of 1867: The comet was already moving away from both the sun and Earth when discovered. Throughout February, observers typically described the comet as very faint, with a strong central condensation. The comet was last seen on April 4.11, when J. Winlock (Massachusetts, USA) described it as very faint in the 38-cm refractor.
  • The first parabolic orbit was calculated by W. Valentiner. Using positions from the period spanning January 25 to February 4, he determined the perihelion date as 1867 January 28.84. The first elliptical orbit was calculated by G. M. Searle during the last days of the 1867 apparition. He took positions spanning the period of January 27 to March 29 and determined the perihelion date as January 20.36 and the period as 33.62 years. A definitive orbit was calculated by L. Becker in 1891. He began with 63 positions spanning the period of January 25 to April 4, and ultimately found a perihelion date of January 20.71 and a period of 40.1 years. Becker recognized that there was some uncertainty in the period because of the relatively short period of visibility. He concluded that the comet would likely return sometime between 1902 and 1912. He provided an ephemeris giving rough positions for every month based on five different perihelion dates. Unfortunately, the comet was not then recovered and was not seen again until accidentally found by L. Oterma in 1942.
  • Apparition of 1942: The comet slowly brightened following Oterma's discovery and was reported as magnitude 9.2 around mid-December by G. van Biesbroeck (Yerkes Observatory, Wisconsin, USA). Around that same time, most observers were reporting a coma about 3 arc minutes across with a well-defined nucleus and a fan-shaped tail. The comet steadily faded thereafter and was last detected on 1943 May 2.12, when van Biesbroeck photographed it with the 82-inch reflector at McDonald Observatory (Texas, USA). He then gave the magnitude as 17 and said the nucleus was hazy and about 10 arc seconds across. There was also an extension of the coma toward PA 120°.
  • Using three precise positions obtained between 1942 November 5 and November 14, Whipple computed an elliptical orbit which was published on November 16. It gave the perihelion date as 1942 December 18.86 and the orbital period as 41.4 years. He added that the orbit was "essentially identical" to that of Stephan's comet of 1867. By December 3, J. Bobone had determined the orbital period as 38.3 years. During the 1970s, the orbit was independently investigated by several astronomers. Their linking of the 1867 and 1942 apparitions indicated a perihelion date between December 19.07 and 19.09, and an orbital period between 38.84 and 38.88 years.
  • Apparition of 1980: Predictions for the 1980 apparition came from L. M. Belous (1980 December 6.79), J. V. Carey (December 7.80), and D. K. Yeomans (December 5.22). Belous and Yeomans had each used positions from the 1867 and 1942 apparitions for their predictions, while Carey had integrated a previously published orbit for the 1942 apparition. The comet was recovered by H.-E. Schuster (European Southern Observatory) on 1980 June 13.35. He confirmed the recovery on June 19.38. Schuster estimated the magnitude as 18 on both dates. The positions indicated the prediction by Yeomans required a correction of only -0.07 day.
  • Close approaches to planets: The comet experienced three close approaches to Earth during the 19th and 20th centuries, and three minor approaches to Jupiter or Saturn. There will be two close approaches to Earth during the 21st century.
    • 0.92 AU from Earth on 1866 November 25
    • 1.66 AU from Jupiter on 1903 January 17
      • increased perihelion distance from 1.575 AU to 1.588 AU
      • increased orbital period from 37.12 to 38.44 years
    • 0.63 AU from Earth on 1942 December 7
    • 0.59 AU from Earth on 1980 December 9
    • 1.95 AU from Jupiter on 1982 March 26 and 1.42 AU from Saturn on 1984 June 1
      • increased perihelion distance from 1.574 AU to 1.588 AU
      • increased orbital period from 37.71 to 37.96 years
    • 0.77 AU from Earth on 2018 December 17
    • 0.70 AU from Earth on 2056 November 3

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