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46P/Wirtanen

Past, Present, and Future Orbits by Kazuo Kinoshita

A. Nakamura image of 46P exposed on 1997 February 9
Copyright © 1997 by Akimasa Nakamura (Kuma Kogen Astronomical Observatory, Japan)

This CCD image was taken on 1997 February 9, using a 0.60-m f/6 Ritchey-Chretien telescope. Two images were overlayed and centered on the comet, which resulted in every star appearing double.

Discovery

     Carl A. Wirtanen (Lick Observatory, California, USA) discovered this comet on photographic plates exposed with the 20-inch f/7.4 Carnegie astrograph on 1948 January 17.32. The comet was described as diffuse, with a central condensation, and magnitude 16. The comet was then at its brightest as it was moving away from both the sun and Earth.

Historical Highlights

  • Computations following the 1948 appearance revealed the comet had passed perihelion on 1947 December 2.9, had a perihelion distance of 1.625 AU, and an orbital period of 6.71 years.
  • Wirtanen recovered his periodic comet on 1954 September 8.50. The comet reached a maximum magnitude of 18.5.
  • The comet was poorly placed at its 1961 apparition. Elizabeth Roemer recovered the comet on 1960 October 26. When last seen on 1961 March 9, Roemer determined the magnitude as 18.0.
  • The comet was well placed at the 1967 apparition, and when recovered by Koichiro Tomita on October 5.73, it was already magnitude 15. It reached magnitude 14 by late October and then observers reported a slow fading during the next month, despite decreasing distances from the sun and Earth. The comet was closest to Earth (0.657 AU) on November 27, at which time the magnitude as near 15. The comet passed perihelion on December 15, and then began to fade. By late 1968 February, it had faded to 18 and when last seen by Roemer on March 24.2, the magnitude was estimated as fainter then 19.5.
  • The comet passed 0.28 AU from Jupiter during April 1972, which reduced the perihelion distance from 1.61 AU to 1.26 AU, and also decreased the orbital period from 6.65 to 5.87 years.
  • The comet was very unfavorably placed for observation in 1974, but Roemer and L. M. Vaughn still managed to recover it on December 20. The nuclear magnitude was then estimated as 21.5. Only one other observation was obtained and that came on 1975 February 6.4, when Roemer photographed a poorly defined spot of mangitude 21.5. The comet had passed perihelion on 1974 July 5.
  • What amounted to the comet's most unfavorable apparition, the 1980 appearance went unobserved. With perihelion coming on 1980 May 22.9, the comet spent the period of March 6 to October 1 within 20° of the sun. During the beginning of March the comet would have been 2.44 AU from the sun, just a little closer than its record distance of 2.54 AU on 1974 February 6. Unfortunately it was then at a much closer solar elongation (22° compared to 109° in 1975).
  • Another close approach to Jupiter occurred in 1984 which further reduced the comet's perihelion distance from 1.256 AU to 1.085 AU, and the orbital period from 5.87 years to 5.50 years.
  • The comet was next recovered on 1985 November 13.4, when A. C. Gilmore and P. M. Kilmartin (Mount John University Observatory) estimated the nuclear magnitude as 19. The comet was closest to Earth on 1986 March 26 (1.5900 AU), and was last detected on April 1.
  • As a result of the orbital changes of the 1970s and 1980s, this comet's best apparition came in 1991. Recovered on 1991 July 8.8, by T. Seki (Geisei, Japan), the comet's total magnitude was 17. The comet passed closest to Earth on September 8 (1.3501 AU), at which time observers were giving magnitude estimates near 10.
  • The 1997 return of this comet was not as favorable as that of 1991, but it still became an easy object for most amateur astronomers. The comet's closest approaches to Earth came on 1996 September 9 (1.4917 AU) and on 1997 March 24 (1.5104 AU). The latter was better as the comet passed perihelion on March 14 (1.065 AU). As 1997 began the comet was reported as between magnitude 12.5 and 13, there was an increase of about one-half magnitude by mid-month. Following the interference of moonlight, the comet became more widely observed at the beginning of February. Magnitude estimates were then averaging 11.5, and estimates of the coma diameter were typically a little smaller than 2 arc minutes. Shortly before mid-February, observers were reporting the comet had become brighter than magnitude 11, while the coma diameter was generally acknowledged as a little larger than 2 arc minutes.
  • The comet could reach naked-eye visibility during its 2018 return. With perihelion falling on 2018 December 12 and the comet passing 0.0776 AU from Earth just four days later, the magnitude should reach magnitude 5-6.
  • Close approaches to planets: This comet made 4 close approaches to Earth and 3 close approaches to Jupiter during the 20th century. It makes 3 close approaches to Earth and 1 close approach to Jupiter during the first half of the 21st century. (From the orbital work of Kazuo Kinoshita)
    • 0.53 AU from Jupiter on 1912 December 31
      • decreased perihelion distance from 1.63 AU to 1.43 AU
      • decreased orbital period from 6.82 to 6.32 years
    • 0.75 AU from Earth on 1914 November 3
    • 0.66 AU from Earth on 1927 December 14
    • 0.65 AU from Earth on 1947 December 4 (contributed to comet's discovery)
    • 0.66 AU from Earth on 1967 November 27
    • 0.28 AU from Jupiter on 1972 April 10
      • decreased perihelion distance from 1.61 AU to 1.26 AU
      • decreased orbital period from 6.65 to 5.87 years
    • 0.47 AU from Jupiter on 1984 February 26
      • decreased perihelion distance from 1.26 AU to 1.08 AU
      • decreased orbital period from 5.87 to 5.50 years
    • 0.92 AU from Earth on 2008 February 17
    • 0.08 AU from Earth on 2018 December 16
    • 0.76 AU from Earth on 2029 October 10
    • 0.56 AU from Jupiter on 2042 November 25
      • increased perihelion distance from 1.08 AU to 1.22 AU
      • increased orbital period from 5.49 to 5.78 years

    Additional Images

    L. Buzzi and F. Luppi image of 46P exposed on 2007 August 5
    Copyright © 2007 by Luca Buzzi and Federica Luppi

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