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21P/Giacobini-Zinner

Past, Present, and Future Orbits by Kazuo Kinoshita

Michael Jäger image of 21p exposed on 1998 October 24
Copyright © 1998 by Michael Jäger

The photograph was taken on 1998 October 24, using a 0.25-m Schmidt camera. This was an 8-minute exposure using TP6415 hypered film. The comet's magnitude was determined as 8.8, while one degree of tail is seen extending toward PA 78°. (Thanks to Gerald Rhemann for permission to use this image. Images by Rhemann, Jäger, and others are located on Rhemann's web site.)

Discovery

     Michel Giacobini (Nice, France) discovered this comet in Aquarius on 1900 December 20.81. It was moving eastward.
     Ernst Zinner (Bamberg, Germany) discovered a comet while observing variable stars near Beta Scuti on 1913 October 23. He described it as about magnitude 10, with a coma 3 arc minutes across and a tail 30 arc minutes long.

Historical Highlights

  • This comet was observed for nearly two months during its discovery apparition. During the latter days of 1900 December, observers estimated the comet's total magnitude as between 10.5 and 11, while the magnitude of the nuclear condensation was about 12. The coma was near one arc minute in diameter, and no tail was reported. The comet faded as January began and it was last seen on February 16.
  • The comet was recognized as a short-period comet during the discovery apparition, and the orbital period was ultimately determined as 6.52 years. A prediction was made that it would return to perihelion during 1907 and that it would be very badly placed for observation. No searches appear to have been made. The comet was next expected to return to perihelion in 1914, when conditions would make the comet impossible to recover. As later calculations revealed, the orbital period in 1900 was actually 6.46 years, so that the prediction for the apparent 1914 return was in error by about six months.
  • The comet was missed at the unfavorable 1920 return, but was next recovered on 1926 October 16 at Bergedorf. The precise position indicated the prediction for this return had been in error by only 5 days. The comet reached a maximum brightness of 11 and a maximum tail length of 2 arc minutes.
  • The comet has been seen at every return since discovery, except for the very unfavorable 1953 return. The 1946 apparition was especially noteworthy as the comet passed only 0.26 AU from Earth in late September. The comet was then near magnitude 7, making this the brightest appearance since its discovery. Interestingly, the comet experienced an unexpected outburst in brightness which caused it to reach magnitude 6 during the first days of October. Following the missed return in 1953, the comet returned in 1959. Unexpected brightness outbursts were detected on August 31, September 23, and October 24, each increasing the comet's brightness by about one-half magnitude. Ultimately, the comet's maximum brightness reached 7 during 1959 and, at one point, the tail was one degree long.
  • This comet is especially noteworthy as it is one of a small number of comets that can produce very spectacular meteor showers under the right conditions. The meteor display is variously referred to as the Draconids, October Draconids, and the Giacobinids. The meteor shower occurs around October 9 of each year, but is usually unrecognizable; however, meteor storms occurred in 1933 and 1946 which produced several thousand meteors within an hour at maximum.
  • The 1998 Return: The comet passed closest to the sun (1.034 AU) on 1998 November 21. The comet was rather faint at the beginning of summer, with a magnitude of 15 as June began, but steadily brightened. It surpassed magnitude 14 in late July, 13 just before mid-August, and then rapidly rose to magnitude 11 by the end of August. It then brightened at a rate of slightly less than one magnitude per month, reaching 10 by the end of September, 9 by the end of October, before peaking at magnitude 8.5 during the middle of November. Thereafter, as the comet moved away from both the sun and Earth, the brightness rapidly declined. It was fainter than 10 at the end of January, near 12 by the end of February, and was near 13 by the end of March. The coma diameter was between 5 and 6 arc minutes when the comet was at its brightest. Interestingly, the comet's meteor shower, most commonly known as the Draconids, reappeared. Observers around the world began watching the skies on October 7, but it was October 8 when rates began rising. The hourly rates peaked over the region between Asia and eastern Europe, with the highest visual rates being about 100 per hour and the highest radar rates being about 500 per hour. All visual observers reported an abundance of faint meteors during the display.
  • Close approaches to planets: The comet experienced seven close approaches to Earth and two close approaches to Jupiter during the 20th century. It will make two 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)
    • 0.88 AU from Earth on 1900 December 15 (contributed to comet's first discovery)
    • 0.51 AU from Earth on 1913 November 14 (contributed to comet's seond discovery)
    • 0.26 AU from Earth on 1946 September 20
    • 0.93 AU from Jupiter on 1958 January 19
      • decreased perihelion distance from 0.99 AU to 0.94 AU
      • decreased orbital period from 6.56 to 6.42 years
    • 0.35 AU from Earth on 1959 November 8
    • 0.58 AU from Jupiter on 1969 September 23
      • increased perihelion distance from 0.93 AU to 0.99 AU
      • increased orbital period from 6.41 to 6.52 years
    • 0.93 AU from Earth on 1972 July 24
    • 0.47 AU from Earth on 1985 September 6
    • 0.85 AU from Earth on 1998 November 27
    • 0.39 AU from Earth on 2018 September 11
    • 0.37 AU from Jupiter on 2029 February 14
      • increased perihelion distance from 1.01 AU to 1.07 AU
      • increased orbital period from 6.53 to 6.70 years
    • 0.53 AU from Earth on 2031 September 5

    Additional Images

    NAOJ image of 21p exposed on 1998 September 20
    Copyright © 1998 by the National Astronomical Observatory of Japan

    This CCD image was taken by H. Fukushima, S. Abe, and T. Nakamura on 1998 September 20, using a 0.50-m telescope. It was constructed by combining 14 CCD images. (Thanks to Dr. Junichi Watanabe for permission to use this image from the NAOJ web site.)


    Yuichi Chimura image of 21p exposed on 1998 October 10
    Copyright © 1998 by Yuichi Chimura (Fukaya, Saitama, Japan)

    This CCD image was taken on 1998 October 10, using a 0.13-m f/6.1 Takahashi refractor and an SBIG ST-7 CCD camera. The image was obtained by combining three 120 second exposures.


    A. Nakamura image of 21p exposed on 1998 October 21
    Copyright © 1998 by Akimasa Nakamura (Kuma Kogen Astronomical Observatory, Japan)

    This CCD image was taken on 1998 October 21, using a 0.60-m f/6 Ritchey-Chretien telescope. The image was obtained by combining two CCD images, which resulted in every star appearing double.


    Masayuki Suzuki image of 21p exposed on 1998 November 5
    Copyright © 1998 by Masayuki Suzuki (Japan)

    This image was taken on 1998 November 5, using a 0.20-m f/9 telescope and a CCD camera. The image is a 60-second exposure. The image measures 16 by 12 arcmin.

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