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C/1999 S4 (LINEAR)

Gerald Rhemann photo of C/1999 S4 exposed on 2000 July 7
Copyright © 2000 by Gerald Rhemann

The 19.5-minute exposure was taken in Austria on 2000 July 7, using a 1.12-m f/3.3 Hypergraph and Kodak TP4415 film. Rhemann gave the brightness as magnitude 7.2 and noted a 50'-long gas tail and a 20'long dust tail.


This is another comet found by the automatic minor planet search program LINEAR in New Mexico. It was found on CCD images obtained between 1999 September 27.40 and September 27.45 and was reported as an unusual moving object. The nuclear magnitude was given as 17.3-17.6. LINEAR obtained additional images on September 28 and October 1. On October 1.40 and 1.43, D. T. Durig (Cordell-Lorenz Observatory, Sewanee, Tennessee) obtained images of the comet with a 0.3-m Schmidt-Cassegrain and a CCD. He noted a total magnitude of 16.3 on the second image, as well as a coma 10 arcsec across and a tail extending 20-25 arcsec toward PA 200°-220°. The cometary nature was confirmed on October 1.89, when J. Ticha and M. Tichy (Klet Observatory) imaged the comet with a 0.57-m reflector and a CCD. They determined the total magnitude as 16.2 and noted a coma 8 arcsec across and a tail extending more than 10 arcsec toward PA 245°.

Historical Highlights

  • The first published orbit came from Brian G. Marsden (Harvard-Smithsonian Center for Astrophysics) on October 1. His parabolic solution, based on observations collected over five days, indicated a perihelion date of 2000 July 18 and a perihelion distance of 0.72 AU. He suggested "this comet might become a naked-eye object next July." Marsden revised the comet's orbit on October 6, using positions collected over eight days. The new perihelion date was 2000 July 24.2 and a perihelion distance of 0.754 AU. His ephemeris indicated the comet's maximum brightness could reach magnitude 4.
  • The comet slowly brightened during the months immediately following discovery. It was slightly fainter than magnitude 14 during November and was brighter than 14 as 2000 began. The total magnitude had reached 13.5 during the first week of March, and the comet was lost in the sun's glare after March 22.
  • The comet was recovered in the morning sky on May 4, by K. Kadota (Ageo, Japan) at a magnitude of about 13. Observations by other observers began around mid-May after the comet had gained more altitude in the morning sky and had become brighter than magnitude 12. By the beginning of June most observers were reporting a brightness slightly fainter than magnitude 10, with a moderately condensed coma about 3 arcmin across. Visual observations of the tail became fairly common in larger amateur telescopes after mid-June, with the length typically estimated as 4-5 arcmin. As June came to a close, most observers found the comet slightly brighter than magnitude 8.5. The coma was then about 5-6 arcmin across and the tail about 10 arcmin long.
  • During June, revised predictions of the maximum brightness of this comet indicated a maximum magnitude of 4 to 6 would be attained around July 22 or 23. Although an apparent change in the rate of brightening occurred on June 22, which indicated the comet was heading for the magnitude 6 prediction, an minor outburst around July 5 to 6 was indicated by visual observers and fathered hopes that the comet might still reach magnitude 4 or 5. This brightening only lasted a few days, however, and by mid-July most observers were giving in to the possibility that this comet would not become brighter than magnitude 6. With respect to the brightening in early July, it is interesting that on July 28, the Space Telescope Science Institute issued press release giving details of observations made by the Hubble Space Telescope on July 5, 6, and 7. The images indicate a brief outburst on the 6th and the appearance of a fragment moving toward the tail from the nucleus on the 7th. Complete details and images can be found at the Hubble Space Telescope web site.
  • The comet continued a slow brightening as July progressed. By July 21, the general consensus was that the comet's brightness was between magnitude 6.6 and 6.8, with a coma 5 to 6 arcmin across and a dust tail 30 to 60 arcmin long. But something happened late on the 21st and by July 22, observers were commonly reporting the magnitude as around 6 and, although the coma diameter and dust tail had not changed, there was a bright, straight, long gas tail. My personal observations on July 22.16 revealed a brighter comet than I had seen a couple of days earlier, which exhibited a straight gas tail extending at least 1 degree. This is exactly why people like me observe comets. Their potential unpredictability makes them very interesting.

  • July 21.87: Hermann Mikuz (Crni Vrh Observatory, Slovenia) obtained this CCD image with a 15-cm, f/4 Maksutov-Cassegrain telescope.

    July 21.88: Gerald Rhemann (Austria) took this 5-minute exposure with a 225/255/435mm Schmidt camera.

  • So, on July 23 I asked the question "what will happen next?" and noted "The gas tail seen on the 21st and 22nd was certainly short-lived and indicates something sudden and violent occurred to the nucleus. It may only prove to have been a new pocket of gas that was released, but observers should be on the lookout for something more within the coma during the next few days and weeks." Well, the comet has indeed undergone changes that became apparent shortly before July ended. The comet's nuclear region became noticeably more diffuse and elongated beginning on July 25, and the comet began fading at a rate much greater than had been predicted. My final observation was made with my 33.3-cm reflector on August 2 and revealed a slightly elongated nebulosity with virtually no condensation. My estimate of the comet's brightness was 9, which was over 2 magnitudes fainter than expected.

    Konrad Horn (Germany) obtained this image on August 1.86. He combined sixteen 60 second exposures obtained with his Genesis 100/500 and a Starlight SX CCD. The comet was then a very diffuse and elongated smudge, at least two magnitudes fainter than it should have been at that time.

  • The Hubble Space Telescope was used to observe the comet on August 5. What it found was more than a dozen tiny comets enveloped by a cloud of dust in the area where comet LINEAR was supposed to be. The image below is only a small section of the photo. The large scale images and complete story are on the Hubble web site. Interestingly, the images were obtained at about the same time as another, wider field image was obtained with the 2.2-meter telescope at the Mauna Kea observatory complex.

  • In a display of how good ground-based observations can be, the European Southern Observatory released an image obtained at Paranal (Chile) with the 8.2-m Very Large Telescope ANTU unit on the evening of August 6. A portion of the image is shown below. The complete image and press release is located on the VLT web site.

    Additional Images

    2000 March 9--0.60-m Ritchey Chretien

    A. Nakamura photo of C/1999 S4 exposed on 2000 March 9
    Copyright 2000 by Akimasa Nakamura

    The CCD image was taken at Kuma Kogen Astronomical Observatory on 2000 March 9.43, using a 0.60-m f/6 Ritchey-Chretien telescope.

    2000 June 17--0.31-m Baker-Schmidt

    G. Sostero photo of C/1999 S4 exposed on 2000 June 17
    Copyright 2000 by Giovanni Sostero

    The CCD image was obtained by G. Sostero and E. Dembitzer (Remanzacco Astronomical Observatory, Italy) on 2000 June 17, using a 0.31-m f/2.8 Baker-Schmidt camera and a Hi-SIS 24 CCD. Five 30-second exposure frames were combined.

    2000 July 7--0.31-m Baker-Schmidt

    G. Sostero image of C/1999 S4 exposed on 2000 July 7
    Copyright 2000 by Giovanni Sostero

    The CCD image was obtained by G. Sostero (Remanzacco Astronomical Observatory, Italy) on 2000 July 7, using a 0.31-m f/2.8 Baker-Schmidt camera and a Hi-SIS 24 CCD. Fifteen 60-second exposure frames were combined.

    2000 July 8--0.31-m Baker-Schmidt

    G. Sostero image of C/1999 S4 exposed on 2000 July 8
    Copyright 2000 by Michael F. Borman

    These images were obtained by Michael F. Borman (Evansville, Indiana) on 2000 July 8.4, using a 0.31-m f/6.3 Meade LX200 and an ST-7E CCD camera. The large image was a 180-second exposure and tracked on the stars. The inset was tracked on the comet and was composed of 10 30-second exposures. The inset was also processed to reveal the nuclear condensation.

    2000 July 10--1.12-m Hypergraph

    G. Rhemann photo of C/1999 S4 exposed on 2000 July 10
    Copyright 2000 by Gerald Rhemann

    This photograph was obtained by G. Rhemann (Austria) on 2000 July 10.02, using a 1.12-m f/3.3 Hypergraph. It is actually two photographs overlayed, one using GPY 400/120 film and the other using GPX 160/120 film. Each exposure was nine minutes long.

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