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C/1995 O1 (Hale-Bopp)

Orbit and Ephemeris by Central Bureau for Astronomical Telegrams

Analysis by Seiichi Yoshida

G. Rhemann image of C/1995 O1 (Hale-Bopp) exposed on 1997 March 27
Copyright 1997 by Gerald Rhemann (Austria)

This image of comet Hale-Bopp was obtained by Rhemann on 1997 March 27.78. He was using a 190/255/435mm Schmidt camera and Kodak Gold 400. The exposure time was 8 minutes. (The image was reduced by the webmaster to save space.)

Discovery

     After several months of no comet discoveries, which marked one of the longest dry spells in recent years, Alan Hale (Cloudcroft, New Mexico, USA) and Thomas Bopp (near Stanfield, Arizona, USA) independently discovered a new comet on 1995 July 23. The comet was found in Sagittarius, not far from the globular cluster M70. It was described as diffuse, with some condensation, and about magnitude 10.5. Both Hale and Bopp had been observing M70 when they noticed the diffuse glow just a short distance away. The discovery was incredible considering the comet was then 7.1 AU from the sun.
     Prediscovery images were later discovered. T. Dickinson (Chiricahua Mountains, Arizona, USA) found the comet on an image exposed on 1995 May 29. Incredibly, R. H. McNaught (Anglo-Australian Observatory, Australia) found a prediscovery image on a plate exposed over two years earlier, on 1993 April 27. The comet's total magnitude was then 18 and the coma was 0.4 arc minute across. The orbit indicated the comet was then 13.0 AU from the sun!

Historical Highlights

  • At the beginning of August, various observers were indicating the comet was magnitude 10.5, about 2-3 arcmin across, and weakly condensed. There was a trace of a tail, or a slight elongation of the coma, towards the north. Another indication that this comet was larger than normal came on August 18 when the one-m Schmidt at LaSilla revealed a coma measuring 9.2x6.0 arcmin. At the comet's distance of 6.2 AU from Earth, this indicated a coma size of 2.5x1.6 million kilometers. The comet had brightened to magnitude 10 when it was lost in twilight at the end of November.
  • After passing about 2° from the sun during the first days of 1996 January, the comet was spotted during the early days of February at about magnitude 9. Terry Lovejoy (Australia) described the comet as "well condensed with a noticeably fan shaped coma and is significantly brighter than last year." The comet took backstage to comet C/1996 B2 (Hyakutake) during March and April and few observations were obtained. Nevertheless, the comet appears to have brightened to magnitude 8.5 around mid-March and was near 8.0 near the end of April.
  • The comet finally became a naked-eye object on May 20, when Lovejoy caught several glimpses of it in very clear skies. With 10x50 binoculars he determined the magnitude as 6.7 and said the coma was 15 arc minutes across. By the end of May a few more observers were reporting the comet visible to the naked eye. Magnitude estimates were then typically near 6.5, while the coma diameter was still in the 10-15 arcmin range.
  • The comet continued to slowly brighten throughout June and had reached a magnitude of 5.5 by the beginning of July. But then something unexpected happened. As July progressed, the comet failed to become brighter. This caused some amateur and professional astronomers to worry that the comet had begun to fizzle. The situation did not improve during August, and, in fact, average magnitude estimates indicated that a slight fading of perhaps 0.3 magnitude had occurred by the end of that month and into September. Shortly after the comet had moved to within 3 AU of the sun on September 21 it showed signs of brightening, and by the end of the month the magnitude had increased to about 5.3. Despite the comet's seemingly strange behavior during this three-month period, observatories around the world were gathering an unprecedented amount of information for a comet so far out. Later analysis led several astronomers to conclude that the fluctuations in brightness were probably normal for comets this far from the sun, as different molecules of different reflectivity warmed and vaporized as the solar distance decreased. The evidence supporting this included the first detection of silicate emission on July 8, the first detection of Methyl Cyanide (CH3CN) during the period of August 14-17, and the first detection of Cyanogen (CN) during August.
  • The brightening continued thereafter. By the end of October observers were giving brightness estimates near 5 and the comet finally surpassed magnitude 4 on December 12, which also marked the date the comet had moved to within 2 AU of the sun. Observations became more difficult thereafter as the normal orbital motion of our planet took it around to the side of the sun opposite that of the comet. This would eventually lead to the comet being visible only in twilight for most of the Northern Hemisphere, except for those in Canada and northern Europe, during late December and early January, with the solar elongation reaching a minimum of 27° on December 21.
  • As the new year began, the comet was still situated in twilight, with observers having their best chance to see it in the morning sky. The comet was still best seen by observers in the northern part of the northern hemisphere, with B. H. Granslo (Norway) seeing it with 10x50 binoculars on the 1st and O. Skilbrei (Norway) seeing it with the naked eye on the same morning. Several additional observers in Norway, as well as The Netherlands, saw the comet during the next few days. By January 6 the average magnitude was 3.2. During this same period of time, Alan Hale (New Mexico) became the most southerly observer by far, though it was not easy. On January 2 he saw the comet with 10x50 binoculars and "guessed" that the magnitude was between 3 and 3.5. By January 10 the comet had increased its angular distance from the sun to 30° and became more widely seen in the morning sky thereafter. By the 19th naked eye observations were becoming quite plentiful and by the 20th observers were estimating the brightness as magnitude 2.5. By the 31st, coma diameter estimates were typically between 15 and 25 arc minutes.
  • The comet continued its brightening early in 1997 as it became a nighttime object that could not be ignored for the next four months, first appearing in the morning sky and then in the evening. It attained a magnitude of 2 early in February, 1 around February 20, 0.5 as March began, and 0.0 by March 7. The tail steadily increased in size from 2° to over 10° during this same period. As the comet climbed higher into the evening sky, observations of the comet increased and so did media coverage. Both local and national news did short news briefs on the comet, and several television shows did interviews with Alan Hale and Thomas Bopp.
  • The comet's next milestone came on about March 15 or 16, when brightness estimates from over 50 individual observers indicated the comet had reached magnitude -0.5. This indicated the comet was suddenly surpassing the predictions. After continuing a steady increase in brightness through the 20th, the comet levelled off at about magnitude -0.7 or -0.8. As March came to an end, the comet was holding quite close to its maximum magnitude of -0.7 or -0.8. Photographs were showing the gas tail still prominent, but more fanned than in February and early March. Sadly, the excitement of the comet was somewhat tempered on March 26, when 39 members of the Heaven's Gate religious cult committed suicide in California believing this would release their souls so that a spaceship they thought was hidden within the comet would take them to the afterlife.
  • G. Kronk images of C/1995 O1 (Hale-Bopp) exposed on 1997 March 19 and 25
    Copyright 1997 by G. W. Kronk (Illinois, USA)

    This image is a composite of two color images obtained by Gary W. Kronk (Troy, Illinois) on 1997 Mar. 19 and 25. They are to the same scale and lined up as close as possible. Each photo was exposed for 20 seconds using a Minolta 35mm camera equiped with a 50mm f/1.4 lens stopped down to f/4. These closely represent how the comet looked to the naked eye.

  • The comet passed perihelion on April 1, at which time experienced observers were estimating the total magnitude as between -1.4 and -0.3, with the average around -0.7. The tail was then extending 15-20 degrees for observers in dark skies. Depending on the observing conditions, some observers saw the gas tail as the longest, while others saw the dust tail as the longest. The comet maintained its peak brightness for a few days before beginning a slow fade. For the most part observers continued to report the comet in negative magnitudes until about the 24th. The gas tail became less prominent as the month progressed, while the dust tail seemed to become even more distinct. The gas tail's problem was not that it was disappearing, but that it was fanning out. By mid-month two distinct streamers could be seen on short exposure photographs, while longer exposures revealed the fan-shaped gas tail was filled with streamers. The dust tail seemed to have maintained its longest length almost until the 15th, when moonlight began interfering.
  • Crni Vhr Observatory images of C/1995 O1 (Hale-Bopp) exposed during March and April of 1997
    Copyright 1997 by H. Mikuz and B. Kambic (Crni Vhr Observatory, Slovenia)

    This image is a composite of several color images obtained by Herman Mikuz and B. Kambic during March and April of 1997. They show how the blue ion tail fanned out during March and April, thus leading it to become more difficult for visual observers. The images were all obtained using a 20-cm f/2 Baker-Schmidt camera and Fujicolor 400 SG+ film. The field of view for each image is 5x3.5 degrees.

  • The comet moved from Northern Hemisphere skies into Southern Hemisphere skies as May progressed and continued to steadily fade. It was magnitude 0.3 as May began, but dropped to 1.0 by mid-month and 2.0 by month's end. Twilight interfered with observations during a large part of June and the comet was even lost in twilight from June 24 until July 9. The comet was consistently described as a 4th-magnitude object during the last half of July and the first half of August. Perhaps the highlight of August was that the astronauts aboard the space shuttle Discovery observed the comet shortly before mid-month. The astronauts saw the 4th-magnitude comet with both the naked eye and with a 7-inch ultraviolet telescope that was mounted to a window.
  • The comet's magnitude faded to 4.3-4.4 by the end of August, 4.9-5.3 by the end of September, 5.9-6.4 by the end of October, 6.9 by the end of November, and 7.6 by the end of December. Despite the comet's continued southward motion, Northern Hemisphere observers got additional glimpses of the comet during late summer and early autumn. Alan Hale was the first of this group to reobserve the comet when he detected it in strong twilight on August 31. Observations continued throughout September and October for those people with a clear southeastern horizon. Magnitude estimates by the northern observers were typically 0.2-0.3 less than those of the southern observers because of the comet's very low altitude. Interestingly, Richard Keen spotted the comet on October 6 and 7 from Alaska, which was the farthest northern observation made. The comet was last seen in the Northern Hemisphere on November 1 by H. Dahle (Hawaii). He caught a glimpse of the comet with 9x63 binoculars and reported the total magnitude as 5.8. Interestingly, a weak outburst occurred early in December. Possibly as a result of this minor outburst, Terry Lovejoy (Moranbah, Queensland, Australia) noted on December 9.71, that the comet "was still just visible to the naked eye at mag. 6.8." Since Lovejoy reported the first naked-eye observation on 1996 May 20, his December naked-eye observation means the comet remained visible without optical aid for 569 days, or about 18 and a half months! The previous record had been set by the Great Comet of 1811, which was about 9 months.
  • The comet continues to be under observation. Since 1997, the magnitude was given as 8 in February of 1998, 12.5 during September of 1999, and 16 in February of 2003.
  • With a well-established orbit, astronomers know the comet probably passed its last perihelion about 4200 years ago, and it will next pass perihelion after another 2380 years.
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