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C/2001 A2 (LINEAR)

This comet became the kind of comet astronomers love--
truly unpredictable in its behavior.

T. Hynek image of C/2001 A2 obtained on 2001 July 12
Copyright 2001 by Tomás Hynek (Ostrava, Czech Republic)

This image was obtained by Tomás Hynek on 2001 July 12.96. The comet was then experiencing another outburst and displayed a sharp nucleus and a tail containing several streamers. The image was composed of eleven 60-second exposures obtained with a 500mm f/5.6 telephoto lens and a Pictor 416XTE CCD camera. The full image, in its original orientation, is below.

Discovery

The LINEAR project announced the discovery of an asteroidal object on images obtained on 2001 January 15.32. The magnitude was given as 15.8. From three precise positions an ephemeris was posted on the Near-Earth Object Confirmation Page of the Central Bureau for Astronomical Telegrams (the clearinghouse for observations of comets and asteroids). P. Pravec and L. Sarounova (Ondrejov Observatory) noted a coma 0.3 arcmin across on January 16.03, while M. Tichy and M. Kocer (Klet) noted the object was diffuse with a coma 10 arcsec across on January 16.87.

A rough, unpublished orbit then linked this comet to an object detected by Lowell Observatory's LONEOS program on January 3.28. Additional images were found from the LINEAR program for the period of January 3.31 to January 6.32. LONEOS gave the magnitude of the January 3 object as 17.8, while LINEAR's images of January 3 to 6 revealed magnitudes of 18.4 to 19.2. The LINEAR images would seem to indicate a minor jump in brightness between January 6 and 15.


Historical Highlights


  • Following the recognition that this object was a comet, Brian G. Marsden (Central Bureau) took the observations spanning January 3 to 16 and calculated a parabolic orbit which indicated the comet would pass only 0.78 AU from the sun on 2001 May 24. This early orbit indicated the comet might reach a maximum brightness of magnitude 10.
  • The object slowly brightened as it approached both the sun and Earth. Shortly after mid-March it surpassed magnitude 13, and it had reached 12.5 early on March 26. The coma was then between 1 and 2 arcmin across.
  • Sometime after March 26.4 observers began reporting that the comet was brightening and, at the same time, increasing in size and becoming more condensed. During the next four days, the comet's brightness increased by at least 5 magnitudes! The comet's strong nuclear condensation (see image below) and the weak diffuse coma caused both magnitude estimates and coma diameters to vary quite a bit from one observer to another; however, it seems likely that the outburst magnitude peaked at about 7.5 on April 1, with a coma about 6 arcmin across. By April 8, the webmaster observed with his 13.1-inch reflector and noted the magnitude was 8.6 and the coma was 4' across. The strong condensation seen during the outburst had weakened considerably. Magnitude estimates made around mid-April generally ranged from 8 to 8.5, which was still 4-5 magnitudes brighter than early predictions had indicated. But the comet then resumed its brightening, from this new level of brightness and by the end of April is was slightly fainter than magnitude 6.
  • With a possible minor outburst in brightness during the first half of January and a major outburst at the end of March already under its belt, this comet added a new surprise as May began--the nucleus was split into two pieces. The announcement came on Circular 7616 of the International Astronomical Union (2001 May 1). The discovery was made by Carl W. Hergenrother, Matthew A. Chamberlain, and Y. Chamberlain (Lunar and Planetary Laboratory, University of Arizona), when an image they obtained on April 30 with the 1.54-m reflector revealed two nuclei separated by 3.5 arcsec. A routine image obtained with the same telescope on April 24 showed only one nucleus. Zdenek Sekanina (Jet Propulsion Laboratory) has determined that the split occurred on March 29.9+/- 1.6 UT.
  • The month of May continued to prove interesting for this comet. It followed a fairly stable brightening trend during April and into the first days of May, when it virually ceased to brighten further, with the brightness holding at between magnitude 5.8 and 6.1 through May 10. On the latter date, Michael Mattiazzo (Adelaide, South Australia) commented, "The comet's rate of brightening has slowed." Before the 10th had even ended, observers in Africa were already commenting on how the comet seemed brighter, and by the time the comet was observed in South America and Australia, observers were typically reporting a brightness of 5.2 to 5.3, indicating another outburst had occurred. The next 10 days were interesting. Where the comet should have brightened nearly a magnitude, it virtually held its ground, with most observers continually reporting a brightness of 5.2 to 5.5. On May 20, the comet had resumed its brightening. During this entire period observers were reporting a coma 5 to 7 arcmin across. The tail was about a half degree in length early in May, but was at least 2 degrees long by the 20th.
  • The European Southern Observatory announced in a press release on May 18 that fragment "B" has now split. Images obtained with the ESO's 8.2-m MELIPAL telescope at Paranal Observatory on May 14 revealed the nucleus "appeared somewhat elongated," while images obtained on May 16 with Paranal's 8.2-m YEPUN telescope revealed the nucleus had definitely split.
  • Moonlight was interfering with observations as June began, but observers were indicating the comet was continuing to slowly brighten, with magnitude estimates around 4.8 on June 1. By June 6, many observers were indicating the brightness was around 4.6-4.7. Moonlight was still interfering during June 7 to 9, but several observers indicated the comet was between magnitude 4.2 and 4.4--indicating the possible beginning of another outburst. On June 11, Andrew Pearce (Nedlands, Western Australia) confirmed an outburst was present and he estimated the magnitude as 3.6. Magnitude estimates of 3.3 to 3.4 were fairly common on the 12th. Interestingly, the coma appeared to have become about 30% larger compared to observations made the previous week. The comet was still shining at magnitude 3.4-3.5 on the 15th. On July 5 International Astronomical Union Circular No. 7656 announced that three additional fragments had appeared during high-resolution imaging of the inner coma during the period of June 16 to 21. It reported that Sekanina had subsequently determined that nucleus "D" had separated from "B" on June 3.5, "E" had separated from "B" on June 9.5, and "F" had separated from "B" on June 11.3. The Circular added, "These breakup events apparently triggered another major outburst, reported by visual observers to have peaked on June 12."
  • This comet became visible to higher northern latitudes during late June, as observations began arriving from Europe and the United States. At low altitude in most of these locations, magnitude estimates ranged from 3.5 to 4.5. Observers generally located in the Southern Hemisphere were reporting magnitudes near 4, with a tail about 0.5 degree long and a coma 20 to 25 arcmin across.
  • As of July 10, the comet continues to fade. The Author saw the comet about 20° from the moon with 20x80 binoculars early this morning. Under very clear skies, the magnitude was about 5.5 and the coma was about 15' across. Overall, the observations of amateur astronomers during the last two weeks have shown the comet to have faded uneventfully, with no apparent surprizes.
  • The comet experienced another outburst in brightness around mid-July. Magnitude estimates were generally between 5.5 and 6.5 on July 11 and into at least the early part of July 12. But sometime after July 12.4 reports began coming in that indicated the brightness was increasing. There were even some reports that a stellar nucleus had appeared, as well as an unusual "hammerhead" structure around the nucleus. By July 13 the brightness was generally estimated as between magnitude 4.3 and 5.3. Although the comet maintained this brightness for a couple of days, it soon dropped into a more normal fading mode and was about magnitude 7 by the end of July.
  • Since the mid-July outburst, this comet has slowly faded because of its increasing distance from both the sun and Earth. Magnitude estimates were near 7 as July ended and near 8.5 by mid-August, while the coma had decreased from 12 arcmin to 6 arcmin over the same period. In addition, the comet had become quite diffuse by mid-August, with many observers reporting on the near absence of a central condensation.
  • This comet proved to be a wild ride for astronomers, with outbursts in brightness and a nucleus which split into several pieces. For other examples of comet outbursts and splits, check out some of the following comets: 3D/Biela, 29P/Schwassmann-Wachmann 1, 73P/Schwassmann-Wachmann 3, and 141P/Machholz 2. The next question is will there be a complete breakup. Such a fate happened to another comet LINEAR (C/1999 S4) during July of 2000.


Additional Images--Whole Comet


Remanzacco Observatory in Italy obtained this image of C/2001 A2 obtained on 2001 February 11
Copyright 2001 by S. Garzia, R. Geretti, and G. Sostero (Remanzacco Observatory, Italy)

This image was obtained by S. Garzia, R. Geretti, and G. Sostero on 2001 February 11.77. The image is composed of 7 180-second exposures obtained with a 0.3m f/2.8 Baker-Schmidt camera and a Hi-Sis 24 CCD camera.


Giovanni Sostero photo of C/2001 A2 exposed on 2001 April 1
Copyright 2001 by G. Sostero and L. Donato (Remanzacco Observatory, Italy)

This image was obtained by G. Sostero and L. Donato on the evening of 2001 April 1, while the comet was in outburst. The image was composed of 20 30-second exposures obtained with a 0.3-m f/2.8 Baker-Schmidt and a Hi-Sis 24 CCD camera.


R. Ligustri and G.Savani photo of C/2001 A2 exposed on 2001 April 14
Copyright 2001 by R. Ligustri and G.Savani (CAST Association, Italy)

This image was obtained by Rolando Ligustri and G.Savani on the evening of 2001 April 14. The comet was still considerably brighter than early predictions had indicated, following its outburst of late March. The image was composed of 10 30-second exposures obtained with a Celestron 8 and an ST9E CCD camera.


Gordon Garradd image of C/2001 A2 obtained on 2001 May 18
Copyright 2001 by Gordon Garradd (Loomberah, New South Wales, Australia)

This image was obtained by Gordon Garradd on 2001 May 18.37. The image shows the fine detail present within the first 1.5 degrees of the tail. The image was composed of six 40-second exposures obtained with a 45-cm f/5.4 Newtonian and an AP7 CCD camera from the Gene Shoemaker Planetary Society NEO grant. The inset reveals two nuclei, of which "A" is the fainter.


Y. Chimura image of C/2001 A2 obtained on 2001 July 1
Copyright 2001 by Y. Chimura (Japan)

This color image was obtained by Y. Chimura on 2001 June 30.76. The image shows a stellar nucleus and large coma. The image was composed of 25 20-second exposures obtained with a 13.0-cm Takahasi refractor and an SBIG ST-7 CCD camera. (The time given on the image is actually Japan Standard Time, which is 9 hours ahead of Universal Time).


T. Hynek image of C/2001 A2 obtained on 2001 July 12
Copyright 2001 by Tomás Hynek (Ostrava, Czech Republic)

This image was obtained by Tomás Hynek on 2001 July 12.96. The comet was then experiencing another outburst and displayed the sharp nucleus and a tail containing several streamers. The image was composed of eleven 60-second exposures obtained with a 500mm f/5.6 telephoto lens and a Pictor 416XTE CCD camera. A larger version of this image, as well as links to other comet images are located on Hynek's comet page.


F. J. Melillo image of C/2001 A2 obtained on 2001 July 13
Copyright 2001 by Frank J. Melillo (Holtsville, New York)

This image was obtained by Frank J. Melillo, the Mercury Section Coordinator of the Association of Lunar and Planetary Observers, on 2001 July 13.77. The comet was then displaying a tail about 1.5° long. The image was a one-minute exposure obtained with a 150mm f/4.5 telephoto lens and a Starlight Xpress MX-5 CCD camera mounted Piggyback to a Celestron C-8.


G. Sostero image of C/2001 A2 obtained on 2001 July 14
Copyright 2001 by Giovanni Sostero (Drenchia, Italy)

This image was obtained by L. Donato, S. Garzia, V. Santini, and G. Sostero on 2001 July 14.92. Several faint streamers can be seen within the ion tail, especially close to the comet's head. The image is composed of 10 30-second exposures obtained with a 0.2-m Schmidt-Cassegrain and an SBIG-ST6V CCD camera.


G. Sostero image of C/2001 A2 obtained on 2001 July 22
Copyright 2001 by Giovanni Sostero (Italy)

This image was obtained by K. Korlevic, M. Juric, and G. Sostero (Visnjan Observatory, Croatia) on 2001 July 22.0. The image is composed of 4 120-second exposures obtained with a 0.4-m f/4.3 Newtonian and an APS.


Giovanni Sostero photo of C/2001 A2 exposed on 2001 September 5
Copyright 2001 by G. Sostero and V. Gonano (Remanzacco Observatory, Italy)

This image was obtained by G. Sostero and V. Gonano on 2001 September 5.88. The image was composed of 8 30-second exposures obtained with a 0.3-m f/2.8 Baker camera, a Hi-Sis 24 CCD camera, and a Cousin I filter.


Additional Images--Multiple Nuclei


C. W. Hergenrother, M. A. Chamberlain, and Y. Chamberlain photo of C/2001 A2 exposed on 2001 April 30
Copyright 2001 by C. W. Hergenrother, M. Chamberlain, and Y. Chamberlain (Lunar and Planetary Lab, University of Arizona)

Observation of the double nucleus of Comet 2001 A2 (LINEAR) taken on April 30.12 UT by C. W. Hergenrother, M. Chamberlin and Y. Chamberlain of the Lunar and Planetary Lab, University of Arizona with the Steward Observatory 1.54-m in the Catalina Mountains northeast of Tucson. The two nuclei were 3.5 arcsec apart and are nearly equal in brightness. Field directions are north is up and east is left.


Paranal Observatory photo of C/2001 A2 exposed on 2001 May 16
Copyright 2001 by European Southern Observatory

Image of the triple nucleus of Comet 2001 A2 (LINEAR) taken on May 16.97 UT with the European Southern Observatory 8.2-m VLT YEPUN telescope. The two brightest nuclei are offically designated "B1" and "B2" and were about 1 arcsec apart and nearly equal in brightness. Nucleus "A" is much fainter and is situated 14.6 arcsec away toward the upper right part of the image. Field directions are north is up and east is left.


LaSilla Observatory photo of C/2001 A2 exposed on 2001 June 17
Copyright 2001 by European Southern Observatory

Image of the bright nucleus "B" and the faint, diffuse nucleus which later became nuclei "E" and "F" of Comet 2001 A2 (LINEAR) taken on June 17.45 UT with the European Southern Observatory's 3.6-m telescope at LaSilla. The diffuse nucleus was then situated 4.6 arcsec from nucleus "B". The scale is 0.3 arcsec per pixel.

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