C/1996 Q1

C/1996 Q1 (Tabur)

Unlike any other body in the solar system, comets can occasionally perform in a completely unexpected manner. This is a fact that drew my attention in the early 1970s and hooked me on observing and researching comets. Comet C/1996 Q1 (Tabur) was a classic example--a comet with a good chance of becoming a nice naked-eye object suddenly faded away near perihelion. This had happened in the past with other comets, but, this being the 90s, both professional and amateur astronomers extensively documented the event.

Discovery (1996 August 19)

Vello Tabur (Wanniassa, Australian Capital Territory) discovered this comet in Eridanus on 1996 August 19.70. He was using a 0.20-m reflector. The comet was then described as magnitude 10, with a little-condensed, circular coma 3 arcmin across. Tabur had a difficult time convincing the proper authorities that he had found a comet. After thoroughly checking several sources, he sent an e-mail to the Central Bureau for Astronomical Telegrams, the clearinghouse for comet discoveries, that morning. He checked his e-mail throughout the day, but failed to hear anything about the comet. Finally at 1:00 a.m. (local time) he received a message from the e-mail delivery system that said his e-mail was deleted because it could not be delivered. A short time later, Tabur asked Steve Johnston to drive with him to Williamsdale to help observe the comet. It didn't take long to confirm that it had moved.

The next day was a stressful one. Tabur received phone calls from Dave Herald and Jack Child stating that their searches for the comet had revealed nothing. A short time after this news, Dan Green of the Central Bureau called and said others had failed to find the comet. That night, Tabur arranged for several members of the Canberra Astronomical Society to meet him at Williamsdale at 2:30 a.m. (local time). He sweated as the skies grew more cloudy as that time approached, but, before he left home, he checked his e-mail one last time and received word that the comet had been confirmed.

H. Mikuz (Crni Vrh Observatory, Slovenia) was the first to confirm the comet when his CCD image revealed the comet as 4.5 arcmin across, with a total magnitude of 11.1 on the 21st. Alan Hale (New Mexico) independently confirmed the comet a few hours later. His 0.41-m reflector revealed a coma 3.5 arcmin across, with a total magnitude of 10.0. Soon afterward, the comet was officially designated C/1996 Q1 (Tabur).

This is one of the earliest images of the comet. It is a 200-second exposure obtained by Gordon Garradd (Loomberah, Australia) on 1996 August 24.74 using a 0.25-m f/4.1 Newtonian and a HI-SIS 22 CCD camera. North is up and the image covers an area of 9 by 7 arcmin. The false color palette fails to reveal a tail, but does show a strong condensation.

Orbit

Brian G. Marsden (Central Bureau for Astronomical Telegrams) computed the first parabolic orbit, which was first published on August 23 on IAU Circular 6460. Based on 15 positions obtained on only three days, it indicated a perihelion date of 1996 November 1.076 and a perihelion distance of 0.78 AU. It indicated the comet could reach magnitude 5.6 during the second week of October because of a close approach to Earth of 0.38 AU. On August 30, IAU Circular 6464 revealed a revised orbit by Marsden. It indicated a perihelion date of November 3.562 and a perihelion distance of 0.84 AU. It indicated the maximum magnitude would probably reach 5.5 just before mid-October and that the closest approach to Earth of 0.42 AU would occur at that time.

Ultimately, Kenji Muraoka took 170 positions obtained during the period of 1996 August 21 to October 16 and determined the following orbit, which has a period of 18,356 years:

Perihelion Date (T)	1996 Nov. 3.52909 TT
Perihelion Distance (q)	0.8398150 AU
Eccentricity (e)	0.9987931
Argument of Perihelion ()	57.40985°
Ascending Node ()	31.40097°
Inclination (i)	73.35731°

IAU Circular 6464 also revealed a surprising fact. It said Jost Jahn (Bodenteich, Germany) had noted a strong resemblance between the orbit of this comet and that of comet C/1988 A1 (Liller). Marsden commented, "it is certain that these objects separated from each other in the general vicinity of their previous perihelion passage." Although no orbital period was then known for comet Tabur, Marsden pointed out that the original orbit of comet Liller had a period of about 2900 years.

Observations

1996 August 23-31:

Numerous observations were made during the remainder of August. Most magnitude estimates were in the range of 9.3 to 10.2, while the coma diameter was estimated as 3 to 8 arcmin across, with an average between 4 and 5. The comet was widely reported as a diffuse object, and there were no reports of a tail.

1996 September 1-15:

With the moon no longer a serious factor in observing this comet, observers resumed their observations on September 5. Magnitude estimates ranged from 7.5 to 10.0 during the period of September 5-9, depending on the type of observing instrument used. Binoculars revealed the brightest magnitudes as more of the coma was being detected. The largest coma diameters being reported at that time was 6 arcmin. During this same period of time, the most experienced observers were giving a magnitude near 8.0. This indicated the comet was about 0.5-magnitude brighter than expected. It is not particularly unusual for magnitude predictions to need revision within the first couple of weeks following a comet's discovery.

This image was obtained by Stefan Beck in Southern France on September 9. He was using a 5.5-inch Schmidt camera. This is a 10-minute exposure on TP2415.

By the 15th of September, the experienced observers were reporting magnitudes near 7.4, thus adding support to the idea that magnitude predictions required an upgrade of 0.5 magnitude. This indicated the maximum brightness would probably be near 5.0. Meanwhile, the coma diameter had increased to 7-8 arcmin, as the comet approached Earth.

1996 September 16-30:

Moonlight interfered during this period and few observations were made during the period of September 19 to the end of the month, except for the 27th when a total lunar eclipse occurred. During totality numerous observers saw the comet, with most determining the total magnitude as between 5.5 and 5.9. Coma diameters generally ranged from 10 to 18 arcmin. A few observers also reported a tail that was visually detected with binoculars. This tail was 1 to 2 degrees long and extended a little north of west.

The above photo was obtained by Marco Antonio De Bellis (Brazil) on September 21. He was using a Celestron 8 and tracked the stars while the comet moved over the period of one hour. This photo was too narrow a field to catch the comet passing only 52 arcmin from the 6th-magnitude open cluster NGC 2169 in Orion, but certainly shows how fast the comet was then moving.

The photo below was obtained by Tim Puckett (Puckett Observatory, Georgia) on the 23rd. It is a CCD composite based on 30 one-minute exposures. He used a 30-cm reflector and an ST6 camera.

1996 October 1-15:

The comet began the month just a little fainter than magnitude 5, but continued to brighten during the first week and was widely noted as a naked-eye object. George Gliba saw the comet on the morning of the 7th and determined the magnitude as 4.8. Many other observers reported the comet a little brighter than normal on that morning, and then noted the comet dropped a little in brightness during the next couple of days. Gliba determined the magnitude as 5.3 on the 11th. Throughout the period the coma was continually estimated as 10-15 arcmin in diameter.

The comet was within the bowl of the Big Dipper during October 13-15, passing only 9 arcmin from the 11.6-magnitude galaxy NGC 3998 on the 14th.

1996 October 16-31:

The comet began fading rather rapidly after mid-month despite the fact that it was continuing to approach perihelion. Although the comet had exceeded early predictions of its maximum brightness early in the month, during its closest approach to Earth, it faded more rapidly than was initially expected during this portion of the month. Predictions had placed the comet around 6.5 by the end of October, but most observers were providing magnitude estimates of fainter than 8 on the 31st. The comet was still rather large, with a coma diameter of 8 to 12 arcmin. Observations by Gary W. Kronk (Troy, Illinois) on the 31st revealed the comet as extremely diffuse in 20x80 binoculars, with no trace of condensation.

1996 November:

This comet's diffuse nature continued to be noted as November began and this caused brightness estimates to vary widely. During the first 8 days alone, observers turned in magnitudes ranging from 7.5 to 10.5, and coma diameter estimates ranging from 3 to 14 arcmin. The magnitudes could generally be correlated to the coma diameters, with larger diameters revealing brighter magnitudes. Despite these apparent discrepancies, most observers were seeing the comet with large binoculars, generally those with objectives around 3-inches across. There were a few observers who distinctly saw the coma as elongated during the first 8 days, an aspect confirmed by photography. H. Mikuz (Ljubljana, Slovenia) said the coma measured 14 by 7 arcmin on the 2nd, while K. Hornach (Lelekovice, Czech Republic) said it measured 12 by 6 arcmin on the 8th.

Following November 9th observations declined. The few people still observing the comet reported the brightness had dropped below magnitude 9, while the coma was at least 7 arcmin across. Telescopes were the only instruments being used to see the comet. The last confirmed visual sighting came on November 23, when Hornach saw it with his 0.35-m reflector. He said it was magnitude 9.4.

Cometography