Comet Hale-Bopp, showing a white dust tail and blue gas tail - February 1997 Comet Hale-Bopp (formally designated as C/1995 O1) was probably the most widely-observed comet of the last century, and one of the most spectacular seen for many decades. It was visible to the naked eye for a record 18 months, twice as long as the previous record holder, the Great Comet of 1811.
Hale-Bopp was discovered on 23 July 1995 at an enormous distance from the sun, straight away raising expectations that the comet could become very bright when it passed close to the sun. Although comet brightnesses are very difficult to predict, Hale-Bopp met or exceeded most predictions for its brightness when it passed perihelion on April 1 1997. The comet was dubbed The Great Comet of 1997.
The passage of Hale-Bopp was notable also for inciting a degree of comet panic not seen for decades. Rumours that the comet was being followed by an alien spacecraft gained remarkable currency, and inspired a mass suicide among followers of the Heaven's Gate cult.
Discovery
The comet was discovered by two independent observers, Alan Hale and Thomas Bopp. Alan Hale had spent many hundreds of hours searching for comets without finding one, and was tracking known comets when he chanced upon Hale-Bopp, shining at 11th magnitude near a globular cluster, M70 in the constellation of Sagittarius. Hale first established that there was no other deep-sky object near M70, and then consulted a directory of known comets, finding that no known objects were in this area of sky. Once he had established that the object was moving relative to the background stars, he contacted the Central Bureau of Astronomical Telegrams, the clearing house for astronomical discoveries.
Thomas Bopp did not own his own telescope, and was out with friends observing star clusters and galaxies when he chanced across the comet while at the eyepiece of his friend's telescope. He realised he might have spotted something new when he checked his star atlases to find out what other deep-sky objects were near M70, and found that there were none.
The following morning, it was confirmed that this was a new comet, and it was named Comet Hale-Bopp, with the designation C/1995 O1. The discovery was announced in International Astronomical Union circular 6187[1].
Early progress
It was soon apparent that Hale-Bopp was no ordinary comet. For a start, when its orbit was calculated, it turned out to be 7.2 Astronomical Units (AU) from the sun, way out between Jupiter and Saturn and by far the greatest distance from earth at which a comet had been discovered. Most comets at this distance are extremely faint, and show no discernable activity, but Hale-Bopp already had an observable coma. An image taken in 1993 was found to show the comet, then an extraordinary 13 AU from the sun, a distance at which most comets are essentially unobservable. Analysis indicated that its nucleus was some 50km in diameter, nearly three times the size of Comet Halley.
Its great distance and surprising activity clearly indicated that Comet Hale-Bopp might put on a spectacular show when it reached perihelion in 1997. However, comet scientists were wary - comets can be extremely unpredictable, and many have large outbursts at great distance only to quieten down later. Comet Kohoutek in 1973 had been touted as a 'comet of the century' and turned out to be quite unspectacular.
Hale-Bopp becomes a Great Comet
Hale-Bopp became visible to the naked eye in the summer of 1996, and although its rate of brightening slowed considerably during the latter half on 1996, scientists were still cautiously optimistic that it would become very bright. It was too close to the Sun to be observable during December 1996, but when it reappeared in January 1997, it was already bright enough to be seen by anyone who looked for it, even from large cities with light-polluted skies.
The Internet was a growing phenomenon at the time, and numerous websites which tracked the comet's progress and provided daily images from around the world became extremely popular. The internet played a large role in encouraging the unprecedented public interest in Hale-Bopp.
As the comet approached the Sun, it continued to brighten, shining at 2nd magnitude in February, and showing a growing pair of tails, the blue gas tail pointing straight away from the Sun and the yellow-ish dust tail curving away along its orbit. On March 9th, a solar eclipse in Mongolia and eastern Siberia allowed observers there to see the comet in the daytime.
As it passed perihelion on April 1 1997, the comet had become truly spectacular. It shone brighter than any star in the sky except Sirius, and its two tails stretched 30-40 degrees across the sky. The comet was visible well before the sky got fully dark each night, and while most great comets are very close to the Sun as they pass perihelion, Comet Hale-Bopp was visible all night to northern hemisphere observers.
As impressive as the comet was, it could have been much more impressive. Had it passed as close to Earth as Comet Hyakutake (C/1996 B2) did in 1996, then the comet's tail would have spanned the entire sky and it would have been brighter than a full moon. However, it made a closest approach to Earth that would have rendered many lesser comets totally invisible and still spanned half the sky with its two tails, although the longest reaches of the tail were too faint to be visible.
After its perihelion passage, the show was over as far as the public were concerned, but astronomers continued to track the comet. The last naked-eye observations were reported in December 1997, which meant that the comet had remained visible without 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. As of 2004, the comet is beyond the orbit of Uranus, but is still observable with large telescopes.
The comet probably made its last perihelion 4200 years ago. A close approach to Jupiter on it way towards perihelion altered its orbit considerably, and it will next return to the inner solar system in 2380 years time.
Paranoia and superstition
In many cultures, comets have historically been viewed as bad omens and viewed with great suspicion. Perhaps because of the very long build-up to Hale-Bopp's passage, and its rare size and activity, the comet became the subject of many bizarre beliefs and theories.
In November 1996, Chuck Shramek, an amateur astronomer in Houston, Texas, took a CCD image of the comet, which showed a fuzzy, slightly elongated object nearby. When his computer sky-viewing program did not identify the star, Shramek called the Art Bell radio program to announce that he had discovered a 'Saturn-like object' following Hale-Bopp. UFO enthusiasts, such as remote viewing proponent Courtney Brown, soon concluded that there was an alien spacecraft following the comet. Later, Art Bell even claimed to have obtained an image of the object from an anonymous astrophysicist who was about to confirm its discovery. However, astronomers Olivier Hainaut and David J. Tholen of the University of Hawaii stated that the alleged photo was an altered copy of one of their own comet images [1] (http://www.sc.eso.org/~ohainaut/Hale_Bopp/hb_ufo_tholen.html).
A few months later, the cult group Heaven's Gate chose the appearance of the comet as a signal for their mass cult suicide. They claimed they were leaving their earthly bodies to travel to the space ship following the comet.
Hale-Bopp's legacy
For almost everyone who saw it, though, Hale-Bopp was simply a beautiful and spectacular sight in the evening skies. It was a record-breaking comet: discovered the furthest from the sun, with the largest cometary nucleus known, and it was visible to the naked eye for twice as long as the previous record-holder. It was also brighter than magnitude 0 for an astonishing 8 weeks, longer than any comet in the past thousand years.
References
- Hale, A., & Bopp, T. 1995, IAU Circular, 6187
External links
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