Comet ISON and Comet Hale-Bopp: an excerpt from my upcoming book

Posted by David Eicher
on Monday, July 22, 2013

My book COMETS! Visitors from Deep Space will be published by Cambridge University Press this October. Here’s a taste of one of the observational chapters, describing history’s last truly great comet, Hale-Bopp.


As we patiently await Comet ISON (C/2012 S1), we all know that comparisons will be made to the last really stunning comet, Hale-Bopp (C/1995 O1). On July 23, 1995, while observing the globular star cluster M70, two observers in the southwestern United States each stumbled on a fuzzy blob near the cluster in their eyepieces. In Cloudcroft, New Mexico, astronomer Alan Hale saw the comet, while 440 miles [700 kilometers] away, in Stanfield, Arizona, Thomas J. Bopp also spied it. Following confirming observations and the calculation of an orbit by Brian Marsden, Comet Hale-Bopp was born.

Incredibly, at its discovery, the comet was a staggering 1.1 billion km [700 million miles] away — between the orbits of Jupiter and Saturn. Although at this distance most comets would appear as tiny specks, Hale-Bopp already showed a coma. Scottish-Australian astronomer Robert McNaught, at the Anglo-Australian Telescope at Siding Spring, found a 1993 image of the comet showing a coma at the very great distance of 2 billion km [1.2 billion miles], between the orbits of Saturn and Uranus. At that distance, Halley’s Comet would have been more than 100 times fainter than Hale-Bopp.

Astronomers, professional and amateur, would be able to view Hale-Bopp for a long time before it moved into the inner solar system. Astronomers were immediately excited by the fact that Comet Hale-Bopp was likely to brighten substantially, reaching a perihelion distance of 136.7 million km [85 million miles] April 1, 1997, and swinging past Earth at a distance of 196.7 million km [122 million miles] March 22, 1997.

No one knew it quite yet at discovery, but Hale-Bopp was destined to become the brightest comet of its era and was perhaps the most widely observed comet in history, given the huge numbers of the general public who looked at it with unaided eyes, binoculars, or telescopes. Calculations showed the comet would peak shining brighter than any star in the sky except for Sirius, the sky’s brightest star. Starting May 20, 1996, Hale-Bopp would remain visible to the naked eye for 569 days — more than 18.5 months, the longest period of unaided visibility of a comet known. That record would double the previous nine-month record set by the Great Comet of 1811.

During the final weeks of 1996, Comet Hale-Bopp established itself as one of history’s Great Comets. It rose to 5th magnitude by Halloween and 4th magnitude by mid-December. By this time, the comet had approached to within 300 million km [185 million miles] of the Sun, twice the orbital distance of Earth, and slinked into the twilight as it lay near the other side of the Sun from our planet.

January 1997 witnessed Hale-Bopp still situated in morning twilight for those in the best position, the Northern Hemisphere. On January 6, observers reported magnitude estimates centered around 3.2, most of them coming from those situated at high northern latitudes. The most southerly observer to record observations of the comet at this time was, ironically enough, Alan Hale himself, who spotted the comet from New Mexico on January 2 and estimated its magnitude at between 3 and 3.5. The morning sky favored a larger number of observers after about January 10, as the comet’s apparent distance from the Sun increased.

Now the stage was set for Comet Hale-Bopp to put on its spectacular show. The comet swung into the evening sky and brightened rapidly, reaching 2nd magnitude in early February and rising to an impressive magnitude 0 by March 7, with a tail stretching 10°. By mid-March, Hale-Bopp brightened further to magnitude –0.5, surpassing the predicted magnitude, and by March 20th reached magnitude –0.8. The incredible display of this amazingly bright comet and its prominently fanned gas tail awed millions of people across the globe, and the media coverage of the comet was enormous. A bizarre twist occurred March 26 when 39 members of a religious cult in California called Heaven’s Gate committed suicide, believing that somehow the comet’s appearance would convey them into a magical afterlife.

Comet Hale-Bopp’s perihelion date arrived on April Fool’s Day, and reliable magnitude estimates then ranged as high as –1.3, with a tail stretching as much as 20°. The amazing gas tail (strongly blue in photographs) and wide, fanned dust tail each measured most of this length, and some observers saw one or the other as slightly longer on different nights. But as April matured, the gas tail faded in prominence, spreading out in width.

Comet Hale-Bopp became one of history’s greatest comets, a path we all certainly hope Comet ISON will follow.

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