Hail, Hale-Bopp, and Farewell

(a revised version of an article that first appeared in Springboard.)

The Extraordinary Outburst of Comet 17P/Holmes
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It came from the dark fringes of the solar system, having last passed our way when the Great Pyramids were young and before Stonehenge was built. It has wowed the public, and provided astronomers with priceless data. It has generated headlines, first as what may well be the cometary treat of a lifetime—the Great Comet of 1997—and then as the “marker” that the Heaven’s Gate cult believed called them to mass suicide. Oblivious to the excitement it has caused on Earth, Comet Hale-Bopp still lingers in the evening sky, slowly growing fainter as it recedes on a journey that won’t take it back to the sun’s neighborhood until around the year 4350.

As April ends, the comet is located in the northwestern sky after dark, and although it will have lost a bit of its luster it should still be an easy sight to the naked eye, and impressive in binoculars. It will appear, as twilight deepens, almost directly above the point where the sun disappeared. On May 1 it will be 20° high—almost a quarter of the way from horizon to zenith—an hour after sunset. But then it will rapidly descend into the solar glare and be gone by the middle of May. When it reappears in August, it will have moved far to the south, near the star Sirius, and from the city, one will probably need binoculars or a small telescope to see it.

Unlike last year’s Comet Hyakutake, which was discovered only two months before its close approach to Earth, astronomers had plenty of time to prepare for Hale-Bopp. The comet was found on July 22, 1995, almost simultaneously by Alan Hale in New Mexico and Thomas Bopp in Arizona—and quite by accident. The two men were observing a distant star cluster by which the comet happened to be passing. Alan Hale bridges the realm between professional and amateur astronomy: an unemployed Ph.D. who specializes in sunlike stars and planetary systems, he also makes an avocation of observing known comets and had spent 400 hours trying to find new ones. It is ironic that his first discovery happened at a time when he wasn’t actually looking for a comet. Bopp, an amateur astronomer, specializes in observing deep-sky objects (distant galaxies, star clusters, and nebulae); in fact, in over 25 years of observing, he had never before even seen a comet.

The astronomical world soon realized that Comet Hale-Bopp was quite unusual. When it was found, it was out beyond the orbit of Jupiter, a record distance from the Sun for a comet discovered by amateurs, and it was several hundred times brighter than Halley’s Comet had been at the same distance from the Sun. Brian Marsden of the Harvard/Smithsonian Astrophysical Observatory compared Hale-Bopp to the Great Comet of 1811 which, although it did not come particularly close to either the Earth or the Sun (as is also the case with Hale-Bopp), nonetheless stayed brilliant for a long time. Astronomers realized that Hale-Bopp was an unusually large and active (in terms of the gas and dust that it throws off as it melts) comet.

As the months progressed, excitement over the arrival over the comet grew. Last spring, the surprise appearance of Comet Hyakutake, which although a much smaller comet came much closer to Earth, was only a prelude to the show that Hale-Bopp was expected to produce. Comets are notoriously unpredictable, as anyone remembering the flop of Comet Kohoutek back in 1974 can attest to, but apart from a brief lull in its rate of brightening late last summer, Hale-Bopp did nothing to let us down. It came closest to Earth on March 22, passing 197 million km away. This is not a particularly close approach for a comet—it stayed considerably farther away from us than we are from the sun. On April 1, the comet passed 137 km from the sun, once again, not a particularly close approach, but throughout this period Hale-Bopp still outshone every star except Sirius. This is a testament to the comet’s great brilliance. If Hale-Bopp had arrived four months sooner, it would have passed almost as near to us as Comet Hyakutake did last year—but Hale-Bopp probably would have been bright enough to cast shadows and be seen in full daylight.

Hale-Bopp has become the most scrutinized comet in history, and data about it is rapidly disseminated. Information on the comet’s brightness, appearance, and other characteristics are updated several times daily on a number of websites. Among the best sites are http://encke.jpl.nasa.gov, a NASA site with a wide range of both technical and nontechnical information about Hale-Bopp and other comets, http://hq.eso.org, the European Southern Observatory’s site which features scientific findings, and http://www.halebopp.com, with information geared to the general public, including discussion forums.

Of course, the Internet can also be used to spread disinformation as well. Last November Chuck Schramek, an amateur observer with a personal website rife with conspiracy theories, took a photograph of what he claimed was a planet-sized object following Hale-Bopp. Although astronomers readily identified the object as a particular distant star, Shramek did not back down. He took his case to both talk radio and the Internet. His supporters alleged that the “Saturn-like object” was an alien spaceship, that the comet was changing course to rendezvous with Earth, and that the many astronomers who stepped forth to debunk this speculation were really agents of a Government conspiracy to hide the truth. When the comet reached perihelion, 39 members of the Heaven's Gate cult killed themselves believing that their souls would be transported to the spaceship they thought was trailing the comet, the worst mass suicide ever to occur on U.S. soil.

In reality, comets are but large chunks of ice and dust, primordial matter that formed at the frozen fringes of the sun’s domain when the solar system was created. While the average comet is a “snowball” a kilometer or two across, Hale-Bopp has proved to be a 30- to 40-km wide bruiser. Comets exist in a huge spherical region called the Oort cloud, far beyond the orbits of the planets. The pull of a passing star occasionally nudges a few of these comets towards the sun, a journey that can take millions of years. Some of these comets will pass close enough to planets to be diverted into shorter orbits. As they approach the sun, their outer layers boil off, forming a cloud (or coma) around the nucleus.

Comet Hale-Bopp has shown a double-tailed structure not unusual among comets: a bright, yellow dust tail, and a fainter, blue gas tail. At their peak, these tails stretched over 100 million km into space. Both tails point away from the sun: the particles in the dust tail are forced outward by the minute but steady pressure of sunlight, and the ionized gases by the sun’s magnetic field. Hale-Bopp’s dust tail is composed of silicates and carbon-based molecules. Water vapor comprises 80% of the gas in the comet, and most of the rest is carbon monoxide and carbon dioxide. Other elements and compounds that have been detected include, sodium, ammonia, sulfur dioxide, hydrogen sulfide, formic acid, and hydrogen cyanide. Through a telescope, Hale-Bopp has shown a spectacular structure, including jets of material spewing back into the tail and concentric semicircular “hoods” surrounding the nucleus.

Astronomy writer and amateur astronomer Fred Schaaf has written a book called
Comet of the Century , published by Copernicus in the fall of 1996. It includes information about the nature, origins, and fate of comets, and a chronicle of great comets throughout history, including Halley and Hyakutake. Attention is paid to comet lore and superstitions over the centuries, how different societies viewed comets, and the inroads that science has made over the past few centuries in unlocking their true nature. The book contains extensive information, including a viewing guide, for Hale-Bopp. The projections Schaaf makes have proven very close to the mark. It is a book that will be a valuable reference, not just for Hale-Bopp’s reign but for years to come.

As the comet races away towards the fringes of the solar system whence it came, let us reflect on its legacy. Is Hale-Bopp indeed the “Comet of the Century”? Other comets have been brighter, such as 1965’s Ikeya-Seki and the Great January Comet of 1910, which were each visible as tailed apparitions in broad daylight near the Sun. Still others have shown more splendid tails. Comet West of 1976 showed a bright, curving dust tail with five distinct branches, and in 1910, Earth actually passed through the tail of Halley’s Comet. But for a combination of brightness and ease of visibility, Hale-Bopp takes the cake. No comet, throughout recorded history, has been visible to the unaided eye for as long as Hale-Bopp, and few have shown a long period of brilliance while easily visible in a dark sky. (Most comets, when at their brightest, are closest to the sun and thus often lost in the twilight. Hale-Bopp’s orbit took it far north of the sun, making for particularly favorable viewing from the Northern Hemisphere.)

For many, though, the comet will be remembered mainly for its connection to the “UFO cult” and its mass suicide. Throughout history, bright comets—those swordlike, fiery-looking visitors that seemingly appear out of nowhere, blaze in our sky for a few weeks while wandering far beyond the bounds of the zodiac, then disappear just as suddenly—have had a powerful effect on the human psyche, inspiring emotions ranging from bliss to religious terror. Comets have often been seen as omens, or precursors of calamity. The late Carl Sagan compared science to a candle in the dark, a light to illuminate the truth in the face of ignorance and superstition. The Heaven’s Gate tragedy underscores the need for scientists to continue their sometimes thankless, often uphill effort to communicate their truths to the general public and to fight the tide of disinformation and unreason that persists, even in this modern age.

E-mail to tonyhoffman [at] earthlink [dot] net