Diversion

Another Comet Cometh

Just past midnight on a starry summer night a year and a half ago, from his backyard in Cloudcroft, New Mexico, Alan Hale spotted the smudge of his dreams.  The founder of the Southwest Institute for Space Research, Hale had been sightseeing in the constellation Sagittarius, killing time until another galaxy rose above the horizon.  Then he noticed a dim, fuzzy object near the Messier 70 star cluster.

"I had spent 20 years looking for a comet, dragging myself out of bed, losing a lot of sleep and never finding anything," recalls Hale, who had finally given up the search.   "But I didn't see what else it could be." 

Just to be sure, however, he flipped his telescope eyepiece to a higher magnification and checked his star charts to verify that he was looking at the right cluster.  Then he made a quick sketch of the position of the smudge relative to the stars.  He ran inside and pored through every catalog of galaxies he had in his home office for a sign of a smear in M70.  While he was at it, he sent an e-mail to the Central Bureau for Astronomical Telegrams (CBAT), the world's comet clearinghouse at Harvard University, to file a claim.  Finally, he peered through the telescope again.  The smudge had moved.  "And that's when I woke up my wife and asked if she was interested in looking at Comet Hale."  (Her reply was unprintable.)

By the following morning, Hale had received good news and bad news.  The good news was that his observations were correct.  The bad news was that he wasn't alone.  Thomas Bopp, an amateur astronomer and a shift supervisor in the parts department of a construction materials company in Phoenix, Arizona, had been at a star party in the desert 90 miles outside of town, when he, too, stumbled across the suspicious smudge.  Thus, the name on the comet's official birth certificate became: Comet C/1995 O1 (Hale-Bopp).

A quick explanation of cometary nomenclature:  Under a system recently codified by the International Astronomical Union, "C/1995 O1" means that this was the first comet found in the second half-month of July (each half-month is given a letter, with "I" being omitted and "Z" not needed).  The C designates comets with a solar-orbiting period of more than 200 years.  (Halley's Comet, with its short 76-year cycle, is 1P/Halley.)  Lastly, although proponents of alphabetical order might assume the comet would be called "Bopp-Hale," according to CBAT's calculations, Hale beat Bopp to the smudge by about 10 minutes, so he got pride of place.

Over the past two years, an average of six new comets were discovered each year.  Of these, the most spectacular was Comet Hyakutake, named for Japanese amateur astronomer Yuji Hyakutake, who spotted his claim to celestial history through a pair of giant 25 x 150 binoculars on January 31, 1996.  But Comet Hyakutake, which pasted a silvery-green-blue blob in the sky from late March through much of April, was only icing on the cake for comet-watchers waiting for Hale-Bopp.  Same time, next year, they said.  Even if you missed Hyakutake, Hale-Bopp was bigger, brighter and close on its tail, scheduled to light up the skies in March, 1997.

Comets have always been cause for excitement.  Until recently, however, most of it was bad.  "Eight things there be a comet brings, When on high at its horrid reign," went an ancient jingle, which proceeded to enumerate just about every catastrophe then known to man:  "War, famine, plague, death to kings, Wind, earthquake, flood and dire change." 

Such was their bad rep that Hannibal, who had so triumphantly crossed the Alps on elephant back, committed suicide after hearing that a comet had appeared to foretell his death.  An apparition of what later became known as Halley's Comet appeared in 1066 and was commemorated on the Bayeux Tapestry by the wives of the Norman conquerors.  Two comets in rapid succession spooked Aztec ruler Montezuma into believing that the great white-beared god, Quetzacoatl, had already booked his return ticket to Mexico to reclaim his empire, leading Montezuma to cave in to Hernan Cortez a lot more quickly than he might have otherwise.  In recent years, Comet Kohoutek's 1973 fly-by led at least one astrologer to predict the downfall of Richard M. Nixon.

What changed people's minds was the most famous periodic comet in history, Halley's Comet.  An ardent astronomer who was trying to crack the secret of planetary orbits, Edmund Halley, Jr. switched his focus after seeing the Great Comet of 1680 blaze over the English Channel while he was en route to France. It was a remarkable time for scientific discovery, and Halley was in the thick of it.  From the director of the Paris Observatory, he picked up the idea that comets, like planets, might have orbits.  From his friend Isaac Newton, he appended the notion that those orbits might be elliptical.  And from his own research into the comet of 1682, compared to previous visitations in 1607 and 1531, he concluded that these comets were not just similar but, in fact, the same. 

The implications of A Synopsis of the Astronomy of Comets were stunning.  Halley's 1705 paper was the first application of Newton's laws of physics, by a scientist other than Newton, to solve one of the mysteries of the universe.  Furthermore, by predicting the return of the comet--and forecasting its re-appearance at the end of 1758 as precisely as Babe Ruth calling a home run--Halley shattered the belief that comets were mystical portents and symbols.  Thanks to Edmund Halley, comets became physical objects suitable for scientific study.

To understand the whys and wherefores of these so-called "hairy stars" requires a fast course in cosmology.  According to accepted theory, the planets in our solar system formed 4 1/2 billion years ago as a result of collisions between much smaller objects, called planetesimals.  The collisions didn't render space squeaky clean, however.  Among the cosmic debris were planetesimals made up of rocky and metallic substances, now called asteroids. Other leftover planetesimals were made of water ice and other frozen liquids or gases--collectively referred to as "volatiles."  We call them comets, from the Greek  kometes, meaning "long-haired," a reference to the way the wispy tail streams out behind the comet's head like tresses floating in a wind.  Comets are essentially cosmic fossils, remnants of the origins of our universe.

While asteroids are confined by gravity to a region between Mars and Jupiter known as the asteroid belt, comets inhabit the extreme outer reaches of the solar system on the edge of interstellar space, a vast spherical shell known as the Oort Cloud.  Occasionally, a gravitational nudge from a passing star or other object causes a comet to break out of the Oort Cloud and begin falling toward the sun. 

As the sun's radiation warms the comet's nucleus, what is basically a dirty snowball of dust, frozen carbon dioxide (i.e., "dry ice"), methane and other volatiles changes in character.  The volatiles steam, or sublimate, outward, venting microscopic particles of dust which are caught in the solar wind.  This creates the fluffy coma, or atmosphere, surrounding the comet's nucleus as well as its distinctive tail, which usually blows out behind the coma in the opposite direction from the sun, like a large windsock.

Not all comets look alike.  The ancient Chinese described them as either broom stars or bushy stars, depending on whether they had tails or a large, fuzzy head.  The more dust in a broom star, the greater the tail created as it sweeps through space. Comet West, an exceptionally dusty comet that came by in 1976, resembled a cosmic explanation point.  "It was the kind of comet that smacked you in the face," recalls Dr. Joseph N. Marcus, a St. Louis pathologist and founder of the Comet News Service.  Comet Hyakutake looked more like a teardrop, tinted an unearthly shade of blue from its high proportion of methane. Comet Hale-Bopp is expected to have a dusty tail.

A comet's reflective powers are as much a function of its distance from the sun as its intrinsic make-up.  Distance is against Hale-Bopp; the closest it will come to the sun will be about 85 million miles (and the closest it will come to Earth will be 120 million miles).  However, it's got familiarity in its favor.  Virgin comets, i.e., first-time visitors to the solar system, are rarely as bright as those that have been around the block a couple of times; that's one reason that the much-hyped Kohoutek was such a snooze in 1973.  Hale-Bopp swings by approximately every 3,000 years, so it has had a chance to burnish its volatiles to a bright sheen.

The question is, how bright?  When it was first spotted, Hale-Bopp was brighter than any other comet ever observed at that distance, a little over seven Astronomical Units from Earth.  (An Astronomical Unit is 93 million miles, the distance between the Earth and the sun.)  Most objects out there can't be seen by any telescope in the world.  But Hale-Bopp was not only visible but visible through a small scope.

Its initial light curve was quickly mimicked by the blood pressure of its trackers, since it suggested that if Hale-Bopp stayed on course it would be one of the brightest comets of the century.  This past summer, however, its luminescence stalled, then decelerated.  Initial predictions that the comet would be as bright as Jupiter faded, with pessimists predicting it would achieve nothing more than the faint glow of Polaris, the North Star.

Hale-Bopp's size could change its ways.  Definitely a heavyweight, Hale-Bopp's nucleus is estimated to be about 25 miles in diameter.  Compare that to Comet Halley's nucleus, which was shaped like a 5-by-5-by 10-mile potato.  The hope is that Hale-Bopp's heft may be due to water weight, which could sublimate as the comet approaches the sun, leaving a brighter nucleus.

Unfortunately, no one really knows.  "Comets are like cats," commented David Levy, an astronomer with a prodigious 19 comets to his name.  "They both have tails and do whatever they want." 

Alan Hale is resigned to the comet's orneriness. "I would like to think it provides an excellent opportunity to turn people's attention skyward so they can see the wonderful things out there."  Meanwhile, like an anxious parent, he checks up on his namesake every two or three nights and encourages it to be the best and the brightest.  "One morning, Hale-Bopp was rising in the east while I could see Hyakutake in the west," he recalls.  "So I said, 'You've got a hard act to follow, but I know you can do it.' I don't know how well it will listen to me," he adds.  "Hopefully, better than my kids." 

 

BOX:  Clicking on Comets

As might be expected, the Internet is absolutely jammed with comet-related sites.  Type "comet" into any search engine and you will get an enormous list.  Our favorites:

Comet Hale-Bopp Home Page.

WWW.HALEBOPP.COM Magazine.  A webzine companion to the Hale-Bopp home page.  http://www.halebopp.com.

SKY Online.  Astronomy-related articles and comet updates from the editors of Sky & Telescope.

TK.  Dan Green, the point man at the Harvard-Smithsonian Center for Astrophysics, got so tired of answering the same questions regarding Comet Hale-Bopp that he compiled this highly useful press information sheet.

 

If you'd rather read a real book, here are three recommendations:

The Mystery of Comets by Fred Whipple (Smithsonian Institute Press, 1985).  A readable explanation from the man responsible for the "dirty snowball" theory.

Comets by Carl Sagan and Ann Druyan (Random House, 1985).  Painstakingly researched but highly entertaining, with great illustrations.

Guide to Observing Comets by Daniel W.E. Green and Charles S. Morris (Smithsonian Astrophysical Observatory, 1996).  The most up-to-date book on the topic, published just last November.  To order a copy, send $15 to International Atomic Quarterly, Smithsonian Astrophysical Observatory, 60 Garden St., Cambridge, MA  02138.

 

BOX:  Photographing Comets

Dr. Mario Motta, a cardiologist in Salem, Massachusetts, has been watching Hale-Bopp since its discovery on a telescope that he built from scratch.  In addition to chasing comets, he also photographs them . (NOTE:  He has many slides and would be delighted to have us use them.  E-mail him at:  mmotta@massmed.org or call him at the office, 508-744-3499.) 

      The key to a good picture is super-fast film.  "Use 1,000 or 1,600 ASA film, such as Previa Fujichrome," he suggests.  A SLR camera is best, one for which you can open the shutter and keep it open for as long as you want.  Motta uses a 50mm lens for close-ups and a 30mm lens for wide-angle shots that show all of the comet's tail.

      Use a good tripod to hold the camera steady.  Focus on the comet, rather than on the stars, because the comet moves against the star background.  To avoid star smears in the background of the photograph, either piggyback the camera on a telescope with a tracking device or invest in a clock drive tripod mount to compensate for the earth's rotation.  These are usually advertised in popular astronomy magazines, such as Sky & Telescope.

      Start exposing the film for a few seconds, then go up to a few minutes or more, depending on the brightness of the comet.  Motta has gone to 15 minutes' exposure with a wide-field camera for dim comets.  When shooting a close-up through a telescope, he generally uses a one-minute exposure.