From Antiquity These Icy Visitors Have Awed and Terrified
Photo of Comet Hale-Bopp by Bill Newcott
Whistling and moaning, a 50-mile-an-hour (80-kilometer-an-hour) wind whipped among the telescope domes atop Kitt Peak. Just a few feet below, turning gray in the dusk, slid a river of clouds that had been rising and dropping all day. And high above, comet Hale-Bopp hung suspended like a feathery fishing lure, its tail curving off a bit, as if blown to the side by the punishing wind.
One by one, stars winked on in a darkening sky. In each of the telescope domes, teams of astronomers prayed that the wind would drop below 40 miles per hour (64 kilometers an hour), the point at which they'd be able to open the sliding doors and get back to work. The sky turned indigo. Then black.
Viewed from the summit, 6,873 feet (2,095 meters) above Arizona's Sonoran Desert, Hale-Bopp's bright dust tail, along with a dimmer, all but transparent blue one, seemed to grow by degrees. Among the brightest comets ever seen, Hale-Bopp had been visible for months from midtown Manhattan, of all places.
But here, on a moonless night in the mountains in the desert, the length of Hale-Bopp's tail became visible—a wispy, delicate veil. Along with eclipses, comets have been the most feared and admired sky spectacles of all. But while astronomers have been able to predict eclipses for thousands of years, only in the 1700s was a comet's return correctly predicted, by Edmond Halley.
Some comets swing around the sun every few years. Others, like Hale-Bopp, may take thousands of years. Most can be seen only with a telescope. But every once in a while—a few times a century, perhaps—an impressive one is visible to the naked eye. And in the past two years the world has witnessed not one but two of them.
Hyakutake in 1996 had one of the longest tails on record, stretching more than halfway across the sky; Hale-Bopp in 1997 had one of the most brilliant heads, nearly as bright as the star Sirius. Add the Jupiter crash of comet Shoemaker-Levy in 1994, Halley's most recent visit in 1986, vivid comet West in 1976, and the scientifically signifiant—if visually disappointing—Kohoutek in 1973-74, and you could say that we are indeed living in the age of comets.
Hovering in the most fragile of gravitational balances, a fleet of dirty, lumpy snowballs numbering in the trillions is barely held in orbit by the pull of the sun. They are stored in the Oort cloud, a huge, diffuse sphere of cometary nuclei in the far reaches of the solar system. Close to the sun, yet still beyond Neptune, circle what may well be their brethren, in a great disk called the Kuiper belt. Comets are leftovers, scraps of material that didn't make it to planethood in the events creating our solar system.
Once, many astronomers believe, the solar system was full of comet nuclei, chunks of ice and dust left over from the formation of the sun. Most clumped together to form planets, leaving a relative handful—averaging perhaps a few miles wide, with temperatures as low as minus 400 degrees Fahrenheit (minus 240 degrees Celsius)—as time capsules of the early solar system. They orbit in a perpetual deep freeze until some subtle gravitational nudge upsets the delicate balance. Then the great fall begins. Imperceptibly at first, a snowball drifts toward the sun and steadily accelerates.
As solar radiation heats the comet, the ice within sublimates, escaping as gas from vents at the surface. Sometimes jets of sublimating ice whirl off the rotating comet nucleus like a fireworks pinwheel. Dust trapped in the ice breaks free. Pushed back by the pressure of the sun's radiation, the dust streams out behind the comet in what appears as a fiery tail.
Now the comet is among the fastest things in the solar system. It whizzes past the inner planets at around 100,000 miles an hour (160,000 kilometers an hour). The tail can stretch a hundred million miles (160 million kilometers). A quick whip around the sun, a long journey back into night, and the show's over. Until gravity calls it back. With each visit to the sun the comet loses more of its ice. Eventually it may become a rocky ghost, its glory days gone for good.
Kitt Peak's solar telescope is bored right into the mountain, with mirrors on top that can be oriented to follow the sun's path across the sky. Solar studies are one of astronomy's few day jobs, and in 1973 Frank Scherb and Fred Roesler, physicists at the University of Wisconsin, got the bright idea of using the telescope at night to track comet Kohoutek. Scherb has been back for every major comet since then, except for Hyakutake in 1996.
"That one just came up too fast," he said, showing me through the cramped corner of the telescope facility that has been made available to him. A few computers whirred in a row, their terminals displaying Hale-Bopp images taken the night before. He punched up a series taken a few minutes apart through filters that brought out various parts of the light spectrum. With each image, the tail changed shape a bit, indicating the presence of different molecules. "The most important new result we've got here is that we're detecting an unusual kind of atomic carbon," Scherb told me. "It's being thrown off by the many types of carbon compounds in the comet."
Besides the bountiful hydrogen and oxygen found in cometary ice, astronomers knew that comets contained a lot of carbon monoxide. But radio, infrared, and ultraviolet measurements have revealed a whole zoo of carbon compounds in comets, some of which are essential building blocks in all known life-forms. Two dozen carbon molecules have turned up in Hale-Bopp, along with trace amounts of nitrogen, sodium, and sulfur.
The growing list of compounds found in Hale-Bopp supports the theory that the ingredients for life were delivered to Earth by comets. Scherb's carbon atom measurements will help determine how much of each compound is present in the average comet and whether or not there is enough to keep the theory alive. I asked Scherb about Hale-Bopp's transparent blue tail, which gave the comet the appearance of a celestial bird-of-paradise flower.
"That's the ion tail," he explained. "As water sublimates off the comet, the sun's radiation strips an electron from the water molecules, creating charged particles. Those ions get picked up by the solar wind, and they drive back to form the ion tail." Ron Oliversen, a NASA scientist working with Scherb, hopes to use the ion tails of comets to get the "weather reports" from the distant solar system. "Ion tails are a kid of wind sock for the solar win," he said. "If we can follow comets far enough, we can get some idea of what the solar wind is doing out there."
Alan Hale was on the phone. Again. Giving driving directions so that a TV crew could find his house. Again. The co-discoverer of comet Hale-Bopp was riding the wave of media frenzy on this, the day his comet would make its closest approach to the sun. His four-year-old son, Tyler, had greeted me at the door of their home and then ushered me into the cluttered living room. As dad intoned some well-worn answers to an interviewer, Tyler proudly showed me his Thomas the Tank Engine toy. "My daddy," he confided, "is a star guy."
Much has been made of the fact that Hale found the comet while looking through a telescope in his driveway. Few accounts note that to reach his driveway, you must follow 9 miles (14.5 kilometers) of unpaved road in the Sacramento mountains of southern New Mexico. Far from city lights, it's the perfect place for comet hunting. The sky was absolutely clear the night Hale made history. It was just after midnight on July 23, 1995.
"Sagittarius was high in the south. I was looking at a star cluster there, and I noticed a fuzzy object. I checked a star atlas, and it showed nothing there. So I knew it was a possible comet." As a trained astronomer who has seen some 200 comets,
Hale knew exactly what to do. He headed inside and sent an e-mail to the Central Bureau for Astronomical Telegrams in Cambridge, Massachusetts, to register his findings. A few hours later he got a response: His new comet was officially designated C/1995 o1. His name would also be attached.
But he was not alone. That same night, about 400 miles (644 kilometers) away, Tom Bopp was out with some pals looking through telescopes in the desert outside Phoenix, Arizona. He, too, noticed something fuzzy in Sagittarius and pointed it out to the friend who owned the telescope he was using. "You might have something there, Tom," the friend said. He was right. Bopp, a manager at a construction materials factory, had never seen a comet. He missed Kohoutek. He even managed not to see Halley.
"I knew I had to contact the Central Bureau, but I didn't have the address with me," Bopp told me. "I had to drive all the way back home and get it." In the wee hours he roused Western Union and fired off a telegram to Cambridge—where its arrival was greeted with the same bemusement that might accompany a horse and buggy at a Porsche service garage. "Nobody sends telegrams anymore," laughed Brian Marsden, who has run the bureau since 1968.
"I mean, by the time that telegram got here, Alan Hale had already e-mailed us three times with updated coordinates." A jovial Brit with a shock of gray hair, Marsden is the Man Who Names the Comets, the leading voice on a committee that has the final say.
"These things can get quite contentious," he added. "Some people try to cheat. They'll see a comet and call a friend, who will then also report it. Usually we can weed them out." Marsden's brow furrowed—the first time in a half hour the smile left his face. "But I'll tell you, there's at least one comet where I'm sure one of the names was faked." Marsden was seething.
Tom Bopp and I were sitting at a restaurant near his home in Glendale, a Phoenix suburb. Neither of us thought of it at the time, but the restaurant was part of the same chain where, one week before, 39 members of a California cult had reportedly eaten their last meal before ritually committing mass suicide. They claimed they were departing on a spaceship that was trailing comet Hale-Bopp.
Weird stuff seems to go with major comets. When a comet appeared in A.D. 60, the people of Rome assumed it meant the impending death of their still new emperor, Nero. He responded by exiling a potential rival. When another comet turned up just four years later, ancient historians say he ordered the execution of hundreds of nobles. It is said that Moctezuma II saw a comet in 1517 that foreshadowed the downfall of the Aztec empire. In 1910, a wave of hysteria swept over the United States amid reports that Earth was about to pass through Halley's tail.
For Tom Bopp, the comet did indeed portend heartbreak. Three days before we met, as his comet reached its most spectacular point, his brother and sister-in-law were killed in a late night car wreck. They had been out photographing the comet. "This," he told me quietly, "has been the best week of my life. And the worst."
David Levy's alarm goes off at 3 a.m. Trying not to disturb his wife, he leaves the house and trudges to a homemade observatory behind his home in Vail, Arizona, near Tucson. Levy and his friends Gene and Carolyn Shoemaker composed a remarkably prolific comet-hunting group before Gene was killed in a car accident in Australia last July. Working individually and together, they discovered 40 comets. "On average a dozen comets are found each year," said Levy. "It is unpredictable."
Even a comet's periodic return can vary. Hale-Bopp had a 4,200-year period, but thanks to a gravity assist from Jupiter it will be back in 2,4000 years. Sometimes a comet such as Shoemaker Levy 9 will go into orbit around Jupiter and act as a moon. Others leave Jupiter's orbit and go back around the sun. Comets are like cats; they both have tails, and they both do precisely what they want to do."
One thing comets do is crash into things. Imagine yourself atop the Empire State Building with the world's most powerful bowling-ball cannon, capable of reaching the general vicinity of Washington, D.C. You are randomly firing bowling balls as a lone Volkswagen bug circles the Washington Beltway. It's not likely that you will hit that Beetle anytime soon, but one of these days, one of these millennia, you will inevitably crush one vintage compact car. The last bowling ball to devastate earth—either a comet or an asteroid—hit about 65 million years ago. It blasted a crater more than a hundred miles (160 kilometers) wide under today's Yucatan peninsula and threw up a global cloud of dust and gas.
The sun was all but blocked for years, dropping average temperatures to near freezing and killing off more than half of the plant and animal species on Earth. The dinosaurs died. On the other hand, many mammal species survived.
"If you want some idea of the devastation that's possible from a comet," said David Levy, "take a look at what just one of the fragments of Shoemaker-Levy 9 did to Jupiter. Here's something as long as two or three football fields, and it left a mark bigger than Earth."
In May of 1983 the comet IRAS-Araki-Alcock got within 3 million miles (4.8 million kilometers) of Earth. At its closest Hale-Bopp was 122 million miles (196 million kilometers) away. Had Hale-Bopp arrived just four months earlier, it would have come within 11 million miles (17.7 million kilometers) of Earth, shone 50 times brighter—and been visible in daytime for about a week.
Comets can bring more than destruction when they slam into a planet. There's evidence that a good storm of comets in a planet's formative stages could provide an ocean or two. We may well be swimming in comet melt-off.
"It could take as few as five Hale-Bopps to fill the Great Lakes," says astronomer Michael U. Mumma of NASA's Goddard Space Flight Center. One of the most popular theories for the origin of Earth's oceans has been that the water was delivered via comets.
The theory has problems though. Studies of Hale-Bopp—and of Hyakutake and Halley before it—show that cometary water contains twice as much deuterium as seawater. If the oceans came primarily from comets, the proportion ought to be closer. While most scientists were envisioning such big splash models, one man was thinking about a cosmic drizzle.
In 1986 Louis Frank, a physicist from the University of Iowa, suggested that each year Earth is bombarded by ten million house-size mini-comets weighting up to 40 tons (36 metric tons) each. Vaporizing high above Earth's atmosphere, these cometlike objects would deliver, in 20,000 years of so, enough moisture to cover the planet with an inch (2.5 centimeters) of water.
Frank's credentials are impeccable. Since 1958 he has worked on scientific payloads for 40 U.S. spacecraft. But his theory was soundly rejected by astronomers. Then in September 1996 NASA's Polar satellite, using cameras and filters designed by Frank, took images of what he says are cosmic snowballs evaporating in long, cloudlike streaks.
"These snowballs are cometlike, but they are just about all water. No dirt," said Frank.
"They're very, very fragile. And between Earth and Jupiter there are probably enough of them to fill all of our oceans with water." After a brief period of uncertainty Frank's critics still don't buy it. "Right now there are a half-dozen papers either published or in preparation that disagree with Lou's interpretation," said NASA's Mumma.
"For one thing, we don't know how far those streaks are from the camera. Some suggest they might be ice flakes coming from the craft itself. This has been a raging controversy for years, and Lou's images have not made it go away."
As Hale-Bopp passed the sun and headed back toward the deep-space icebox, the biggest news seemed to be the discovery, with the Isaac Newton Group of telescopes in the Canary Islands, of a third tail. Visible only with a sodium light filter, the tail was 400,000 miles (644,000 kilometers) wide, 30 million miles (48 million kilometers) long, and made of sodium atoms.
David Levy remains more enamored of a 1986 comet Halley finding that bore both scientific and philosophical significance.
"Halley showed carbon, hydrogen, oxygen, and nitrogen in similar proportions as found in the human body," he said. "I think it just goes to show that we are, as Gene Shoemaker used to say, the progeny of comets."
The winds had died down when I left Kitt Peak an hour after sunset, and Hale-Bopp was already low in the sky. I drove down the winding road and pulled over at the mountain's base.
The starts shone like, well, like only stars can shine. To the west, above where the sun had set, a dim, misty light climbed into the darkness. It was the zodiacal light, the reflection of sunlight off the disk of dust that floats between the planets—the scattered remains of comet tails, slowly drifting toward incineration by the sun.
The sound of crunching rocks startled me. A few feet away three wild horses, black on black against the nightscape, wandered past. They never glanced skyward at the gossamer swath of Hale-Bopp nor at the wondrous spectacle that is the night sky on a clear night, comet or no.
It felt good to be human.
A somewhat shorter version of this article appeared in the December 1997 issue of National Geographic Magazine