Canadian David Levy co-discovered a major comet that is expected to crash into planet Jupiter
BY D’ARCY JENISH
To the police who patrolled his Montreal neighborhood after dark, David Levy was known as the kid with the telescope. Occasionally, the officers would stop to chat with Levy as he gazed at the night sky from his parents’ backyard on Belmont Avenue in tony Westmount. Three decades later, at 46, Levy still scours the heavens with a backyard telescope. But now he lives in Tuscon, Ariz., writes about astronomy for a living, and has discovered eight comets on his own. And despite his lack of formal training, he has also shared in one of the most startling astronomical findings of the century. In March, 1993, Levy, along with the husband-and-wife team of Eugene and Carolyn Shoemaker, found a comet orbiting Jupiter that had fractured into 21 pieces. Scientists now expect the fragments to crash into Jupiter between July 16 and 22—creating one of the most spectacular celestial events on record. “All of astronomy is going to stop,” said Levy. “Every major telescope in the world will be looking at Jupiter.”
As they prepared for the event, astronomers scrambled for words to describe their excitement This will be the first time mankind has witnessed a major cosmic collision involving a planet and a large object such as a comet. Many scientists believe that a similar cataclysmic impact occurred on Earth 65 million years ago, wiping out the dinosaurs and most other forms of life. The effects on Jupiter—a gaseous giant 300 times larger than Earth—are uncertain. Some experts warn that the comet fragments could simply disintegrate when they hit Jupiter’s atmosphere. But others predict that the fragments—possibly as large as four kilometres across and travelling at a rate of 60 km/second— could trigger a series of explosions each equivalent to two million hydrogen bombs and unleash fireballs that surge more than 1,000 km above the planet’s atmosphere. “Fortunately it’s not happening on Earth,” says Levy with studied understatement “If it were, we’d have more special effects than we would know what to do with.”
But even if there are few spectacular visual ef-
fects, most astronomers believe the impacts will be a major scientific event. The comet fragments are expected to smash into the dark side of Jupiter and, in most cases, scientists will have to wait at least 15 minutes before the planet rotates and the impact sites can be seen from Earth. Nevertheless, hundreds of professional and amateur astronomers around the world—in places as diverse as China, Chile, India and Antarctica—will be watching Jupiter. So will the Hubble Space Telescope and several other space-based observatories. “We’ve marshalled the whole planet’s resources to look at this thing,” says Keith Noll, an astronomer at the Space Telescope Science Institute in Baltimore.
Scientists expect that the spectacle will yield reams of new information about Jupiter—the solar system’s most imposing planet and one of its most mysterious. Jupiter is believed to be a gaseous sphere with a small solid core, surrounded by several decks of cloud that could be 1,000 km thick. But astronomers have not been able to peer into the atmosphere because of its chemical composition. Some experts compare Jupiter’s atmosphere to a tequila sunrise, a layered drink with a creamy, opaque surface. The fragments will disturb the atmosphere, just as an ice cube chums up a cocktail, and should allow astronomers a unique glimpse into Jupiter’s middle and lower atmospheres.
Comets, too, are among the most intriguing objects in the solar system (page 40). Astronomers believe that trillions of comets were formed of ice, gases and cosmic dust when the solar system was born 4Va billion years ago. Occasionally, comets passing through the inner solar system are visible from Earth. They have brilliant halos and tails millions of kilometres long—created when the sun vaporizes the icy nucleus. And they have been the source of fear, superstition and even poetry. “I see comets as very romantic objects,” says Levy. “A comet that is very bright one night will have a different appearance the next. It’s almost like their moods change.”
Finding a comet requires extraordinary patience. Levy likens it to uncovering a needle in a haystack—when the needle is moving. Fewer than 900 comets have been discovered since recordkeeping became systematic over the past two centuries, notes Brian Marsden, director of the Central Bureau for Astronomical Telegrams in Cambridge, Mass., the quaintly named agency that keeps track of celestial discoveries. He says hundreds of amateurs like Levy scan the skies every night, weather permitting, using the oldfashioned technique of looking through telescopes. They still make up to 30 per cent of the finds—making astronomy perhaps the only scientific field in which amateurs can still compete with professionals in cutting-edge discoveries.
Professionals now make most discoveries using photographic telescopes, which take pictures of sections of the sky. They later scan negatives for comets, which resemble fuzzy smudges. But amateurs and professionals share the same passions—a love of the night sky and the thrill of discovery. ‘When I look at my film, I’m out there in this strange and wonderful universe,” says Levy’s collaborator Carolyn Shoemaker, flVtSHTa^l who works almost exclusively with a photographic telescope at the Palomar
Mountain Observatory in Southern California. “There’s a joy in finding something and seeing it when you know nobody else has ever looked at it before.”
David Levy became enthralled with the mysteries of the universe in the summer of 1960 when he was 12 years old and broke his arm in a bicycle accident. A cousin gave him a book on the planets and by the time he had the cast removed, his father, Nathaniel, a lawyer, and mother, Edith, a doctor and geneticist, had bought him his first telescope. Levy, the third of four children, was never athletic and did not make friends easily as a child, says his older brother Richard, 51, a Toronto real estate salesman: “I think he found looking through a telescope and connecting with the stars a spiritual experience.”
By the time he was 17, David Levy knew enough about the stars, the permanent inhabitants of the night sky, to begin searching for comets, which are mere passersby. But his high-school math and physics marks were not high enough to let him study astronomy at university. Instead, he took an undergraduate degree in English at Acadia University in Wolfville, N.S., and completed an MA on the 19th-century poet Gerard Manley Hopkins at Queen’s in Kingston, Ont. Levy married a Montreal woman in 1976 while attending university but, by 1980, the marriage had ended and he has remained single.
Despite his attraction to literature, studying the stars remained his passion, and after receiving his master’s in 1979, he moved to Tuscon primarily because its clear night skies are ideal for observing.
He found odd jobs as a research assistant in local observatories. And he continued to search the skies from his own backyard with a 400mm telescope. In November, 1984, he finally discovered his first comet.
“I felt an enormous surge of excitement,” he recalls. “I’d spent 19 years on this and finally had something.”
By the late 1980s, Levy had established a career as a science writer.
He had a column in the American magazine Sky & Telescope, a monthly publication for amateur astronomers, and published the first of 11 books and guides on astronomy. (His latest, Quest for Comets, appeared earlier this year.) In March, 1988, Levy found his fifth comet, and it changed the direction of his life. The Shoemakers, who live in Flagstaff, Ariz., had tracked the same object while observing at Palomar Mountain. Levy and the Shoemakers soon formed a lasting partnership. They now spend seven to eight weeks a year working together at Palomar Mountain, and have jointly discovered 13 comets.
Eugene Shoemaker, 66, a former geologist with the U.S. Geological Survey, is the only trained scientist among the three. He spent much of his career studying craters on Earth caused by impacts from comets and their celestial cousins, asteroids and meteoroids, which are rocky, ice-free objects travelling in space.
Shoemaker’s wife, Carolyn, 65, who holds an undergraduate degree in history, spent most of her life as a homemaker but began working with her husband in 1980 after the youngest of their three children left home. At first, she visually scanned images from photographic telescopes for asteroids and comets, although she has since developed her own innovative and sophisticated techniques. And within two years, she had made her first discovery. “It was on its way out of the solar system, and the images were so faint that I could hardly see them,” she recalls. “It was so exciting knowing that I could find them.”
In early 1993, the southwestern United States experienced unusual-
HT=i n ly wet weather that severely disrupted the work of astronomers. Several times in January and February, heavy rain and cloud cover prevented Levy and the Shoemakers from following their normal observation schedule. On the night of March 23, they were sitting in the Palomar Observatory, waiting for a break in the clouds. And it was on that night, through a combination of diligence and good luck, that they made the observations that led to their big discovery.
Levy, anxious to get something done, convinced his partners that they should prepare their telescope in case the skies cleared, even briefly. He also suggested using some film that had been damaged around the edges when someone inadvertently exposed it to light. They all expected the clouds to roll back in before they were done observing and did not want to waste good film. They focused on a section of the sky that included Jupiter, then at its farthest point from the sun.
Two days later, on the afternoon of March 25, Carolyn Shoemaker was scanning the images captured on the damaged film on that unsettled night. The moment she spotted the unusual-looking object near Jupiter is still fresh in her mind. “I was very startled and tremendously excited when I looked at the film,” she says. “I turned to Gene and David and said ‘I don’t know what we’ve got but it looks like a
Relative sizes: the diameter of Jupiter at its equator is 142,743 km, more than 11 times the diameter of Earth
squashed comet.’ ” Levy also vividly recalls his first impression: “We saw this fabulous-looking comet with multiple heads, multiple wings and multiple tails. Nobody had ever seen anything like it before.”
The trio immediately sent a computer message to comet counter Marsden in Cambridge to establish their claim as discoverers. The strange new object, formally named Periodic Comet Shoemaker-Levy 9 (1993e), quickly became the subject of intense study. Marsden and others calculated that it was orbiting Jupiter, which made Shoemaker-Levy 9 very unusual since comets normally revolve around the sun. On May 22, 1993, Marsden startled astronomers around the world by announcing that the comet would likely collide with Jupiter. Other observers, working on more powerful telescopes than those available to Levy and the Shoemakers, discovered that the comet contains as many as 21 nuclei stretched out like a string of pearls.
But there were still many other unresolved questions. Donald Yeomans and Paul Chodas, two experts in orbital dynamics at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., unravelled some of the comet’s recent history by studying 276 photos of the comet taken after the discovery. They believe that Shoemaker-Levy 9 had been just another cosmic traveller whizzing through the solar system until about 1970, when it passed close to Jupiter and was captured by the giant planet’s gravity. Yeomans and Chodas calculated that the next milestone in the comet’s 4V2-billion-year existence had occurred on July 7, 1992, when it passed within 30,000 km of Jupiter. Even at that distance, the planet’s gravitational force was strong enough to pull the comet apart and turn it into a trail of fragments. In its newly disrupted state, Shoemaker-Levy 9 began travelling away from Jupiter in a long elliptical orbit and by July, 1993, had reached the far end of its journey, 50 million km away. Since then, the comet has been travelling back towards Jupiter, and scientists have continued studying it to determine precisely when it would slam into the planet. Chodas says astronomers are convinced that they know within 20 minutes— plus or minus—when the fragments will hit, and that a near miss is impossible. They now predict that the first fragment will hit Jupiter shortly before 4 p.m. EDT on July 16.
Despite the intense public and scientific interest, astronomers concede that they can only speculate about the impacts. There are, in fact, two big questions that they cannot answer. They have no way of determining whether each nucleus is composed of a single chunk of material or a swarm of debris. More important, they have not been
able to calculate the size and mass of the fragments. Estimates have ranged widely—from one-tenth of a kilometre to four kilometres in diameter. In simple terms, the bigger the object, the bigger the explosion. “The key word right now is uncertainty,” says Harold Weaver, an astronomer at the Space Telescope Science Institute. ‘We don’t know what the heck is going to happen on Jupiter.” However, even when scientists rely on conservative assumptions, and estimate the fragments at one kilometre across with the density of ice, they can still foresee explosions on a scale that is difficult to comprehend. Mordecai-Mark Mac Low, an astronomer at the University of Chicago, says the fragments should hit Jupiter at 60 km/second and heat the impact sites from -150° C to 30,000° C. The hot, expanding gases from the comet and from Jupiter’s atmosphere should explode with unimaginable force—10 million times greater than that of the atomic bomb dropped on Hiroshima.
The impacts, and their aftereffects, will be measured and recorded in dozens of different ways by hundreds of scientists all over the globe, and may yield one of the biggest scientific windfalls of the century. Weaver says Comet Shoemaker-Levy 9 has already answered questions that have intrigued astronomers for years. For example, some of Jupiter’s moons, along with some of the planets, are pockmarked with strings of craters, and scientists now think they were caused by impacts from shattered comets like Shoemaker-Levy 9. “People were at a loss to explain where they came from till this object came along,” he said. “For those of us who study comets, this has already been a spectacular event.”
For Levy and the Shoemakers, the discovery and subsequent scientific interest have been a sweet reward for the hundreds of nights spent doggedly searching for comets—those thin wisps of light in the solar system. The trio will spend what has been labelled Impact Week at observatories and science centres in the Washington area and will be able to see images of the collisions as they are sent in from around the world. Levy is bringing his 76-year-old mother from Montreal to Washington for the event. “It’s by far the most exciting time of my life,” he says.
And the afterglow may well illuminate the rest of their lives. “I always daydreamed about finding the comet of the century—one of the great visual comets with a tail streaking across the heavens that everyone can see with the naked eye,” says Carolyn Shoemaker. “I haven’t found one like that and neither has anyone else. But I do think we found the comet of the century, nevertheless.” At this point, no one is arguing with that assessment. □