COVER

CELESTIAL MYSTERIES

Some scientists fear a collision between a comet and the Earth

MARK NICHOLS July 18 1994
COVER

CELESTIAL MYSTERIES

Some scientists fear a collision between a comet and the Earth

MARK NICHOLS July 18 1994

CELESTIAL MYSTERIES

Some scientists fear a collision between a comet and the Earth

MARK NICHOLS

Towering 14,000 feet above sea level, the summit of Hawaii’s Mount Mauna Kea is the site of 10 telescopes. On July 16, scientists at most of them will aim a variety of optical lenses and reflecting mirrors, infrared detectors and radio antennas towards a single distant target—the planet Jupiter. At the Canada-FranceHawaii reflecting telescope, one of the largest on Mauna Kea, John Caldwell, an astronomer from Toronto’s York University, is scheduled to use the observatory’s 3.6-m telescope in the hope of catching a glimpse of the debris flung out when fragments from the comet Shoemaker-Levy 9 begin crashing into the planet. At a neighboring radio telescope, University of Lethbridge, Alta., astronomer David Naylor planned to use the highly accurate infrared spectrometer, which he designed, to analyse radiation from the impact. The data could provide clues to the molecular composition of the comet—and of Jupiter itself. “This is terrifically exciting,” said Naylor. “If the comet punches holes deep into Jupiter’s atmosphere, then we may be able to gather valuable information about the lower atmosphere of the planet But nobody really knows what will happen.”

The intense activity on Mauna Kea will be repeated in observatories around the world as scientists strive to glean every scrap of information from a once-in-a-lifetime astronomical event. Much of the fear and mystery historically associated with comets has faded. But scientific interest in comets and asteroids is growing. Experts are debating whether a comet was responsible for the disappearance of the dinosaurs 65 million years ago. And there is concern on the part of some American scientists and politicians over the possibility that someday a comet might be involved in a disastrous collision with Earth. Some American experts have even proposed preparing nuclear-tipped rockets as weapons against incoming comets.

Certainly the planet has experienced massive comet and asteroid collisions in the past Eugene Shoemaker will follow events from the Space Science Telescope Institute in Baltimore as the comet he co-discovered with his wife, Carolyn, and Canadian David Levy strikes Jupiter. According to Shoemaker, a disastrous strike on Earth is unlikely in the immediate future. “The odds against it happening in the next couple of years are very small—about one chance in 50 million,” Shoemaker

told Maclean’s. “But it could happen.”

For now, scientists hope that the rendezvous between Shoemaker-Levy 9 and Juiter will shed light on the solar system’s largest planet—a giant ball of hydrogen and helium with a diameter more than 11 times greater than the Earth’s—and on the nature and composition of comets.

Astronomers believe that comets originate in a belt of space debris that lies on the fringes of the solar system—about 5 trillion miles from the sun. Every so often, some event—the gravitational tug of a nearby star or an interstellar gas cloud—shakes loose a chunk of the debris and sends it roaming through the solar system. Scientists know that comets are essentially composed of frozen gases and interstellar dust. “The reason that scientists are excited about the collision with Jupiter,” says Jaymie Matthews, a University of British Columbia astronomer, “is because we believe comets represent a kind of primordial material—the things that were around when the solar system was first created.”

Comets once inspired not curiosity, but fear. In many societies, they were believed to herald evil events—the death of a monarch, famine, pestilence or war. In 1066, Halley’s comet appeared before invading Norman forces defeated English soldiers in the Battle of Hastings. In 1456, when a comet appeared at a time when Turkish forces menaced Christian Europe, Pope Calixtus III reportedly excommunicated the comet. When two comets blazed in the skies above Mexico, the Aztec emperor Montezuma became so demoralized that he abdicated his throne rather than oppose the tiny force of Spanish invaders under Hernán Cortés who arrived in 1519. As recently as 1910, another appearance of Halley’s comet so terrified members of a religious sect in Oklahoma that they attempted to sacrifice a young woman to pacify the comet.

In the 20th century, the unreasoning fear of comets largely subsided—just as scientists were learning that comets and asteroids strike the Earth far more often than they once believed. Until about 30 years ago, experts thought that most of the craters that pockmark the planet were caused by volcanic eruptions. Then, Shoemaker and a handful of other researchers launched a series of investigations that found telltale mineral and chemical clues proving that many craters were actually caused by the searing impacts of incoming asteroids and comets. The world’s largest known crater is the buried 250-km-wide depression near Sudbury, Ont., probably caused by a comet nearly 2 billion years ago.

Controversy still rages over the event that probably involved one of the largest space objects ever to crash to Earth: the comet or asteroid that many scientists now believe plowed into the planet about 65 million years ago. The awesome impact, say the experts, was so powerful that it created fire storms followed by freezing cold and acid rain that killed vegetation—and doomed the dinosaurs that dominated the planet. During the 1970s, scientists uncovered geological evidence to support the theory, and set off a search for the crater that the mighty collision must have left. In 1991, Alan Hildebrand, now an Ottawa-based research scientist with the

Geological Survey of Canada, played a prominent role in showing that a vast crater was buried under Mexico’s Yucatán Peninsula— and was probably caused by the impact that doomed the dinosaurs.

Just how big was the comet that may have killed off the dinosaurs? Calculating the answer depends partly on the size of the Yucatán crater, and scientists are currently locked in a debate over its exact dimensions. Hildebrand and Mark Pilkington, another Geoglogical Survey scientist, maintain that the crater is about 170 km in diameter, while a group of American scientists argue that it is much larger—between 250 and 300 km from rim to rim. Shoemaker estimates that the comet probably would have had to be 12 to 15 km in diameter to scoop out a crater 180 km wide. That, adds Shoemaker, would mean that the comet from 65 million years ago may have been just a little bigger than ShoemakerLevy 9 before it broke into about 21 pieces during its journey past Jupiter in 1992.

Eugene and Carolyn Shoemaker and thencolleague Levy have been in the forefront of efforts to create an inventory of the asteroids and comets that cross the orbital path of the planets—and could at some point collide with Earth. Asteroids—the rocky leftovers from the creation of the solar system 4V2 billion years ago—are the most likely culprits. Using one of the telescopes on California’s Palomar Mountain to scan the skies, Shoemaker’s team has concluded that there are probably about 2,000 potentially Earth-threatening asteroids with diameters of one kilometre or more orbiting within the solar system—and that, on average, one of them strikes the Earth’s surface every 125,000 years. Comets can be much larger, and they are far more numerous. Scientists calculate that there may be more than 100 billion comets of all sizes clustered in the Oort Cloud, a region on the edge of the solar system. Of those, only about three large comets, with average diameters of about 5 km, cross the Earth’s path every year. And every 100 million years or so, one of them strikes the planet.

What saves the Earth from experiencing more comet strikes is the benevolent role played by the giant planets Jupiter and Saturn, whose powerful gravitational pulls lure comets towards them—and usually send the interlopers spinning off into outer space. Jupiter alone, says Shoemaker, “ejects most of these objects.” If it were not for Jupiter, he adds, Earth would experience comet impacts equal to the one that exterminated the dinosaurs “every million years or so. It’s unlikely that under those circumstances higher forms of life would have evolved on Earth.”

Still, the growing awareness of the swarms of asteroids and comets that might some day collide devastatingly with the planet persuaded the U.S. government to consider protective measures. In April, 1992, a committee of experts set up by the National Aeronautics and Space Administration (NASA) recommended a $50-million program to build six new observatories to scan the heavens for incoming asteroids and comets. And some scientists proposed that, if necessary, nuclear-tipped rockets might be used to destroy or deflect any sizable space object that appeared likely to strike Earth. So far, NASA has agreed only to provide a fraction of the requested funding for the space-watch maintained by Shoemaker’s group and two other teams of American astronomers.

The idea of employing nuclear weapons to deflect menacing space objects away from the planet prompted ridicule from some critics— and expressions of concern from others. In a letter to the London-based journal, Nature, in April, astronomer Carl Sagan of Cornell University in Ithaca, N.Y., suggested that the idea could be a double-edged sword, because an unscrupulous government could employ the same principle to tum an asteroid into a mega-weapon by deflecting it towards Earth. Sagan argued that “in the light of well-established human frailty and fallibility,” the proposal could create dangers greater than the risks posed by asteroids and comets.

A different kind of controversy flared during the late 1970s when the Nobel Prize-winning British astrophysicist Fred Hoyle and a Sri Lankan scientist, Chandra Wickramasinghe, proposed that comets may have been responsible for bringing life to Earth in the form of cells—along with disease-causing viruses and bacteria. While most scientists reject Hoyle’s proposals as impossibly farfetched, there is a growing consensus that the debris from a comet bombardment billions of years ago did in fact bring vital materials to Earth, including oxygen, nitrogen, carbon and amino acids— key building blocks of biological life. Some scientists now believe that by doing that, the comets indirectly bestowed on Earth the breath of life—a gift that at intervals of millions of years, the comets return to threaten. □