Look Out!

By LEON JAROFF

THE FOSSIL RECORD IS clear. Time and again during the nearly 4 billion years that life has existed on Earth, it has been assailed by global catastrophes that have caused the wholesale extinction of animals and plants. Over the past decade evidence has been mounting that many of these calamities were caused . not by long-term climatic changes, volcanism or disease, but by large asteroids or comets smashing into Earth.

These impacts blasted enough dust into the atmosphere to shroud the entire globe for months on end, blocking sunlight and causing temperatures to plummet. In the cold and dark, plants and animals perished. Compelling evidence of such cataclysms was revealed last summer: scientists confirmed that a giant crater, 176 km (110 miles) across, discovered under the northern tip of Mexico's Yucatan Peninsula was the likely impact point of a huge object, probably a comet, believed to have wiped out the dinosaurs and other forms of life 65 million years ago.

Could such mass extinctions happen again? Astronomers have no doubt; the solar system is littered with flying debris, and they say it is only a matter of time before another large celestial object bears down on Earth. Reminders of that potential for disaster occur frequently. Early in January, for example, NASA released several radar images of the 6.4-km-long (4-mile) dumbbell-shaped asteroid Toutatis taken when it sped within 3.5 million km (2.2 million miles) of Earth -- a hairbreadth by astronomical standards. And while the warning that the 10-km-wide (6-mile) Comet Swift-Tuttle might slam into Earth in 2126 has now been retracted, it briefly caused genuine concern among many scientists.

But life on Earth may no longer have to wait helplessly for the next catastrophe. In a paper titled "Cosmic Bombardment," scientists at Lawrence Livermore National Laboratory declare that "terrestrial life now has a representative (mankind) capable of actively defending it from the bombardment -- after four eons of simply enduring it."

The defense that scientists have in mind involves a contemporary version of beating swords into plowshares: using Star Wars technology and rockets to benefit humanity. How? By spotting and then deflecting or destroying threatening asteroids or comets before they can hit Earth. That is the recommendation of two NASA-sponsored workshops, one that proposed detection techniques for identifying incoming objects, another that recommended ways of intercepting and dealing with them. While the proposals have the ring of science fiction, they are closer to reality than most people realize; the workshops were authorized by Congress.

Scientists at the detection workshop focused on what they called "the greatest risk" -- the possibility of impact by asteroids with diameters larger than 1 km (3,300 ft.) and impact energies ranging from 100,000 to many millions of megatons, blasts that would have global effects. Though astronomers have found 100 or so of these hulks that can pass through Earth's orbit -- and that might someday pose a threat -- they estimate that there are some 2,000 large "Earth-crossing" asteroids (ECAs) still awaiting discovery.

To hunt down these objects, the workshop proposed the construction of six 2.5-m (98-in.) telescopes, three located in the northern and three in the southern hemisphere. Each would be equipped with advanced versions of the charge-coupled device, a kind of electronic camera, already being used by Tom Gehrels, a University of Arizona astronomer who heads one of three U.S. teams independently searching for asteroids. The CCDs, which record electronic images of celestial objects, would feed into computers that could speedily identify and track asteroids and comets against the background of fixed stars. "It's the right time to do it," says David Morrison, the nasa scientist who chaired the detection workshop. "If we had proposed this project 10 or 15 years ago and tried to do it with photography, it would have been completely impossible."

A continuing survey with the new telescopes, the panel predicted, would discover most of the large Earth-crossers within a decade, and virtually all of them in 25 years. And while the survey was hunting its larger prey, it would also spot many of the estimated 300,000 ECAs larger than 100 m (330 ft.), which could cause regional, but not global, disaster. One proposal, to use orbiting sensors and lasers for detecting smaller objects, was rejected by the panel as unneeded, prohibitively expensive and probably futile. Astronomer Gehrels estimates that 100 million asteroids larger than 20 m in diameter are on paths that can cross Earth's orbit. "So," he says "There is no way in the foreseeable future you could detect all of these objects."

Proceedings at the interception workshop were tumultuous. But there was general agreement about the basic strategy: detect the threatening object and dispatch a warhead-tipped rocket to intercept it and explode, nudging it into a new orbit that would carry it safely past Earth. For a small asteroid detected years and many orbits before its destined collision, the solution would be straightforward. "You apply some modest impulse to it at its perihelion, or closest point to the sun, using conventional explosives," explains Gregory Canavan, a senior scientist at Los Alamos National Laboratory. "The slight deflection that results will amplify during each orbit, ensuring that the asteroid misses Earth by a wide margin."

But scientists calculate that for objects having diameters of 100 m or more that are spotted late in the game and intercepted at a distance any closer than about 150 million km (93 million miles), only nuclear explosives pack enough wallop to avert disaster. At that distance, the energy needed to deflect a 2-km-wide (1 1/4-mile) object enough to spare Earth is about the equivalent of a 1-megaton nuclear explosion. If the object gets to about a tenth of that distance, the energy required is 100 megatons, more powerful than any nuclear device yet exploded.

More likely than not, a threatening asteroid of that size would be spotted earlier. But so-called long-period comets (those making their first appearance or returning at intervals of greater than 200 years) are another matter. Appearing without warning as they streak in from the outer reaches of the solar system, they usually become visible to astronomers only from a few months to two years before passing Earth. Should one suddenly appear on a collision course, traveling as fast as 217,000 km/h (135,000 m.p.h.) relative to Earth, defenders would not have the luxury of years of observation and of using a small explosion to deflect it. A quick nuclear bang would be needed.

Still, the very notion of having high-megaton missiles at the ready, either on Earth or in orbit, was unsettling to many at the workshop, who feared that they could be turned against fellow humans rather than cosmic interlopers. They simply "did not want to talk about very large amounts of energy," says Canavan. "And therefore they wanted to ignore the problem." Some suggested heatedly, in leaks to the press, that pro-nuclear Star Wars scientists, frustrated by the down-sizing of their projects, were using the asteroid and comet threat as an excuse for revitalizing their jobs.

Less controversial was the proposal that terrestrial defenders should know the exact nature of their target before acting. Responding early to a worrisome asteroid, they would send a "precursor mission," an instrumented spacecraft, to fly by or orbit the object and determine its size, shape and composition. One such "practice" mission, code-named Clementine, has already been budgeted by the Defense Department in coordination with NASA. It will fly an instrument package past the approaching asteroid Geographos in 1994 to test the kind of sensors and navigational devices that someday may be needed to help cope with a real threat.

Once the nature of the approaching object is determined, explains physicist Edward Tagliaferri, a U.S. space program consultant, "it becomes easier to decide if you want a standoff explosion, a surface explosion or a subsurface explosion," If the asteroid or comet is small, it can be vaporized with a subsurface explosion, but for larger bodies, says Tagliaferri, "you'll probably have to nudge them into a new orbit." For an asteroid consisting largely of iron, he says, "you'd probably want to have a surface explosion to do the job."

In attacking a large comet or stony asteroid, however, the interceptors would have to take care not to blast their quarry into many large chunks, each of which would be a potential city killer. One way of avoiding that, workshop scientists suggested, is to use the neutron bomb, a weapon that delivers most of its energy in the form of speeding neutrons rather than an explosive blast. The neutron warhead would be detonated when the missile approached to about a distance equal to the radius of the asteroid. "The neutrons penetrate deeply into the near side of the asteroid," Canavan explains. "They heat and vaporize the material, which expands at a high velocity and blows out of the side of the asteroid," thrusting it into a new, non-threatening orbit.

What about costs? The price list submitted by the workshops included $50 million for the telescope network and $10 million to $15 million annually to operate it. Adding research on defense technologies and possible space-based sensors would run the annual costs to "a few tens of millions." And "a few hundred million dollars could develop and test the robotic spacecraft missions" needed to scout any threatening object. An effective way of reducing later costs, says Eugene Shoemaker of the U.S. Geological Survey, would be to put aside a handful of the missiles now being dismantled by the U.S. and Russia and modify them for the intercept program. "It's not huge bucks," he says.

Antinuclear, anti-Star Wars scientists were not reassured. Some campaigned through last summer against even the mention of any nuclear deterrence in the final draft of the interception workshop's report. Then came word of Comet Swift-Tuttle. "Nothing so clears the mind as the sight of the gallows," quips Canavan, who oversaw the final report. "Even though Swift-Tuttle turned out to be a false alarm," he says, "it brought everyone's thinking into focus. There was no longer the kind of disagreement you saw earlier about nukes versus non-nukes." Compromises were made, and the long-delayed interception report was finally distributed in November.

How will Congress respond to the NASA proposals? "There was a high giggle factor when they first heard about it," says a congressional aide. No longer. California's George Brown, chairman of the House Science, Space and Technology Committee, is convinced that "a significant possibility of an impact exists that would have major consequences" and that "we can do something about it. It can't be some hare-brained scheme that would cost umpteen billion dollars for an immediate mission," he stresses, "but we can do all of the precursor planning and prepare the kind of launch vehicle necessary to do the job." Brown's words carry weight; he will preside over hearings on the subject during the current session of Congress.