Look Out!

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 And while the warning that the 10-km-wide (6-mile) Comet Swift-Tuttle might slam into Earth in 2126 has now been retracted, it briefly caused genuine concern among many scientists.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.