When it comes to asteroids' wreaking disaster on Earth, the real question is not if, but when. Two hundred or so large craters and a geological record stretching over billions of years provide ample evidence that, time and again, explosive impacts by asteroids or comets have devastated large parts of the planet, wiped out species and threatened the very existence of terrestrial life. Astronomers are all too aware that more large hulks are out there, hurtling through space, some of them ultimately destined to collide with Earth.

As scary as this seems, disaster is not inevitable. For after nearly 4 billion years of life on Earth, a species has evolved that can prevent the next catastrophic encounter — if it has the will to do so. That species is us. Why worry? After all, the most notorious impact of them all, the one that caused the extinction of the dinosaurs, occurred 65 million years ago. Really ancient history.

Yet if you want to get contemporary—on a geological scale, of course — it was only 49,000 years ago that an iron asteroid blasted out Arizona's 3/4-mile-wide Meteor Crater, almost certainly killing any living creatures for hundreds of miles around. And as recently as 1908, a small, rocky asteroid or chunk of a comet exploded five miles above the Tunguska region of Siberia, felling trees, starting fires and killing wildlife over an area of more than 1,000 sq. mi. Had the blast, now estimated at tens of megatons, occurred over New York City or London, hundreds of thousands would have died.

And what about near misses? As recently as 1996, an asteroid about a third of a mile wide passed within 280,000 miles of Earth — a hairbreadth by astronomical standards. It was the largest object ever observed to pass that close, and had it hit, would have caused an explosion in the 5,000-to-12,000-megaton range.

What was particularly unnerving about this flyby is that the asteroid was discovered only four days before it hurtled past Earth. All the more reason for a detection system that will discover asteroids early, plot their paths and predict, many years in advance, whether they will eventually threaten Earth.

The good news is that just such a detection system, after a slow start, is rapidly gearing up. Four small groups of dedicated astronomers in Arizona and California, totaling fewer than the number of employees at an average fast-food restaurant and using mostly off-the-shelf equipment for their telescopes, have been mapping the heavens and steadily adding to the number of known near-Earth objects.

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