In 1997 the asteroid-hunting pioneers were joined by a precocious upstart, a joint Air ForceÜM.I.T. Lincoln Laboratory group supported by generous Pentagon funding. Using an Air Force satellite-spotting telescope in New Mexico and a camera equipped with an advanced M.I.T.-designed charge-coupled device, the totally automated, computerized operation quickly began discovering more asteroids and comets, large and small, than all the other groups combined. Getting further into the spirit of the game, the Air Force has deployed a second asteroid-hunting scope, is lending another to astronomers and musing, unofficially, about launching a fleet of microsatellites for even better asteroid detection.

What to do if an Earth-bound comet or asteroid is discovered? Early detection, preferably many years in advance, would enable us to send out exploratory spacecraft to determine the nature of the interloper, much like the spacecraft near's current investigation of the asteroid Eros. Scientists at the Los Alamos and Lawrence Livermore National Laboratories are already dreaming up a variety of ingenious defenses against an incoming asteroid. Depending on its mass and composition, they would use tailor-made nuclear explosions to pulverize small asteroids or deflect larger ones. Given enough time, and under the proper circumstances, less drastic measures would be needed. Some schemes call for conventional explosives alone, or anchoring a rocket motor or a solar sail on an asteroid to alter its orbit enough to allow it to safely bypass Earth.

At the beginning of 2000, only about half the estimated 500-to-1,000 near-Earth asteroids six-tenths of a mile across or larger—big enough to cause a global catastrophe—had been detected. One of the unknowns could even now be on a collision course with Earth. The sudden appearance of long-period comets, usually larger and with better than twice the impact velocity of asteroids, presents an even greater menace. Such objects (comet Hale-Bopp was one) are usually not spotted until they begin to flare somewhere out near the orbit of Jupiter or closer, only a few to 18 months before they pass Earth's orbit. That doesn't leave much time for defensive measures. Then, too, only a tiny fraction of the more numerous and smaller neos, some of them potential city killers and tsunami producers, are yet known.

Someday in the foreseeable future, the first thing that strollers out for an evening walk might see would be a sudden glow on the horizon. Then, in short order, they would feel the ground shake, hear a thunderous roar and be incinerated by an onrushing blast of superheated air. All the more reason to identify and track every single near-Earth object and prevent a nasty surprise.

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