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But what really worries astronomers is the devil they don't know. While they estimate that perhaps as many as 2,000 asteroids larger than a kilometer (six-tenths of a mile) across either cross or come close to Earth's orbit, they have discovered and tracked fewer than 200 of them. "We simply don't know where the other objects are," says JPL astronomer Helin. "But the ones that have been discovered," she warns, "certainly suggest that we could someday face a surprise encounter with a large, unseen object." The significance of the kilometer size? An impact of anything that large, scientists believe, would cause not just a regional but a global catastrophe.
Almost as worrisome are the estimated 300,000 asteroids larger than 300 ft. wide that also come perilously near or intersect Earth's orbit; each could inflict Tunguska-like damage over a large region. The number of Earth-crossing asteroids larger than 60 ft. across, says University of Arizona astronomer Tom Gehrels, could be as high as 100 million. A hit by any one of them could destroy a large city.
Gehrels heads Spacewatch, Scotti's astronomy group and one of two such teams dedicated exclusively to the discovery of threatening "near-Earth objects" (NEOs). The other group, called NEAT (for near-Earth asteroid tracking), is run by Helin and uses an Air Force telescope atop a mountain on Maui, Hawaii.
Strapped for funds--NASA contributes only $1.8 million annually to asteroid hunting--astronomers fear it will take decades to discover most of the larger objects. With only a few million more dollars a year, they say, and with access to the other two Air Force satellite-tracking telescopes, most of the kilometer-wide and larger asteroids could be identified and tracked within 10 years.
What if one or more of these asteroids are found to be a serious threat? Scientists generally agree on the best strategy for avoiding disaster: launch a rocket to intercept the intruder and, at the very least, change its orbit. If the asteroid is small and detected many years and orbits before its predicted impact, the solution would be straightforward. "You apply some modest impulse to the asteroid at its closest approach to the sun," says Los Alamos' Canavan. "The slight deflection that results will amplify during each orbit, ensuring that the asteroid misses Earth by a wide margin." That little push, he notes, could be provided by conventional high explosives.
For objects 300 ft. or larger and detected late in the game, however, nuclear weapons may well be the only answer. If XF11 had been discovered only 90 million miles away and on a beeline toward Earth, for example, the equivalent of a 1-megaton explosion would have been necessary to shove it into a safe orbit. Had it first been spotted at just a tenth of that distance, a 100-megaton blast would have been needed to turn it away.
If the incoming asteroid is composed largely of iron, a nearby or even a surface explosion would present no problem. But if the asteroid is rocky, a blast, particularly an ill-planned one, might well shatter it into chunks, each a potential danger to a terrestrial region or metropolitan area.