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For that reason, Earth's defenders, if they have the luxury of time, would prefer to send a robot craft to rendezvous with a threatening asteroid and determine its composition and mechanical strength before dispatching a nuke to the scene. Physicist Edward Teller suggests that this is what we should do, just for practice, when XF11 passes far from Earth two years from now. Other defensive plans being bandied about at the Los Alamos and Lawrence Livermore national labs involve more exotic devices, such as neutron bombs or netlike arrays of interconnected tungsten balls.
Yet none of these defensive measures can be effective without adequate warning. And given the large numbers of undiscovered NEOs still out there, says David Morrison at NASA's Ames Research Center, an asteroid strike could take place with far less than a 30-year warning. Indeed, says Morrison ominously, "the most likely warning time would be zero."
--With reporting by Dan Cray/Los Angeles and Dick Thompson/Washington