Incoming!

Of the 600-plus large NEOs tracked thus far, only 1950DA poses any threat at all. But at this stage of the search, there are an estimated 400 potential global killers left to find, not to mention over a million hard-to-spot smaller asteroids capable of regional destruction. (A rock that exploded over Tunguska, Siberia, in 1908 leveled a thousand square miles of remote forest; it was a mere 60 meters wide.) Making the tallying work more tricky are a few long-period comets, which only swing by every few hundred years and are much more difficult to track.



The search is only the beginning, and as Jay Melosh, a planetary scientist at the University of Arizona points out, "The question is, If we find one with our name on it, can we do anything?"



NASA's search effort receives a paltry $3 million per year, just a fraction of the $25 million that NASA earmarked last year to fix the doors on the Kennedy Space Center's vehicle-assembly building. "I'd like to see more money spent," says David Morrison of NASA's Ames Research Center. But as yet, there's no official program either to build or to test asteroid-deflection technologies. If Earth gets whacked by a significant asteroid within the next few centuries, survivors might find themselves marveling that their ancestors, with tools in hand, did little to prevent a cataclysm.



The asteroid interception and diversion experts are mostly hobbyists—planetary scientists, astronomers and engineers who think up these strategies on their own time. But the ideas are plentiful: As our gatefold shows, the path from detection to mitigation could include low-thrust engines, solar sails, standoff nuclear explosions and more.



Melosh, for example, has been focusing on the use of solar collectors, which could concentrate sunlight on an asteroid, vaporizing enough material to gradually nudge the rock off course. Until recently, this idea amounted to little more than a series of conceptual sketches
bolstered by calculations. Then Melosh learned of L'Garde, a California company that makes smaller versions of the exact collectors he needs. With a few adjustments, he says, his strategy could be put to work tomorrow.



It would take years for sunlight to redirect an asteroid, however, so
advance notice is absolutely critical. Ditto for tactics that would involve painting an incoming asteroid or covering its surface with white glass beads—both approaches would make the asteroid more reflective, increasing the tiny reaction forces produced when sunlight is radiated back into space. Over several centuries, the cumulative effect of these slight forces would alter the asteroid's velocity and cause a miss. "You let the Sun do the work," says Jon Giorgini of NASA's Jet Propulsion Laboratory (JPL), one of the scientists who projected 1950DA's orbit out to 2880.



"The key," says Donald Yeomans, who heads the NEO Program Office at JPL, "is you've got to find them early. If they're on an approach trajectory and you've [only] got a few months, there's not much you can do."



Given ample time, an effective defense strategy might require that a probe be launched to study the structure of the incoming body. Not all asteroids are the solid objects familiar from museum meteorite displays. Some are porous, others are collections of rubble loosely held together by gravity. Exploding a nuclear bomb nearby might nudge a dense asteroid off track, but it could break a brittle one into pieces, effectively multiplying the threat by creating smaller but still lethal rocks.



Each threat, in other words, requires an adjustment of strategy. "You need to find out [the asteroid's] density, find out its mass, its porosity, its composition, because all these things are important if you want to effect some kind of mitigation or deflection," says Yeomans.
One approach uses so-called kinetic kill vehicles—numerous small spacecraft placed in an asteroid's path. Hit by hit, they slow it down enough that Earth will pass through the projected collision point before the asteroid does.