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Once any object has been found and logged, its overall risk is calculated on what's known as the Torino Scale. Named after a 1999 conference in Torino, Italy, where it was adopted, the scale is essentially a grid with the probability of impact--from effectively zero to effectively 100%--on its x-axis and the size of the object on the y-axis. It is then ranked on a five-color chart--think the Department of Homeland Security's now defunct terrorism-threat level--going from white (no hazard) through yellow (decorously described as "meeting attention of astronomers") to red (certain collision, capable of causing at least regional devastation of a kind seen only every 10,000 to 100,000 years). The Torino color for 2012 DA14 was a comforting white, since astronomers knew it would miss us, but there is an ever present risk of any number of bright red rocks to be reckoned with. "We've got a big system that processes this data for us," says NASA research scientist Paul Chodas, who has developed much of the near-Earth object-tracking and analysis software. "There are a lot of asteroids out there."
Earth Strikes Back
A close brush with an asteroid may be scary, but the fact is, it's as good as a distant miss. The rocks that take a direct bead on us are a different matter, and they will have to be wrangled. The cold truth is that we have no way of protecting ourselves at the moment, but we're getting close.
NASA has been dancing with asteroids and comets for a while. The Dawn spacecraft orbited the asteroid Vesta from 2011 to 2012, then peeled off for its next target, the asteroid Ceres, which it will reach in 2015. In 2001 the NEAR Shoemaker spacecraft actually touched down on the asteroid Eros. And in 2005 the Deep Impact spacecraft fired a cannonball-like projectile into comet Tempel 1 to blow out a bit of debris and study its composition. All that practice comes in handy in space defense.
Simply flying to an asteroid and destroying it while it's still deep in space seems like the most direct way to go, but it would be a logistical nightmare. A blast big enough to get rid of the largest rocks in a single go would require a nuclear weapon--something no sane nation would mount on top of a rocket and fire into space. Conventional explosives could, in theory, do the trick, but that would be awfully hard to translate into practice. "Astronauts would have to land on an asteroid and drill into it, and that would take years," says Johnson. Also, either method might just shatter one massive asteroid into several dangerous ones, essentially turning it into a cluster bomb.
A much simpler approach would be to reprise the Tempel 1 impact model. A projectile need not carry an explosive at all if it can hit an asteroid with enough force to change its trajectory. It would take a big bullet to move a big rock--one on the order of a couple of metric tons--but spacefaring nations have been throwing heavy loads into the sky for decades. What's more, the farther an asteroid is from Earth when the collision takes place, the greater the difference even a tiny shift in its trajectory would make by the time it reaches us. "You might only need to alter the object's speed by a few centimeters per second to get it out of the way," says Johnson.