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Observations by NASA's infrared WISE space telescope recently allowed astronomers to estimate that there are about 981 of these mountain-size asteroids on orbits that bring them relatively close to Earth.
That number may sound scarily large. But sky-scanning researchers have already found and catalogued nearly all of them, Johnson said.
Preventing an impact
It's possible, of course, that one of the huge, as-yet-undetected near-Earth asteroids does pose a threat to our planet sometime down the road. But that threat could be mitigated, or even eliminated, if we spot it soon enough.
Researchers have several ideas about how to avert a devastating impact, given enough lead time.
One is the so-called gravity-tractor method, in which a small, unmanned probe is launched to rendezvous with the asteroid in deep space. The spacecraft would travel with the space rock, exerting a tiny but constant gravitational tug that would eventually nudge the asteroid into a benign orbit.
We could also be much more aggressive with our rendezvous craft, slamming it into the asteroid to knock it off course. We have the know-how to perform both of these missions; multiple spacecraft have met up with asteroids in deep space, and in 2005 NASA smashed an impactor into the comet Tempel 1 to determine the object's composition.