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This isn't the first such rendezvous with a fast-moving space rock. In July 2005, a NASA probe collided with comet Tempel 1 to get a better look at what it's made of.
One probe, Hidalgo, will crash into the asteroid while the other, Sancho, will orbit the rock for several months to monitor any changes.
Experts first narrowed the field of test asteroids in February 2005. They dismiss worries that the test could cause problems for Earth, saying the probe should make only a slight deviation in the asteroid's orbit.