Images: Target practice on asteroids

Still, the scare motivated the European Space Agency to launch the Don Quijote asteroid-deflecting project. The agency wants to try to deflect the orbit of an asteroid (either 2002 AT4 or 10302 1989 ML), with the target to be chosen in 2007.

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.

Scientists classify asteroids according to their orbital path. Ones that cross Mars' orbit but do not cross Earth's orbit are called Amors. Apollo asteroids cross Earth's orbit and may pose a threat. Aten asteroids travel mostly inside Earth's orbit but cross it before disappearing back into the glare of the Sun. See the ESA Web site for more information on the Don Quijote project.