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In 2004, astronomers predicted a newly discovered asteroid, called MN4, was on a near collision course with Earth. Further studies showed an MN4-Earth catastrophe to be extemely remote.
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.