Images: Target practice on asteroids

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.

Photo by ESA 2002. Illustration by Medialab