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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.