NASA counts down to first test of "planetary defense" against asteroids

NASA is launching a small probe Wednesday that will crash head-on into a small asteroid next fall at some 15,000 mph to test the feasibility of one day nudging a threatening body off course just enough to prevent a catastrophic impact on Earth.

The $330 million Double Asteroid Redirection Test, or DART, "will be historic," said Tom Statler, mission program scientist at NASA Headquarters. "For the first time, humanity will change the motion of a natural celestial body in space."

The 525-foot-wide target asteroid, known as Dimorphos, is actually a small moon orbiting a half-mile-wide parent body named Didymos. Neither poses any threat to Earth, either before or after the DART encounter.