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So, what is NASA doing about it?
Just last week they've launched the Near-Earth Object observation program to study the orbits of these potential threats. Right now, scientists can only track asteroids, which usually travel between 27,000 and 33,000 miles per hour, and warn the public of close calls. NASA has also started serveral basic research and technology demonstration projects to study asteroids and find ways to prevent them from striking Earth.
One of the possibilites reminds us of the movie, "Armegeden" with the use of nukes that could deflect a killer asteroid. Other methods to change an asteroid's course include hitting it with a heavy projectile traveling at high speed (tested by the Deep Impact mission which hit Comet Tempai 1 with an 850 pound copper slug), or using a gravity deflector from a spaceship near enough to slightly change its course. Other research being conducted now include improved Solar Electric Propulsion (SEP) systems that could push or pull an asteroid for an extended time, or the use of grappling mechanisms.
NASA has also built a 230-foot Goldstone antenna, near Barstow which is part of NASA's Deep Space network.