How serious does an asteroid threat have to be before we take action?

As the world waited for today's close flyby of the asteroid 2012 DA14, a small asteroid broke up over Chelyabinsk Russia this morning, blasting in city windows with its sonic boom and injuring hundreds. Meanwhile, NASA's Near-Earth Object Program has revised the odds of asteroid 2011 AG5 hitting the Earth in 2040 to… »2/15/13 12:10pm2/15/13 12:10pm