The head of NASA spoke at a Congressional hearing this week about protecting the Earth from asteroids.  And he had some sobering things to say. 

First, an asteroid with a diameter of about six-tenths of a mile could wipe us all out.

Two, 95% of the asteroids near Earth ARE that big.

And three, if an asteroid is coming and we don't detect it until it's three weeks away?  Quote, "Pray."  (Good advice.)

In other words, it we don't see it until then, we won't any chance of doing something about it until it's too late.