http://www.nasa.gov/feature/nasa-spo...ar-astronomers
Hmmmm 21 days to scramble a divert/destroy mission IF the trajectory had been a few 100,000 km different ? ?
Not likely to have succeeded, so in the IF scenario; we might have had a new 6 km crater.
On the bright side; great science is being done from 'home' rather than by robotic space craft.