Scientists believe the dinosaurs were wiped out from the collision of an asteroid (about 7-8 miles wide) with the Earth. The resulting volcanic activity and dust cloud blocked sunlight and plants/animals died. A planet killer would be around 60 miles wide.
However, a half mile wide asteroid would also ruin your day as its impact would be roughly that of 100 billion tons of TNT and cause wide-spread earthquakes.
If you're like me, an asteroid colliding with the Earth is not on your DVR list of scheduled events anytime soon. Ditto for Netflix coverage in the next several thousand years. Hopefully, we will be vacationing on Alpha Centauri by then. Anyway, it doesn't look like there are any likely planetary colliders in the neighborhood.
But even if there were, NASA has a new program called the Asteroid Redirect Program that will land on a large asteroid, grab a boulder to study and then redirect the asteroid into a stable orbit around the moon. Although it seems like a new Bruce Willis or Matt Damon film, this mission is set to launch by 2020.
Sounds like the operational outcome of all the work astronomers have been doing for years. Now instead of just shouting, "Incoming!" We'll soon have correctional space technology and a plan to divert disaster. Go Science!