Would we be told if there was an asteroid that was in a collision course with our planet and was large enough to do damage? If we were told, how long before civilization broke down? I don't know if they would tell us. Why would they if the governments of the world could avoid all the mayhem that would come with a declaration like that.
According to astronomers, it would be impossible to keep a oncoming collision quiet because they would all see it, and at least one of them would talk. Plus, they supposedly have all asteroids mapped in our solar system.
Yet, they keep finding new ones...
So asteroid 2012 DA14 is flying by us on February 15, and will swing by at its closest, about 17,200 miles (inside the geosynchronous orbit of our satellites) and it is the size of half a football field. That's a pretty good sized rock. Make sure you duck. Check out:
Here is a list of more fun rocks in space at, Near Earth Object Program
Mint Chip Ice cream, Krispy Kreme donuts, homemade pizza, pralines, Chinese chicken salad...mmmmm!