Some asteroids have not been detected until they have passed earth. Just last year one missed us, and came very close. It was not detected until it had actually passed us.
I always get a kick out of people laughing this off, thinking it's all nonsense.
But look no further than the lunar surface and the surface of other planets and moons in our solar system. Thousands of those lunar and planetary impacts are massive. Even with our atmosphere, many large objects can easily penetrate and cause a cataclysmic event.
It's only a matter of time, until earth sustains a massive strike. It will be interesting to see how so called "modern" man reacts prior to impact, and after.
They can't even predict the path of a hurricane 24 hours in advance.
As far as how people will react should it come to fruition...the good will help their fellow man to the best of their ability, the bad will take advantage of other's misfortune. Not too tough to figure out. Just look at Katrina for a microcosm of what will happen in any catastrophe.
Now I'm going to go worry about something really important...is my elderly neighbor's sidewalk shoveled yet.