What would happen to the safety of humans and other living species if an asteroid half the size of mountain Everest (like the asteroid 1998 OR2 which will skirt through earth at 3.9 miles on April 29th 2020) decides to hit earth and NASA and other space scientist notice late?

If this happens, which i am pray not to experience, we will have to evacuate the region that might be affected. On the other hands it would be cool if we colonize another planet to ensure human safety.