First and foremost, we must know be able to detect, track, and characterize the effect of the asteroid so that we aren't uncertain but more accurate in modeling and decision making. Second, set up ground-based operations intended to scan the skies for asteroids to better determine when and where the asteroid will strike. Thirdly, set up a rapid-response team if consequences are imminent. Fourthly, establish an international co-op to prepare the rest of the world for the hazard. This plan may have flaws, but I think it's a good bet.
