With sufficient time there are plenty of realistic options, ranging from evacuating the impact zone, to nudging it off-course with a "gravitational" space tug (

With sufficient time there are plenty of realistic options, ranging from evacuating the impact zone, to nudging it off-course with a "gravitational" space tug (surprisingly feasible), all the way to deflection with nuclear weapons. (Pushing the asteroid enough to nudge it off-course is always a lot easier than blowing it up, Armageddon-style.)