If recognize years in advance that a good-sized asteroid is to strike Earth, there are methods of deflection. But what if the notice is much shorter? From an Ask Me Anything at Reddit by Ed Liu, a former U.S. astronaut who’s now in the asteroid-nullification business:

“Question:

Theoretically, if we learned that a moderately large asteroid was going to impact populated land on earth in 72 hours, would we do anything about it or do we not have the capability right now?

Ed Liu:

Evacuation would be our only option then, and depending on the size of the asteroid, that may not even be possible. Best not to get into this situation and instead lets find out years or even decades in advance. BTW, I was one of millions of people who evacuated Houston for a hurricane back in 2004. Believe me, evacuating that many people is not an easy thing! It took us nearly 12 hours just to drive to Austin.”