When it comes to dangerous asteroids striking Earth, it's not a matter of if, it's a matter of when. We have begun to track projectiles large enough to destroy our planet, and we are in the clear for the foreseeable future. However, there are countless asteroids large enough to take out an entire city that we cannot see.

Originally released by Knowledgemotion, 2016. Streaming video file encoded with permission for digital streaming by Infobase on February 21, 2020.

5 Ways to Stop a Killer Asteroid (8:31); .