This National Geographic documentary examines the asteroid impact theories of Eugene and Carol Shoemaker. This couple discovered the Shoemaker-Levy 9 comet, and they also put forth a theory that a one-mile-wide asteroid, if it hit earth, would destroy all life on the planet. National Geographic: Asteroid - Deadly Impact shows how the Shoemakers came to their conclusion by examining existing craters. Several noted experts in the field offer their opinions on the likelihood of a fatal asteroid hitting Earth. Robbie Robertson narrates.