It’s the stuff of movies: an asteroid is on a collision course with Earth, and we have to do something to stop it—or die. Disturbingly this scenario is also the stuff of the actual universe. Scientists have discovered more than 2,000 “potentially hazardous” asteroids. To merit that official NASA designation, an object must be at least 460 feet in diameter and have an orbit that passes within about 4.7 million miles of our planet. Protecting Earth from these rocky hazards falls under a field...