The next time you’re on a flight and the seat-belt sign clicks off, glance out the window. Six miles below, the landscape looks peaceful and small. Now picture a single rock—silent, ancient—so massive it could stretch from the ground up to that cruising altitude. That was the size of the asteroid that ended the dinosaurs’ reign.
That kind of disaster isn’t just ancient history. Every couple of weeks, something big enough to flash across the sky as a brilliant fireball—astronomers call it a “...