A relatively small asteroid struck Earth 65 million years ago. It left a 100-mile-wide crater in the ocean floor and wiped out the dinosaurs and two-thirds of all species on Earth. There are more asteroids still out there, many on Earth-crossing paths. Several are large enough to wipe out all life on Earth, from humans to single-celled organisms. A number of large asteroids have hit Earth in the past … and, inevitably, another will hit Earth in the future.