In between the extinction-level asteroids and the mostly harmless ones is a medium-size class of asteroids about the size of a football pitch. Those are the sc

In between the extinction-level asteroids and the mostly harmless ones is a medium-size class of asteroids about the size of a football pitch. Those are the scary ones. They are small enough to be hard to spot, and there are enough of them that one of them hits earth every 10,000 years or so. They have not been a big deal in the past. They don't cause mass extinctions, but they are more than big enough to bring about the end of civilization. If one of them hits an ocean (which is the most likely scenario) it will cause global tsunamis that will make the Fukushima event look like a small ripple. We're talking waves many hundreds of meters high. Not a single coastal city bordering the ocean where the asteroid hits would survive.