Dinosaur killers, 10 kilometers across, hit every 100 million years on average. Globally devastating asteroids, one kilometer or larger, come every half a million years or so. City destroyers, 50 meters across, strike perhaps once a millennium.

The Spaceguard Survey has found just over 700 kilometer-size bodies, none posing a threat over the coming centuries. The rate of discovery is tapering off, suggesting the survey has found about 75 percent of the total.

The chance of an impactor among the remaining 25 percent is small, but the consequence would be large. Statistically, the risk amounts to an average death toll of 1,000 people a year. Smaller ones kill maybe 100 a year on average.