A collision with a Near-Earth asteroid is believed to have wiped out the dinosaurs 65 million years ago. Near-Earth asteroids are usually piles of rock from the Mars-Jupiter asteroid belt whose orbit has been disturbed by a collision in the asteroid belt or by the gravitational pull of Jupiter. As asteroids are exceptionally faint visible objects, they are extremely difficult to detect. Rabinowitz and his colleagues came to their new estimate using a new technique for collecting and analysing the images of an automated 1 metre telescope in Hawaii. They then created a computer model to predict the total number of Near-Earth asteroids in the Solar System by extrapolating the number of asteroids discovered in the past ten years to those in the future.

Other astronomers are more concerned about asteroids smaller than 1 km in diameter. In a related article, David Jewitt of the Institute for Astronomy in Hawaii points out that there are 100 times as many such small asteroids as there are kilometre-sized asteroids. This will make it "a hundred times more likely [that they will] collide with Earth," he says. He adds that a 300 m diameter asteroid could cause over 100 000 people to die in such a collision (Nature 403 145).