According to a new report from the National Aeronautics and Space Administration, some 100,000 asteroids and comets routinely pass between the Sun and the Earth’s orbit. About 20,000 of these orbit close enough to us that they could one day hit the Earth and destroy a major city.

But the really worrying news from NASA is that over a thousand of these things are large enough (almost a mile wide in diameter) and their orbits close enough to us as to pose a real potential hazard of crashing into the Earth with enough force to end most life on this planet. Scientists think this is what killed off the dinosaurs about 65 million years ago.