“Altogether it is thought – though it is really only a guess, based on extrapolating from cratering rates on the Moon – that some two thousand asteroids big enough to imperil civilized existence regularly cross our orbit. But even a small asteroid – the size of a house, say – could destroy a city. The number of these relative tiddlers in Earth-crossing orbits is almost certainly in the hundreds of thousands and possibly in the millions, and they are nearly impossible to track.”
