This is where NASA’s Wide-field Infra-red Survey Explorer (WISE) space telescope comes in. Anything that has a temperature emits infra-red radiation, and WISE is especially good at seeing those emissions. After its primary mission to map the entire sky in infra-red ended, it was given a new mission (NEOWISE) to use its sensitive detectors to look for asteroids, and the results of its first survey have just come in: There are 19,500 mid-sized asteroids in near-Earth orbits, almost halving the previous estimate.
Mid-sized, incidentally, means a diameter of between 100 meters, and 1 kilometer. This is not quite on the scale of the asteroid that wiped out the dinosaurs, but would certainly be big enough to eliminate entire metropolitan areas. Our biggest nuclear weapons are nothing compared to a good solid impact from a mid-sized asteroid.
So what does this mean? Well 19,500 is still a lot of asteroids. NASA’s spaceguard effort, responsible for finding and tracking asteroids so that we’ll have some warning about future impacts, has found 5,200 of them – that means that three quarters of them still need to be found. But that’s fine. The actual chances of a serious impact within our lifetimes is small – space is very very big. But we must find them, because it WILL happen eventually and we need to be prepared.