In 1900 there were only 21 known NEOs, all but one of which were comets. By 1990 the number of NEOs detected had risen to 170, the majority of which were asteroids. As of December 2011 there were 8,513 NEOs known. Of these, 1,273 were classified as Potentially Hazardous Asteroids.

There are far more small NEOs than large. On a global scale it is the large NEOs, with diameters greater than 1 kilometre, which pose the biggest threat. There are estimated to be around 1,000 such NEOs. As of December 2011 astronomers had detected 834 of these large objects.