A reduced estimate of the number of kilometre-sized near-Earth asteroids

Near-Earth asteroids are small (diameters < 10 km), rocky bodies with orbit s that approach that of the Earth (they come within 1.3 AU Of the Sun). Mos t have a chance of approximately 0.5% of colliding with the Earth in the ne xt million years. The total number of such bodies with diameters > 1 km has been estimated to be in the range 1,000-2,000, which translates to an appr oximately 1% chance of a catastrophic collision with the Earth in the next millennium(1,2). These numbers are, however, poorly constrained because of the limitations of previous searches using photographic plates. (One kilome tre is below the size of a body whose impact on the Earth would produce glo bal effects(3).) Here we report an analysis of our survey for near-Earth as teroids that uses improved detection technologies. We find that the total n umber of asteroids with diameters > 1 km is about half the earlier estimate s. At the current rate of discovery of near-Earth asteroids, 90% will proba bly have been detected within the next 20 years.