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.