In 1994 NASA committed itself to discovering, within ten years, all near-Earth asteroids greater than 1 km in diameter. Such 1-km near-Earth objects (NEOs) could cause global devastation if they struck the Earth. A new study suggests that the number of 1-km NEOs is only half what we thought. But most of the threatening NEOs (between 100 m and 1 km) have yet to be discovered.
