To date, astronomers say they have discovered between 90 and 95 percent of the approximately 1,000 near-Earth asteroids estimated to be larger than half a mile across.
In 2005, Congress instructed NASA to hunt for smaller asteroids – setting a goal of finding 90 percent of near-Earth asteroids 500 feet wide and larger by 2020.
But as the Chelyabinsk asteroid demonstrated on Feb. 15, objects far smaller can inflict damage. At about 55 feet across, and with a mass estimated at 10,000 tons, the asteroid exploded high over the Ural mountains. The shock waves damaged an estimated 4,300 buildings and injured nearly 1,500 people.
With tens of millions of objects this size orbiting the sun, the recurrence rate for collisions with a Chelyabinsk-like object averages once every 100 years, according to Paul Chodas, with NASA's Near-Earth Objects Program Office at the Jet Propulsion Laboratory in Pasadena, Calif.