NASA’s Spaceguard survey has succeeded in mapping about 95 percent of the largest NEOs (larger than 1 km), yet due to the limitations of searching for asteroids using mostly ground-based optical telescopes, there may be a million or more smaller, but still dangerous NEOs orbiting the sun in the inner solar system. These smaller asteroids can pose quite a threat, as evidenced by the Tunguska impact on June 30, 1908, which devastated 800 square miles of Siberian forest.
The 1908 Tunguska event, an impact by a stony asteroid, about 40 meters in diameter, entered the atmosphere over Siberia and exploded over a remote and unpopulated region. Recent work has determined that the explosive energy was equivalent to a 3-5 megaton bomb. Had the impact occurred in a more populated region of the planet, say New York or London, millions of people would have been killed without any warning.
About a million of these near Earth objects are estimated to be equal to or larger than the asteroid that struck Tunguska in 1908, and about 300 times more powerful than the atomic bomb dropped on Hiroshima.