NASA’s Jet Propulsion Laboratory has estimated that a Tunguska-size asteroid (just 90 feet across, leveling some 800 square miles of forest in Siberia) will enter Earth’s atmosphere once every 300 years and says there may be 375,000 objects of such size out there.

NASA only tracks potential “civilization killers” of 1 kilometer (.62 miles) or more in diameter that have orbits coming within 30 million miles of the Earth’s.