What is the true long-term threat of Near Earth Objects? NASA defines “potentially hazardous” as a Near Earth Object that will pass within .05 AU from Earth and is at least 140 meters in diameter. (http://neo.jpl.nasa.gov/neo/pha.html)

But consider the damage left by a 30-meter object in the famed Siberian impact of 1908:

“Recent scientific studies by meteorite researcher Christopher Chyba have estimated that the Tunguska event may have been caused by the explosion of a stony meteroid about 30 meters in diameter traveling at about 15 km/s. Compare the energy released by such an object with that of an atomic bomb such as those dropped on Japan in World War II.”
See: http://www.psi.edu/projects/siberia/siberia.html

The truth is no one really knows how many asteroids this size or larger are out there. According to NASA sources, the population breaks down as follows:

100 meters in diameter: 300,000
500 meters in diameter: 10,000
Over one kilometer in diameter: 500-1,000

The good news is that eight projects are at work to search for them, including NASA’s NEO-Wise space telescope, and more are coming on line soon. The bad news is that fewer than 8,000 of these have been discovered so far.