Search for Asteroid Threats

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.