A New Count of Potentially Hazardous Asteroids
May 16, 2012: Observations from NASA's Wide-field Infrared Survey Explorer (WISE) have led to the best assessment yet of our solar system's population of potentially hazardous asteroids. Also known as "PHAs," these asteroids have orbits that come within five million miles (about eight million kilometers) of Earth, and they are big enough to survive passing through Earth's atmosphere and cause damage on a regional, or greater, scale.
The asteroid-hunting portion of the WISE mission, called NEOWISE, sampled 107 PHAs to make predictions about the population as a whole. Findings indicate there are roughly 4,700 PHAs, plus or minus 1,500, with diameters larger than 330 feet (about 100 meters). So far, an estimated 20 to 30 percent of these objects have been found.
http://science.nasa.gov/science-news/science-at-nasa/2012/16may_pha/