The NEO threats are smaller and more numerous!

Previously, Tunguska was thought to have been at least 50 meters...

• But the size of the Tunguska object was perhaps 30-40 meters in diameter - or loosely the size of the White House - and the process to track most of those will take longer, he said.
  


• "In the process of finding the bigger objects, you find some of the smaller ones," Schweikart said. "But there are perhaps 400,000 objects that are 45 meters in diameter out there, and we've found less than 1 percent of them so far."
  


• Prior estimates suggested that Tunguska-like events happened every 1,000 years or so. New estimates put that closer to the 300-400 year range, Boslough said.
  


• "The smaller ones at most could hit perhaps every 100 years or so," Boslough said. "And the last one was 99.5 years ago, but any estimates are based on a lot of uncertainty."
  
  http://www.abqtrib.com/news/2007/dec/22/killer-asteroids-more-common-was-thought-sandia-sc/


• See animations here:
  http://www.sandia.gov/news/resources/releases/2007/asteroid.html
  http://blog.wired.com/wiredscience/2007/12/small-asteroids.html