astroengine writes "The meteor that exploded over the Urals region of Russia in February was a violent reminder that our planet exists in a cosmic shooting gallery. Now, astronomers are focusing on these mysterious small and possibly dangerous objects in the hope of understanding what they are made of and what kind of threat they pose in the future. However, a recent paper accepted for publication in the Astrophysical Journal has identified a possible 'Achilles Heel' of visible light surveys. Using data from NEOWISE (the near-Earth object-hunting component of NASA's Wide-field Infrared Survey Explorer mission), there appears to be a bias in visible light asteroid surveys against finding small (100 meters) dark space rocks. 'With our previous NEOWISE studies, we found that about a third of NEOs larger than 100 meters are dark. It's possible that a population of smaller dark asteroids exists, but we don't have the right sample to test that theory with what we've done so far (in this research),' NASA JPL scientist and NEOWISE principal investigator Amy Mainzer told Discovery News. 'In my opinion it is probable that a similar fraction of small NEOs are dark, but the visible surveys are biased against finding them. They do find some but not many.' On considering the impact of the small Chelyabinsk object earlier this year, it is perhaps sobering to realize that while around 90 percent of NEOs with diameters larger than 1 kilometer are thought to have been discovered, less than one percent of asteroids the size of the Chelyabinsk meteor (17-20 meters in diameter) have been detected."