London Telegraph 
Feb 13, 2013
Although the 150ft (45m) rock will zoom harmlessly by our planet on its record near approach this week, it is just one of up to a million “near-Earth objects” which astronomers believe could one day pose a serious threat.
A program set up by NASA to monitor so-called “NEOs” a decade and a half ago has so far tracked fewer than 10,000, meaning the vast majority are still hidden.
Most of the larger specimens, which could have the potential to wipe out entire continents, have been found but even small asteroids like 2012 DA14 could be powerful enough to destroy an entire city if they plunged down to Earth.
Now experts are calling for greater monitoring of small NEOs measuring less than a kilometre across, and for a contingency plan in the event of a likely impact.