An astersoid zipped past Earth as close as 3.3 million miles, just two weeks ago.  The asteroid was originally thought to be about the size of a city block.  The closeness of the pass allowed scientist to get a much closer look using the radio telescope dish at Arecibo in Puerto Rico.  The results suggested it is actually about twice as big as they originally believed; more that half a mile across at the widest point.  This asteroid was initially discovered for the first time in early June, and initially considered potentially dangerous, but with more time and observations, scientists have concluded it would not be at risk of hitting earth, for another 750 years

"This object turned out to be quite a bit bigger than we expected, which shows how important radar observations can be, because we're still learning a lot about the population of asteroids," said research team member Ellen Howell, of the Arecibo Observatory, in a statement.

http://www.space.com/16263-asteroid-2012lz1-size-earth-flyby.html