>> paul chodas: none. >> cooper: paul chodas is a scientist at nasa's jet propulsion laboratory in pasadena don yeomans, have been trying to track near- earth objects for decades. >> chodas: we didn't see it coming. it was coming from the general direction of the sun, so it was in the daytime sky as it approached. >> cooper: so how did you find out about it? >> chodas: twitter and youtube, when we... when we first saw the images. >> cooper: so the first people at nasa that heard about it was twitter? ( laughter ) >> chodas: exactly. >> cooper: chodas says an object this size hits earth once every hundred years, on average. yet the same day, purely by chance, another asteroid twice as large came within 17,000 miles of earth, passing between us and the satellites that are bringing you this broadcast. the only reason there was any advance warning was because an amateur astronomer in spain, an oral surgeon by day, noticed it just before it moved out of view. >> amy mainzer: we know about some of the most distant galaxies in the known universe, and yet, we don't really know everything that's rig