NASA radar checks out ECAs, Earth-orbit crossing these, in case one is about to wipe out civilization
An asteroid 1 mile across could snuff out civilizations, while one 
that is 3... [DOC] [TLE] asteroid comet impact: Topics by Science.gov [PAR] In the case of comets, the rock and ice are finely distributed, thus the mixture ..... 
We now believe that while there are about 2000 earth orbit crossing rocks .....