Asteroid 4769, Castalia, is a little over one mile long.  If it were to impact with Earth, it would end civilization.  Castalia's next close approach to Earth will occur on August 26, 2046.  It will pass a mere 10 Earth-Moon Distances.  Castalia has the shape of two lobes that have been fused together.  The cleft between the two could swallow a 27-story building.  The original model of this asteroid was created by Scientists Dr. Scott Hudson and Dr. Steven Ostro using Radar Imaging.