 FACT: The chunk which hits San Francisco was reported to be  traveling at about 32,000 miles per hour. Thats very credible for  an orbital collision. However, the firey blob we see impact with the  city was clearly traveling at about 500 miles per hour. Something  traveling at 32,000 will not be seen before it hits. There certainly  wont be any time for someone to hear a rumble (speed of sound is  only about 700 miles per hour 