Subject: Is the panspermia hypothesis credible?

Some believe that the Earth was "seeded" with life from space - living 
cells or the molecules needed for life to start travelled here inside 
comets or asteroids. This is called the panspermia hypothesis; Francis 
Crick is one of its supporters. A variation of this theory is that life 
first originated on Mars and pieces of Martian rock, blasted off by 
asteroid impacts, rained down on Earth, bringing life to our planet.
  Is this hypothesis credible? I guess a very large amount of amino acids 
(or other organic molecules) must have been brought here if Earth was 
"seeded" from outer space - could this really have happened the way Crick 
and others believe? Space is mostly empty, would enough 
life-carrying asteroids be able to hit us? Wouldn't anything inside an 
asteroid or a comet be destroyed when it hit the ground?   And what about 
the harsh conditions in space - anything travelling to us would be exposed 
to extreme cold, vacuum and radiation for millions of years.
  Are there other flaws in this theory?  How is the panspermia hypothesis 
viewed by the majority of scientists? Is it criticized, considered 
interesting, or accepted? Please be as comprehensive as possible in your 
answer. 
                

Re: Is the panspermia hypothesis credible?