== Asteroids == 
 http://www.dodlive.mil/index.php/2013/02/armed-with-science-saturday-science-fiction-the-aperture-for-science-future/ 
 “A fact that most people don’t appreciate is that all of our rare Earth metals come from asteroid strikes.” 

 :Huh? First I heard of it. I'd add it, but it's USAF and so a really dubious source.