Space mining has been a longtime staple of science-fiction films — and the companies are almost always the villains. The transport ship in "Alien" was towing a load of ore when sinister, corporate overlords diverted it into the clutches of the galaxy's baddest monster. "Avatar" was all about saving the inhabitants of Pandora from thugs clawing their home planet to shreds in search of "unobtanium." Nevertheless, a group of entrepreneurs received a hero's reception Tuesday in Seattle, when they unveiled a new company with the goal of extracting platinum, gold and other valuable resources from asteroids. Director James Cameron, who...