Asteroid mining sounds improbable, on grounds of technology and economics alike. But lots of things we take for granted once seemed just as unlikely.
Outer space economics
You can tell from the names, Deep Space Industries and Planetary Resources, that the two companies have been started by true believers in manned exploration of our solar system. "Visionary" hardly begins to describe their business plans: mine asteroids - hundreds come through Earth's neighbourhood each year - to get metals and water for space-based manufacturing, for colonisation and even for shipping back to Earth.
It's an invigorating idea, although we should perhaps be careful of what they wish for: in the 16th century, Spanish imports of New World gold and silver touched off decades of steep inflation across Europe.
Planetary Resources is backed by rich dreamers, including moviemaker James Cameron, and Larry Page and Eric Schmidt of Google; Deep Space is looking for investors to back its ambitious, just-announced plan to begin unmanned prospecting of certain asteroids by 2015.
Projects like these may pay off, or not. Plucking gold or platinum from the skies might sound enticing, but hauling back a spaceship full of it would flood markets and diminish returns. Rather, these explorers should hope for a yet-to-be-identified-but-soon-to-be-invaluable material. Dilithium, perhaps? Finding that would be worth the trip.