If mankind is ever to become an interplanetary species, our outward expansion across the solar system probably can't be fueled by NASA funding alone. Why did the first humans venture out of Africa? What made the Europeans sail into the unknown? What drove Americans to expand across the continent? Curiosity and an adventurous spirit, yes, but more importantly: resources--be they riches, food, or fertile farmland.

Similarly, resources may be the only thing that can lure us from the comforts of Earth. Mining for lunar water could make it up to 90 percent cheaper to colonize the moon. And extracting platinum and other minerals from asteroids could propel mankind to travel beyond low Earth orbit.

At least two companies—Planetary Resources and Deep Space Industries—are openly planning to mine asteroids.