It is believed that 20% of meteorites contain free metals such as iron, nickel, and platinum. Iron meteorites are even more metal rich, at about 99%. Carbonaceous asteroids (the most abundant type) are proposed to have 5-20% water. This would have obvious utility in future space exploration and lunar colonization.
Planetary Resources is one company planning to exploit asteroids for their precious materials. Their main competitor, Deep Space Industries Inc., announced only last week that they aim to launch their fleet of Fireflies (small spacecraft) by 2015 to explore these resource-rich space objects. They hope to retrieve asteroid-mined samples and bring them back to earth. By 2016, using larger 70 pound-air crafts (the Dragonflies), they expect to seize 60-150 pounds of asteroid samples.
Images on their website also propose future technological directions, for example the Harvestor, wich would be a full-scale commercial operation utilizing major launch vehicles to yield thousands of tons per year of water, propellant, metals, and building materials that could potentially support something like a lunar station.
But how do we even go about landing and extracting these appealing resources? Obviously getting up there would be expensive, even though we would target the closest objects, needing less fuel than for a moon landing. However, once there, the temperamental orbits of asteroids can cause problems for the diggers. It has been suggested that attaching a rocket to the asteroids could stop the spin, but this is as yet untested.
Miners would likely use similar techniques to those we currently use on earth. One method would be to scrape desired material off the asteroid, and tunnel into areas of specific substances. Strip mining, for example, would pull out valuable ore that drifts off the asteroid, which could be captured by a canopy of sorts. And because gravity is very low on asteroids, any human workers would have to be tied to the surface. But on the up side we would need less power to move the mined samples because they would be floaty light!
So it seems plausible then? Probably not the mass sourcing of gold, but if they can just work out the funding for such activities and then design the appropriate technology to make it happen, then maybe. The need for a lunar colony, however, will have to be a topic for another day.