Outer space has extremely rich mineral resources, especially in the form of nickel and iron from iron-nickel asteroids. The asteroid 16 Psyche, a 200 km (125 mi) wide asteroid that makes up 1% of the material in the asteroid belt, contains 1.7 x 1019 kilograms of ore, enough to supply the 2007 world production requirement for millions of years. A more modest asteroid, just a kilometer in size, could contain billions of tonnes of iron-nickel ore. For reference, the annual world production of iron ore is about a billion tonnes. About 800 billion tonnes of iron ore resources exist worldwide, which seems like a lot, but if consumption rates continue to grow exponentially, it might be completely dug up in under a century, necessitating iron from elsewhere.
The concept of exploiting asteroids for minerals has been called asteroid mining. Sufficient resources exist in the asteroid belt to sustain the iron and nickel needs of our civilization for many years to come, but the high costs of space launches make their current recovery prohibitively expensive. Other resources exist on asteroids in smaller quantities, including most metals with an atomic number lower than iron. Substantial fresh water resources exist on Jupiter-family comets, but desalination is likely to be cheaper than bringing in an asteroid, even in the long run. Large Helium-3 resources exist on the surface of the Moon, which could theoretically be used to power fusion reactors.
For asteroid mining to be economically feasible, extensive robotics would need to be developed, preferably self-replicating robotics, as space mining would be a laborious and somewhat dangerous affair. Heavy in situ resource utilization would be needed to minimize the weight of material to be launched from Earth. The first asteroid mining would likely occur on Near-Earth asteroids, which are rarer than their cousins in the asteroid belt, but large and numerous enough to provide serious resources for early-stage space colonization and economic exploitation. One possible target would be 4660 Nereus, a 1-km wide asteroid whose trajectory relative to the Earth means it would require less energy to make it there than the Moon, but the trip would be longer, as it is about three times as far as the Moon at its closest approach.