Near-Earth asteroids (NEAs) are asteroids that pass near to Earth and they provide attractive targets for the extraction of resources to support the expansion of activities in space. It is estimated that there are about 1000 NEAs larger than 1 km diameter and about 100,000 NEAs larger than 140 m diameter. For as many as 60 percent of the NEAs, the amount of energy required to make a round trip from the Earth to a NEA is less than a round trip from the Earth to the surface of the Moon. A 1 km diameter metallic asteroid is expected to contain a few times more platinum group metals (PGMs) than all that has been mined on Earth.
The large distance between the Earth and an asteroid means that a round-trip time for communications can be several minutes or more and transporting the extracted materials back to Earth can take up to a few years. As a result, one good strategy before mining a selected asteroid is to capture the asteroid and transport it into a stable orbit around the Earth. Having an asteroid in orbit around the Earth makes the resources on the asteroid a lot more accessible and effectively removes the problem of round-trip time for communications. A spacecraft with a solar-electric propulsion system can be used to haul in an asteroid since such a propulsion system contains many times more propulsive energy per unit mass than chemical propulsion systems. The strategy of capturing a NEA is likely to occur in 2 stages. Stage 1 involves transporting the asteroid until it is sufficiently close to the Earth for it to rendezvous with the Earth’s gravitational sphere of influence. Stage 2 involves placing the asteroid into a stable orbit around the Earth. With proper timing, a gravitational flyby with the Moon can be used to slow an incoming asteroid into a stable orbit around the Earth.
Materials extracted from an asteroid can have many useful applications both in space and back on Earth. Volatiles such as water can be extracted for use as rocket fuel and for biological uses. Precious metals such as gold, platinum and PGMs are in demand in the manufacturing of numerous products such as fuel cells, high-end electronics, catalytic converters and advanced medicines. These precious metals are vital to our advancing technological civilization but they are extremely rare on Earth because very little remains in the Earth’s crust as most of them have sank into the Earth’s core. However, asteroids are pristine objects that have undergone very little modifications. For this reason, a typical chunk of asteroid material is expected to have hundreds of times greater concentration of precious metals than in the Earth’s crust and tens of times greater concentration than in the richest mines on Earth. In addition to water and precious metals, other materials such as iron and aluminium can serve as useful materials for constructing structures in space. Even the leftover bulk material of the asteroid can serve as radiation shielding for long duration manned missions.
Mining an asteroid, especially one that has been transported to the vicinity of Earth reduces the need to launch large amount of materials from Earth to support the expansion of activities in space. Precious metals extracted from asteroids can reduce the environmental stresses that result from the mining of these metals on Earth as it creates another avenue of supply. One thing to note regarding the mining of precious metals from asteroids is that the profit margin needs to be high enough because an outside source of precious metals can lower prices sufficiently to doom the asteroid mining venture.