Asteroid Mining

Future large-scale space operations, including space hotels, solar power satellites, and orbital factories, will require volatiles such as water, methane, ammonia, and carbon dioxide.^* These materials can be used to produce propellant, metal for facility construction (such as nickel-iron alloy), semiconductors for manufacturing photovoltaic power systems (such as silicon, arsenic, and germanium), and simple mass for ballast and shielding . The cost to transport these commodities from Earth today is $10,000 per kilogram. In the future, the extraction of these materials from easy-access asteroids will become a competitive option.

All of these resources are present in asteroids. About 10 percent of the near-Earth asteroids (NEAs) are more accessible than the Moon, requiring a velocity increase (delta-v) from low Earth orbit of less than 6 kilometers per second (km/s; 3.75 miles per second) for rendezvous, with a return departure delta-v of 1 km/s or less. A few are extremely accessible, only marginally more demanding to reach than a launch or a satellite to geostationary orbit .

The return of asteroidal materials using propellant derived from the target asteroid will enable potentially unlimited mass availability in low Earth orbit. That will break the logistical bottleneck and cost constraints of launching from Earth. Asteroid-sourced raw materials will enable and catalyze the development of an Earth-Moon space economy and humankind's expansion into the solar system.

The growing recognition of the "impact threat" to Earth has prompted several successful NEA search programs, with approximately 1,800 NEAs now identified (as of April 2002), up from about 30 NEAs twenty years ago. Some 400 are classified as potentially hazardous asteroids (PHAs) that come to within 7.5 million kilometers (4.7 million miles) of Earth orbit on occasion. New potential mining targets are found every month.

Based on meteorite studies, astronomers recognize that NEAs have diverse compositions, including silicate, carbonaceous and hydrocarbon-bearing, metallic, and ice-bearing materials. Some may be loose rubble piles held together only by self-gravity.

Insights from comet modeling, studies of orbital dynamics, and observation of comet-asteroid transition objects indicate that 30 to 40 percent of NEAs may be extinct or dormant comets.

There has recently been major work on modeling of the development on comets of a crust or regolith of dust, fragmented rock, and bitumen that has been prompted by the Giotto spacecraft's observations of Halley's comet in 1986 and other comets. This insulating "mantle," if allowed to grow to completion, eliminates cometary outgassing , and the object then takes on the appearance of an inactive asteroid. These cryptocometary bodies, if in near-Earth orbits, will stabilize with a deep-core temperature of about −50°C. The deep core would probably be depleted of CO and CO₂ and highly porous but would retain water ice in crystalline form and in combination with silicates as well as bituminous hydrocarbons. This ice could be extracted by drilling and circulation of hot fluid or by mining with subsequent heat processing.

Photographs of the asteroids Gaspra, Ida and Dactyl, Mathilde, Braille, and Eros by various space probes and radar images of Castalia, Toutatis, 1998 KY26, Kleopatra, 1999 JM8, and Geographos reveal a varied, bizarre, and poorly understood collection of objects. Many images show evidence of a thick loose regolith or gravel/sand/silt layer that could be collected easily by scooping or shoveling. Eros shows slump sheets in the sides of craters where fresh material has been uncovered, a lack of small craters, an abundance of boulders, and pooled dust deposits in the bases of craters.

Eros and Mathilde have improbably low densities, suggesting that they have large internal voids or are highly porous; Mathilde has craters so large that their generating impacts should have split it asunder. Both Toutatis and Castalia appear to be contact binaries: twin asteroids in contact with each other. Eros and Geographos are improbably elongated, shaped like sweet potatoes. Kleopatra is a 140-kilometer-long (87.5 miles) dog-bone shape. 1998 KY26 is tiny and spins so fast that any loose material on its surface must be flung off into space, implying that it must be a monolithic solid object under tension.

Mining Concepts

The choice of mining and processing methods is driven by what and how much is desired, difficulty of separation, duration of mining season, and propulsion demands in returning the product to the nominated orbit. Minimization of project cost and technical risk, together with maximization of returns in a short timeframe, will be major factors in project planning.

If the required product is water, which can be used as the propellant for the return journey, underground rather than surface mining will be required because of the dryness of the asteroid surface. Some sort of tunnelborer will be needed, or a large-diameter auger-type drill. If the product is nickel-iron metal sand, surface regolith collection by scraping or shoveling is indicated. Surface reclaim is threatened by problems of containment and anchoring. In situ volatilization (melting and vaporizing ice at the bottom of a drill hole for extraction as steam) has been proposed for mining comet matrix material but is subject to fluid loss and blowouts.

The processing methods depend on the desired product. If it is water and other volatiles, a heating and condensation process is essential. If it is nickel-iron sand, then density, magnetic, or electrostatic separation will be used to produce a concentrate from the collected regolith. Terrestrial centrifugal grinding mills and density-separation jigs can be adapted for this work.

Initial asteroid mining operations will probably be carried out by small, low-cost, robotic, remotely controlled or autonomous integrated miner-processors designed to return a few hundred to a few thousand tons of product per mission, with propulsion systems using asteroid-derived material for propellant.

Conclusion

The knowledge and technologies required to develop the resources of asteroids and enable the industrialization and colonization of the inner solar system will provide humankind with the ability to protect society and Earth from threats of asteroid and comet impacts.

see also Asteroids (volume 2); Close Encounters (volume 2); Getting to Space Cheaply (volume 1); Hotels (volume 4); Impacts (volume 4); Solar Power Systems (volume 4); Space Resources (volume 4).

Mark J. Sonter

Bibliography

Binzel, Richard P., et al., eds. Asteroids III. Tucson: University of Arizona Press, in press.

Gehrels, T., ed. Hazards Due to Asteroids and Comets. Tucson: University of Arizona Press, 1994.

Lewis, John S. Mining the Sky. Reading, MA: Helix/Addison Wesley, 1996.

Lewis, John S., Mildred Shapley Matthews, and Mary L. Guerrieri, eds. Resources of Near-Earth Space. Tucson: University of Arizona Press, 1993.

^*Volatiles easily pass into the vapor stage when heated.