Economical extraction of platinum from main belt asteroids

Abstract

Metallic M−class asteroids may contain 20 to 100 parts per million (ppm) of platinum-group metals (PGM), dissolved within a nickel–iron matrix. Economic extraction of these resources has been elusive. This work introduces two new architectures to produce 43 metric tons (MT) of PGM from a Main Belt asteroid within a 12-month mission duration. A cornerstone of this capability is the transmutation of lunar thorium to the isotope uranium-233. Fission reactors loaded with lunar-sourced fuel pins are used for rapid transport as well as for electric power to refine the metals. A key safety bonus with fission fuel produced on the Moon is that no radioactive materials need to be launched through Earth’s atmosphere. Neutrons are required for transmuting thorium, so a non-radioactive compact accelerator neutron source (CANS) is designed for this purpose, as well as for heating and refining of the asteroidal ore. The concept of operations begins with a precursor mission of five-months duration to the lunar surface, followed by soft landing of an ascent rocket to loft the U-233 to Low Lunar Orbit (LLO). A rendezvous in LLO with Earth-launched hardware, including a (not yet radioactive) nuclear thermal rocket (NTR), is followed by a three-month transit to a 250 m diameter asteroid orbiting Sol at 2.5 to 3.5 AU. A unique feature of this class of architecture is a circumferential track around which a processing bogie orbits. Crucibles containing molten metal face inward, towards the asteroid. As the transition metals are boiled off at 1728 K, they impinge on the regolith surface. As a result, the asteroid becomes (partly) encased in a thick layer of steel. When evaporation is complete at this temperature, a nugget of PGM remains held within the crucible, ready for extraction. Techno-economic analysis shows that one of these architectures can achieve economic self-sufficiency upon completion of the first one-year mission. The other architecture requires two missions to reach cost breakeven, but is more easily re-used. Both approaches build an enduring lunar infrastructure that can support many such missions, plus a wide range of habitation needs and additional in situ resource utilization (ISRU) applications.