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There are numerous types of asteroid classified based on spectral analyses of their surfaces. E-type, or enstatite asteroids, are composed predominantly of the pyroxene mineral enstatite (MgSiO3), P-types are believed to contain organic-rich, potentially hydrated, silicate material while S-types are stony, composed of well-known terrestrial rock-forming minerals such as olivine [(Mg,Fe)SiO4]. M-type, or “metallic”, asteroids are of particular interest due to emission spectra and albedo being consistent with metallic iron.
Numerous iron-rich meteorites, i.e. asteroids that have made landfall on Earth, contain abundant nickel with the most Ni-rich, at up to ~17 wt% Ni, as well as up to ~140 parts per million (ppm) of platinum and palladium and iridium contents in the 10’s millions ppm range. These metallic bodies are believed to be the remnant cores of differentiated planetesimals, i.e. small proto-planets that were never accreted into larger bodies that were subsequently fragmented by impacts. Siderophilic elements that easily dissolve in molten iron, like the Platinum Group Metals (PGMs), have been sequestered in the denser metal-rich core regions of these early Solar System bodies.
The PGMs consist of ruthenium (Ru), rhodium (Rh), palladium (Pd), Osmium (Os), Iridium (Ir) and Platinum (Pt) and, together with iron (Fe), Nickel (Ni) and Cobalt (Co), make up a solid block of elements in the d-block of the periodic table. Due to their siderophilic nature, they have mostly been trapped in the Earth’s core and are therefore rarely enriched at the Earth’s surface. Estimations of PGM contents in terrestrial crust vary widely with ranges, for each individual PGM, of between <1 parts per billion in upper continental crustal sediments and up to 100’s ppb of Ir and Rh, and 1000’s ppb of Pt, Pd, and Rh, in metal-enriched ultramafic complexes.
M-type asteroids, therefore, represent a potential source of highly-enriched PGM-bearing metal.
Main Belt Asteroids
Main-belt asteroids, i.e. those located between the orbits of Mars and Jupiter, are hard to reach and therefore not economically viable targets in the short-term, whereas near-Earth objects, including asteroids and comets, are more viable in terms of potential for exploiting their mineral content. Approximately 20,000 iron-nickel (Fe-Ni) near-Earth asteroids are larger than 100m in diameter with 10’s of millions larger than 20m in diameter representing 1000s of billions of tonnes of metallic ore.
Mining of asteroids will most likely be done remotely, using robots, with a number of companies developing both the hardware and software for precision mining of asteroids. The Japanese company, ispace, is developing small rovers, weighing only 4kg, specifically designed for prospecting and exploration of potential targets. Kleos Space is another firm working towards prospecting developing robotics that manufactures parts for mining and exploration machinery in space. OffWorld is yet another company working in the robotics side of space-mining developing AI systems for autonomous industrial robots.
Mining of Asteroids
Two companies have been at the forefront of pursuing asteroid mining, for now generating revenue through more typical means. Planetary Resources have focused on detecting water-bearing asteroids rather than metallic bodies. In 2018 they put their orbiter, Arkyd-6, equipped with infrared sensors, into terrestrial orbit. Their hardware can detect both water and hydrated minerals.
Deep Space Industries, acquired by Bradford Space Group, had been developing strategies for reducing the cost of space exploration including researching a water-propulsion system.
In 2019 the Hayabusa2 spacecraft, built and operated by the Japanese Aerospace Exploration Agency, touched down on Ryugu, a near-Earth asteroid with a 1 km diameter, firing a projectile into the surface and collecting dust for analysis. The probe was also equipped with a sampler horn that would have sampled the surface had the projectile system failed. The probe is due to begin its return to Earth towards the end of 2019, with a total mission time of more than five years.
Such technologies will become integral to making asteroid mining a viable source of metals in the future. Concerns surrounding bringing vast bodies back to Earth and the expense and energy investment of doing so most likely mean than asteroid mining will be performed either in situ or proximal to the ore-bodies The aim would be to harvest materials for construction of machines and equipment for further space exploration.
Sources and Further Reading
- Bacuta GC, Kay RW, Gibbs AK, Lipin BR (1990) Platinum-group element abundance and distribution in chromite deposits of the Acoje Block, Zambales Ophiolite Complex, Phillipines. Journal of Geochemical Exploration. 37: 113-145.
- Castelvecchi D (2019) Japan’s Hayabusa2 craft touches down on asteroid Ryugu - Nature News.
- Cornish C (2017) Interplanetary players: a who’s who of space mining. Financial Times.
- Davies JK, Eaton N, Green SF, McCheyne RS, Meadows AJ (1982). The classification of asteroids. Vistas in Astronomy. 26: 243-251.
- Elvis M (2014) How many ore-bearing asteroids? Planetary and Space Science, 91: 20-26.
- Hawley FG (1939) The occurrence of platinum in meteorites. Contributions of the Society for Research on Meteorites.2: 132-137.
- Park JW, Hu Z, Gao S, Campbell IH, Gong H (2012) Platinum Group element abundances in the upper continental crust revisited – New constraints from analyses of Chinese loess. Geochimica et Cosmochimica Acta. 93: 63-76
- Rivkin AS, Howell ES, Lebofsky LA, Clark BE, Britt DT (2000) The nature of M-class asteroids from 3-μm observations. Icarus. 145: 351-368.
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