Let’s learn how to extract space resources closer to home.
The UK Daily Mail recently published a piece extolling the benefits of asteroid mining (before lightly tripping over some mundane, yet critical, technical details). The article leads with the headline: Single asteroid worth £60 trillion if it was mined – as much as world earns in a year. Should we chide them for such blatant sensationalism? Then again, is it blatant, or are they merely following an established pattern?
Asteroid mining is a field with lots of hype but little sober consideration. To redeem the technique of in situ resource utilization (ISRU) from the realm of ridicule and science fiction and make it a routine aspect of space mission architectures, we must honestly discuss the difficulties of extracting useful product from raw asteroid debris.
As with every Solar System body of interest and potential use, I am firmly convinced we will eventually mine asteroids. In truth, if we do not take up these formidable technical challenges, there is little hope for any permanent and extensive human presence in space. As long as we confine ourselves to launching everything we need for spaceflight from the bottom of the deepest gravity well in the inner Solar System, we will remain mass- and power-limited and thus, capability-limited.
Essential, low-information density material – spaceflight’s “dumb mass” of propellant and consumables – should be obtained from sources in space, rather than long-hauled (at great cost) from Earth. Only complex, high-information density items not easily made in space should be brought up from Earth. For most missions beyond LEO, the amount of dumb mass vastly exceeds the complex mass. For example, in a chemical propulsion human Mars mission, propellant makes up more than 80% of the total mass of the vehicle.
Media attention tends to focus primarily on two aspects of asteroid mining: extraction of water and platinum group metals. While water is probably the most useful material for consumption in space, platinum is often cited as a material whose principal value lies with its return back to Earth. The composition of asteroids – inferred from remote sensing (primarily precision measurements of the objects’ color) and laboratory studies of meteorites (pieces of various near-Earth asteroids) – informs us that both materials are to be found in these objects.
While quite rare in most meteorites, water can occur in amounts of up to 10-20 wt.% in some meteorite types. Platinum (symbol: Pt) is a trace element in most meteorites but it can make up about 20 parts per million (ppm) of the metal fraction of meteorites. Although this sounds like a miniscule amount, it is orders of magnitude greater than the average abundance of Pt in the Earth’s crust (~0.005 ppm), where we find it only in rare ore bodies (most of which might ultimately be related to meteorite impact).
Water is the most useful near-term space product and needs to be targeted as locally obtained dumb mass. All accessible near-Earth objects orbit within a couple AU of the Sun (1 AU = 150 million km), inside the “frost line” of the Solar System (~ 5 AU, the zone beyond which water ice is stable). Some known asteroids contain spectral evidence of water ice, but they’re in the Main Belt – the zone between the orbits of Mars and Jupiter. Water in near Earth asteroids is chemically bound in clay minerals (geologists call these complex structures phyllosilicates). To extract water vapor, one cannot simply heat the raw asteroid material to 100° C, as we do with water ice.
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