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. The chemical bonds that attach oxygen and hydrogen atoms within the clay crystal structures require considerably more energy (> 500° C) to break than the simple phase change of ice to vapor. Moreover, at very high temperatures, the released water from clay structures is highly reactive and does not remain as “free water” for long – it quickly combines with reduced components in asteroidal debris, such as troilite (FeS) and graphite (C), both common meteorite minerals. The sum of these effects results in an extremely low yield of water from asteroid processing, on the order of much less than 1% by mass and worse.
Low yield is not a problem if you have a high throughput of feedstock and a lot of it to draw on. Presumably the latter would be available at an asteroid, but the problem is getting processing equipment to the object. Hundreds of kilowatts of energy will be needed to make significant amounts (i.e., multiple tons) of product. This will require either enormous solar arrays or a prohibitively expensive (and yet-to-be-devised) nuclear reactor. Solar thermal energy (concentrated by concave mirrors) could provide the needed heat for processing, but the movement of material, the collection and storage of the product, and the power needed to operate the robotic systems supervising the processing stream, all require substantial electrical power. Then there is the need for these operations to be controlled by an intelligent operator (possibly remotely, if the time-delay in communications isn’t too long). For most near-Earth objects, time delays (running into minutes) will require significant automation and intelligent robotics. Again, such systems can be envisioned but do not yet exist.
Given these realities, it makes sense to develop the technologies and operational procedure for remote mining on the Moon rather than on asteroids. At 3 light-seconds round trip, the Moon is close enough that complex machines could be easily operated from Earth. There, we would learn how to handle large amounts of granular materials, collect and store the extracted product, and work out the difficulties of early resource utilization. Experience in conducting lunar ISRU is directly applicable to asteroid processing, including both the equipment needed and the procedures to be followed. Water on the Moon is present in the chemically unbound form of ice and requires only modest heating (to 100° C) for vaporization and collection. When attempting something as potentially revolutionary as ISRU, it makes programmatic good sense to start with the easy stuff first (lunar polar ice) and work up to the more difficult things (asteroid mining) later.
Platinum extraction from asteroids is even dicier. In asteroids, platinum is intimately mixed with other iron-loving metals (called “siderophile” elements). To extract Pt from asteroidal material, we must remove its alloyed iron and nickel (which is 99.9% by mass of the asteroid metal). One approach is the Mond process, named for Ludwig Mond, the chemical engineer who developed it (curiously, Mond means Moon in German). In this time-tested technique, heated carbon monoxide is passed over metallic granular material at modestly high temperatures (~100° C) under pressures of up to 10 atmospheres. The gas interacts with the iron-nickel and forms vaporous iron and nickel carbonyls, which can then be removed from the particle bed and condensed as metal films. The residue from this process forms a dust enriched in Pt (about 0.5 wt.%) along with many other siderophile elements. After collection, this enriched residue dust would be sent back to Earth for further processing.
Once again, this procedure is simple in principle, but doing such processing in space, millions of kilometers from the Earth, raises many difficult questions, the answers to which are mostly unknown. How could we collect and store the gaseous iron and nickel carbonyls? With no gravity, magnetic field separation might be useful, but this again requires high power and complex machinery to separate the components. The containment vessel must be isolated from other components and unreacted feedstock must be cleared and recycled or discarded; can such delicate and complex operations be automated? Having humans in the control loop might answer a lot of these problems, but the most valuable asteroid might not be close to the Earth – out of reach for human missions, at least in the early stages of asteroid mining.
I outline these difficulties not to cast doubt on the feasibility of mining in space, but rather to point out that in complex fields of endeavor, we should crawl before trying to walk and walk before attempting to run. Extracting and making useful materials from space resources is an engaging challenge, one whose mastery can change the paradigm of spaceflight. We are fortunate to have within our near grasp, a Moon that possesses abundant “dumb mass” – those resources needed to both create new space faring capability and to perfect the skills and techniques we will need to reach, secure and use the wealth of the Solar System.
For additional information on the Moon vs. asteroids as our next space destination, please see my three-part series: