Space Prospecting

It’s been an enticing subject for more than half a century now and continues to capture the imaginations of extremely ambitious engineers and powerful capital allocators alike. It’s also somewhat viable: although most asteroids (and almost all of those that are considered to be “near-Earth”) have ore concentrations lower than those found terrestrially on Earth, there is a class of “metallic” (M-type) asteroids likely rich with platinum group metals (PGMs). In high demand, PGMs like Pd, Os, and Ir are difficult & pollutive to mine and thus are among the most expensive materials known to exist at >$500,000/kg.

Companies like AstroForge have recently raised millions on the business case for returning such metals prospected from asteroids, partly because of the enormous down mass potential (~20-50t) of SpaceX’s Starship. But the asteroid mining field is littered with bad ideas and failed projects. For one, Planetary Resources, a 2010s venture backed by Larry Page, thought it viable to extract volatiles from asteroids and robotically produce chemical fuel for in-space depots. Their money made it as far as a few test launches before running out and their assets acquired by a crypto company. It turns out it’s a bad idea to mine near-Earth asteroids for pretty much anything. Water is mixed in with rock and must be heated >400° C to use. Volatiles are not thermally stable and outgas in the vacuum. Metals are hit and miss. Asteroids are also more like piles of rubble, difficult to land on, and overall very small – so you’d want to land on quite a few.

To make the case for asteroids worse, we don’t currently have a ton of conviction in their elemental composition. Statistical issues mar researchers: we don’t understand the distribution of samples we have (i.e. how often they’re from the same asteroid), how measurements are affected by the atmosphere, and old studies have extrapolated seemingly erroneous values [1]. According to leading researchers like Kevin Cannon, PGM concentration on M-type asteroids can vary from 6-230 ppm, which is still higher than most Earth ore concentrations at the bound.

Besides M-type asteroids, which have material that could be economically shipped back to Earth, there are asteroids with modest concentrations of materials that are only useful for supplying projects off-world, in-situ. These are volatiles like water that could be used to generate fuel, as well as Fe, Mg, Al, and Si. The latter are elements that could be used to build structures or photovoltaic material, which is crucial for powering activities in space.

And then the ultimate rub with asteroid mining. Everything needed for in-situ utilization and even healthy Earth-like industry can be mined with comparatively less power and effort in one place: the moon.

Mining Energy Requirements

Mining is the process by which useful geological materials are extracted from the Earth or other astronomical objects. It is extremely important for the function of modern society. Electronic devices are made with materials obtained across the globe and we currently produce about 500 kg of iron ore per year for every person on the planet [2]. We dug up about 68 million tonnes of aluminum just last year. Production of iron and aluminum ore has tripled in the last 30 years. To add to the pile, the coming transition to clean energy infrastructure is more mineral intensive, not less. Batteries require copper, lithium, nickel, manganese, rare earths – all of which need to be dug out of the ground in the next 30 years at rates never before seen.

Not only does mining grievously endanger workers and immutably scar local environments, it is extremely energy intensive to pull this much raw material out of the ground. Mining consumes somewhere around 6% (+/- 3%) of global final energy consumption, around 3500 TWh. This is roughly equivalent to the energy consumption of every household in America, every year.  

Mining Ore on Earth

On the home planet, mining an ore such as iron requires one of two basic methods. Either the material is obtained through open-pit mining, which mine ore by carving giant steps into the earth, or by underground mining, where a system of vertical and horizontal shafts cut through a vein of ore deep underground.

Typically, the series of processes for mining are something like: drilling, blasting/crushing, loading, hauling, and grinding. High energy consumption is rampant throughout this entire procedure and the most intense part can vary based on the site, technique, and ore quality (aka grade, which is just a proxy for energy required). Often the most energy intense aspects besides the large equipment needed for hauling is the power needed for the pumps for water management or HVAC systems, in the case of underground sites. This can be up to 25% of total power requirements.

What requires by far the most power per mine site is comminution, or the process of breaking down the particle size of the ore until the useful mineral has been separated. Milling or grinding the rock is extremely energy inefficient, with most of it dissipated as heat. Generally, this is anywhere from 40-80% of a mine’s energy consumption [3].

Mining on the Moon

Because mining, in particular the comminution of material, requires so much energy, it is rather difficult to pull off anywhere in space, much less the moon. Massive solar arrays are needed to supply power. If they are located anywhere besides permanently sunny regions like the crater rims of the south and north poles, they’ll need to be accompanied by massive battery cells capable of 14-day duty cycles. To make matters worse, volatiles like frozen water that could be made into fuel are located at the often permanently shadowed bottoms of craters.

Researchers are constantly considering techniques to circumvent this paucity of power, including microwave or solar sintering the lunar regolith (dirt) into bricks at around 800 W/g, 3D printing structures with melted regolith, and electrolytic decomposition of molten regolith (MRE) at increasingly low temperatures. Not unlike what is used on Earth, the latter would use a series of electrolytic cells to evolve oxygen from the regolith and then extract other useful elements like Fe, Si, Al, Mg, and Ca for use. It is critical that these systems utilize loose, fine grain regolith as feedstock to minimize the mechanical energy needed to break down the rock.

Luckily, most lunar regolith on the first 0.5 meters or so of the surface is extremely fine-grained, with an average particle size of 70 μm, it doesn’t pack densely, and has a bulk density around that of sandy soil. Many novel excavation techniques to mine regolith for processing are being developed based on techniques here on Earth. These include discrete systems like a scraper (~0.1 Wh/kg) or continuous systems that vary from the established, like the bucket wheel (~0.5 Wh/kg), to the experimental, like the pneumatic with no moving parts and therefore far less maintenance (~3 Wh/kg).

Three techniques for gathering lunar regolith, a scraper and two bucket drum concepts

The feasibility of any lunar excavation concept is still very much unsettled and robotic explorers in the next decade will likely pursue developing this nascent field. What is likely is that the systems will need to be power efficient, minimize moving parts, and mobile, which will allow them to roam after depleting a given area’s surface layer of loose, fine-grained lunar regolith.

Building Homebase

To illustrate how resource extraction might go on the moon, let’s imagine using just one mine to obtain iron in lunar regolith. With this iron, people on the moon could construct the structure for a lunar habitat, not unlike NASA’s inflatable concept. This structure seems to be about 4x3x3 m.

For power, let’s assume that we deliver the single largest solar array that can fit in a Starship fairing, about 162 m^2. We’ll position this array on the edge of Shackleton’s crater on the lunar south pole, which will receive near continual sunlight and should be near ample iron-rich lunar regolith. For reference, a space-grade solar panel like Spectrolab’s generates about 330 W/m^2 at the beginning of its life. Finally, let’s assume that the moon mining is 10x less efficient than mining on Earth (about 500 MWh in a year). This is generous. Mining operations on Earth are done at scale, lunar regolith particles damage machinery on the moon, and the ore quality on the moon is still quite poor.

If we run our mine continuously for two full years, we assimilate just enough iron to construct the lunar habitat structure (with walls about 2.5 cm thick). Of course, this doesn’t include maintenance or the time to set up the mine or construct the structure from the material. If it turns out mining is more like 100x less efficient than on Earth, we need a lot more power generation to extract materials on useful timeframes.

Building a structure on the moon like this, in comparison to shipping one from home, is a pretty nonsense choice. An architecture like Starship can launch 20-40 tons into lunar orbit at something like $50,000 $/kg. An entire habitat, structure plus life support systems, could be served up on the moon for less than $1 bn and in about a month [4]. As rockets get larger and launches more efficient still, resource extraction at scale on the moon or other planetary bodies might start to look increasingly less appetizing.

Industrial Transcendence

One question that a lot of people (myself included) struggle with is the “why.” Why spend billions of dollars lofting tens of tons into orbit for something like a better glimpse of some surface in space. Why spend billions of dollars just to figure out how hard it is to survive in space? One thing worth noting is that it is unlikely that any commodity resources in space will ever be sold on Earth, even PGMs [5].

One of the best reasons for doing this whole space mining thing is that solving seemingly impossible problems, like going to the moon, requires the funding of development and production of seemingly impossible technologies, like the integrated circuit. NASA bought more than half of all IC’s produced globally in the first ~5 years after the tech was invented, playing a crucial role in their commercialization.

Okay then, why go to the moon specifically? Especially if mining in space has no proper business case?

Mining for ore the way we do it on Earth – at a scale with hundreds of humans, thousands of tons of heavy machinery, terawatts of power and square kms – simply will not do on the moon. Industry like mining will need to be more energy efficient. Compactified. Humans will be removed from the process as much as possible and resource extraction on the moon will be as programmable as possible. This sort of “industrial compactification” is where the value of such an endeavor truly lies.

Trees on the Moon

It is hard to deny the sheer magnitude of resources, energy and material alike, that is ripe for the taking in our immediate solar system, even without the “energy-Malthusian” outlook purported by those who are of the view that Earth is simply too finite to allow for a sustainable civilization of 10 billion humans, each using ~100-500 kWh every month. As robotics and computers have miniaturized significantly in the past 50 years and launch costs by 90-95% in the same period, the feasibility of bootstrapping heavy industry off-world is tangible. Some optimistic assessments think this could be done by landing only between 10-50 tons of specialized, teleoperated robotic equipment on the moon which could grow into a lunar-scale autonomous industry capable of producing even the sophisticated computer components needed to build new machines [6].

Plants like trees are instances of nature perfecting industrial compactification. They’re self-replicating, self-repairing, self-reproducing factories that use only the chemical and radiative energy immediately adjacent to them. They tower hundreds of feet in the sky and start as a hardy collection of material and biological code that fits in your palm. If we can figure out how to create and tap into a factory like this, one that sustains itself with little to no interference, perhaps it even replicates itself to some extent, and produces a resource – be it electricity or metal or a little bit of everything – we are well on our way to accessing the power equivalent of a Type II civilization, and likely abundant energy for all along the way.

Augmentation

[1] Kevin M. Cannon, Matt Gialich, Jose Acain, “Precious and structural metals on asteroids”, Planetary and Space Science, https://www.sciencedirect.com/science/article/pii/S0032063322001945#bib30

Jeffrey S. Kargel, “Metalliferous asteroids as potential sources of precious metals”, Journal of Geophysical Research, https://doi.org/10.1029/94JE02141

[2] Good visual here. Iron ore is the vast majority of material mined on Earth – it’s extremely abundant in the crust and we have great use for it (steel). Some people think we will run out this century but it’s up in the air.

[3] Average comminution power requirements are around 6.7 KWh/kg.

[4] Cheap and fast relative to NASA standards

[5] See Casey Handmer’s excellent post on the matter in his series countering space journalism misconceptions. Regarding PGMs, a lot of the cost is due to failure to develop efficient supply chains due to low, specialized demand. A supply shock wouldn’t exactly keep things status-quo. Also, he points out that a lot of the market for exotic materials like those found in space is due to speculation on energy generation, which could ironically be rendered moot by advances in technology driven by space prospecting. 

[6] Philip T. Metzger, Anthony Muscatello, Robert P. Mueller, James Mantovani: “Affordable, Rapid Bootstrapping of the Space Industry and Solar System Civilization”, 2016, Journal of Aerospace Engineering 26.2, 18-29 (2013); arXiv:1612.03238.