One thing you see often in the media is the concept that perhaps in the future we can solve our resources problem by mining asteroids. Hopefully, that is fine for science fiction, and I use that word “hopefully” because my next piece of science fiction, currently in the editing mode, includes collecting asteroids for minerals extraction. However, what is the reality?
We know we have a resource problem. An unfortunately large and growing number of elements are becoming scarcer and harder to obtain. As a consequence, ores are getting less concentrated, and so much material has to be thrown away. As an example, the earliest use of copper at around 7,000 BC used native copper. All the people had to do was take a piece and hammer it into some desirable shape. Some time later someone found that if something like malachite was accidentally in a fireplace, it got reduced to copper, and metallurgy was founded. Malachite is 57.7% copper, while if you were lucky enough to find cuprite you got a yield of almost 89% copper. Now the average yield of copper from a copper ore is 0.6% and falling. The rest is usually useless silicates. So, you may think, if we have worked through all the easily available stuff here, nobody has worked through the asteroids. There we could get “the good stuff”.
At this point it is worth contemplating what an ore is and where it came from? All the elements heavier than lithium were made in supernovae or through collisions of neutron stars. Either way, if we think of the supernova, the elements are made at an extremely high temperature, and they are flying away from the stellar core at a very high velocity. The net result is they end up as particles that make the particles in smoke look big. This “smoke” gets mixed in with gas clouds that end up making stars and planets. To get some perspective on concentrations, for every million silicon atoms you will get, on average, about 900,000 iron atoms, almost 24,000,000 oxygen atoms, 5420 chlorine atoms, 52,700 sodium atoms, 522 copper atoms, almost half a silver atom, 0.187 gold atoms, 1.34 platinum atoms and about 0.009 uranium atoms.
So what happens depends on whether the elements react in the accretion disk, so that molecules form. For example, all the sodium atoms will either form a chloride or a hydroxide, but the gold atoms will by and large not react. About half the iron atoms form an oxide or stay as the element, and the oxides will end up as silicates (basalt). What happens next depends on how the objects accrete. That is not agreed. Most scientists say they simply don’t know. I believe the bodies are accreted through chemistry. If the former, we have to assume the elements end up as a mix that have those elements in proportion, except for those that make gases. If the latter, then some will be more concentrated than others.
On earth, elements are concentrated into ores by geochemistry. The heat and water processes some elements, and heat and volcanism concentrates others. Thus gold is concentrated by it dissolving in supercritical water, together with silica, which is why you often find gold in quartz veins. The relevance to asteroids is that processing does not happen in most because they are not big enough to generate the required heat. The relevance now is that the elements you want will either be bound up with silicates, or be scattered randomly through the bulk. To get the metals out, you have to get rid of the silicates, and if you look at the figures, the copper content is actually less than in our ores on earth. Now look at the mining wastes on Earth, and ask yourself what would you do with that in space? (There is an answer – build space stations with rocky shells.)
So why do we think of mining asteroids. One reason comes from asteroid Psyche. One scientific paper once claimed asteroid had a density as high as 7.6 g/cm cubed. That would clearly be worth mining, because the iron would also dissolve nickel, cobalt, platinum, gold, etc. You will various news items that wax on about how this asteroid alone would solve our problems and make everyon extremely rich. However, other papers have published values as low as 1.4 g/cm cubed, and the average value is about 3.5 g/cm cubed (which is what it would be if it were solid basalt).
Why the differences? Basically because density depends on the mass (determined by gravitational interactions) and volume. The uncertainty in the volume, thanks to observational uncertainty due to the asteroid being so far away and the fact it is not round, can give an error of up to 50%. The mass requires very accurate measurements when near something else and again huge errors are possible.
So the question then is, if someone wants to get metals out of asteroids, how will they do it? If the elements are there as oxides or sulphides, what do you do about that? On Earth you heat with coal and air, followed by coal. You cannot do that in space. On Earth, minerals can be concentrated by various means that use liquids, such as froth flotation, but you cannot do that easily in space because first liquids like water are scarce, and second, if you have them, unless they are totally enclosed they boil off into space. Flotation requires “gravity”, which requires a centrifuge. Possible, but very expensive,If you were building a giant space station, yes, asteroids would be valuable because the cost of getting components from Earth is huge, but we still need technology to refine them. Otherwise the cost of getting the materials to Earth would be horrifying. Be careful if you see an investment offering.