Most readers will have heard that there are a number of proposals to go mine asteroids, or maybe Mars. The implication is that Earth will become short of resources, so we can mine things in space. However, if we mine there for the benefit here, how would we get such resources here, and in what form. If the resources are refined elsewhere, then there is the “simple” cost of getting them here. If we bring them down in a shuttle, we have to get the shuttle back up there, and the cost is huge. If on the other hand, we drop them (and gravity is cheap) we have to stop whatever we send from burning up in the atmosphere, so to control the system we have to build some sort of spacecraft out there to bring them down. Overall, this is unlikely to be profitable. On the other hand if we build structures in space, such as space stations, or on Mars for settlers, then obviously it is very much cheaper to use local resources, if we can refine them there.
So, what are the local resources? The answer is it depends on the history. All the solid elements are expelled in novae (light elements only) or supernovae (all). The very light elements lithium, beryllium and boron are rather rare because they tend to be destroyed in the star before the explosion. The elements vary in relative amounts made, and basically the heavier the element the less is made, and elements with an even number of protons are more common than elements with odd numbers. Iron, and to some extent nickel, are more common than those around them because the nuclei are particularly stable. The most common elements are magnesium, silicon and with iron about 10% less. Sulphur is about half as common, calcium and aluminium are about 6 – 8% as common as silicon, while the metals such as copper and zinc are about 100,000 times less common than aluminium. The message from all that is that unless there is some process that has sorted the various elements, an object in space is likely to have the composition of dust, which are mainly silicates, i.e. rock. There may well be metal sulphides as well, as there is a lot of sulphur there.
So what sorting could there be? The most obvious is that if the body formed close enough to the star during primary accretion, the heat in the accretion disk could be sufficient to melt the element, if it were there as an element. It appears that iron was, because we get iron meteorites and iron-cored meteorites. The accretion disk, of course, was primarily hydrogen, and at the melting point of iron, hydrogen will reduce iron oxides to iron, also making water. So we could expect asteroids to have iron cores? Well, we are sure most members of the asteroid belt do not, and the reason why not is presumably it did not get hot enough to melt iron where they formed. However, since the regolith (fine “soil”) on the Moon has iron dust in it, perhaps there was iron dust where the asteroids formed. However, the problem is what caused them to solidify. If they melted, steam would be created, and that would oxidise iron dust, so the iron then would be as an oxide, or a silicate.
The ores we have on Earth are there due to geochemical processing. For example, in the mantle, water forms a supercritical fluid that dissolves all sorts of things, including silica and gold. When this comes to the surface, it cools and deposits its solids, which is why gold is found in some quartz veins. The big iron oxide deposits we have were formed through carbon dioxide weathering iron-containing silicates (such as olivine and pyroxene) to make ferrous and magnesium solutions in the oceans. When oxygen came along, the ferrous precipitated to form goethite and haematite, which we now mine. All the ore deposits on Earth are there because of geochemical processing.
There will be limited such processing on Mars, and on the Moon. Thus on the Moon, as it cooled some materials crystallised out before others. The last to crystallise on the Moon was what we call KREEP, which stands for potassium, rare earths and phosphate, which is what it largely comprises. There is also anorthite, a calcium aluminosilicate on the Moon. As for Mars, it seems to be mainly basaltic, which means it is mainly iron magnesium silicate. The other elements will be there, of course, mixed up, but how do you get them out? Then there is the problem of chemical compatibility. Suppose you want rare earths? The rare earths are not that rare, actually, and are about as common as copper. But copper occurs in nice separate ores, at least on Earth, but rare earths have chemical properties somewhat similar to aluminium. For every rare earth atom, there are 100,000 aluminium atoms, all behaving similarly, although not exactly the same. So it is far from easy to separate them from the aluminium, then there is the problem of separating them from each other.
There is what I consider a lot of nonsense spoken about asteroids. Thus one was reported to be “mainly diamond”. On close questioning, it had an infrared signature typical of carbon. That would be typically amorphous graphitic carbon, and no, they did not know specifically it was diamond. Another proposal was to mine asteroids for iron. There may well be some with an iron core, and Vesta probably does have such a core, but most do not. I have heard some say there will be lots of platinum there. Define lots, because unless there has been some form of sorting, it will be there proportionately to its dust concentration, and while there is more than in most bits of basalt, there will still be very little. In my opinion, beware of investment opportunities to get rich quickly through space mining.
Several space mining companies just went bankrupt, at least another got bought up, fate unknown. That probably has to do with the US government abandoning Obama’s idiotic idea of a manned mission to nowhere (namely an asteroid). Instead the Trump administration wisely just dumped 500 millions on NASA to specifically make Moon landers expeditiously, and advanced going back to the Moon to 2024, from 2029 (this has to do with Chinese space scientists asking the PRC government to allow a Moon landing project). Trump wants to dump the ridiculous NASA SLS, and instead use private companies (a good inheritance from Obama, I admit) .
What’s big in space right now is low altitude communications. SpaceX for the FAA’s authorization to launch nearly 10,000 small satellites… And they are launching, and others are too (some using SpaceX for launches…) The idea is worldwide videophones and internet.
The structures of asteroids are a question mark. Even for defense against them, space rocks, it’s not too clear what and how they are made. An article just published suggested 10 kilometer wide asteroid would hold together even after a collision with one kilometer fast impactor.
As you point out mining will be useful if we build in space. Another use may be Helium 3 mining on the Moon (where it forms from solar wind bombardment)… Or on the giant gas planets (where it is gravitationally trapped). Although it requires higher temperatures, because of the higher Coulomb barrier, He3 +He3 fusion produces only protons (which can be herded through electromagnetic fields). Thus it will not damage reactor walls (the reaction has been produced in the lab). Total energy consumption of the USA in a year is equal to the fusion of 6.7 tons of Helium 3….
Yes, low altitudes small satellites are a big thing right now. There is even a New Zealand company that has designed a rocket and has launched a couple of them – and is open for business.
Asteroids are clearly solid because we have some images of them with fairly sizeable impact craters already. The crater is solid, and from energy considerations, the rest has to be solid too. Needless to say [:-)] my theory of planetary formation has a mechanism for them – in fact several mechanisms, depending on where they formed. It is commonly held that they formed in the asteroid but but have been dislodged, and while that will be true for some/most, it cannot be completely right because the somewhat rare enstatite chondrites have been knocked off something that could not have formed there.
Helium 3 would certainly be valuable if there was plenty of it and easy to isolate. One problem we have is we tend to only have information on the top millimetre of regolith, as I suspect the samples from the Apollo period did not really concentrate on this issue and any regolith they have will have equilibrated itself anyway.