# Could you liquefy an asteroid to collect its resources?

Suppose you could pump a load of energy into an asteroid at a safe distance. As energy gets added to the object, does it make sense that this could be done in a way to get the asteroid to differentiate into different liquid rock/metal layers long enough to collect it?

Might a miner or a mining corporation consider this an easier way to collect material if they have the means?

Some of the things Im considering are whether you'd need to evenly heat the thing, maybe it would be easier to vaporize it and then condense it for collection, maybe it would cool too quickly to collect.

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• You're confusing mining and smelting. Heating something is much more efficient when it has a lot of surface area. That's why smelters crush the ore before processing. – RonJohn Nov 1 '18 at 15:47
• Sure, I modified the title. Im interested in the collection of the pure or at least better differentiated material. – Chris W Nov 1 '18 at 15:50
• On the Earth, where there's a lot of gravity and atmosphere, where hot, light gases rise higher than other gases, you might -- theoretically -- get away with heating big rocks so hot that the constituent minerals (or even elements!) themselves in something akin to a petroleum cracking tower. That wouldn't work in space, though. – RonJohn Nov 1 '18 at 15:56
• Could you be more specific? What exactly do you want to mine and what is the composition of the asteroid? Also, how long do you think this is going to take? I don't know how you plan to heat it, but my guess is that you're gonna evaporate a lot more than you will melt. How do you keep it molten? Please share more details before one can state if this is worth it. Btw, I once discussed this with a geologist and concluded that asteroid fields are not worth the effort, better concentrate on some planet, but I don't think I'm expert enough to summarize why. – Raditz_35 Nov 1 '18 at 15:56
• "Cooling too quickly" is not going to be the problem, unless a couple centuries is not enough time for whatever you're doing. – Roger Nov 1 '18 at 16:05

It depends on the size of the asteroid. For something the size of the Earth or moon, where gravity is generated, a molten mass will differentiate with heavier metals descending to the center and lighter material migrating to the crust. It would still be a "rough sort", elements of similar mass might not be able to separate into clean layers.

You can't just spin the asteroid, it will fly apart.

For small asteroids of "mining" size, the gravity will be too slight, even when molten, to separate like that. Elements won't overcome surface-tension and viscosity enough to move.

A more likely mining scenario is to melt the asteroid parts and introduce them to a centrifuge, where with spin you can apply hundreds or thousands of centrifugal gravities until the molten parts cool into very distinct layers.

You can make the centrifuge out of tungsten, the metal with the highest melting point (6192F) and highest tensile strength.

You might want to start the heating slowly, so you can capture some of the elements released by heating; they will be the first to liquefy and then vaporize. This is basically a form of distillation: Alcohol has a boiling point of 173F, water is 212, so if you heat a mixture of alcohol and water to about 190F, the "steam" vapor contains much more alcohol than water; and this can be repeated a few times to extract nearly pure alcohol (95.5%) from a fermented mash. (See azeotrope for details of why it can't get more pure without more complex methods, like pressurization or other ingredients).

By heating the asteroid slowly, increasing the temperature a few degrees at a time, we can do a similar thing, capturing elements that vaporize at different temperatures.

Slower heating will also help to prevent explosions. For the latter, first cold-grinding the asteroid to sand-size bits will help as well, breaking any large pockets of frozen gas or water trapped in the rock.

All of this can be done with concentrated solar energy, as a typical scenario for actually mining asteroids, for everything from oxygen to heavy metals (precious or not). One of the good products of asteroid mining is iron, for construction steel, which is very expensive to lift from a planet surface, but practically free if mined in space using giant mirrors to focus the sun.

Edit: Note that in micro-gravity space, these can be many paper-thin polished aluminum parabolic mirrors, a hundred yards in diameter with quite minimal support to provide all the power necessary. They can also be used to drive either photo-voltaics or Stirling engines to generate electrical power, which is useful both for the habitats and further distillation or breaking of compounds by electrolysis (like separating water into hydrogen and oxygen).

Edit: Once the molten asteroid cools in the centrifuge; you will have a wheel of layers of rings. For further processing, you would peel (or grind) the layers off. At the boundaries where two materials mix, you can direct the grind to a separate catch-bin, when you have enough of that you can re-melt, re-centrifuge, and do this again, grind off the pure parts and leave any mixed part in the catch-bin. The point being, you don't have to be razor accurate in separating the rings in order to have a pure product.

Edit: For fun, here is a nice wiki page that lets you sort the elements by melting-point or boiling-point, to help you see the order in which you can retrieve them. Of course some will be bound in compounds; typically lowering their melting/boiling point.

Edit on Feasibility: On the mirrors: At the distance of the middle of the asteroid belt (400 million km), there is about 190 Watts per square meter of solar radiation to be captured (barring shadows). (Compare to Earth, 1370 W/$$m^2$$ hits our upper atmosphere, about half makes it through the air to the surface.)

So a single 100 meter diameter parabolic mirror could focus 1.5 MW of power on a "point" (IRL perhaps 50 centimeters wide). But we can have hundreds or thousands of these focused on a single point. Space has plenty of space!

EDIT: For cooling, note Stirling engines are heat pump engines that can also cool things; but in general the trick to cooling the molten mass is to make the heat do some kind of physical work. A steam engine is a primitive example of this. (There are closed cycle steam engines and Stirling engines, BTW, that could be engineered to work in a vacuum). Trivially it could be used to help heat the miner's habitat; but also for mechanical tasks like cold-grinding asteroids to a powder and grinding previously cooled wheels of separated ore, compressing gases back to liquid, making electricity for atomic accelerator propulsion, etc.

• +1. Nitpick: "this can be repeated a few times to extract pure alcohol from a fermented mash": not really. A solution of 95.4% (by mass) ethyl alcohol and water is an azeotrope which boils at 78.2° C, lower than the 78.4° C boiling point of pure ethyl alcohol; this means that distillation won't be able to concentrate the solution further. – AlexP Nov 1 '18 at 17:19
• @AlexP Interesting. How do they distill it further? – Amadeus Nov 1 '18 at 17:20
• Various methods, involving molecular sieves, reverse osmosis, extraction with supercritical carbon dioxide and other exotic techniques. But this is rarely done, because pure ethyl alcohol is extremely hygroscopic and will immediately capture some water from the environment if exposed to air. – AlexP Nov 1 '18 at 17:23
• @AlexP Yeah, the azeotrope link covered some of those. Probably the easiest in space would be simple pressurization methods; also since pressure influences the azeotrope ratios, I'd guess that high (centrifugal) gravity environments also affect the azeotrope ratios. Fun stuff. I corrected my answer about the distillation. – Amadeus Nov 1 '18 at 17:35
• @AlexP and by your second link; in space (which to be clear I was not talking about when describing alcohol distillation as a metaphor for asteroid separations) we actually can do a vacuum distillation to get pure ethanol! :-) – Amadeus Nov 1 '18 at 17:59

A paper submitted to the International Society of Plasma Chemistry describes separating minerals via ion-cyclotron resonance.

The concept calls for a machine to crush the asteroid to a fine powder, then utilizes microwave energy to create a plasma state of target minerals. The plasma is then sent through various magnetic fields for separation based on the electrical properties and atomic weight of each element. The metals are then deposited out of the plasma back to the solid state on to collector plates.

Several sci-fi universes use a similar concept in their lore, like Eve Online, Star Citizen, and a myriad of novels. They call for the total asteroid to be converted to plasma state where it is magnetically separated and cooled. Such visions typically have to hand-wave the energy and thermal management requirements.

• +1 Cool. I think this could be incorporated as a secondary stage of refinement in my own (lower tech) answer. – Amadeus Nov 1 '18 at 18:27

I doubt it would be economically viable, for the following reason:

• As RonJohn pointed out in their comment, heating something is much more efficient when it has a lot of surface area. That's why smelters crush the ore before processing.
• Once the thing is molten, and assuming that there are no chemical reactions in the molten mass, to separate different substances you need gravity. Considering the low gravity of an asteroid, the process will take a lot of time.
• Even assuming you manage to wait until the thing is stratified, you find yourself with a load of molten metal to handle. Not really handy.

Summing up, you would need a lot of energy to be supplied, a lot of time to wait for stratification and again a lot of energy to dissipate before handling. All in all I think it is still easier to drill and mine in the old fashioned way.