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Soki comments on Politicians stymie human colonization of space to save make-work jobs - Less Wrong
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Are there actually any materials on Earth that are so rare and precious (and perhaps in danger of running out in the foreseeable future) that it would make sense to mine them from space?
By the way, the claim about aluminum sounds highly implausible to me. Aluminum accounts for about 8% of the Earth's crust by weight, and even if most of it is difficult to access, I would expect that more than the amount present on Eros would be extractable with methods much easier than any conceivable sort of asteroid mining.
Helium-3 could be mined from the moon. It would be a good fusion fuel, but it is rare on earth so it makes sense to get it from space.
Now that's interesting! I didn't know that the prospects for helium-3 fusion are allegedly that good. Still, given the previous history of controlled fusion research, I'm inclined to be skeptical. Do you know of any critical references about the present 3He fusion research? All the references I've seen from a casual googling appear to be pretty optmistic about it.
I have no reference, but as far as I understand, deuterium-tritium fusion is easier to achieve than deuterium-helium-3. But deuterium-helium-3 seems cleaner and the energy produced is easier to harvest.
So I think that the first energy producing fusion reactor would be a deuterium-tritium one, and deuterium-helium-3 would come later.
The primary reason that D-T is considered to be more easily viable than others is that it has the best numbers under the Lawson criterion. This is also true under the Triple Product test. While Wikipedia gives a good summary I can't find a better reference that is online (The Wikipedia article gives references including Lawson's original paper but I can't find any of them online). The real advantage of He3 Deuterium fusion is that it is aneutronic, that is it doesn't produce any neutrons. This means that there's much less nasty radiation that will harm the containment vessel and other parts and that much less of the energy will be in difficult to capture forms. This is especially important for magnetic confinement since neutrons lack of charge makes them not confined by electromagnetic fields. This is a non-technical article that discusses a lot of the basic issues including the distinction between fusion types, although they don't go through the level of detail of actually using Lawson's equation.