Helium-3 is often seen as a profitable material to “mine” the Lunar regolith for – it’s a potential fusion fuel, but currently a fuel without a market. No current reactors in the works (i.e. ITER) are big enough (!) to burn the stuff, and no 2nd and 3rd Generation Fusion reactors are likely before c.2100, at current pace of development.
BUT let’s assume there was a market – He-3 burns quite well in IEC reactors only a bit bigger than D-T burning IECs, so once Doc Bussard’s Whiffle-Ball is demonstrated a market might appear over-night (5-10 years.) If so, how much is on the Moon? According to this reference there’s 2.5 million tons embedded in the upper layer of Lunar regolith (typically 4-12 metres deep, depending on locale.) Sounds like plenty, but you have to process a lot of moon-dust to get at it – there’s 38 million sq. kilometres of Moon and so just ~ 66 kg He-3 per sq.km, some ~ 8 million cubic metres of regolith to process for just that.
How much is 66 kg of He-3 worth then? Fusing He-3 generates ~ 57 million kW.hr of energy of which about 60-80% can be electricity with the right converters. Call it ~ 60% and 66 kg of He-3 is 2.266 billion kW.hr of power – about $113 million @ $0.05/kW.hr. Using current technology this would be unprofitable, but a few things could be done to improve the economics. For example, Jerome Pearson’s Lunar Beanstalk would eliminate the need for rockets to launch material to Earth, and could deliver ~ 200kg per trip to the Earth-Moon L1 point to be retrieved by low-thrust inter-orbital vehicles for return to LEO. Mining would have to be fully automated, of course, and processing millions of cubic metres of soil per year would be required, but this might not be onerous.
Eventually the supply will run-out. Globally we currently use ~ 15 TW of power, with growth steadily heading up, even with efficiency gains. If everyone used energy like an American or Australian (11 kW/capita) then currently 74 TW would be needed. That’s 74 billion kW.hr per hour, some 649 trillion per year. Some 286,000 tons of He-3 per year. The Moon would be exhausted in a decade. That’s a rather unlikely rate of use, but it does show the Moon’s resource potential is very limited. Within ~ 100 years we would be looking further afield. So where next?
Bryan Palaszewski’s 2006 study for NASA (available via the Glenn Technical Reports Server) looks at the options for mining the Gas Giants. For Uranus and Neptune, which have quieter atmospheres, should be accessible to balloon-borne factories, like the Daedalus report advocates – Bryan actually uses that design for analysis. On Jupiter and Saturn, with greater turbulence, actual aircraft will be needed. What does seem problematic is getting the stuff into orbit as that requires sustained hypersonic flight by the vehicles, something yet to be achieved reliably.
All that could change, at least on Earth, as Alan Bond’s Reaction Engines Limited advocates a hybrid SSTO called SKYLON, and a non-orbital version for hypersonic passenger flight. If SKYLON were developed successfully an immense amount of hypersonic experience would be gained, ultimately allowing mining of the Gas Giants. SKYLON would also enable other power-sources, like SPS, so it’s worth pursuing by itself.