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.
3 responses to “Mining the Gas Giants”
Excellent article. Much of what I’ve read about mining He-3 on the moon makes it out to be the new “gold rush” and this article is a helpful reality check. Other estimates of how long lunar He-3 would last for terrestial use ranges from 43 to 1,00 years. So a decade’s worth is a suprisingly low estimate.
Do you see the initial mining of He-3 as being economically profitable? We don’t have any econimically viable fusion reactors at present as so the market now is limited. But might a relatively small amount (let’s say a ton) still be very valuable in terms of what people would pay for research and development of 2nd generation fusion reactors?
And another question, why do the Chinese and Russians talk about their desire to mine He-3 if there is not a market for it? I for one would think that it would be healthy to have a new space race to develop the moon. Sort of like how San Francisco really developed as a result of the gold rush.
Do you have any numbers on what % of the atmospheres of Neptune and Uranus are He-3? Would the balloon-borne factories have to separate He-3 from He-4 before sending it back? And would this be a difficult process? Would the hypersonic vehicles be needed on all four of those planets or only Jupiter and Saturn?
Your questions are answered mostly by the reference I’ve sourced my data from, so have a look. But to save you the time I’ll try to answer.
Firstly, He-3 is a tiny fraction of the regular helium present in Gas Giant atmospheres – typically 22% by mass of atmosphere, similar to the Sun. But because the mass of helium is so vast that doesn’t limit the supply very much.
Second, separating the two will be needed, or else we’re wasting a lot of energy to lift regular helium-4 from the Gas Giants. The abundance according to Wikipedia is ~0.000137% which means 730 tons of regular He-4 has to be space-lifted for every kg of He-3 if we don’t. Better to do it insitu if we can.
Third, difficulty is yet to be explored in detail as we’re not used to handling so much helium here on Earth. The two isotopes have different fluid properties so some sort of separation will be possible at ultra-cold temperatures based on that. Fortunately the proposed factory sites (the 0.1 bar level) are cold already, so there’s a decent heat-sink available.
Fourth, hypersonics will be needed on all the gas giants to deliver the processed He-3 to Earth-return vehicles in orbit. Uranus has the lowest orbital speed – about 12 km/s – but even that is much tougher for reusable vehicles than Earth’s mere 9 km/s re-entry speed. The more experience we gain here, the better things will be there. Hydrogen/helium gas mixtures are more forgiving than N2/O2 so perhaps it will prove an easier task than Earth than initial expectations indicate.
Another thought. Using deuterium+He3 is more energetic than straight He3, but makes occasional neutrons via D+D reactions. If that can be handled then the He-3 content on Neptune is about 7.3 trillion terawatt-years of energy equivalent. Similarly on Uranus, even more so on Jupiter/Saturn. But at 2.6% energy usage growth it’ll all be gone in less than 1200 years.