Steelmanning Helium-3

The little isotope packs a punch...maybe an important one?

Helium-3 is often tacked on at the end of lists of potential space resources in a half-serious sort of way. In more respectable company it’s looked at with derision, perhaps best summarized in “The helium-3 incantation” by Dwayne Day in The Space Review. The main arguments in that piece (and in other forums) include:

• Lack of demand. There are no operational fusion reactors that have achieved an energy gain factor of Q=1, let alone Q~20 needed for commercial viability. Most current reactors are exploring deuterium-tritium (D-T) reactions, not He-3, and medical or other uses can be satisfied by scaling up He-3 production across national labs.

• The extraction tech stack is difficult, and even basic prototypes have not been demonstrated. As usual with space resources, most researchers tend to gravitate toward the easy parts around the periphery of the problem instead of tackling the more difficult core. With He-3, the easy part is liberating it from the regolith. The harder part is separating parts per billion He-3 from the He-4 and all the other gases that have been implanted by solar wind and will be liberated with it. I’ve seen no end-to-end business case involving the hardware it will take to do this.

• There’s not actually that much He-3 on the Moon. In the movie “Moon”, the fictional Lunar Industries LTD is producing 70% of Earth’s energy needs from lunar-mined helium. Some quick back of the envelope math using He concentrations from Apollo shows that we would have to harvest vast areas of the mare regions to supply current energy demands, let alone future ones. The optics of large strip-mining operations on the Earth-facing side are not pretty, and He-3 is a non-renewable resource.

If some combination of these is the strongest case against helium-3, what’s the strongest case for it? Here’s my attempt at laying out a steelman case for this special isotope, then poking at those arguments. In summarized form:

Fusion is inevitable and we should aim to progress from D-T to aneutronic fusion; He-3 is involved in many aneutronic reactions and the respectable supply on the Moon provides a way to develop the learning curve before we can mine it from the ice giants.

Breaking this down in more detail:

1. Fusion is probably the best energy source of the future, and it’s closer than most people think. Fusion offers a clean, safe supply with an energy density an order of magnitude above fission, eclipsed only by antimatter. We should be taking fusion more seriously and many folks are starting to: since Day’s article in 2015 there’s been significant action in the commercial sector, with Commonwealth Fusion raising $1.8B and Helion raising $2.2B. Ignition has likely been achieved already, and practical breakeven (in the commercial sense) may be years rather than decades away.

To counter this, it can still be argued that He-3 won’t be used in first-gen and possibly second-gen commercial reactors. For the small number of researchers working in space resources, it’s a more valuable use of their time to work on ice- and regolith-based resources rather than He-3.

2. He-3 reactions are a promising source of low neutron or aneutronic fusion. While harder to achieve than D-T fusion, aneutronic fusion offers significant benefits and we’ll definitely want to upgrade from D-T to aneutronic fusion when the right advances in plasma physics and materials have been made. 3He-6Li and 3He-3He both meet the definition of aneutronic; D-3He does not, but still puts out a very low fraction of neutrons. Aside from technical difficulties, low availability of He-3 is seen as a barrier to pursuing these fuel mixes, and a lunar supply would help overcome this barrier to improve fusion’s potential.

Midjourney.

As a counterargument, there are other aneutronic reactions that have much easier-to-source fuels like Proton-Boron-11. Even if He-3 fuels offer some benefit over p-11B, improvements at the margin may not justify the effort involved in pursuing He-3 fusion as a separate line of development.  

3. There are much richer sources of helium-3 in the outer Solar System, and we can use lunar He-3 which is more abundant than previously thought to work out the learning curve before transitioning to these more plentiful supplies. In this view, the overall amount of He-3 on the Moon isn’t all that important, as long as it’s enough to figure out how to use it as a fuel. And recent reports from the Chang’E-5 returned samples suggest there are bubbles rich in He within the rims of ilmenite grains. I’d like to see independent work on Apollo samples to confirm. But if taken at face value, the higher concentrations reported in this work mean we wouldn’t have to devastate the nearside to find reasonable quantities of He3. It also suggests a path to beneficiation that will make extraction easier.

Bryan Palaszewski at NASA Glenn seems to be the only person taking the mechanics of ice-giant-atmosphere-mining seriously. Assuming some semblance of his architecture doesn’t violate basic physics, then aerostats in the atmosphere of Uranus or Neptune could be used to slurp up helium-3 and other isotopic goodies. If this can be realized long before the lunar supply is used up, then the Moon is simply a way to bootstrap an expansion to the outer reaches of the Solar System.

Midjourney.

On the flipside, it’s not clear how much He-3 we’d need to launch a development effort and build reactors that use it as a fuel. Maybe not that much? The US keeps a stockpile of 25 kg; more can be made essentially on demand. And in reaction chains like p–6Li → 3He–6Li → 3He–3He the helium is not needed as an initial product but is formed as an intermediary. So perhaps we can still achieve aneutronic fusion using 3He fuels without digging up a single shovelful of lunar regolith.