Lunar Helium-3

What is helium-3 and why is it important? Well, it's an isotope of helium, containing one neutron instead of two, and serves as one of the fuels for the following nuclear fusion reaction:

The Deuterium - Helium-3 Reaction:

Current fusion reactors use the D-T reaction (deuterium-tritium) which yields 17.6MeV. The D-He3 reaction is better, not only because it yields more energy ( it has the highest energy/mass ratio of any known substance apart from antimatter), but also because it produces virtually no neutrons (which can damage the reactor and slowly generate radioactivity in the surrounding metal). That's the good news. The bad news is that helium-3 does not exist on the Earth, except at the core-mantle boundary, thus making it inaccessible. There is some on the Moon, but it's wrapped up in the lunar regolith in quantities of about four parts per billion, implanted there by the solar wind over millions of years.

To give you an idea of what this means, if you could excavate a square kilometre of the lunar surface down to a depth of 10cm you would obtain roughly 250,000kg of raw material. After processing (boiling out the helium-3 and other volatiles, isotope separation etc.) you would be left with 1kg of helium-3. That's an awful lot of work for a single kilogram (although you could also extract other solar-wind-implanted elements from your 250,000kg of lunar regolith, such as nitrogen, hydrogen and carbon).

Is it worth it? In his book Entering Space, Robert Zubrin estimates that a fusion reactor using D-He3 could produce 100 million kilowatt-hours (kWh) of electricity per kilogram of helium-3 burnt. This would make our kilogram of helium-3 worth roughly $6 million, 400 times more valuable than gold. Of course the actual sale price would be greatly reduced by the cost of running such an operation, not to mention the huge investment required to transport all the equipment to the Moon and get it up and running. Even so, the He3 would still be extremely valuable.

Realistically, mining the lunar surface for helium-3 isn't viable now, and it won't be for a long time. However, as terrestrial launch costs are reduced and people return to the Moon for other reasons, the potential of helium-3 will be hard to ignore. And with the Earth's fossil fuel resource predicted to dry up within the next century, we might not have a choice anyway. Fortunately the Moon has a surface area roughly the size of Africa, so even in such minuscule concentrations as 4 ppb, there's enough helium-3 locked up in the lunar regolith to power the human race for maybe up to a thousand years; a very long time indeed...

For more information about lunar helium-3, refer to this page, courtesy of the Artemis Society.

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