It’s not worth doing. There is enough helium 3 to last us for around 200 years if we mined the entire surface of the Moon. But there is no point in harvesting it as a source of energy. . Crawford says (page 25) that to supply all of our energy from Helium 3 would mean mining 5000 square kilometers a year on the Moon, which seems ambitious (and would mean the whole Moon would only last 200 years). So, even if we develop Helium 3 based fusion, and it turns out to be a valuable export, it's probably not going to be a major part of the energy mix.
Even more telling, he also calculates that covering a given area of the Moon with solar panels would generate as much energy in 7 years as you'd get from extracting all the Helium 3 from that region to a depth of three meters. It’s obviously far easier to just make solar panels to cover a square meter of the lunar surface than it is to mine it to a depth of three meters and extract all the Helium 3 and return that to Earth. And the solar panels would continue to produce that power year after year into the future. You could have solar panels on both sides of the Moon for continuous power and beam it back using microwaves collected by a large rectenna in LEO. After 21 years you have already produced three times as much power as you could from the Helium 3 from that same area of the Moon for much less effort, and after fifty years you’ve produced seven times that power.
In addition Helium 3 is a fuel for a technology we don’t have yet. There are ideas for Helium 3 power plants but no such plant exists yet. The normal methods proposed for fusion power would not work with Helium 3 which requires much higher temperatures. And we don’t have fusion power at all yet. And when we do, it might involve something completely different anyway.
Helium 3 does however seem likely to be a useful byproduct of mining and other operations on the Moon. Valuable and low mass, so one of the few materials that might be worth returning to Earth but it doesn’t seem likely that it would be worth mining the Moon just for its Helium 3. That’s especially so since it may be very easy to make solar panels on the Moon. The lunar dust has large amounts of nanophase iron in it which makes it easy to melt it to glass using microwaves, as easy as boiling the same mass of water in a kettle. The high vacuum on the Moon would then make it easy to manufacture solar cells in situ. This suggests you could actually build a “solar panel paving” robot on the Moon.
These are extracts from my book Why Humans on Mars Right Now are Bad for Science.President Obama, if you love science, Please protect Mars life from contamination from Earth
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