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There is a certain attention given to the moon in order to use its helium-3 to supply nuclear fusion on earth.

But there is some problems involving energy there that concerns me. A day on the moon is equivalent to 26 days on Earth. Assuming that helium-3 is actually usable in nuclear fusion reactors, and we build a mining base there, how could we power it?

Well, one would suggest to use the very own helium-3 to power it, but it would be extremely difficult to actually send the parts and build a nuclear fusion on site. We are doing some of those on Earth and it will take decades.

I thought of nuclear batteries, but their energy output is small though constant. And a nuclear fission reactor is a no-no to send in a spacecraft, considering all the risks.

Solar panels are really the only easy option, and circulate the entire satellite with it seems unlikely.

There are also Stirling motors: they work with differences in air temperature. So astronauts could use their own body wasted heat to continuously power it. But the energy generation of said motors is incredibly small. They could use it in the border of craters to use the side exposed to the sun to heat it and the dark side to cool. But this would be meaningless since everything would be in pure darkness.

There is hydrogen in there, they could use hydrogen cells to power it?

Anyway, what I'm saying is that there should be other bodies in the solar system with helium-3 or just as interesting. Maybe Mercury, since it has a tidally locked orbit and the twilight zone could be easily maintain a base.

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  • $\begingroup$ What do you mean by "how we could energise it"? How would we power the mining facility? $\endgroup$
    – AlexP
    Commented Mar 22, 2021 at 14:14
  • $\begingroup$ @AlexP no, this belter abandoned the moon, and thinks elswhere will be better, and asks for the direction where to go $\endgroup$
    – MolbOrg
    Commented Mar 22, 2021 at 14:18
  • $\begingroup$ Related question on Astronomy Stack Exchange: astronomy.stackexchange.com/q/34616/2153 $\endgroup$
    – HDE 226868
    Commented Mar 22, 2021 at 14:40
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    $\begingroup$ "it would be extremely difficult to actually send the parts and build a nuclear fusion on site": Mining lunar helium-3 just to supply the energy needs of the US would require processing billions of tons of regolith a year. As hard as fusion is (even accounting for how much harder helium-3 fusion is), building some fusion reactors on the moon to power it all would not be the hard part. $\endgroup$ Commented Mar 22, 2021 at 16:03
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    $\begingroup$ Refining HE3 on the Moon means having to melt ~16.7 metric tons of sand to get a single gram of HE3. That means investing ~26 GJ per gram of product... you are not going to be producing that kind of heat with a reasonably sized solar reactor. $\endgroup$
    – Nosajimiki
    Commented Mar 22, 2021 at 17:30

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Ash has already answered the main part of your question, but I'd like to point out that your assumptions that have led you to writing off the moon as a source of useful minerals may be incorrect.

there is some problems involving energy there that concerns me. A day on the moon is equivalent to 26 days on Earth.

The lunar terminator moves at 10 miles an hour. If you had a mobile stripmining facility and a sensible route planned out, you could stay in the sunlight. There are also multiple "peaks of eternal light" near the poles where solar power could be maintained continuously.

Well, one would suggest to use the very own helium-3 to power it, but it would be extremely difficult to actually send the parts and build a nuclear fusion on site. We are doing some of those on Earth and it will take decades.

Not a whole lot harder than building an entire off-planet strip mine, refinery and interplanetary shipping facility, I suspect!

If you can't already throw together fusion reactors with ease, you've got no business trying to get into mining the moon for fusion fuel. We've got deuterium and lithium and boron on Earth, and we can make tritium if we want. Helium 3 is useful stuff, to be sure, but it isn't the be-all and end-all of fusion power. Remember that it is noticably harder to ignite D-3He fusion than it is to ignite D-T fusion, cos that extra proton makes the coulomb barrier that little bit harder to break through.

And a nuclear fission reactor is a no-no to send in a spacecraft, considering all the risks.

The risks are overblown. Fuel rods that have never been put in a reactor and used are very low radiation... vastly lower than the 25kg of extremely radioactive plutonium we just sent to Mars in Perseverance.

If civilisation is at the point where big off-planet industrial facilities are a thing that can be built, safely lifting well packaged uranium fuel rods or pellets out of Earth's gravity well should be possible. And who knows, maybe you could mine thorium up there whilst you're extracting 3He, and not fly any fissile materials out of Earth at all?

Solar panels are really the only easy option, and circulate the entire satellite with it seems unlikely.

Why not use orbital powersats? Gather the sunlight in space, beam power down as microwaves, problem solved.

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    $\begingroup$ "and we can make tritium if we want": the same is true for helium-3, since that's what tritium decays into. Half of any tritium stockpile will have transformed into helium-3 in 12.3 years. If you can produce 6-7 t of tritium per year to replenish the stockpile, that's equivalent to mining and processing a couple billion tons of lunar regolith. $\endgroup$ Commented Mar 22, 2021 at 17:15
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    $\begingroup$ @ChristopherJamesHuff that's a good point, though tritium breeding is practical for power generation but the long lead time means that 3He breeding isn't quite so convenient. Also, D-T fusion gives you lots of nice fast neutrons which are good for tritium breeding from lithium fission, but D-3He does not, which slightly limits the practicality of breeding it for fuel. $\endgroup$ Commented Mar 22, 2021 at 17:32
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    $\begingroup$ certainly, you'd breed the tritium with fission or D-T fusion reactors, the D-3He reactors would only consume it. It raises the question of why you want D-3He fusion so bad, but if you do, a bunch of D-T reactors on the moon (not that it's a good place to put them, just for the sake of comparison) are more effective at getting it than mining the moon. They'd also be a useful source of other radioisotopes, like those needed for RTGs. $\endgroup$ Commented Mar 22, 2021 at 17:52
  • $\begingroup$ @ChristopherJamesHuff if you could breed it in sufficient quantities, it would make for a nice rocket fuel. $\endgroup$ Commented Mar 22, 2021 at 18:06
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    $\begingroup$ If you can fuse it at a high enough rate, that being one of the major problems with it as a fusion fuel. p-B11 might be a better bet, and doesn't require heroic efforts to synthesize the fuels required. $\endgroup$ Commented Mar 22, 2021 at 18:21
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The best stocks of Helium 3 in the solar system, after the moon, is Earth, but the answer you're probably looking for is Uranus.

(I'm excluding the sun because it's way too hard to mine)

7% of some Natural Gas deposits have helium 3 in them, there's up to ~40kg of it in the USA natural gas stockpile right now. There's an estimate of 100,000 - 1,000,000 tonnes of it in the mantle of Earth. Volcanos erupt it, its dissolved in our oceans, and a trace gas in the atmosphere.

3rd best is Jupiter, which has ~100 ppm helium 3. But that's quite hard to mine due it's 60km/s gravity well.

NASA has a report for gas giants helium 3 mining Saturn, Uranus, and Neptune all have some in the range of several parts-per-million. Uranus is probably the easiest to mine. That report linked goes into details of a proposed Uranus helium 3 mining operation.

The way NASA is thinking of doing it is to have an orbital station with a refinery, aerostats (balloons) with scoopers that capture gas and filter it as best they can, an automated ship comes down, replaces the gas reservoir, and takes it the full one up to the orbital station for processing into fuel.

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To begin with, you use batteries. Mining & processing - the major power draw - takes place during the day, and the batteries are charged. During the night, only the life support & maintenance functions are performed.

As your enterprise expands, and you collect some copper or aluminum as byproducts of your mining, you site solar panels further east & west of your mining site, which allows you to extend your mining time. Eventually you have a power grid circling the moon, and can operate continuously.

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The sun has far more helium-3 than any other body in the solar system. But it’s a bit hard to get at it, especially since it’s mostly in the solar core.

To expand, the sun is 8% helium. Helium-3 abundance in the local interstellar cloud is about 2.5 parts per 10,000, and that’s a reasonable estimate for what’s in the sun — the solar wind has rather more, about 4 parts per 10,000. The mass of the sun is 2E30kg, so it contains about 1.6E29kg of helium, of which 4E25kg is helium-3. For comparison, the total mass of Jupiter is 2E27kg, so it would have to be at least 2% helium-3 to have more than the sun, which it obviously isn’t.

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    $\begingroup$ Perhpas you can expand on this? $\endgroup$
    – rek
    Commented Mar 22, 2021 at 16:09
  • $\begingroup$ @rek I have done so. $\endgroup$
    – Mike Scott
    Commented Mar 22, 2021 at 16:25
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    $\begingroup$ Whilst this answer is facetious, it isn't wrong. I don't think it really deserves quite that many downvotes. $\endgroup$ Commented Mar 22, 2021 at 18:08
  • $\begingroup$ @MikeScott you may find that there's a fairly steady rate of production and consumption of 3He in the Sun. The total amount available at any given time might be rather less than you've suggested; it should fuse readily at core temperatures and pressures. $\endgroup$ Commented Mar 22, 2021 at 18:10
  • $\begingroup$ @StarfishPrime, check the original version $\endgroup$
    – L.Dutch
    Commented Mar 22, 2021 at 20:37

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