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Richard of Chelmsford
2004-Jul-18, 08:50 PM
I'm sure this has cropped up before but I can't find much about it.

Could we go back to the Moon to collect helium?

At first sight it might seem a bit stupid to spend all that money returning to old Luna in a very expensive spaceship just to fill up some kids' balloons at a fair (that's just a joke :D ).

Helium is of course quite rare on the Earth, though not in space, so we have to collect what we can. We need it in the supercool areas of science of course and we might need it more and more as time goes on.

Now, Old Sol has been blasting Luna with helium for time immemorial so moon dust could be soaked with the stuff. (What would the chemical or physical description be?)

Shall we go and get some?

Just as a vague aside I was thinking about rare minerals the other day. I read somewhere that they've estimated francium to be the rarest element on Earth. Just 17 or so atoms on the entire planet.

Though I've read other things to counteract this.

We need rare minerals just as we need plentiful ones.

So what about the Helium??

DoktorGreg
2004-Jul-18, 09:21 PM
http://www.space.com/scienceastronomy/helium3_000630.html

The article in a nut shell, says that H3 is the near perfect fuel, but for its rarity and our inability to make an over unity fusion reaction with existing technology. But its a long way off.

Gsquare
2004-Jul-18, 09:25 PM
Could we go back to the Moon to collect helium?...
... We need it in the supercool areas of science of course and we might need it more and more as time goes on.



I say we go for Tritium....much more needed here on earth.. :D :lol: :wink:

G^2

Richard of Chelmsford
2004-Jul-19, 08:47 AM
http://www.space.com/scienceastronomy/helium3_000630.html

The article in a nut shell, says that H3 is the near perfect fuel, but for its rarity and our inability to make an over unity fusion reaction with existing technology. But its a long way off.

Better for the time being that we go for it than that we go to Mars?

Another funniosity about liquid helium is that if you stir it it won't stop slopping around, will it?

TinFoilHat
2004-Jul-19, 01:06 PM
I have read an estimate that you need to process 200 million tons of lunar soil to retreive 1 ton of He3. This seems like a lot, but the energy content of the He3 is so great that it more than pays for itself.

Assuming, of course, that we can build a reactor to fuse He3, and extract enough useable energy in the process to be commercially viable. Right now we can't even make commercially viable deteurium-tritium reactors, and those are easier to fuse than He3.

It's a pity that, other than He3, there's really nothing on the moon to mine that we can't get easier on Earth. What valuable materials the moon has plenty of (aluminum, titanium, silicon) are also easy to find on Earth.

I have also ready plans for harnessing He3 from the outer gas giants. It's a much longer trip, but seperating He3 from already gaseous feed is easier than working with lunar soil, and the total available amounts are much greater.

wedgebert
2004-Jul-19, 02:08 PM
Twenty five tons of He3 would be enough to power the United States for a year. As of a few years ago, that gave He3 a value of 3 trillion dollars per ton.

While the other minerals on Luna are easier to get to on Earth, it's easier to get them into space from Luna. Don't forget that the lunar regolith is over 40% oxygen by mass. Oxygen is a major source of mass for current rockets and the shuttle. Reducing the O2 required to leave Earth to the bare minimum and resupplying with lunar oxygen could go a long way to reducing spaceflight costs.

Besides, if you have to process all that regolith to get the He3, you might as well extract every useful thing from it at the same time.

daver
2004-Jul-19, 06:00 PM
He3 is of course useless until we get commerically practical fusion.

Even after we get commercially practical fusion, there are other reactions--D+D fusion produces He3, H3, H, and neutrons. Presumably a breeder plant could be built which would have a lithium layer to absorb the neutrons and which would harvest the He3 and H3 for use in other aneutronic fusion plants. So it's not all that clear that lunar He3 would be particularly economical.

Lunar oxygen isn't going to help with the cost of getting stuff to orbit. Once in orbit, maybe it would do some good, but let's look at the lunar oxygen journey:

1. Lunar rock is harvested and heated or electolyzed to extract the oxygen (and other stuff).

2. The oxygen is liquified and put in tanks. The tanks are put in a reentry shell.

3. The tanks are launched by the lunar catapult into an orbit that intersects earth's atmosphere. This takes about 1.1 km/sec delta V.

4. The tanks aerobrake, shedding most of their velocity. The heat shield is presumably ejected at this point.

5. The tanks circularize their orbit and rendezvous with the space station.

So, in order to get oxygen from the moon, we've needed to establish a lunar base, establish lunar mining, build a lunar catapult, build an He3/oxygen/titanium/aluminum manufacturing plant, build a plant capable of manufacturing oxygen tanks, reentry shells, circularization and rendezvous rockets, possibly build plants to manufacture the electronics for the above (although some small stuff could presumably be shipped from earth). We also need power supplies and repair and refurbish equipment for all the above.

My guess is that this would require shipping hundreds of thousands of tons of stuff to the moon before we get our first bit of lunar oxygen. Now, once established it gets cheaper, and some of this can be leveraged from the He3 mining equipment, but by the time we have the technology to ship hundreds of thousands of tons to the moon my guess is that the economic incentive for lunar oxygen will have evaporated.

Ilya
2004-Jul-20, 03:12 AM
D-D fusion is easier to achieve and produces more energy than He-3 fusion. The main advantage of He-3 is that it produces no radiation. Neither type produces raioactive waste - the bugaboo of nuclear fission, - but any deuterium reactor will eventually become "hot" and have to be disposed of. Helium-3 will be only economical if the cost of entombing/burying spent deuterium reactors exceeds the literally astronomical cost of mining helium-3. Which I am not at all sure it will.

The cost of mining deuterium from Earth's oceans is negligible in the above equation.

Gullible Jones
2004-Jul-20, 04:03 AM
You got it backwards. Deuterium fusion emits only gamma rays, while He3 or He3-deuterium fusion emits protons. Deuterium fusion, on the other hand, is harder to start than He3 fusion, IIRC.

dvb
2004-Jul-20, 04:27 AM
Could mining all these materials and transporting them to earth affect the moons orbit in any way? Since the earth would be gaining mass, and the moon would be losing mass. Or is the moon too small for there to really be any effect?

ZaphodBeeblebrox
2004-Jul-20, 05:15 AM
Could mining all these materials and transporting them to earth affect the moons orbit in any way? Since the earth would be gaining mass, and the moon would be losing mass. Or is the moon too small for there to really be any effect?

More like, The Moon is Too Big.

Any Mass Exchanged, would be Pretty Much Neligable.

Philistine
2004-Jul-20, 07:32 AM
The great advantage of fusing He3 is the fact that the only particles produced are alpha particles and protons, both of which can be steered by magnetic fields. This means that they can be channeled in one direction and used for thrust in a fusion rocket. Most other fusion fuels produce neutrons, which cannot be manipulated.

An earthbound reactor has no mass restrictions and only needs to produce a lot of heat to be harnessed for power. It can do this by absorbing neutrons in a water jacket. Deuterium would work very well this way, can be extracted from water on earth, and fuses more easily than Helium-3. He3 therefore has no great benefit for power.

The water in the water jacket would become radioactive over time. Why? Because it's forming Tritium, also a fusion fuel! The jacket would form Deuterium as well.

If we ever had a compelling use for He3, we might be better off extracting it from near-earth asteroids. The greater distance would be inconvenient, but the energy required for transport would be far, far less.

TinFoilHat
2004-Jul-20, 01:30 PM
You got it backwards. Deuterium fusion emits only gamma rays, while He3 or He3-deuterium fusion emits protons. Deuterium fusion, on the other hand, is harder to start than He3 fusion, IIRC.

Deuterium can fuse in two different ways:

D + D -> T + p
D + D -> He3 + n

In a fusion reactor, both will occur, with equal probability. (There are actually some much lower-probablility reactions, such as D + D -> He4, but the reaction will be dominated by the above two.) The Tritium and He3 produced will also likely go on to fuse with the deuterium in the following reactions:

D + T -> He4 + n
D + He3 -> He4 + p

So the ultimate product of deuterium fusion is He4, protons, and neutrons. Over time the neutrons irradiate the core structure and make it all radioactive.

Helium-3 fuses in the following reaction:

He3 + He3 -> He4 + 2 p

Thus produsing only He4 and protons - no neutrons are released from the reaction, and none of the end products are radioactive or likely to fuse further with each other.

The D + T -> He4 + n reaction is the easiest fusion reaction to trigger. It is also one of the dirtiest as far as neutron emission. He3 fusion is cleaner but harder to achieve, taking much higher temperatures.

The following pages make for interesting reading:

http://home.earthlink.net/~jimlux/nuc/reactions.htm
http://www.faqs.org/faqs/fusion-faq/section1-physics/

wedgebert
2004-Jul-20, 02:05 PM
The biggest problem with He3 + D fusion is that it requires more energy than straight D + D fusion.

The downside isn't the energy cost however, it's the fact that occasinally you'll still get a D+D reaction, so a He3 reactor would still need some radiation shielding and would eventually become radioactive itself.

It would just take a lot longer because it produces much much less radiation.

Long March
2004-Jul-22, 02:20 PM
About this time last year, part of my dissertation work included translating an article by Ouyang Ziyuan from the Chinese Academy of Sciences, and he certainly thought it was possible to harvest helium 3.

It wasn't a very sophisticated article (or else I wouldn't have been able to understand it) - aimed at increasing the masses' interest and enthusiasm for Chinese space projects, rather than informing people with a scientific background - and he went into absolutely no specifics about how the helium 3 was going to be obtained (plainly, you can't just go outside your lunar lander with with a bucket!). In fact, if Mr. Ouyang is to believed, the Moon is a treasure house filled with all kinds of exciting and useful things - it's just a question of getting there and figuring out how to harvest them.

I hope he was right. It's certainly an exciting thought.

DoktorGreg
2004-Jul-22, 02:28 PM
The biggest problem with He3 + D fusion is that it requires more energy than straight D + D fusion.

The downside isn't the energy cost however, it's the fact that occasinally you'll still get a D+D reaction, so a He3 reactor would still need some radiation shielding and would eventually become radioactive itself.

It would just take a lot longer because it produces much much less radiation.

It seems like (I am not a physicist) the engineers of the reactor would simply select a material that would make less dangerous isotopes.