From: ederd@bcstec.ca.boeing.com (Dani Eder)
Subject: Re: Interstellar travel (magsails, solar sails, whatever)
Date: Jun 19 1995
Newsgroups: sci.space.tech

LNCR02A@prodigy.com (Jackie Reynolds) writes:

>Presumably, before such interstellar distances are traversed, an outpost 
>or mining camp would be established on the moon.  Why not just mine 
>Helium-3 from lunar regolith, stick it in a fusion reactor with deuterium,
> and put the whole reactor on the back of a ship?  Would that not be an 
>efficient engine for just such purposes.


For the same reason you don't normally mine gold in your back yard -
the ore concentration is terrible.  Lunar regolith has a very low
concentration of He, of which a fraction is He3.  Uranus' atmosphere
is 15% helium, about 10,000x higher concentration.  Any mining engineer
will tell you that ore concentration is critical to any mining operation.

The planet Uranus not only has the highest concentration of Helium, 
it is the smallest of the gas giants, thus relatively easy to mine.
Assuming you have a need for He3, you have fusion powered spaceships,
so getting to Uranus shouldn't be such a big deal.  Another good
thing about Uranus is the atmosphere is cold enough that the energy
to liquefy hydrogen is pretty low, leaving you with helium gas.  This
is then liquefied and the He3 separated by superfluid separation.

A nuclear reactor can be used both for a power supply, and later as
propulsion - a nuclear thermal ramjet/rocket can ascend by using the
uranus atmosphere as working gas first, then some of that hydrogen
you liquefied in the previous paragraph and had the good sense to
store in a tank.

Dani Eder

From: "Paul F. Dietz" <dietz@interaccess.com>
Newsgroups: sci.physics.fusion,sci.physics.particle,sci.space.science
Subject: Re: Deuterium
Date: Thu, 29 Apr 1999 21:39:29 -0500

Jonathan A Goff wrote:

> What if some of the He is He3?  In that case, it would only fuse
> to Be-7, but that is stable.  And when that gets hit with an
> alpha particle it breaks a neutron free correct?  Just wondering...

7Be is not stable, it decays by electron capture to 7Li.
This is one of the side chains of the pp cycle in the
sun.

In the sun, the 7Be that doesn't decay in this way
reacts with protons to make 8B, which beta decays
back to 8Be and then 2 4He.  This is an even rarer
side chain of the pp cycle, but one that produces
particularly energetic neutrinos, making it of
interest for solar neutrino detectors.

The rate of the (p,gamma) reaction on 7Be should
be much larger than the (4He,n) or (4He,p) reactions,
since the latter have a much higher Coulomb barrier
and a more massive bombarding particle (reducing
the tunneling rate.)  In the absence of protons,
3He will quickly make some by the 3He + 3He -->
4He + 2p reaction.

About the only place you get a lot 3He building
up will be in the cores of very light stars, where
the p+p and p+d reactions can go but the reaction
of 3He with itself is very slow.  Near the ends of
their lives (which hasn't happened yet, the universe
is too young) these stars can accumulate a lot
of 3He in their cores.

	Paul

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.tech
Subject: Re: Does the Moon have nuclear fuel?
Date: Tue, 20 Jul 1999 00:47:36 GMT

In article <378B2E5E.E189846A@statsbiblioteket.dk>,
Christian Petersen  <cp@statsbiblioteket.dk> wrote:
>With D-T fusion theres also a problem with the energetic neutrons which
>iradiates the reactor core - I think this particular problem is solved with
>D-D fusion...

Alas, not so.  What you get out of D+D is not 4He, but either T+H or
3He+n.  Either way you get to 4He with the next step, but either way it's
bad:  the T+H branch puts out neutrons when the T reacts, and the 3He+n
reaction puts out a neutron directly (although at least it is rather less
energetic than the D+T neutrons).  A lower fraction of the total energy
comes out as neutrons, but it's something like 40% as opposed to 80%:
better, but still a huge problem.

In fact, one of the problems with the 3He+D reaction is minimizing the
D+D side reaction.  Depending on reactor design, that might not be too
hard to do, by keeping the reaction 3He-rich and D-poor.
--
The good old days                   |  Henry Spencer   henry@spsystems.net
weren't.                            |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.history
From: henry@spsystems.net (Henry Spencer)
Subject: Re: Harrison Schmitt's Moon Formation Heresey
Date: Tue, 28 Sep 1999 20:29:19 GMT

In article <vR7I3.502$d92.115849@typhoon1.gnilink.net>,
Robert Pearlman <robert@collectspace.com> wrote:
>Apparently, MRI's are incapable of imaging the human lung -- however, if a
>person inhales a bit of He3, the MRI can not only image the lung, but the
>result is a much more enhanced view than anything we have seen before.

Indeed a useful application... but this is the sort of thing where it's
all too likely that alternate methods can be developed if you try hard
enough.  MRI machines are improving, and techniques are progressing, at a
remarkable pace.

>The sample used to discover this application of He3 was obtained from
>Russia, as a by-product from one of their nuclear projects. The US refuses
>to sell its by-product He3.

Note, though, that this gives the wrong impression:  the US won't sell it,
but will *loan* it.  3He does have research uses in a few areas, notably
cryogenics.  Very little of it gets *used up*, because it's chemically
inert and people recycle it very carefully.

(By the way, the Russians almost certainly get it from the same place the
US gets it:  decay of tritium used in nuclear bombs.)

>If this medical application is to become popular we will need a much greater
>source of He3 -- and I wonder where that could come from?

Existing supplies of 3He might well be adequate for this, given careful
recycling.  As noted above, the stuff doesn't get used up; nothing short
of nuclear processes will actually consume it, so it's just a matter of
controlling leakage and processing exhaust flows thoroughly.  The loss
rate would not be large.

(An analogy here is medical uses of radium.  There is *no* current source
of radium; nobody is refining more.  But it's long-lived, and it gets
recovered very carefully because it's dangerously radioactive, so existing
supplies are perfectly adequate.)

If more 3He has to be made, making tritium in reactors or accelerators is
cheaper than mining the Moon, for small quantities.  Only if you start
*using it up* in bulk does the Moon become an attractive source.
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)

From: henry@spsystems.net (Henry Spencer)
Newsgroups: sci.space.science
Subject: Re: Magnetic Field Speculation
Date: Wed, 16 Feb 2000 20:54:31 GMT

In article <88e352$3ju$1@nnrp1.deja.com>,  <cray74@hotmail.com> wrote:
>Good point. Do you think a lunar atmosphere even as thin as
>Mars' would significantly impact helium-3 retention?

Undoubtedly.  There's plenty (relatively speaking) of 3He on the Moon only
because the solar wind hits the surface more or less undecelerated, so the
helium nuclei (which promptly pick up electrons and become atoms) embed
themselves in the regolith.  Hitting even a thin atmosphere will stop
them, and they'll wander back out into space again relatively promptly.
--
The space program reminds me        |  Henry Spencer   henry@spsystems.net
of a government agency.  -Jim Baen  |      (aka henry@zoo.toronto.edu)

Newsgroups: sci.space.policy
From: henry@spsystems.net (Henry Spencer)
Subject: Re: He3
Date: Tue, 19 Sep 2000 04:17:32 GMT

In article <39C66772.1F7348F2@hotmail.com>,
Anthony Roberts  <anthonyr@hotmail.com> wrote:
>I saw a web page about compact He3 refrigeration unit...
>I have no idea how they got the stuff, but it doesn't react with
>anything, and escapes into space pretty easily... The only thing I can
>think of is Tritium decay.

Exactly.  There is essentially no natural 3He on Earth.  The trace of
helium in Earth's atmosphere, and the somewhat larger amounts in some
natural-gas fields, ultimately derive from alpha particles released by
radioactive decay within the planet; that process produces only 4He.
Helium is light enough that it's easily lost from Earth's atmosphere, and
it does not combine with anything to stabilize it, so any primordial 3He
is long gone, and there's no significant natural source on Earth.

The one source of 3He is transmutation. :-)  Tritium is made both
deliberately and unintentionally in nuclear reactors, it's retained
because it's useful and somewhat dangerous, and its decay yields 3He.

>Of course, the the US has had a large stockpile of warheads with Tritium
>in them for some time... At least 2-3 half lives.

And nuclear-warhead tritium needs to be purified regularly, because 3He is
a neutron poison.
--
Microsoft shouldn't be broken up.       |  Henry Spencer   henry@spsystems.net
It should be shut down.  -- Phil Agre   |      (aka henry@zoo.toronto.edu)