Re: Proposed sample return mission to Phobos

On Feb 17, 4:49 am, "Paul F. Dietz" <d...@xxxxxxx> wrote:
Michael Turner wrote:
"The added advantage of the microwave thruster is the potential
duality of the system. In 1985, Dr. T. Meek of the Los Alamos
National Laboratory (LANL) demonstrated the ability to extract water
from simulated lunar soil using 2.45 GHz microwaves [12]. Microwaves
readily couple to the water trapped in soils. According to Meek,
using conventional means to heat the soil would require 10,000 times
more energy than using the microwaves to selectively heat the water in
the soil.

First, I didn't write that, the authors of the paper did.

This is absurd. Microwaving lunar soil will not selectively
heat water.

No, but it will heat the water even as it heats other things in it.
Water is 10-20%, presumably, of Phobos. What's the fraction of other
compounds subject to heating? Is it significant?

Microwaves at that frequency will predominantly interact
with lunar soil by heating extremely small metal inclusions.

Is that better or worse for getting water out? Also consider context:
whatever Meek said, he was talking about lunar soil, not Phobos
regolith, which is assumed to be water-rich for purposes of this
discussion.

What water there is in lunar soil is trapped in minerals, not
in macroscopic droplets, so any heat deposited into it would
cause bulk heating anyway.

Presumably with some loss of efficiency. I'm no big fan of trying to
get water out of lunar soil anyway -- it's very dry. However, if
you're talking about some substance that's 10-20% water, shouldn't the
behavior be different? I.e., you'd get bulk heating, but wouldn't you
get a lot more water release? Maybe rather than fusing into something
like glass, as with lunar soil, it would popcorn somewhat, releasing
water vapor in the process?

Oh, and in case you didn't know, 2.45 GHz microwaves don't
heat water by some resonance effect.

Yes, you're right, it's dipole heating. But whatever -- let's go back
to the question: can you use microwaving to get significant amounts of
water out of carbonaceous chondrites with relatively little energy
input compared to other methods?

The main industrial purpose for extracting water from rocks, AFAIK, is
in drying coal before burning it. They don't use microwave heating to
do that, probably because it's both inefficient (more energy would go
into it than would be saved) and a combustion threat. However, water
is not a precious substance on Earth.

-michael turner




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