[ExI] The Moon's Cold Embrace

Keith Henson hkeithhenson at gmail.com
Wed Aug 12 18:56:24 UTC 2020

Rafal Smigrodzki <rafal.smigrodzki at gmail.com> wrote:

### Greenhouses on the Moon would need relatively small openings at the
top, with diffusers and UV filters, to provide for plant growth.

I think we are stuck with photosynthetic efficiency of around 1%
Humans burn around 100 watts, by the time you have run some of the
plants through animals, 25 kW of light on the plants seems like a good
first cut.  That makes your greenhouse opening around 25 square meters
per person.  An optimal mix of red and blue LED light is more
efficient, but you have to factor in power generation efficiency.

should be easy. Of course you would also need energy storage for the lunar

And how are you going to do that?  Import power walls?

### but the area of solar cells needed to harvest this energy (at 1.36
kW/sq.m) would not be insanely large.

5 x for typical solar cell efficiency, times 3 x for storage and
losses, so it is a considerable area.

### Yes, maintaining greenhouse
illumination at night would warm up the greenhouse but active radiative
cooling at night would be pretty effective.

It would be.  But getting rid of waste heat is more a daytime problem.
Not hard to do, costly in materials.

### Being surrounded by a large mass and having access to practically unlimited
amounts of mass to construct a settlement on the Moon is a big advantage
over orbital habitats.

For humans and the industrial process we use, it's a seriously wrong
selection of elements.  Some students were once given a stack of
high-temperature fire bricks as regolith simulant.  No carbon, no
chlorine, no lithium.  The volatiles were all cooked out.  The lunar
poles, which humans have not yet sampled, might have a better mix
including water

### If you need to radiate heat, you just build a bigger
radiator farm on the surface and run it all night long.

Storing cold for two weeks takes lots and lots of mass.  And not just
mass, but fluid to run through it and you don't want leaks.

### It's much more
difficult to build large radiators if you need to launch the building
materials and machines from somewhere far away.

> Free space O"Neill type colonies are a lot less difficult with respect
> to energy (light) for plants and heat sinks.

### How so? The solar constant is the same on the Moon and in orbit.

For most of the moon, the solar constant is zero for two weeks as a time.

### Heat sinks are heavy, brittle,

Why should a heat sink be brittle?  I have been involved with design
studies since 1977 and that was never a consideration.  The most
recent ones for power satellites are around a kg/kW.  They are tapered
plastic tubes filled with low pressure condensing steam at 20 deg C
and 2.4 kPa.  They depend on zero-g and would not work on the lunar

### it should be easy to plonk down on the lunar
ground rather than lug it all in space.

### OTOH, a lunar beanstalk is feasible using existing materials, so delivery
from the Moon would be actually pretty cheap.

It's worth looking at the minimum startup mass for a lunar elevator if
you have not done so.  It's daunting.  Using climbers, the throughput
is just awful.  A loop moving cable does better, but even at 2000
km/hr, it takes 25 hours to get out to L1.  Of course, mass drivers
are as bad or worse.

### I don't know of any major showstoppers to either lunar settlements or
habitats but I think at least at small scale the Moon would be easier.

### Maybe you could mention the problems that don't have recognized solutions.

The worst problem is why?  Why are people needed on the moon at all?
None of this is impossible, just expensive.  ISS costs $100 B.  A
minimal 100 person moon colony might cost one or two trillion.

The next worse problem is radiation.  There are solutions, but it is
not obvious that people can live underground


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