Technologies for Moon mission useable for missions further out

"I wouldn't think that kind of insulation would be helpful on Mars."

Assuming you go with surface greenhouses, it appears that any such
insulation would not be so much "helpful" but *necessary*.

-mt

On Jun 29, 11:30 am, Alain Fournier wrote:
Michael Turner wrote:
This paper

http://www.marshome.org/files2/Hublitz1.pdf

Thanks for the interesting link.

is very suggestive of the difficulties. Note Fig. 8: even if you
assume efficient multi-layered insulation covering the surface
greenhouse at night, you have net heat loss except for a brief noonday
period. Also note: this is, they say, when "the sun is at zenith" --
i.e., high summer.

Consider the logistics and the risks of adding and taking off that
insulating cover, every dawn and sunset, with dust blowing around.
With a thin transparent shell underneath, how long before you're
looking at serious abrasion of it? They don't talk about that.

Right, I wouldn't think that kind of insulation would be helpful
on Mars. On the Moon maybe because of longer nights and because
MLI is more efficient in vacuum than in the Martian atmosphere.

Alain Fournier

"I think the easiest supply of water for a colony on Mars would be
extracting it from the atmosphere."

At 0.03% of a very thin atmosphere, you're looking at a lot of energy
input for a rather small yield of water. The H2O is sublimated
already, into a very cold atmosphere, so obviously you can't
efficiently precipitate it simply by freezing it out. You'd have to
compress large volumes. In the process, this would heat the
compressed gases, and you might get to the dew point for efficient
liquid precipitation.

If you're already hunkered down in a lava tube, grinding up volcanic
rock simply to make decent soil (so as to avoid the toxicity and other
dangerous properties of surface dust), well ... almost all of the
gaseous content of lava is water vapor, a few percent by weight. The
lava that flowed through lava tubes will tend to be higher in H2O
content, since higher H2O means lower viscosity, leading to more rapid
flow.

There's good evidence to suggest that the water content of lunar magma
was as high as the Earth's:

http://www.scientificamerican.com/ar...e-harbored-wat
er

If there were significant water flows on the surface of Mars at some
point, the ends of lava tubes would have been a natural collection
point for them. Subsequent lava tube collapses might have the effect
of sealing those collections against sublimation losses.
Alternatively, ash from volcanic eruptions might filter into the tube
and form a protective layer over accumulated water ice.

I suspect that on Mars, as well as on the Moon, most water will be
mined, one way or the other.

-michael turner


On Jun 29, 11:31 am, Alain Fournier wrote:
Michael Turner wrote:
Rebooting this discussion:

Let me suggest that the question be turned on its head: given the
likelihood that survival modes on both the Moon and Mars would be very
similar, what technologies would be location-unique?

I think these technologies will mostly be resource-determined, but
with some exceptions. Would you consider "proximity to the Earth" a
"resource"? Well, in a way: close proximity opens up much vaster
*human* resources. Certain operations might be practical to
teleoperate from Earth, in the case of the Moon, that would definitely
NOT be candidates for terrestrial teleoperation in the case of Mars.
However, in general, by "resource", assume that I mean immediately
available minerals, gases, energy sources, etc.

Examples:

Mars lacks:
- strong sunlight
- high daytime temperatures

The Moon is significantly lacking in:
- carbon (CO2 is most of Mars' atmosphere)
- nitrogen (N2 is around 3% of Mars' atmosphere)

Both are problematic for water supply, but Mars has ice caps, at
least.

I think the easiest supply of water for a colony on Mars would be
extracting it from the atmosphere. The colony would more likely
be in equatorial regions than in polar regions and even though
water vapour is only a small fraction of the atmosphere, you don't
need to move your equipment around to extract it. You just set
up a water extractor and let it run continuously, it can run
automatically.

The Moon? Over the long run, the cheapest place from which to
fetch water for a lunar base might actually end up being Phobos, of
all places, via the Interplanetary Transport Network. (That is, IF
Phobos' carbonaceous chondrite content is confirmed, and IF it's the
right H2O-rich type of carbonaceous chondrite.) Actually, mining and
shipping from Phobos might be an area of technology overlap: useful in
the long term for a Moon base, for supplying water and other volatiles
and minerals that are trace on the Moon; but also useful to reduce
short-term costs of Mars base set-up, until Mars surface ISRU can get
into full gear.

Phobos is still quite deep in the Martian gravity well. Why not go
get it in the main asteroid belt?

Mars might still be volcanically active:

http://www.space.com/scienceastronom...sm_041222.html

The Moon? Decidedly dead in this respect.

So on Mars, you might use geothermal sources of energy, conveniently
continuous and proximate to lava tube habitats, instead of two-week
solar heat buffers as you would on the Moon. If you lucked into a
superabundance of heat from geothermal, even if it's low grade heat,
the Mars surface greenhouse idea starts to make sense again, because
keeping it warm through nights and winters wouldn't cost so much.

Geothermal is not so easy on Earth. It would be much more difficult
on Mars. I think it would be easier to bring along a chunk of radioactive
material for a heat source than to do geothermal. But if you are in
a lava tube, you might not need much of a heat source, your problem
is going to be radiating your excess heat out. I'm not saying this
is a difficult problem, just that heating a habitat inside a lava tube
and in what is basically a vacuum is not a problem.

Alain Fournier

"Geothermal is not so easy on Earth. It would be much more difficult
on Mars."

*Everything* will be much more difficult on Mars (except accidental
death, of course.) What matters is how various approaches trade off
against each other ON MARS, not how easy or hard they are on the
Earth.

Note that, on Earth, "geothermal power" typically translates to
"geothermal electricity generation." You wouldn't have such a
requirement of geothermal on Mars. You'd be happy to get ordinary
room temperature on a round-the-clock basis from warm rock. In fact,
you don't even need rock that human being would call "warm". if you
had batteries, using banked electricity to run a heat pump against
rock that was below freezing temperature might be worth it, if that
rock was nevertheless "warmer" than anything else around it.

"I think it would be easier to bring along a chunk of radioactive
material for a heat source than to do geothermal."

Given that geothermal is ultimately generated by radioactivity, we're
talking about a natural ISRU approach to using nuclear power versus a
less sustainable source imported from Earth ... somehow. Brought down
to the Martian surface with all its heavy shielding. Through a very
thin atmosphere. Intact.

Would it be hard to find geothermal energy on Mars? Maybe not. The
real good news about methane might be not so much that it signals
bacterial life (a rather remote possibility anyway) but that it
signals where you can find natural sources of warmth.

"Methane on Mars: Extremophiles or Geothermal?"
http://ourundiscovereduniverse.com/blog/?p3

"I'm not saying this is a difficult problem, just that heating a
habitat inside a lava tube and in what is basically a vacuum is not a
problem."

IF you assume heat in the first place. I don't. On the Moon, you can
assume heat during the day -- lots of it. If you can store heat (or
derived energy) for two weeks, you can be OK. On ISS, you're as close
to the Sun as anybody on Earth (and the sunlight is harsher.) On
Mars, in midsummer, at high noon, well -- it's still what most humans
would call "cold". And that's as good as it gets.

You can wave your hands and say "let there be nuclear power", but you
just waved a whole host of *other* problems into the picture. RTG's?
That's an awful lot of "chunks of radioactive material" to take along,
if you want to keep crops from freezing to death out in the open. If
you really have to bring along a nuclear reactor (after all, it might
have some propulsion role in the craft that took you to Mars), the
best approach might be to leave it in Mars orbit, and beam power down
to the surface periodically. Sounds pretty iffy to me, though. I
think you'd more likely drill for pockets of liquid CO2, and power a
generator from the energetic gas release, to get your kilowatts.

-michael turner


On Jun 29, 11:31 am, Alain Fournier wrote:
Michael Turner wrote:
Rebooting this discussion:

Let me suggest that the question be turned on its head: given the
likelihood that survival modes on both the Moon and Mars would be very
similar, what technologies would be location-unique?

I think these technologies will mostly be resource-determined, but
with some exceptions. Would you consider "proximity to the Earth" a
"resource"? Well, in a way: close proximity opens up much vaster
*human* resources. Certain operations might be practical to
teleoperate from Earth, in the case of the Moon, that would definitely
NOT be candidates for terrestrial teleoperation in the case of Mars.
However, in general, by "resource", assume that I mean immediately
available minerals, gases, energy sources, etc.

Examples:

Mars lacks:
- strong sunlight
- high daytime temperatures

The Moon is significantly lacking in:
- carbon (CO2 is most of Mars' atmosphere)
- nitrogen (N2 is around 3% of Mars' atmosphere)

Both are problematic for water supply, but Mars has ice caps, at
least.

I think the easiest supply of water for a colony on Mars would be
extracting it from the atmosphere. The colony would more likely
be in equatorial regions than in polar regions and even though
water vapour is only a small fraction of the atmosphere, you don't
need to move your equipment around to extract it. You just set
up a water extractor and let it run continuously, it can run
automatically.

The Moon? Over the long run, the cheapest place from which to
fetch water for a lunar base might actually end up being Phobos, of
all places, via the Interplanetary Transport Network. (That is, IF
Phobos' carbonaceous chondrite content is confirmed, and IF it's the
right H2O-rich type of carbonaceous chondrite.) Actually, mining and
shipping from Phobos might be an area of technology overlap: useful in
the long term for a Moon base, for supplying water and other volatiles
and minerals that are trace on the Moon; but also useful to reduce
short-term costs of Mars base set-up, until Mars surface ISRU can get
into full gear.

Phobos is still quite deep in the Martian gravity well. Why not go
get it in the main asteroid belt?

Mars might still be volcanically active:

http://www.space.com/scienceastronom...sm_041222.html

The Moon? Decidedly dead in this respect.

So on Mars, you might use geothermal sources of energy, conveniently
continuous and proximate to lava tube habitats, instead of two-week
solar heat buffers as you would on the Moon. If you lucked into a
superabundance of heat from geothermal, even if it's low grade heat,
the Mars surface greenhouse idea starts to make sense again, because
keeping it warm through nights and winters wouldn't cost so much.

Geothermal is not so easy on Earth. It would be much more difficult
on Mars. I think it would be easier to bring along a chunk of radioactive
material for a heat source than to do geothermal. But if you are in
a lava tube, you might not need much of a heat source, your problem
is going to be radiating your excess heat out. I'm not saying this
is a difficult problem, just that heating a habitat inside a lava tube
and in what is basically a vacuum is not a problem.

Alain Fournier

On Jun 29, 11:29 am, (Steve Willner) wrote:
Sorry, I dropped a couple of decimal places there and also used too
low a temperature. The correct figure is about 400 W/m^2 _if_ the
greenhouse is a perfect emitter. That's still less than the power
needed for artificial lighting but not by as large a factor as I
found originally.

The paper I mentioned

http://www.marshome.org/files2/Hublitz1.pdf

has peaks of almost 80kW heat loss for Mars night even in mid-summer,
for a 90 m^2 area. That's a little less than 1 kW /m^2. But if
you're going to leave your plants up there on the surface overnight,
radiating away through an uncovered shell, you have no choice but to
offset most of that heat loss, to keep the plants from freezing. You
can't just average it out and call it a 300-400W power requirement.
Once a plant has frozen to death, it's dead. There's not much point
in heating it up again (unless perhaps to eat it, or use it for fiber
value.)

Bringing in sunlight from outside requires about a square centimeter
of light pipe or fiber per square meter of greenhouse; that part was
correct.

And depending on the length of the light pipes, the mass could add up
to considerably more than you'd need for transparent greenhouse
material conducting the same amount of daylight to plants on the
surface. But that's not the point -- there are lots of *costs* to
being on the Martian surface, material and otherwise, most of them
related (directly or indirectly) to keeping plants from freezing to
death.

At least with light-guides (and perhaps PV cells on the surface
powering LEDs underground), you can reduce some of the material costs
of sheltering the plants from low temperatures, UV and solar storms.
Possibly much more important: you can distribute the resulting light
optimally. This is something a surface greenhouse can't do.

For example, depending on the crop, Mars noontime light might actually
be more efficiently used if it were diffused over a wider area than it
would otherwise fall upon. For these crops, having LEDs hanging over
them at such times is a dead-weight loss.

You might want to use midday excess photons for spurts of production
of microalgae, which, with enough engineering, feature a far higher
rates of photosynthesis than you can get with any ordinary plants.
The algae yield would make a good feedstock for other purposes. Do
you need more airtight insulation for expanding your acreage? There
are processes that can make plastic out of algae.

http://www.popularmechanics.com/scie...gae-to-plastic

And plastic has other uses besides airtight insulating shelters for
plants. Like, stuff that's going to wear out. Especially stuff
exposed to highly abrasive dust.

Since you're going to need electricity for many other purposes anyway,
I like the idea of using LEDs in both lunar and martian agriculture.
PVs convert light at many frequencies. For plant growth, the LEDs
need only emit in frequencies that plants actually need. You can
easily move the LED banks from one plant-bed to the next, if need be.
And you can get a lot more watts through a wire, across much longer
distances without significant losses, and through much narrower
apertures, than you can with typical light-guides. This matters, the
farther you get from a lava tube skylight. It could save you the need
to drill holes from surface to tube, as agriculture gets extended
along the tube.

Ideally, the PVs needed to feed enough of the right photons to plants
via LEDs to keep a human being fed would also produce enough of an
electricity surplus to meet all remaining needs of that human being.
I suspect, however, that you can only make it work on Mars if
everything is well insulated, and if you also invested in solar
concentrators and heat storage (assuming you can't find convenient
geothermal sources.) The prospects for bulk heat storage ISRU on
Mars, at least, might be good: salt has high specific heat, and Mars
appears to have a lot of salt on its surface:

http://news.bbc.co.uk/2/hi/science/nature/7302591.stm

Probably not much salt lying around on the Moon, so that might be
another appreciable difference. But you do have a lot more surface
heat (at times) on the Moon, which has a mean surface temperature 50
deg C higher than Mars does. And there's no shortage of rocks

If you have a way to make big bags of plastic insulation (say, from
microalgae; see above), and big places (lava tubes) to store big bags
of warm rocks, maybe a rather low-tech approach to lunar night-time
warmth becomes plausible: a hoist over the skylight is used to lift
heat-storage rocks up into lunar sunlight. When they reach some
temperature that's not quite hot enough to melt a bag, bring the rocks
back down, bag them, and haul them to storage (say, a dead-end in the
lava tube complex, perhaps itself lined with plastic insulation.)
Repeat throughout the lunar day. (A continuous loop should be
possible.)

Keeping surface operations limited to points within or just above a
lava tube skylight would help keep dust exposure to a minimum. This
could be a factor on Mars as well as the Moon. Consider heating rocks
with a mirror array lifted above a lava tube skylight on Mars, where
airborne dust will be a problem. The suspension might have dust-
mitigation value. (Cleaning solar concentrators that sit on the
ground would mean crossing dust-covered ground; eventually the dust
gets tracked around in areas where it could be a problem.) The mirror
array could be lowered at night, to clean the dust off and to limit
further dust accumulation from night-time winds. The Mars version of
this system would naturally collect a lot less heat per unit of
skylight area, but since the heat wouldn't need to last nearly as long
as a lunar night, this might not be such an issue.

-michael turner
On Jun 29, 11:29 am, (Steve Willner) wrote:
Sorry, I dropped a couple of decimal places there and also used too
low a temperature. The correct figure is about 400 W/m^2 _if_ the
greenhouse is a perfect emitter. That's still less than the power
needed for artificial lighting but not by as large a factor as I
found originally.

The paper I mentioned

http://www.marshome.org/files2/Hublitz1.pdf

has peaks of almost 80kW heat loss for Mars night even in mid-summer,
for a 90 m^2 area. That's a little less than 1 kW /m^2. But if
you're going to leave your plants up there on the surface overnight,
radiating away through an uncovered shell, you have no choice but to
offset most of that heat loss, to keep the plants from freezing. You
can't just average it out and call it a 300-400W power requirement.
Once a plant has frozen to death, it's dead. There's not much point
in heating it up again (unless perhaps to eat it, or use it for fiber
value.)

Bringing in sunlight from outside requires about a square centimeter
of light pipe or fiber per square meter of greenhouse; that part was
correct.

And depending on the length of the light pipes, the mass could add up
to considerably more than you'd need for transparent greenhouse
material conducting the same amount of daylight to plants on the
surface. But that's not the point -- there are lots of *costs* to
being on the Martian surface, material and otherwise, most of them
related (directly or indirectly) to keeping plants from freezing to
death.

At least with light-guides (and perhaps PV cells on the surface
powering LEDs underground), you can reduce some of the material costs
of sheltering the plants from low temperatures, UV and solar storms.
Possibly much more important: you can distribute the resulting light
optimally. This is something a surface greenhouse can't do.

For example, depending on the crop, Mars noontime light might actually
be more efficiently used if it were diffused over a wider area than it
would otherwise fall upon. For these crops, having LEDs hanging over
them at such times is a dead-weight loss.

You might want to use midday excess photons for spurts of production
of microalgae, which, with enough engineering, feature a far higher
rates of photosynthesis than you can get with any ordinary plants.
The algae yield would make a good feedstock for other purposes. Do
you need more airtight insulation for expanding your acreage? There
are processes that can make plastic out of algae.

http://www.popularmechanics.com/scie...gae-to-plastic

And plastic has other uses besides airtight insulating shelters for
plants. Like, stuff that's going to wear out. Especially stuff
exposed to highly abrasive dust.

Since you're going to need electricity for many other purposes anyway,
I like the idea of using LEDs in both lunar and martian agriculture.
PVs convert light at many frequencies. For plant growth, the LEDs
need only emit in frequencies that plants actually need. You can
easily move the LED banks from one plant-bed to the next, if need be.
And you can get a lot more watts through a wire, across much longer
distances without significant losses, and through much narrower
apertures, than you can with typical light-guides. This matters, the
farther you get from a lava tube skylight. It could save you the need
to drill holes from surface to tube, as agriculture gets extended
along the tube.

Ideally, the PVs needed to feed enough of the right photons to plants
via LEDs to keep a human being fed would also produce enough of an
electricity surplus to meet all remaining needs of that human being.
I suspect, however, that you can only make it work on Mars if
everything is well insulated, and if you also invested in solar
concentrators and heat storage (assuming you can't find convenient
geothermal sources.) The prospects for bulk heat storage ISRU on
Mars, at least, might be good: salt has high specific heat, and Mars
appears to have a lot of salt on its surface:

http://news.bbc.co.uk/2/hi/science/nature/7302591.stm

Probably not much salt lying around on the Moon, so that might be
another appreciable difference. But you do have a lot more surface
heat (at times) on the Moon, which has a mean surface temperature 50
deg C higher than Mars does. And there's no shortage of rocks

If you have a way to make big bags of plastic insulation (say, from
microalgae; see above), and big places (lava tubes) to store big bags
of warm rocks, maybe a rather low-tech approach to lunar night-time
warmth becomes plausible: a hoist over the skylight is used to lift
heat-storage rocks up into lunar sunlight. When they reach some
temperature that's not quite hot enough to melt a bag, bring the rocks
back down, bag them, and haul them to storage (say, a dead-end in the
lava tube complex, perhaps itself lined with plastic insulation.)
Repeat throughout the lunar day. (A continuous loop should be
possible.)

Keeping surface operations limited to points within or just above a
lava tube skylight would help keep dust exposure to a minimum. This
could be a factor on Mars as well as the Moon. Consider heating rocks
with a mirror array lifted above a lava tube skylight on Mars, where
airborne dust will be a problem. The suspension might have dust-
mitigation value. (Cleaning solar concentrators that sit on the
ground would mean crossing dust-covered ground; eventually the dust
gets tracked around in areas where it could be a problem.) The mirror
array could be lowered at night, to clean the dust off and to limit
further dust accumulation from night-time winds. The Mars version of
this system would naturally collect a lot less heat per unit of
skylight area, but since the heat wouldn't need to last nearly as long
as a lunar night, this might not be such an issue.

-michael turner

On Jun 29, 11:29 am, (Steve Willner) wrote:
In article , I wrote:
How
do you propose to illuminate your greenhouses if not by direct
sunlight? You need either something like 10 MW/hectare if you do it
with efficient electric lighting

The Hublitz et al. paper referred to elsewhere
http://www.marshome.org/files2/Hublitz1.pdf
gets 2.3 kW/m^2, a factor of two worse than my estimate. Their
estimate was for high pressure sodium lights while mine was for LEDs,
which are more efficient.

Heat loss of a surface greenhouse won't be trivial, but it's only
about 30 kW/hectare

Sorry, I dropped a couple of decimal places there and also used too
low a temperature. The correct figure is about 400 W/m^2 _if_ the
greenhouse is a perfect emitter. That's still less than the power
needed for artificial lighting but not by as large a factor as I
found originally.

Bringing in sunlight from outside requires about a square centimeter
of light pipe or fiber per square meter of greenhouse; that part was
correct.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123 swill...@
cfa.harvard.edu
Cambridge, MA 02138 USA

Phobos is still quite deep in the Martian gravity well. Why not go get [w
ater] in the main asteroid belt?

Intuition is a poor guide to costs of interplanetary transportation,
in delta V terms. I know I don't trust my intuition. Not after
looking at this:

http://clowder.net/hop/railroad/deltaveemap.html

Starting from LEO: 5261 Eureka, a Mars Trojan, is "obviously" closer
than the main belt, and much farther out of Mars' gravity well, than
Phobos. Surely, it's easier to get to and from? Yet going to and
from Phobos is cheaper in delta-V terms than going to 5261 Eureka.

SM84 is an NEA, and closer in delta V terms to LEO than Phobos. But
not by all that much. And reasonable rendezvous opportunities are
rarer than they are for Phobos.

Going out beyond Mars means going farther out of the *Sun's* gravity
well. That's going to cost you. Sending stuff back might be done
with rotating slings powered by PVs on the surface of the asteroid.
For free. Sort of. Except that the farther out you are from the Sun,
the weaker the sunlight, and the more PVs you need to take along to
power the sling that sends stuff back.

Phobos is *very likely* carbonaceous chondrite (perhaps the "wettest"
meteoritic material), and might be very water-rich chondrite. If so,
and if getting water out of the Moon itself turns out to be
insuperably hard, Phobos is almost certainly the "closest" place (in
delta-V terms at least; transit times will be long) from which much
water might be gotten.

Phobos would almost certainly be an important staging point as well,
if the ultimate goal is Mars. Even with all the resources Phobos is
likely to have, getting set up on the surface of Mars will be very
hard. But if Phobos has a lot of water, it could solve one of the
hardest problems of setting up on Mars: repeated rocket-assisted
landing, and supplying a Mars surface base during its setup. You can
put a small rover on the surface of Mars using aerobraking,
parachutes, and inflatable cushioning. But putting significant mass
down there, especially of anything delicate (like, uh, a human being?)
will be a big problem. Mars has much higher gravity than the Moon,
but still not much atmosphere to help slow you down. How nice to have
a source of LH and LOX in low Mars orbit. Much as the Zubrinistas
like to talk of making their own fuel on Mars for the return trip, it
might make more sense to make it on Phobos instead (the hydrogen part
anyway) and send it down in batches. Much as the Zubrinistas like to
talk about going from Earth to Mars in one go, I think the progression
is more likely this:

(1) Learn to live on the Moon.
(2) Learn to extract resources and import them from Phobos
(robotically) to make living on the Moon cheaper.
(3) Get set up on Phobos and learn to live there,
(4) Study Mars, looking for ideal base locations while refining the
requirements.
(4) Set up Mars bases from Phobos, mostly teleoperatively.
(5) Land people on Mars.

Maybe in 50 years, with a constant international push, plus some
incentive from space tourism.

-michael turner

On Jun 29, 11:31 am, Alain Fournier wrote:
Michael Turner wrote:
Rebooting this discussion:

Let me suggest that the question be turned on its head: given the
likelihood that survival modes on both the Moon and Mars would be very
similar, what technologies would be location-unique?

I think these technologies will mostly be resource-determined, but
with some exceptions. Would you consider "proximity to the Earth" a
"resource"? Well, in a way: close proximity opens up much vaster
*human* resources. Certain operations might be practical to
teleoperate from Earth, in the case of the Moon, that would definitely
NOT be candidates for terrestrial teleoperation in the case of Mars.
However, in general, by "resource", assume that I mean immediately
available minerals, gases, energy sources, etc.

Examples:

Mars lacks:
- strong sunlight
- high daytime temperatures

The Moon is significantly lacking in:
- carbon (CO2 is most of Mars' atmosphere)
- nitrogen (N2 is around 3% of Mars' atmosphere)

Both are problematic for water supply, but Mars has ice caps, at
least.

I think the easiest supply of water for a colony on Mars would be
extracting it from the atmosphere. The colony would more likely
be in equatorial regions than in polar regions and even though
water vapour is only a small fraction of the atmosphere, you don't
need to move your equipment around to extract it. You just set
up a water extractor and let it run continuously, it can run
automatically.

The Moon? Over the long run, the cheapest place from which to
fetch water for a lunar base might actually end up being Phobos, of
all places, via the Interplanetary Transport Network. (That is, IF
Phobos' carbonaceous chondrite content is confirmed, and IF it's the
right H2O-rich type of carbonaceous chondrite.) Actually, mining and
shipping from Phobos might be an area of technology overlap: useful in
the long term for a Moon base, for supplying water and other volatiles
and minerals that are trace on the Moon; but also useful to reduce
short-term costs of Mars base set-up, until Mars surface ISRU can get
into full gear.

Phobos is still quite deep in the Martian gravity well. Why not go
get it in the main asteroid belt?

Mars might still be volcanically active:

http://www.space.com/scienceastronom...sm_041222.html

The Moon? Decidedly dead in this respect.

So on Mars, you might use geothermal sources of energy, conveniently
continuous and proximate to lava tube habitats, instead of two-week
solar heat buffers as you would on the Moon. If you lucked into a
superabundance of heat from geothermal, even if it's low grade heat,
the Mars surface greenhouse idea starts to make sense again, because
keeping it warm through nights and winters wouldn't cost so much.

Geothermal is not so easy on Earth. It would be much more difficult
on Mars. I think it would be easier to bring along a chunk of radioactive
material for a heat source than to do geothermal. But if you are in
a lava tube, you might not need much of a heat source, your problem
is going to be radiating your excess heat out. I'm not saying this
is a difficult problem, just that heating a habitat inside a lava tube
and in what is basically a vacuum is not a problem.

Alain Fournier

SW The correct figure is about 400 W/m^2 _if_ the
SW greenhouse is a perfect emitter.

(The above is just the Stefan-Boltzmann law for a temperature of
300_K. The area to be considered is the radiating area, which in
general will differ from the useful area of the greenhouse.)

In article ,
Michael Turner writes:
http://www.marshome.org/files2/Hublitz1.pdf

has peaks of almost 80kW heat loss for Mars night even in mid-summer,
for a 90 m^2 area. That's a little less than 1 kW /m^2.

Or about 900 W/m^2, about double the simple estimate. They seem to
be assuming a perfect emitter and a factor of 2.25 for the ratio of
radiating surface to greenhouse area. These strike me as very
conservative assumptions.

Since you're going to need electricity for many other purposes anyway,
I like the idea of using LEDs in both lunar and martian
agriculture.

Notice that the electricity you have to supply to the LEDs, depending
on assumptions, is more than you would have to supply to heaters.
Take a look at Fig 9 in the Hublitz et al. paper.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA

Michael Turner wrote:
"But if you are in a lava tube, you might not need much of a heat
source, your problem
is going to be radiating your excess heat out."

You keep ignoring how cold it is there, and how vast these lava tubes
can be. Even assuming you did need some kind of heat rejection, you
have the planet's own surface into which it can be conducted. You
can't do that on a spacecraft.

I didn't mean that you would need to radiate excess heat from the
greenhouse. It is the habitat which would presumably have equipment
generating heat that would need to radiate heat out. On the Moon
where the lunar soil is a great insulator so the heat inside the lava
tube doesn't escape very well and doesn't dissipate into the soil either,
if your habitat is running equipment that generates heat you will need
to get rid of heat. The greenhouse seems to be a sensible place to pump
your extra heat. And since your greenhouse is also in the lava tube
and it also has few places where it can lose its heat, that might
very well be enough to keep the plants from freezing.

On Mars, I don't know about the thermal conductivity of Martian soil.
I wouldn't expect it to be as good an insulator as lunar soil. Plus
even the thin atmosphere of Mars does transport heat. So if you don't
have equipment generating lots of heat, you will most likely not
have to get rid of excess heat in the habitat. But it is not inconceivable
that you would have equipment generating lots of heat. Imagine if
you are operating a small smelter.


Alain Fournier

Michael Turner wrote:

"I think the easiest supply of water for a colony on Mars would be
extracting it from the atmosphere."

At 0.03% of a very thin atmosphere, you're looking at a lot of energy
input for a rather small yield of water. The H2O is sublimated
already, into a very cold atmosphere, so obviously you can't
efficiently precipitate it simply by freezing it out. You'd have to
compress large volumes. In the process, this would heat the
compressed gases, and you might get to the dew point for efficient
liquid precipitation.

Since your water is recycled, you don't need to extract a lot.
You will want to compress some atmosphere just to compensate
the gas leakage from the habitat and the greenhouse. Once you
compressed the Martian atmosphere, just let it chill for water
to precipitate. So, since you need a compressor, you kind of
already have all you need to extract water from the atmosphere,
just let the compressor run a little longer if you need more
water.

If you are already processing the atmosphere, to extract O2 as an
oxidizer for rockets, you can get the water simply as a by-product.

If you're already hunkered down in a lava tube, grinding up volcanic
rock simply to make decent soil (so as to avoid the toxicity and other
dangerous properties of surface dust), well ... almost all of the
gaseous content of lava is water vapor, a few percent by weight. The
lava that flowed through lava tubes will tend to be higher in H2O
content, since higher H2O means lower viscosity, leading to more rapid
flow.

There's good evidence to suggest that the water content of lunar magma
was as high as the Earth's:

http://www.scientificamerican.com/ar...e-harbored-wat
er

But you can't only use the water from the grinding of volcanic rock for
soil. You need more than the water needed to have your soil moist enough
for farming. Volcanic rock on Mars if not given extra water is not moist
enough for farming. So you would be processing more rock than what you
need for soil. Not a major problem, but not a free lunch.

If there were significant water flows on the surface of Mars at some
point, the ends of lava tubes would have been a natural collection
point for them. Subsequent lava tube collapses might have the effect
of sealing those collections against sublimation losses.
Alternatively, ash from volcanic eruptions might filter into the tube
and form a protective layer over accumulated water ice.

But if subsequent lava tube collapses have sealed the hydrated rock
against sublimation, then you are talking about a major mining operation
to get that water.

I suspect that on Mars, as well as on the Moon, most water will be
mined, one way or the other.

If you aren't processing the atmosphere for rocket propellant and
getting water as a by-product you may be correct.


Alain Fournier

Michael Turner wrote:


"Geothermal is not so easy on Earth. It would be much more difficult
on Mars."

*Everything* will be much more difficult on Mars (except accidental
death, of course.) What matters is how various approaches trade off
against each other ON MARS, not how easy or hard they are on the
Earth.

Note that, on Earth, "geothermal power" typically translates to
"geothermal electricity generation." You wouldn't have such a
requirement of geothermal on Mars. You'd be happy to get ordinary
room temperature on a round-the-clock basis from warm rock. In fact,
you don't even need rock that human being would call "warm". if you
had batteries, using banked electricity to run a heat pump against
rock that was below freezing temperature might be worth it, if that
rock was nevertheless "warmer" than anything else around it.

Rock doesn't conduct heat all that well. If you extract heat from
rock after a short while, the rock is cold. Usually, when a house
is heated by geothermal heat, the heat is extracted from some
underground water. I have a friend who heats her house that way,
the equipment needed to set up her heating system was not small
and light. Therefo

"I think it would be easier to bring along a chunk of radioactive
material for a heat source than to do geothermal."

Given that geothermal is ultimately generated by radioactivity, we're
talking about a natural ISRU approach to using nuclear power versus a
less sustainable source imported from Earth ... somehow. Brought down
to the Martian surface with all its heavy shielding. Through a very
thin atmosphere. Intact.

Well if you are going to bring humans, I hope you can bring a chunk
of uranium intact.

You can wave your hands and say "let there be nuclear power", but you
just waved a whole host of *other* problems into the picture. RTG's?
That's an awful lot of "chunks of radioactive material" to take along,
if you want to keep crops from freezing to death out in the open. If
you really have to bring along a nuclear reactor

A nuclear reactor might make sense, but if all you want is heat, your
"reactor" can be quite simple. You could just bury uranium 1m below
the soil of the greenhouse. Of course you would do something a little
more sophisticated than that. But you can heat an awful lot of greenhouse
that way before the mass of the uranium is greater than the mass of
the equipment that came to install my friend's geothermal heating
system.


Alain Fournier

Michael Turner wrote:

At least with light-guides (and perhaps PV cells on the surface
powering LEDs underground)

LEDs aren't particularly useful in this case. The reason why LEDs
use much less electricity than old fashioned light bulbs for the
same amount of light is that LEDs emit very little heat. So, the
inefficiency of less efficient lights is very efficient in this
case, they heat the greenhouse. LEDs probably are still the best
choice because you can choose your wave lengths more precicely and
because they last for such a long time, which is useful because an
artifically lighted greenhouse needs so much lights that changing
the light bulbs can be a waste of time. But there advantage is minimal.


Alain Fournier