Mars domes (was feedback needed on TEDx talk)

wlm_efn@... [spacesettlers] wrote:

> In The Millenial Project, written in 1994, Marshall Savage talks about domes
> on both the Moon and Mars, with a water layer. The "domes" he envisioned
> were more like flat surfaces built on top of craters. The atmospheric
> pressure pushing up/outward is several times the force that gravity is pushing the
> water-filled dome downwards. And in fact the domes need to be secured down
> rather than kept up.

That's interesting. The Millenial Project contains a number of
problems, I think, but that he could have been right on that point. (Or
not, at the moment I can't say.)

> He used a figure of a tonne of mass of something per square meter, which for
> water is about 1 meter thickness.

That's much too low. The NASA studies assumed 5 tons per square meter,
but even that turns out to much lower than necessary for the radiation
target they had set at the time (5 mSv/year).

In last year's radiation shielding paper
(http://space.alglobus.net/papers/RadiationPaper2014.pdf), Al Globus and
I argued that you could get probably get away with as much as 20
mSv/year... but even that requires 10 tons/m^2 of regolith, or about 7
tons/m^2 of polyethylene radiation shield. Water would be in between
these, so call it 8 tons/m^2.

That's for a colony in free space... having a planet blocking the sky
certainly helps. It effectively cuts the number of cosmic rays you have
to deal with in half, so as a rough heuristic, we could say you need 4
tons/m^2.

Now that I think about it, though, that's a pretty gross simplification.
On a planetary surface you're getting no cosmic rays from one
direction, but full-strength cosmic rays from the other direction.
Stopping those cosmic rays will be no easier than it would be in free
space... and if you only *partly* stop them (for example, with shielding
only half as thick), you can actually make matters quite a bit worse,
due to secondary radiation.

But I think it's safe to say that a Mars dome would need to have between
4 and 8 tons of mass per square meter.

> > There's no way atmosphere under a dome would have anywhere near
> > enough mass to make any noticeable difference to cosmic rays.
>
> 8.4 km of atmosphere under a dome provides the same shielding as the
> earth's.

I hadn't heard this one before, so I had to work through it:

Air at sea level masses about 1.2 kg/m^3. So if we had a uniform 8400
meters of that, that's about 10 tons/m^3. That must be where that
figure comes from.

A column of air on Earth masses about 1.03 kg/cm^2, or (also) about 10
tons/m^3.

Of course on Earth we're protected primarily by the magnetosphere; the
mass of the air is secondary. But yeah, I agree, that much air would
provide adequate (not Earth-equivalent, but adequate) protection. I
stand corrected.

> Granted this would have to be a huge dome, the equivalent of
> an Island Three.

True.

> A figure of 3-4 meters still means the atmospheric pressure pushing up/out
> is several times the weight of the dome pushing down. No I haven't seen any
> "serious" proposals. But then I haven't checked, I'm not a mars advocate.

Fair enough. The proposals I've seen are either for early-stage
(MarsOne-style) outposts, where people simple live in habitat modules on
the surface until they get a chance to bury them; or for living underground.

> A quick google search provided this:
>
> http://voitlab.com/courses/thermodynamics/index.php?title=Mars_Greenhouse_Dome

This appears to be for 1-mm-thick low-pressure inflatable greenhouse
domes. There's no significant protection from cosmic rays here (in
fact, it will probably be slightly worse inside than out).

[Re. big living domes]

> It would certainly require some engineering, but I doubt very much
> that it would require advanced nanotech.

Granted. I probably won't have time to work all this into the talk, but
if it comes up in questions I will now have a more nuanced view of the
possibilities -- thank you.

Best,
- Joe