Pressurised natural caves could offer a home from home on the Moon | Hacker News Reader

Pressurised natural caves could offer a home from home on the Moon

economist.com

22 pointsMoSattler48 comments

This idea is unlikely to work unless the entirety of the pressurized volume is encased in a giant "balloon" or some sort of sealant coating the entirety of the cave's surface.

Lava tubes are formed from flowing molten basalt, and basalt shrinks when it cools, forming contraction cracks. If we're producing breathable atmosphere and pumping it into lava tubes, it's destined to leak from innumerable cracks in the cave walls. And it'd presumably leak out faster than we can produce new breathable atmosphere.

This differs from proposed deep "carbon capture" strategies, where we'd pump large amounts of CO2 underground for storage; in that case, the depth provides an airtight seal due to significant overburden, impermeable clays/shales, and perched aquifers above it. But lava tubes, by their very nature, are always very near the surface. That's because lava tubes - unlike other types of caves - form on the surface as molten basalt flows over it. They generally don't survive for long (geologically speaking) nor survive significant sedimentation which would bury them deeply.

Lava tube caves can certainly provide shelter, and can significantly buffer temperature changes. But it would be extremely difficult to use them as an airtight biosphere.

Source: I'm former cave resource manager for the National Park Service at an NPS unit with 700+ lava tube caves which was used by NASA for training and ongoing research.

Yeah, that’s likely the idea. The rock is more to protect from radiation, cosmic rays and punctures than to capture the atmosphere. I’d probably build it in multiple layers too. Low pressure on the outside and mounting pressure in internal domes.

The big problem with Moon colonies is that the Moon lacks carbon and nitrogen. Hydrogen is also hard to come by.

Let's say we find a lava tube that is 15 meters in diameter and 100 meters long. That is about 70 thousand cubic meters, and would require about 70 tons of nitrogen to create an artificial atmosphere similar to the one we have here (we can ignore the oxygen, or which there is plenty in the lunar regolith). Bringing 70 tons of stuff from Earth is not impossible, but far from easy, even given Musk's Starship.

It is much more likely that initially we'll build something similar in volume to the International Space Station, where we were able to keep a breathable atmosphere for 20+ years. Which, by the way, has a volume of only about 1000 cubic meters (out of which only about 400 m3 habitable).

My impression was that it's mostly about particle pressure and 100% O_2 at 0.2 atmospheres would also be possible. ISS keeps with the normal atmospheric pressure and 80% nitrogen but from what one reads on the internet, that is more for compatibility reasons and not because we humans cannot sustain a pure oxygen, low pressure environment, once you go all in (and stay there for prolonged times).

A 100% O2 atmosphere would produce oxygen toxicity in the human body. Plus, it would create a huge fire hazard.

Now, it's possible that we could be ok with a gas mix that's different from the traditional 21% oxygen 78% nitrogen we have here. But we don't like to experiment on humans. A different gax mix would need a lot of long term testing on animals before we'd ever consider using it.

I believe you may be incorrect, the fraction is not the problem [1]. Apollo also ran with pure O2, even after an early accident under higher pressure [2].

> In such applications as extra-vehicular activity, high-fraction oxygen is non-toxic, even at breathing mixture fractions approaching 100%, because the oxygen partial pressure is not allowed to chronically exceed 0.3 bar (4.4 psi).

1. https://en.m.wikipedia.org/wiki/Oxygen_toxicity

2. https://www.popsci.com/why-did-nasa-still-use-pure-oxygen-af...

Is there any data about long term effects?

That's why I am writing 'at 0.2 atmospheres'! Under that condition, you do get neither any poisoning (which the internet claims appears at 1.4 atmospheres pure oxygen), nor a big fire hazard. You create other problems due to the lower air pressure like lower boiling temperature and more body heat retention but those might be engineered away... On SkyLab, astronauts supposedly experienced 75% oxygen, 25% nitrogen for upto 84 days (again at lower pressure). That's not really long term, but not short either.

100% oxygen at turned the crew of Apollo 1 into ash. It makes stuff like concrete and aluminum as flammable as wood.

That was 100% oxygen at 100% atmospheric pressure.

The lunar module was 100% oxygen at 5 PSI (atmospheric pressure is about 14 PSI.

I’m not a chemist but I think it’s not the percentage, but the number of atoms that counts.

https://history.nasa.gov/alsj/CSM08_LM_&_SLA_Overview_pp61-6...

The Apollo engineers knew this and it factored into their choice. For whatever reason not enough consideration was given to full up ground tests where the same atmosphere would be used, but at 1atm.

100 O2 at regular or reduced pressure?

That’s not true. Regolith contains plenty of nitrogen and carbon. The moon is basically a giant Bussard collector.

“Similarly, carbon and nitrogen have also been implanted from the solar wind. The amount of nitrogen in a cubic meter of lunar regolith is similar to the amount of hydrogen-about 100 grams, or 3 percent of the nitrogen in a human body. The amount of carbon is twice that, the carbon beneath each square meter of the lunar surface is some 35 percent of the amount found tied up in living organisms per square meter of the Earth's surface.”

https://space.nss.org/settlement/nasa/spaceresvol4/lifesuppo...

So 70,000 tons of regolith? I'm not a moon engineer or anything, but I think given the choice of bringing 70 tons of nitrogen, or the equipment to process that amount of regolith over a reasonable scale you'd just bring the nitrogen.

Nice catch. Only it would not be 70000 tons, but one million tons of regolith.

On the other hand this is about 0.1 sqkm of lunar surface stripped and processed. This feels like a pretty small quarry mine on earth. Given that, it feels like it would probably be worth shipping the materials one would need to operate a quarry plus build the ISRU as without material environmental concerns you could process considerably more in a sustainable fashion than shipping elements from earth. The ISRU could extract all sorts of other useful materials -

Oxygen

Silicon (can be used to make plastics)

Iron

Aluminum

Magnesium

Sodium

Potassium

Phosphorus

Etc

Power would be abundant to fuel the reactions via solar radiation. So I wouldn’t write it off, I think regolith processing would be foundational for establishing any meaningful presence. Likely it can be entirely automated as well.

You could cut this by about a third by using helium as a buffer gas. And cut it even further by reducing pressure and increasing oxygen percentage

TIL; when researching breathing gases I found "Hydrox". And yes, it's exactly what it sounds like and divers have successfully used it.

My calculations for the volume of a 15m x 100m cylinder would be 17,671 m³ (π * (Diameter/2)² * Length). I think you calculated it with 15m as its radius.

For an atmosphere similar to the Earth's at sea level, you would only need to bring a single Starship with liquid nitrogen (~12k kg) or ammonia (~22k kg, which you could use the ammonia breaking process) to fill it up the volume. Starship can hold 100 tons in its cargo configuration. With a 15m radius you would still only need ~49k kg of liquid nitrogen or ~80k kg of liquid ammonia, which is doable in a single-ish Starship flight (maybe two with the ammonia if you couldn't use all of the cargo capacity).

This seems extremely doable on smaller sizes, and pretty doable for larger sized lava tubes (albeit expensive).

Isn't lunar dust an absolute nightmare made of sharp shards of material melted/compressed from large object impacts, or something of the sort? That has to be a nightmare on the skin or soft body tissues. And devices.

The idea is fun, but I'm definitely not moving there. I mean uuuuh "it is my duty to let the younger generations populate the new frontiers while I hold the fort in their absence". Yeah, let's roll with that.

Knowing my luck I'd be the first Selenite to die from lung lacerations infected from recycled air because the single live native bacterium of the moon decided to get cozy in my air filter, just the day the neighbor decided to clean the rugs. Or I'd get the only non-functioning internet connection in the planet. "Satellite's fried, kid, can't do nothin' until next rocket comes with da spare!".

Jokes aside, is there some sort of good reference about the internals of the Moon? I never gave it a lot of thought but it makes sense it'd have some internal structure. While I don't think there's a lot of lunar spelunkers on the job, maybe we got something with scanning or something. I don't even know where to look.

Correct. Moon dust is also terrible for rubber seals and other things used to create airtight environments.

What is a "home from home"? I'm not familiar with this phrase.

The British version of "home away from home" if you've ever heard it said like that. Basically a place that's not home but you can feel comfortable/familiar as if it were anyways.

"home away from home" makes sense, but if you take away the away, I personally can't make sense of it anymore - if it's "from home", you are doing it (whatever "home" may mean when used as a verb) while you're at home, like in "work from home", so... ?!

I think it's the same 'from' as 'far from home', but I'm not British so take that with a grain of salt.

Even 'work from home' is weird. The only way I can read it if I really think about it is your work is coming from home.

Pretty clearly a typo

Going full circle back to cavemen.

I get this is a joke but the reason we find stuff in caves isn't that they _lived there full time_ it's that stuff was only preserved there. Try building a wood house on the shores of the ancient Mississippi and seeing if you can find a trace of it in 30,000 years.

Astrocavemen will need to first create air, water, and food. Then they will be able to create fire. Now we've gone full circle.

I recommend the Heinlein short story entitled gentleman be seated

The Heinlein novel "The Moon Is A Harsh Mistress" also features pressurized living spaces on the moon heavily.

I still think this was part of Elon's motivation for investing in his Boring Company. Those machines could be used to create subsurface chambers for both habitation and transportation, while generating plenty of crushed rock for surface construction.

Elon only gets into industries with medium-hanging fruit and huge payoffs. The best ground drillers in the world moved at something like 3"/hr. It's hard to go faster than that, but if you could, the payoff (building tunnels for trains, construction projects, mining, etc.) would be insane.

> Elon only gets into industries with medium-hanging fruit and huge payoffs.

what's the huge payoff with Tesla supposed to be?

or Twitter?

> what's the huge payoff with Tesla supposed to be?

Huh? It's few hundred billion dollar company that's the biggest EV company and by some metrics the largest car company.

OK, it's true that its stock price is high enough that the market has priced it above the rest of the entire automotive industry. but that price will undoubtedly crash, and it's not the biggest car company by any realistic or stable metric.

they've been slashing their prices and they make cars which catch fire. they've been around a decade and they still haven't turned a profit.

and again, what's the huge payoff going to be with Twitter?

The payoff has already happened, he is literally the richest person on Earth. With Twitter I don't know, he's probably expecting massive social power for himself or something.

> they've been slashing their prices

And yet still have margins of >20%, dramatically higher than any other manufacturer.

> and they make cars which catch fire

At a rate roughly 10% of gasoline-powered cars.

> they still haven't turned a profit

They've been profitable since Q2 2020.

Market cap is a meaningless metric.

It's a car company that makes about 13% as many cars as Toyota. The fact it's priced at the sum total of every other car company combined isn't really relevant to the facts on the ground.

He has a giant company providing money to do extensive research into motors and batteries. A breakthrough in either could potentially be worth billions.

Twitter is one of the most-visited sites (probably top 20 eh?) in the world and it wasn't profitable. If you could make a few changes and make it profitable, that would be ideal. Now that it's somewhat integrated into society and used quite a bit, that becomes easier to do.

> I still think this was part of Elon's motivation for investing in his Boring Company.

Given what I've seen on twitter.com I think you may be overthinking it. IMO he legitimately thought he could 'defeat traffic' and make everyone buy a Tesla if he dig tubes around town. Which I mean you can pretty quickly disprove with a quick prototype: take a lane of highway and put a hat on it, see if traffic gets better.

But why?

The Moon will probably be colonized by robots. Robots can be built to operate in vacuum. Off earth, humans are stuck indoors except for occasional excursions in space suits.

If we ever have any sort of base on the moon, even an automated one, inevitably humans will want to move there too. Might as well think about how that might work with time to plan ahead.

Nobody lives in Antarctica for its own sake, and that's a lot closer to home. You can breathe the air there. If we could replace McMurdo base with robots, nobody would weep.

Wouldn't expect that to change until/unless we find some natural resources worth exploiting down there.

Plenty of people go to Antarctica for its own sake.

You can buy a cruise ship ticket to Antarctica for $30,000.

To fix the robots. Moon dust is evil -- unweathered, sharp, and electrostatically suspended in clouds due to solar wind charging it up. Until robots get good at repair work, some poor humans are probably going to get stuck with this job.

To do repacking/refueling logistics for more ambitious manned missions.

To unpack and deploy infrastructure.

To shoot at people who try to jack the infrastructure :)

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