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This question focuses on an artificial atmosphere on the moon and the achieved amount being the limiting factor for the size of habitats.

I got a lot of beef for the suggestion to terraform the whole moon in the style of omg-the-amount-of-energy-needed-for-that, and apparently no ones believes we can do magic in the future except me, so lets turn the question around and ask, how much terrain can we populate within the realm of believable possibilities.

We are in the future and have created technology that allows us to move masses around in our solar system. Assume our technology literally skyrockets in the next 1000 years (including the connection of our knowledge beyond internet-capabilities). We can bring a lot of resources from the Kuiper belt and other "convenient" places relatively easy to the moon (sure it takes time, but we have really many and big transportations). Added as guideline: let's say we 100.000 times better at transporting things in space than we are now.

We have the knowledge to maintain an earth-like atmosphere on the moon; there is a dome of some kind.

How big an area can we possibly make habitable on the moon? Since this is of course a very complex question with a lot of variables, let's focus only on the creation of atmosphere and leave out that humans need more than air.

If you can express an answer in area, please convert your answer into km2. Alternativly an estimate in percent would be appreciated.

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    $\begingroup$ sorry, but with a loose definition like "technology skyrocketed" any answer is possible, from the few square meter of the LEM to the entire lunar surface. $\endgroup$ – L.Dutch - Reinstate Monica Sep 19 at 7:53
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    $\begingroup$ The Kuiper belt isn't exactly what I'd call "convenient" to the moon, or anywhere else really. $\endgroup$ – Cadence Sep 19 at 7:58
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    $\begingroup$ If you're going to do magic in the future, use it to make awesome magical things like Orbitals (or the nearest thing that your materials technology can plausibly manage). Covering the moon in frozen poop and wondering how best to farm it isn't my idea of magic ;-) $\endgroup$ – Starfish Prime Sep 19 at 9:49
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    $\begingroup$ @Cadence energetically, it isn't far away. Lead times are long, but once the deliveries start coming in there's no reason you couldn't manage regular and sizeable shipments of various useful things you might find there. $\endgroup$ – Starfish Prime Sep 19 at 9:51
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    $\begingroup$ I think with the technological scenario you describe it would actually be possible to terraform the whole Moon. $\endgroup$ – Renan Sep 19 at 12:15
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We (as a K2.something-civilisation) could make the entire surface area of Luna perfectly habitable. It would be titanic effort and have little, one might argue negative, payoff but we could certainly do it. It would take centuries unless you want to make the moons surface liquid and boil of the atmosphere you are building with the impact energy of the endless streams of comets which would have to hail down onto the moon daily and would let the worst nuclear war scenarios look like child-firework. Massive mirrors and sunshades will be needed to keep the climate under control. Luna would be unusable during the terraforming prosses. Terraforming is an extremely destructive process. I would recommend this video "Springtime On Mars" by Isaac Arthur to you so you can gain some perspective on what you are proposing.

Then, after centuries of work, an enormous material investment and rendering the prime industrial real estate Luna used to be all but unusable for this purpose you can claim your price:

3.793 * 10^10 $m^2$ of habitable surface area

Sounds great, right? Well, only until you consider that Earth has a habitable surface area of:

1.04 * 10^11 $m^2$

We are one order of magnitude short of Earth's habitable surface area. This still sounds fine, right? Until you consider that you could have had way more useful surface area with significantly less effort per $m^2$ if you just industrialized the moon and used this industry to build O'neil cylinders. If one where to mine the upper 100 km of the moon's surface and assume that the habitats require 10 $t/m^2$, which are quite conservative estimates, one could get:

1.138 * 10^18 $m^2$

Of not only habitable surface area, but extremely uncontrollable and extreme weather free surface area.

This is the main issue with terraforming. Building spin habitats is just more efficient. Advanced civilizations wight very well terraform places, but not to get Lebensraum. Terraforming projects will be art and vanity projects, even and especially for civilizations with "magic technology".

I don't want to ruin your setting or bash you with this in any way, but you would have to answer the question why your civilization decided to turn amazing industrial real estate into extremely expensive and inefficient Lebensraum.

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Why not underground?

Assuming easy transport of materials as described and the technology peripheral to that, plus the incentive. Humans could riddle the whole moon with pressurised tunnels etc,.

Without terraforming the surface you could cover the surface with pressurised domes up to whatever engineering limits your high tech materials have in that scenario.

Utilising both strategies you'd ultimately have more inhabitable space than Earth has right now.

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The main problem of all this dome structures is not buiding them, but maintaining them. It means that size of artificial structure is maxed when all the transportation we have is spent on maintance.

Today we deliver about 100-200t of payload to orbit each year. It means that in this scenario we deliver about 10 million t of payload each year.

Project biosphere was about 0,01 km^2 and has a weight about few thousands tones and lasts for about 2 years before it needed maintance. Lets say we have superlight construction materials and thus 1000 t per 0,01 km^2, 100 000 per km^2. And lets asume this domes require equivalent of total rebuild once in 10 years.

All this means that we are capable to deliver 10 km^2 of domes per year, and support 100 km^2 of domes on surface.

You can play with numbers. Say asume 30 years dome lifespawn. This would give you more area (300 km^2). But few thousands of km^2 (0,1% of Moon surface) is the max.

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So the one hard number you offered is that we are "100,000 times better at moving material around in space than we are now."

I'm going to give all the benefit of the doubt to the numbers that make your case the best. Let's use the cost it currently takes to put a kg of matter on the moon. Current costs are around 1-1.2 million per kg landed on the surface. But let's be generous and say that with the next generation of reusable rockets we can reduce that to 100,000 per kg. That's nice, because if we assume we get 100,000 times better, we can use the nice even number that one day we can move mass to the moon for $1/kg. I'm assuming that the 100,000 times number includes getting the mass from somewhere other than Earth.

So...

The moon has a radius of roughly 1737 km. Therefore, the surface area is roughly 3.79×1013 square meters.

On Earth, a column of air that provides sea level pressure is about 1.03kg per square centimetre, or roughly 10,000 kg per square meter. On the Earth, because of 1/6 G we would need six times the mass. But let's assume we can get by with half the pressure., so we only need 3 times as much. So, it will cost us about 30,000 per square meter of the moon's surface, or 11.1 X 10^18 dollars.

The annual GDP of the Earth is 80.68 X 10^9 dollars. Dividing the two, and you find that it would take about 137.5 million times the Earth's annual GDP to pay for it.

Dropping comets on a body is the standard proposed way to terraform it. This would not work for the Moon, because any comets of a size big enough to matter impacting it would throw a whole lot of debris at the Earth. Bombarding the Moon with the thousands of comets required would likely devastate the Earth. Also, when a comet hits an airless world, the impact will vaporize it, and a lot of that vapor will be lost to space. That's a serious problem, but we'll skip if for now to make things look the best for your scenario.

Then you have the problem that the Moon's soil is full of unoxidized iron which would bind with oxygen in the atmosphere, that the moon is also depleted in necessary chemicals like carbon and nitrogen, and the long lunar days and nights would likely play havoc with your new atmosphere.

There is a much better solution. There are likely hundreds to thousands of 'lava tubes' on the moon, and some are so large that you could put entire cities in them. For example, the Marius Hills lava tube has been partially radar mapped and is at least 50 km long, 70m deep, and 500m to 1km wide. Even pressurizing this would be a monumental task, but at least it's in the realm of feasibility in your future world.

Purdue university has calculated that a stable lava tube on the moon could be as wide as 5 km with a ceiling 1.5 km high. Some lava tubes could be hundreds of kilometers long. That's thousands of cubic kilometers of living space. In one lava tube.

The GRAIL mission discovered that the moon's crust is about 12% void space. A lot of that is small, but there is bound to be more living space underground than we would need for a very, very long time.

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