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Moon's surface is about thirty million square kilometers large. That's a lot of plastic, but no bigger effort than many other terraforming projects we have discussed.

Description:

A global bubble of plastic floating at a height of eight thousand meters over the moon's surface, sustained by an atmospheric pressure of 1 bar. Not much more advanced materials technology than the present.

Some questions:

1.- How to make it.

Would it be better to extend the plastic on the surface and then inject the gas? Or would it be better to place plastic sheeting in orbit and go creating a cocoon? This cocoon would then decay from the orbit for being sustained by the atmosphere.

2.- How to maintain it.

How long will an ordinary plastic or PVC last in the space? How long will take for an small meteoritic impact to have irreparable consequences if not fixed?

3.- How to live with it.

Best designs for access portholes. Best ecological choices for the lunar biosphere. Best social choices for a human society that would live in a near earthly, but low gravity environment with a monthly day-night cycle.

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    $\begingroup$ Moon's radius is 1,737,000 m, plus 8,000 m of the plastic ceiling height over each Moon's side, is a total surface area of 38,600,000,000,000 m2. Given an approximate density of a standard PVC panel of 3.5 kg/m2 that is a total mass of 135,100,000,000,000 kg. With a fleet of Saturn V launchers, with a payload to Trans Lunar Injection weight of 48,600 kg, that's 2,779,835,391 Saturn V needed to lift enough PVC to cover the Moon. Are you sure that it is "no bigger effort than many other terraformings we have discussed"? PS: Re-commented due to miscalculations in my previous comment, sorry. $\endgroup$ – JordiVilaplana Apr 6 '16 at 11:08
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    $\begingroup$ We have discussed terraformings that may take thousands, even millions of years. Earth needn't to be the only source of materials. There may be other sources of PVC in the Solar System: Titan, or carbonaceous asteroids. Are you sure that an advanced civilization won't be able to wrap a moon in plastic if they have millions of years to make it? $\endgroup$ – Ginasius Apr 6 '16 at 11:11
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    $\begingroup$ I assumed (wrongly, of course) that your setting was present day civilization. If they are advanced enough, they can do whatever they want given enough resources. Please, give us any hint of how advances is the civilization in your setting, that would give us a better idea. $\endgroup$ – JordiVilaplana Apr 6 '16 at 11:27
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    $\begingroup$ The whole project is not viable, as the answers below indicate, but if I were going to fiction it into place with a much smaller leap of logic, I'd use Lunar-manufactured glass, a low ceiling, and an array of bulwarks to keep the wind down. $\endgroup$ – J.D. Ray Apr 6 '16 at 18:23
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    $\begingroup$ FYI, this style of terraforming project is known as a World House, in case you want to look up more information on the idea in other sources. A not-bad place to start is orionsarm.com/eg-article/484746e824a3a $\endgroup$ – Logan R. Kearsley Jul 31 '17 at 2:57

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I saw several answers here close to what I wanted to say, but none of them quite do. So:

Plastic Wrap Not Needed

If you supply the Moon with an atmosphere, that atmosphere will not be instantly lost to space. On human time scales the Moon holds most atmospheric gases quite well.

However, it will slowly bleed water away to space. So the important question is, "how long can the Moon hold onto its atmospheric water?" According to my calculations (table below), atmospheric water has a half-life of almost 400,000 years.

Half-life of gases on significant bodies: Half-life of gases on significant bodies

Key:

  • Black letters, red background - gas half-life < $1 \cdot 10^8$ years.
  • Orange letters, yellow background - $1 \cdot 10^8$ years < gas half-life < $4 \cdot 10^9$ years.
  • Green letters, green background - $4 \cdot 10^9$ years < gas half-life.
  • Blue lettered "liquid" - substance is a liquid at these conditions.
  • Green lettered "solid" - substance is a solid at these conditions.
  • White letters, brown background - two phase solid/gas $4 \cdot 10^9$ years < gas half-life.
  • Yellow letters, brown background - two phase solid/gas $1 \cdot 10^8$ years < gas half-life < $4 \cdot 10^9$ years.
  • Red letters, brown background - two phase solid/gas gas half-life < $1 \cdot 10^8$ years.
  • White letters, blue background - two phase liquid/gas $4 \cdot 10^9$ years < gas half-life.
  • Yellow letters, blue background - two phase liquid/gas $1 \cdot 10^8$ years < gas half-life < $4 \cdot 10^9$ years.
  • Red letters, blue background - two phase liquid/gas with gas half-life < $1 \cdot 10^8$ years.

NOTE: I consider the substance to be two phase when its partial pressure at these conditions exceeds 0.01 bar (1% of Earth's atmospheric pressure). Anything below that and the loss of gases to space will be significantly reduced due to the small amount of the substance in the atmosphere. Also because it was exceedingly difficult to find the conditions required for partial pressures lower than 0.01 bar for many of these substances.

So for a long-lived technologically sophisticated civilization, it may just be easier to periodically refresh the Moon's surface water inventory than to build and maintain a plastic wrapping.

How to accomplish it anyway

Assuming you want to throw caution to the wind and construct the Moon's plastic wrapper anyway...

Materials

Start with composite materials.

For the matrix material, you will want a durable plastic that's not reactive to either the conditions in space or the atmosphere. The plastic must also be transparent. Something like this PVC (polyvinyl chloride) that's been mixed with a number of other plastics to make it inert.

Then embed either glass (for transparency) or carbon (for strength) fibers. You'll want to lay-up the fibers at 60 degree intervals to accommodate stresses in all directions.

Structure

Rather than making one continuous sheet of plastic across the Moon. Make this as a bunch of interconnected smaller domes. Select a standard dome size and make this the size of each segment.

Each dome will consist of a minimum of two layers of dome material. Separate the layers by about 32 feet. Fill the inter layer region with pure water. This provides your terraformed habitat with 3 essential things:

  1. Radiation shielding.
  2. Mass to counter (some of) the pressure of the atmosphere
  3. A thermal reservoir to help mitigate some of the extreme temperature swings from the 28 day long light/dark cycle.

Shape the structure so that the carbon fiber reinforcing strands reach down to anchor points in the Moons surface. The carbon fiber strands anchored to the surface of the Moon provide the rest of the necessary force to hold the gases in.

Each of the dome's sides will include something like "tent flaps". This allows a dome segment to retain atmosphere when an undomed (or depressurized dome) section is adjacent to it. But the flap will be raised when there is an adjacent dome and it is pressurized. Ideally the raising and lowering of flaps will occur automatically.

Domes will include anchor & sealing points on their exterior surface. This will allow a "dome" patch to extend from the 6 adjacent domes and cover a dome that has been damaged or needs maintenance.

Construction

To construction this in sections, make each segment a hexagonal dome. As you add segments, attach them to the appropriate face the adjacent dome. When first added, the plastic layer will extend all the way down to the surface.

Making a tight seal with the surface of the Moon will be difficult. It probably requires a special construction effort to create a rim made of some concrete or plastic analog at the dome edges. This rim probably needs to extend down into the Lunar bedrock and be formed of an inert material too.

The designers & builders would embed airlocks in the dome rim structure so they wouldn't have to put holes in the dome structural materials. Each connection face would likely have at least one airlock connection.

Timing

This is a massive effort. Before you finish the project, the original domes will probably require complete refurbishment. Some method of sealing domes away from the vacuum of space and each other must be included in the structure. That enables the construction crew to repair dome segments without total depressurization.

After completing a dome segment, the terraforming crew will pump the proper hydrospheric and atmospheric substances into the segment. Expect the Moon's surface to react, possibly violently, with the materials for a while. I presume that additional oxygen and water will need to be pumped back in after a time since the initial mix will likely react with the Lunar surface.

Including "port holes" in the structure

Simply leave some hexagonal segments empty. Then the adjoining segments would provide the airlock facilities required to access the area of vacuum.

Similarly, industrial process which could take advantage of the vacuum of space, could leave adjoining segments empty too. However, if they're exhausting corrosive substances, the builders will want to coat the dome materials with extra substances to make it inert to the caustic materials being exhausted at that point.

Living in this biosphere

Unfortunately, we really don't have any data for how things will do in this environment. Therefore, the biologists in charge of the effort might keep several of the domed hexagons sealed off from each other and experiment with different biospheres in each segment. Once the find some biospheres that work, then they might simply open the "tent flaps" and allow that one to spread across the surface.

Of course the main problem will be the day-night cycles. This will be extremely difficult for the plant life to live with. Therefore, the domes will likely possess an artificial light source to provide light during the long lunar night.

The temperature extremes will also pose a problem. The water trapped between the two dome layers will help but won't solve the problem. Including large bodies of water (water filled craters, anyone?) will help moderate temperatures but probably the Lunar surface will require additional heating and cooling. I expect the dome structures as I've envisioned them to interfere with natural atmospheric convection that might otherwise help moderate those temperatures. However, the surface could have built in heat pumps. During the long day, they exchange heat between the surface and the bottom of nearby crater-lakes - cooling the surface. During the long night, they do the same in reverse - heating the surface.

Fluffy Lunar Atmosphere

Because the Moon's gravity is so much less than Earth's, the Moon's atmosphere will be much deeper. You will also need 6x the mass of atmosphere in a column to make the same pressure that you see on Earth. What this means is that the Moon requires nearly the same mass of gases as the Earth does for a given pressure.

  • Earth's atmosphere mass = $5.1 \cdot 10^{18}$ kg
  • Moon's atmosphere mass (for 1 bar pressure) = $2.3 \cdot 10^{18}$

In fact, the Moon's atmospheric profile ought to look very similar to Titan's (at least the pressure would - probably need to add ~200 K to the temperature though):

Titan Atmosphere Profile Titan Atmosphere Profile

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  • $\begingroup$ Will the water freeze at "night"? Is the resulting expansion likely to cause a problem? $\endgroup$ – Joel Apr 8 '16 at 1:45
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    $\begingroup$ The people engineering the thing will accommodate water's expansion - I can think of several ways to do this. I'm not sure whether it will freeze or not. I was thinking that circulating the water in this region might be a great way to pump heat around the Moon. It'd be an interesting problem though. $\endgroup$ – Jim2B Apr 8 '16 at 2:48
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    $\begingroup$ The "fluffiness" of the atmosphere is irrelevant in this case, as the atmosphere does not extend upwards to arbitrarily low pressures. It gets capped off by the plastic roof, and the weight of the roof replaces the weight of all of the additional atmosphere that would've been needed to maintain the desired surface pressure in the roof's absence. Ergo, you need about the same mass of stuff, including the roofing material, but you don't need that much gas specifically. The gas requirements can be freely adjusted by changing the altitude of the roof. $\endgroup$ – Logan R. Kearsley Jul 31 '17 at 2:52
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Actually, there are multiple reasons it can't be done as you hoped

Trying to make a single air-supported structure to cover the entire moon is a rather insane idea. For safety and other reasons, large areas would be necessarily be broken into a (possibly connected) series of pressurized areas. Losing pressure (planned or accidental) would limit the affected area. Some maintenance would certainly be required, and that implies depressurization in some cases -- replacing an old section, etc.

Building in sections is also a huge advantage in that you don't have to pay to envelop the entire moon at one time; you can pay as you go while you expand. The 8000 meter high is a pretty strong constraint on the smallest individual dome size (as the vertical extent is 8000m, as you would probably want the horizontal dimension to be at least 8000m, so you are probably 100 sq. km. or so for the individual domes -- thus you need about 380,600 domes to cover the moon.

We have zero experience trying to make such large structures, esp. for huge air supported structures. While this does not necessarily mean it can't be done, it will certainly raise the difficulty of doing so.

Your suggested internal air pressure is far in excess of the air pressure actually used in existing inflated structures (well over 100 times as great). This actually is a huge problem too; even a 1 km square structure would require impossible strength to keep the roof (or the sides of the dome) from destruction as it would have a net lift of over 10 million tonnes.

Standard PVC does not have near the operating temperature range that you would need for lunar use. -- it becomes soft and distorts, and may even melt at the top daily lunar temperature (2 week long days and nights).

Even without temperature softening problems, PVC would not last long due to oxygen degradation from the inside of the dome, esp. at high temperatures that accelerate the oxygen reactions, and photo degradation from the abundant ultraviolet light during the day. Micro-meteors and ablation from solar wind won't help the dome much either.

Other materials would be more durable, but are going to be more expensive and likely require more research. For example, glass has many desirable characteristics including transparency at visible wavelengths. Some modern flexible glass display materials might serve or could be adapted. On a large scale, air pressure could certainly be able lift such a structure, were it not for the high pressure that would destroy it.

Even if you make a pure oxygen atmosphere at 15% of standard atmospheric pressure, the internal pressure will still destroy any huge pressure supported structure. You could (as is actually done with some such structures), build internal support bracing that connects to the roof, but this is rather far from your stated design, i.e., it tends toward a more typical rigid roof design, esp. given the high pressures required for a breathable environment.

Just thought of another big problem -- the huge windstorms inside the dome. Due to the month long days and corresponding temperature changes, there will be very active internal storms systems.

To be specific re: your questions.

1) How to build -- no need to bother, since it will break apart once you try to inflate it

2) How to maintain -- you must build it in sections, or invent new tech to repair your roof in situ, but for safety you must build it in sections. Actually, even when built in sections, you will still want the automated repair bots to handle most of the maintenance without closing the sections.

3) Any sort of airlock design would be acceptable for access to the pressured region. Presumable a mix of large and small airlocks of various designs would ultimately be desirable.

How about a better idea. It would be better just be forget the dome entirely, and just add an atmosphere to the moon.

8000 meters worth of atmosphere means you already have a significant fraction of the atmospheric mass needed for the whole moon. On earth, about 65% percent of the atmosphere is in the lowest 8000 meters. Due to the lower lunar gravity, the pressure drop with altitude will be slower than on earth, but if you add enough air, you eventually get a nice breathable atmosphere with earth normal pressure at ground level.

Sure, it leaks into space, but the rate will slow enough to be usable perhaps. Some people have calculated that you would only need to top off such a lunar atmosphere only once per 10,000 years or so. You could add a solar wind shield to the moon to slow the rate of air loss too, though there is disagreement on how much difference it actually makes.

Terraform the moon by dropping plenty of comets, etc. selectively to get the lunar spin back to a decent day duration that provides the oxygen, nitrogen, etc. If you have high enough tech, you get all the oxygen you needed from a icy comets, moons, etc. You also want inert gas to add to the overall mass of the atmosphere. There is probably no better choice for this than nitrogen and there is plenty of ammonia on some planets, moons, and comets to fill this need.

There are a few big complications, once you add atmosphere, the oxygen will start reacting with the existing lunar surface -- think lots of rust being formed, so you many need to wait a few thousand years after initial terraforming for things to settle down. That's not too bad, since you have to wait a long time for the moon to cool from the cometary bombardment anyway.

But, wouldn't it be grand to look up in the sky and see another blue marble.

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    $\begingroup$ re: air pressure: 10 million tonnes of force is distributed over 1 square km of plastic. I think you're misunderstanding pressure. If 1 m^2 of a material can hold back a pressure of 1 bar, 1 km^2 of the same thickness of the same material can still hold back 1 bar. The problem with scaling up isn't static pressure, it's turbulence and stuff that could create local spikes in pressure. $\endgroup$ – Peter Cordes Apr 6 '16 at 15:52
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    $\begingroup$ @PeterCordes - sorry, but the lift will definitely pull apart the material. Lift is proportional to area, but the tension load must be resisted by a section of material that is proportional to perimeter. Classic square / cube law problem. Making the material weight the same amount as the pressure load does not help as the static load still applies to the sides of the dome. $\endgroup$ – Gary Walker Apr 6 '16 at 16:40
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    $\begingroup$ Pressure is force per unit area. Having more area doesn't increase the force on each 1m^2. The material is under tension in two directions, in the plane of the material. There's nothing here that scales up cubically, except the volume of air. But that's volume, not pressure. The ideal gas law is PV = nRT. Pressure and volume are independent variables. $\endgroup$ – Peter Cordes Apr 6 '16 at 17:36
  • $\begingroup$ @PeterCordes. Consider a square 1 km on a side. Assume dome is 1 cm thick PVC (much thicker than used in air domes on earth), So the 4 km perimeter has a total area of 40 square meters, so it must have a tensile strength of 250,000,000 kg (earth gravity) per square meter, (this is about 0.355 million psi) - this is 4.4 times more tensile strength than the strongest steel and 50 times the tensile strength of PVC. It is called the square-cubed law even when you are only compared linear (perimeter) and area. And a 1 km square dome is very small in this application $\endgroup$ – Gary Walker Apr 6 '16 at 18:15
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    $\begingroup$ @GaryWalker: oh, I finally see what I was missing, thanks. You're right, each unit area of the surface transmits the tension to its neighbours, so the total area does matter. And I agree that using heavy material to counter the force doesn't work unless you're covering the entire moon so it's always perpendicular to gravity. Or maybe if the sides aren't vertical even near the ground? Instead of a hemispherical dome, use more of a lenticular dome. Or use thicker material in the sides. $\endgroup$ – Peter Cordes Apr 6 '16 at 19:02
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You might run into another problem. Even if you have no holes air will gradually pass through a thin sheet of plastic. Even over the course of decades you'd lose a hell of a lot of air through it.

Also seconding the people saying it's not strong enough. Imagine the effects of some air getting heated in one spot. You end up with a plume 8KM high. at the top it hits the plastic and distorts it upwards, it could be doing it over thousands of square miles. the plastic would be rippling like waves on the surface of the ocean but at larger scales. PVC would disintegrate from all the constant pulling in different directions even if it somehow survived the forces involved in the first place.

I would suggest something a lot more modest. Perhaps a few thousand square miles of reinforced airtight greenhouses on the lunar surface.

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  • $\begingroup$ According to Jim2B's numbers, only 0.005% of a nitrogen atmosphere would be lost in 100 years without the plastic (slightly less for a nitrogen-oxygen atmosphere). The plastic will only decrease this further. $\endgroup$ – Charles Apr 26 '16 at 17:54
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It will crash into the planet if not supported.

In addition to the problems other people have listed, the air present inside your bubble won't stop it from crashing into the planet. The air on the 'day' side of the planet will start to expand from the heat, while the air on the 'night' side contracts due to cooling. This will create a significant net force on the bubble, pushing it towards the sun.

While the air inside the bubble will slow its collapse, it will not stop it. Air will stream from the cold side of the bubble to the warm side, ripping up buildings on the surface and throwing around boulders with sustained, hurricane force winds. Then, the plastic on the cold side of the moon will slam into the surface, crushing any remaining structures before rupturing. The force of the moon stopping the plastic from moving further will then propagate across the plastic on the sunny side as a shockwave, which will grow progressively stronger and more concentrated as it moves across the sunny side, which will rupture, crack, and break apart, releasing the rest of the trapped air into space. Some bits of the plastic sphere will likely reach escape velocity and fly off into space, possibly towards Earth where they will cut swathes of destruction through our orbital satellites before burning up in the atmosphere. The rest of the plastic will fall back to the surface of the moon, where it will form a clear plastic sarcophagus for the lifeless corpses of the once-hopeful lunar settlers who once tried to live beneath it.

TLDR: Plastic falls. Everybody dies.

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    $\begingroup$ You can solve this problem with tethers that hold the plastic to the surface. (Holding down the parts that want to go up, rather than holding up the parts that want to go down.) Tethers take much less material than pillars, because they don't have to resist buckling stresses. $\endgroup$ – Peter Cordes Apr 6 '16 at 16:04
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    $\begingroup$ I suspect you may be overestimating the force of winds generated by the sun in this instance. After all, Earth has a "night side" and "day side" as well, and that doesn't (usually) result in hurricane force winds "ripping up buildings on the surface and throwing around boulders". $\endgroup$ – Ajedi32 Apr 6 '16 at 21:10
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    $\begingroup$ On earth, there is more distance between cold and hot sides, as well as shorter days. How that would play out in practice I'm not sure (Does day length make these forces more or less extreme, for instance). Moon-wide hurricane, not sure. Significant storm? Almost certainly. $\endgroup$ – wedstrom Apr 6 '16 at 22:23
  • $\begingroup$ Hmm, it would take some calculations to confirm or refute this scenario. Yes, the side facing the sun would heat and the air would expand. But how much? If air is flowing under the plastic, that would reduce temperature differentials. On Earth, difference between day and night air temperature is typically what, maybe 20 F? Anyway not hundreds or thousands of degrees, but tens. Moon's day is longer, so maybe it would be more extreme, but, etc. $\endgroup$ – Jay Apr 26 '16 at 16:29
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To create what you suggest will take far more advanced technology than a bubble of plastic.

First of all, plastic isn't a suitable material anyway, lacking strength, being susceptible to breaking down due to the action of oxygen and ultraviolet light as well as temperature extremes and generally permeable to air.

The substitute will have to be far stronger and more stable. I would suggest using diamond as the substrate, and possibly one or more over layers of materials like gold to protect the diamond from oxygen and reflect some of the unwanted solar energy from the Moon. This would still resemble a thin, flexible sheet on the scale we are talking about.

The next issue is that there needs to be active control. The sheets of diamond might be held together with flexible graphine gaskets, which have some sort of electrical or mechanical controls to adjust for deflections from the atmosphere below. Further control would have to come from a elaborate system of tethers to the surface, which help keep the bubble overhead at a constant altitude.

The natural circulation of the atmosphere might also need to be adjusted, since the "hot spot" at local noon will deliver a huge amount of energy into the atmosphere, causing potentially hurricane force winds. Selectively adjusting the reflectivity would keep the temperature extremes more reasonable. On the cold side, the opposite problem would occur, as heat leaks from the atmosphere into space. The bubble might have to have multiple layers to provide insulation from both heat and cold, and active circulation of fluid between the layers to equalize temperatures.

OF course, even without a bubble covering the entire moon, the Moon has enough gravity to hold an atmosphere for tens of thousands of years on its own, longer than any recorded civilization on Earth.

Terraformed Moon

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    $\begingroup$ the Moon has enough gravity to hold an atmosphere for tens of thousands of years on its own Source? I always assumed it didn't have enough gravity to hold an atmosphere on any appreciable time scale. Holding an atmosphere for thousands of years would be a game changer. $\endgroup$ – Martin Carney Apr 6 '16 at 22:29
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    $\begingroup$ slate.com/articles/technology/future_tense/2014/07/… $\endgroup$ – Thucydides Apr 7 '16 at 0:42
  • $\begingroup$ @Martin carney Most of the article is rubbish, eg. the idea that we could change moon's rotation by crashing comets into it. But lunar surface gravity & escape velocity are about 1/6 earth's. Moon probably can hold an atmosphere for 10000 years, given that Earth has done so for billions. But the mass that would be required to create and maintain an atmosphere it is huge. If we crashed a large comet like halley's comet (3E14kg) into the moon and it all vaporised (which it wouldn't) it would give about 12 bar pressure. You best not make a mistake steering something that big close to earth! $\endgroup$ – Level River St Apr 7 '16 at 12:29
  • $\begingroup$ Adding enough energy and momentum will change the rotation of any body (the Earth's rotation was radically changed when a giant protoplanet impacted it and created the Moon in the first place). It is all a matter of scale. $\endgroup$ – Thucydides Apr 7 '16 at 12:34
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    $\begingroup$ @Thucydides I just realised my calculation is out by a million - we'd actually need a million halley's comets to get a (theoretical) 12 bar pressure. But even that would be only 3E20kg, which is still 1/200 of the mass of the moon, not enough to drastically impact the moon's rotation, even if delivered perfectly. I would accept this if it were presented as fiction, but your linked article presents this as if it were something the Russians might do in the foreseeable future. $\endgroup$ – Level River St Apr 7 '16 at 14:04
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Everyone is forgetting the biggest hazard on the moon - radiation. Without a magnetosphere, the moon is constantly bombarded by huge amounts of radiation from the Sun. and PVC is not particularly good at shielding from the intense radiation you'd be getting. There's a reason the Apollo missions were all relatively short and nobody got to go more than once. Your best bet for long term colonization is heavily shielded underground bunkers. Granted it's not as sexy as walking around in the open air on the surface, but it's far more viable.

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  • $\begingroup$ The PVC is not much a shield, but 8000 meters of air is an awesome radiation shield, except for the UV, but this is enough bulk atmosphere to support a fair bit of ozone too. $\endgroup$ – Gary Walker Apr 6 '16 at 16:47
  • $\begingroup$ @GaryWalker - I still feel like the lack of magnetic field would subject people to far more radiation than they would receive on Earth, even with an artificial atmosphere. $\endgroup$ – Darrel Hoffman Apr 6 '16 at 16:49
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    $\begingroup$ The magnetosphere only deflects charged particles. All charged particles hitting the earth are reliably stopped by air by a few meters air at most (at standard pressure) Free neutrons, Gamma and x-ray will also be stopped by 8000 meters of air (the earth atmosphere is only about 50% greater in total). It is in fact as I said, that U/V is really the only radiation problem, primarily soft U/V as atmosphere stops U/V harder than the U/V A/B then gets through the atmosphere. There will be a only a slight increase in surface radiation, maybe somewhat more skin cancer due to the U/V. $\endgroup$ – Gary Walker Apr 6 '16 at 17:48
  • $\begingroup$ The reason the Apollo missions were relatively short was not radiation, but the limited amount of consumables (food, water, oxygen, spacecraft power...) that could be carried. $\endgroup$ – jamesqf Apr 6 '16 at 19:37
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    $\begingroup$ 8000 meters of full air inside a dome would have the same total atmospheric shielding as living at about 3300 meters, the highest city with population greater than 1 million is the aptly name El Alto Bolivia (over 4000 meters) could not even find an article on their cancer rates, so no-one has been terribly concerned about it. Found a study for altitude vs cancer in USA; comparing Colorado to Florida, Rhode Island, etc., the cancer rate was actually lower in Colorado. I had expected to find a least a minor cancer increase at high altitude. $\endgroup$ – Gary Walker Apr 7 '16 at 2:46
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Ordinary plastic like PVC would not have the necessary strength to resist either an inflation or an orbit maneuver for scales of that size.

This means that if you pulled one part (or even several) of the plastic piece it would not move the entire piece. It would just rip apart.

Assuming that this plastic piece was already somehow in place notice that 1 Bar is almost the pressure we have, on Earth, at sea level. This is quite a bit. For your structure to support itself without collapsing (it would be in free fall after all) it would need to orbit the moon at speeds far from Selenosynchronous orbit requirement (I doubt 8000 meters is enough). In any case this isolation "plastic" would rip apart with the forces produced by the speed difference between plastic surface and moon surface.

I do not think you can get away with using such a conventional material, or such a simplistic (one piece?) structure, for such an unconventional use. Neither the implementation or maintenance would work. Off the top of my head I would say the deterioration would be immediate.

I think pursuing this idea in similar molds to the ones you have stated would require other type of structure (or composition of structures).

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Serban Tanasa Apr 7 '16 at 15:13
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While it's certainly not practical there's a fix that addresses most of the points brought up in this thread:

Put your sheet of material and then on top of it put 10 meters of water and then another sheet to keep that from boiling away into space.

Radiation: You'll have less than on Earth.

Strength: No super materials needed--the net force on the inner sheet is zero, the net force on the outer sheet is only the vapor pressure of the water--and you can keep it just above freezing to keep this down. The water balances the pressure of the atmosphere underneath, the outer layer can be heavy enough to balance the vapor pressure. You only need support to keep things in place.

Micrometeors: They'll be absorbed in the water. A little hole gets punched in the upper sheet that has to be repaired, that's all.

We still don't have a good construction material for the sheets and the sheer scale of the project is far beyond anything we could do at present. A water roof would be far more practical over a crater than over the whole moon.

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    $\begingroup$ The only problem is if that incase the innershield gets lower a little, it will cause a avalanche effect in that the water pressure on that part will be higher, and it will result in even more downward force. $\endgroup$ – Ferrybig Apr 7 '16 at 7:46
  • $\begingroup$ @Ferrybig Which is why you need a support sufficient to keep it basically level. If you put a grid high enough above the city it will be invisible to the naked eye--you'll see the sky (admittedly dimmed and tinted by it's passage through the water), not a roof. $\endgroup$ – Loren Pechtel Apr 7 '16 at 23:22
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One thick sheet of plastic strong enough to hold back 1 bar isn't going to be very practical, as other answers point out.

(source for this idea: Neal Stephenson's Seveneves.)

Some (but not all) of the problems can be addressed by a design using multiple layers of plastic, like an onion. The pressure differential between any two layers is small. This means each layer is thin enough to fold. It also has the huge advantage of defence in depth and maintainability. You can remove and replace one layer without a huge increase in pressure on the other layers.

The whole moon is too big to depressurize quickly through a meteor puncture. You'd need to make sure you used a material where holes don't grow quickly from the force of air rushing out through it. You need to patch the outer layer before the pressure differential on an inner layer gets too big.

1 bar of air pressure is enough to hold up a layer of water 62m thick, there's no chance of using enough material for the weight to actually balance the pressure, whether we use many layers of plastic, or just several thick-ish layers. It will take a huge amount of material, but maybe less than with one thick layer.

(Water has a density of $1000 kg/m^3$, and thus on the moon a weight of $1.62 \cdot 1000 N/m^3$. $1 bar = 100kN/m^2$, which is enough pressure to hold up a layer of water $62m = \frac{100kN/m^2}{1620N/m^3}$. Note how the units mostly cancel out, leaving just meters.)

Tether the inner layer to the ground. Stopping it from getting too far away anywhere will also stop it from getting to close on the other side. Tethering the outer layers to each other may not be necessary, other than at airlocks, since it doesn't really matter how the outer layers move around relative to each other.

It would even be possible to have bladders of water sewn in so the weight could help balance some of the pressure. (This probably works better with a single thick layer than with many thin layers, so maybe think of this as more of an alternative idea.) This works as meteor and radiation shielding, too. (But this idea is much more feasible for a local dome than a moon-enclosing bubble.)

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  • $\begingroup$ I thought logistics for this would be a pain in the rear, but theoretically it is possible. I even considered it myself (though I never read Steveneves). Didn't think it changes the fundamentals by much, a meteor will likely puncture all of the layers at the same time, and patching 100 or 1000 layers sounds more complicated than patching a single layer. But, is this examined in detail in the book, i.e., is it worth reading to understand it in some depth? $\endgroup$ – Gary Walker Apr 6 '16 at 18:25
  • $\begingroup$ @GaryWalker: if there's a decent air thickness between layers, a micrometeor can fragment and burn up before reaching the next layer. I think the main advantage is lower forces on each layer so tears won't propagate as easily. You'd need patch robots moving around between every layers, or something, for a moon-size application of this idea. In Seveneves, it was just used for a person-sized spacecraft, and doesn't receive a lot of attention. Definitely not wrt. long-term maintenance. I'd highly recommend Seveneves as a good read in general though! $\endgroup$ – Peter Cordes Apr 6 '16 at 18:31
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Everybody so far is forgetting one simple problem: meteorids. There is a plenty of them, they are fast (50 km/s) and they will shred your plastic cover to pieces.

According to this astronomer researching asteroids hitting Moon asteroid 30g or bigger will hit in average area size of Hong-Kong every year or so.

So yes, even if you can wrap the Moon, wrap will not last. Earth is protected by the atmosphere (meteroids will burn), Moon - not so much.

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It is impossible to make a spherical bubble of that size. Many small bubbles may be more feasible for several reasons.

Consider an arbitrarily long inflated cylinder. The tension in pounds around a short ring is the pressure (say, 14.7 pounds/square inch) times the width and circumference (in inches). When this ring becomes large, it will rupture. Physics doesn't allow arbitrarily large inflated habitats.

But that doesn't mean you can't have inflated domes across the surface.

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Where you gonna get enough nitrogen to make the air? The lunar regolith is nearly 50% oxygen, but whence the nitrogen? You would need an ammonia-rich asteroid so large that hitting the Moon with it would split the Moon open and shower the Earth with dinosaur-killing fragments. Even a simple concrete dome with a 1:4 height-to-width ratio and 1 kilometer in diameter (vastly smaller than your idea) would require about 100 million cubic meters of nitrogen. An asteroid that size hitting at a speed of 3 klicks per second (slightly over escape velocity) would make a bang in the near-one kiloton range. A mini Hiroshima. And that's for one little dome a kilometer across. Domes, in general, don't work on the Moon unless you have a lot of free air. They are very wasteful of air. And this 'wrap it in plastic' idea is like a dome on super-steroids. Completely un-doable.

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  • $\begingroup$ While this is a valid concern, this presumably isn't a story set now, so you don't need to worry about that as it's an implicit assumption in the question that there's enough gases $\endgroup$ – Mithrandir24601 Sep 3 '17 at 8:12
  • $\begingroup$ @Mithrandir24601 Sorry, but the answer has a point. Even in future you cannot create nitrogen out of nothing and Earth has not so much of it to waste it for the moon. So the story must acknowledge this fact. $\endgroup$ – Thorsten S. Sep 3 '17 at 22:28
  • $\begingroup$ @ThorstenS. I agree there's a point, but it's not the answer. Unless I'm missing something, the OP has made a set of implicit assumptions that includes "There's enough gas" and "we can get it to the moon" as these aren't asked. It's worth mentioning in an answer, sure, but that doesn't explain how to make, maintain or live with a massive plastic dome, which is the point of the question $\endgroup$ – Mithrandir24601 Sep 3 '17 at 22:36

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