Domes are over-rated

Part of my series on common misconceptions in space journalism.

It is an unwritten rule of space journalism that any article about Moon or Mars bases needs to have a conceptual drawing of habitation domes. Little scintillating blisters of breathable air clustered between pointy antennas.

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Look, I get it. Domes are cool. I’ve built several. And while I don’t regard myself as an expert on Mars urban planning, I believe domes are not a very good solution for building cities on Mars.

I’m going to motivate this post by describing constraints on “the mission”. There is a time and place for discussion of short term exploration missions with a few plucky astronauts, but as far as I’m concerned, the SpaceX Mars vision is the biggest, baddest vision for exploration and industrialization, and the obvious design reference for this blog.

The goal is to build a self-sufficient city on Mars. This will require enormous quantities of money, time, cargo, and people. The goal is to understand ways to reduce the requirements while increasing the probability of success. Given that human capacity to solve problems isn’t infinite, a good mission architecture allows the technical teams to focus on the core self-sufficiency problem rather than working around counterproductive constraints. As an example, one of the main weaknesses of the Lunar Gateway is that so much engineering effort is required to build the space station, which diverts attention and resources away from the more direct challenges of building landers and surface base hardware. Similarly for Mars – the last thing anyone needs is an architecture that requires undue design work and places hard limits on future growth.

For generations, engineers have internalized a hard mass constraint on Mars missions, resulting in scores of proposals that struggle to keep a handful of humans alive for a few years in a spaceship that weighs just dozens of tons. But while it may be *just* possible to do an Apollo-style human Mars landing with a few SLS-loads of stuff, building a city requires either non-existent self-replicating robots or much, much more cargo. Hence the significance of the SpaceX Starship, which plausibly can launch not tens, but millions of tonnes of cargo to Mars.

The Starship provides a mechanism to retire the Mars city mass constraint, just as Starlink can retire the capital constraint. What are some other important constraints?

Consider the raw material constraint. A self-sufficient city can be either open or closed, but total recycling of all materials with either huge reserve stocks or perfect efficiency, such as would be required for a slow interstellar spaceship, is much much more difficult than an open system. In an open system, an effectively infinite supply of natural raw materials can be used, and wasted, as needed. The Mars city solves this problem by building on the surface of a planet that is made of all the raw materials it could need.

The next constraint of concern is the labor constraint. Self sufficiency means different things to different people, but a sufficiently healthy and well-trained individual human can survive indefinitely in many places on Earth. Indeed, small groups of cooperating humans have been capable of survival since the dawn of technology, and even crossed oceans before the wide-scale adoption of metals.

That said, the surface of Mars is next level in terms of its sheer hostility to life. It’s a pitiless frozen vacuum. The Earth’s south pole in the middle of winter is closer to a beach in Hawaii than the nicest place on Mars on the nicest day of the year. So when we think of self-sufficiency on Mars, the traditional “pioneer with 20 acres and a mule” just won’t cut it. Like the bottom of the ocean or the stratosphere, human survival is possible only with advanced technology. Self sufficiency on Mars means the ability to build all the requisite advanced technology, and to do it faster than it wears out.

Instead of a small group of generalists, the Mars city will need teams of specialists to replicate every part of the modern industrial stack, from raw material extraction and processing through to advanced lithography, and everything in between. This process is enormously labor intensive, requiring on the order of 100 million humans on Earth. On Mars, SpaceX hopes to get by with “only” a million people and a lot of manufacturing automation. Even then, the high marginal cost of keeping an extra human alive changes almost everything we take for granted about how jobs are done here on Earth. For more in this vein, check out the relevant chapter in my book in Mars industrialization.

For the purposes of this blog, however, we need merely agree that humans building a self-sustaining city on Mars will be extremely busy. And so we come to the space constraint.

No matter what people are doing on Mars, the mission designers will do everything they can to make their jobs as easy as possible, to maximize productivity. The next most important constraint on productivity is space. On the domestic level, Marie Kondo can help us make the best of our inability to avoid accumulating worthless possessions. On an industrial scale, it turns out that manufacturing difficulty is exacerbated by not having enough space. Factories need room to move things, store things, and lay out assembly lines.

When cars were first built, factories were multilevel buildings in cities with elevators and total vertical integration. Today, most manufacturing plants are laid out over a single level in areas with lower land value, and cover millions of square feet.

So it must be on Mars. We cannot industrialize a new planet in pressurized trailer homes or tuna cans or subterranean tunnels. Nor can we operate efficient factories in space suits. The city will need unimaginably enormous climate controlled spaces to enable millions of people to work efficiently in a shirt sleeves environment.

What about domes?

A prefabricated dome assembled on Mars would certainly have more interior volume than a lander or short tunnel. On the other hand, domes have significant drawbacks that are underappreciated by their advocates, many of whom haven’t actually ever tried to build one!

Domes feature compound curvature, which complicates manufacturing. If assembled from triangular panels, junctions contain multiple intersecting acute angled parts, which makes sealing a nightmare. In fact, even residential dome houses are notoriously difficult to insulate and seal! A rectangular room has 6 faces and 12 edges, which can be framed, sealed, and painted in a day or two. A dome room has a new wall every few feet, all with weird triangular faces and angles, and enormously increased labor overhead.

It turns out that the main advantage of domes – no internal supports – becomes a major liability on Mars. While rigid geodesic domes on Earth are compressive structures, on Mars, a pressurized dome actually supports its own weight and then some. As a result, the structure is under tension and the dome is attempting to tear itself out of the ground. Since lifting force scales with area, while anchoring force scales with circumference, domes on Mars can’t be much wider than about 150 feet, and even then would require extensive foundation engineering.

Once a dome is built and the interior occupied, it can’t be extended. Allocation of space within the dome is zero sum, and much of the volume is occupied by weird wedge-shaped segments that are hard to use. Instead, more domes will be required, but since they don’t tessellate tunnels of some kind would be needed to connect to other structures. Each tunnel has to mate with curved walls, a rigid structure that must accept variable mechanical tolerances, be broad enough to enable large vehicles to pass, yet narrow enough to enable a bulkhead to be sealed in the event of an inevitable seal failure. Since it’s a rigid structure, it has to be structurally capable of enduring pressure cycling across areas with variable radii of curvature without fatigue, creep, or deflection mismatch.

Does this sound like an engineering nightmare? High tolerances, excessive weight, finicky foundations which are a single point of failure, major excavation, poor scaling, limited interior space, limited local production capability. At the end of the day, enormous effort will be expended to build a handful of rather limited structures with fundamental mechanical vulnerabilities, prohibitively high scaling costs, and no path to bigger future versions.

Is there a better alternative? I think so.

What we need is a method for pressurizing vast areas of the Martian surface with relatively little hassle, labor, and raw material. For a long time, I thought the key might be gigantic masonry vaults, but I’m increasingly convinced that tensile structures are inherently better due to much lower mass requirements. The same goes for digging tunnels, which is so labor intensive that almost no-one lives underground. In contrast, a thin, flexible tensile membrane supported by its own pressure seems to be a step in the right direction. But how would this work?

UV resistant polymers as as ETFE are routinely used for exterior cladding of structures that need transparent, curved, lightweight, waterproof barriers. Some of these structures are even pressure stabilized. A multilayer ETFE fabric incorporating Kevlar fibers is an ideal material for both high performance yacht sails and transparent pressurized structures on Mars. This material can be thermally or chemically welded in the field for ease of integration, modification, and repair.

To transmit pressure load into the ground, a pressurized ETFE membrane must be periodically anchored to the ground by steel cables that can be arbitrarily long, supporting ceilings high enough to permit cloud formation! Unlike a dome foundation, cable anchors can be pile driven from the surface with generic hardware and without trench excavation. Both ETFE and steel can be readily produced from local resources. Importantly, the cables are located in discrete areas and offload all the pressure, so that couplings to the surface at the perimeter don’t have to resist enormous pullout forces and can be done with a set of relatively simple buried steel membranes.

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Conceptual sketch of section of tented area.

Within the pressurized structure, membrane tunnels, shelters, and bulkheads can be readily deployed to ensure any desired level of redundancy or compartmentalization. Tall structures can be supported from the ceiling in tension, reducing material requirements.

Despite its light weight and versatility, inflatable tensile structures have a long history and is not as exotic as it may sound. It is used for both Zodiac dinghies and inflatable mattresses! It has even been used to make a fully functional inflatable plane!

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Such an approach has many advantages over rigid dome construction, but the most important one is that it crushes the space constraint. What is the per capita pressurized area requirement on Mars? In a future post I’ll estimate this more rigorously but I believe it’s on the order of 10,000 sqft. If a Mars base is doubling its population every launch window, then the 5000->10000 person increment requires the addition of about a thousand acres of enclosed area, in just two years. This is about 3000 standard prefabricated domes (roughly one per person!), or less than two square miles of additional membrane, with distributed concurrent anchor construction. Only one of these methods scales easily!

Within the pressurized volume, people can build houses, schools, factories, farms, forests, or anything else they want. The space is versatile and flexible. For more in this vein, see my earlier post on Mars urban planning.

What are the potential drawbacks of such an approach? Thanks to great comments on Hacker News and Twitter, I have a good idea of what people are worried about.

Anchoring. Some have suggested anchoring to a second membrane on the ground, to avoid anchors and to isolate from Mars dirt (that may have toxic chemicals) and to avoid perfusion of gas through the ground. While enclosing membranes are great for bridges, corridors, and air handling, I think building directly on the dirt is the best approach. Provided the perimeter walls are deep enough, the risk of substantial leaks is low. All pressure structures leak, so the key is to ensure that there’s a ready supply of new gasses to make up the difference. As for perchlorates, they decompose in water at room temperatures. In other words, spraying a newly pressurized area with warm water should be adequate to neutralize the extremely thin layer of dust. Finally, farming on directly covered land won’t require moving millions of tonnes of dirt through an airlock.

Radiation. As explained in this blog post, I believe that unshielded radiation exposure on Mars is not one of the major problems to deal with. While it’s possible to build some kind of laminated inflatable structure with pockets of transparent water for shielding, in practice living and sleeping spaces will have modest shielding, and exposure in the “outdoors” will be part of life, just as excessive sun exposure on Earth can cause increased risk of cancer.

Mars’ atmosphere’s effective thickness as shielding is about 10 cm, compared to 10 m on Earth. But 100 m of 1 atm atmosphere under the pressure surface, condensed, would add another 10 cm of shielding! If it is decided that 50 cm of shielding is the minimal requirement, that can be met with 400 m ceiling heights. The only marginal increase in pressure structure cost is longer steel anchor cables.

Underground living. Quite a number of readers pointed out that natural caverns, canyons, and Boring Company-dug tunnels are options. While I agree these have their uses, they also have significant drawbacks. Underground spaces are incredibly labor and energy intensive to build – to the point that basically no private underground structures of any size are routinely constructed. Even natural caverns are location specific, difficult to expand, difficult to ensure structural stability, and lack natural light. I think TBMs will be used extensively to operate pressurized underground mines for minerals not abundant on the surface. But in general the easiest level to build is at ground level.

How does redundancy work? What happens if there’s a hole? Fiber-reinforced ETFE has really good ripstop properties, so a hole won’t suddenly spread over the whole area, leading to instant catastrophic failure. Still, the air will leak out stopped only by choked flow. If the leak rate is lower than the maximum replacement rate, no problems. The hole is found and patched. If the leak rate exceeds the maximum replacement rate, then the pressure will start to drop – which also reduces the leak rate. If the pressure drops below the level needed to tension the anchoring cables, then the whole ceiling will gradually deflate like a bouncy house at the end of a party. As the pressure drops, occupants will need to evacuate to “air shelters”, which would take the form of sealable corridors within the main structure. The membrane will gradually drape itself over whatever structures/former trees/non-spikey supports remain.

Additionally, the pressurized volume must be segmented by vertical bulkheads into separate compartments, so even total collapse (or toxic chemical leak) in one section doesn’t affect other sections. A collapsed membrane could still be repaired by robots or workers in space suits, and once the hole is patched, repressurized.

For particularly important areas, multiple membranes or layered membranes can be deployed to reduce the odds of a leak all the way through.

How much steel is required? Pressure at sea level on Earth is around 100 kPa, or 100 kN/m^2. On Mars, we can make do with lower pressure and enriched oxygen, reducing structural pressure loads, but the gravity is also lower so the total amount of rock needed to anchor is about the same. A 40 kPa atmosphere on Mars needs about 3-4 m of rock to completely react out the pressure load, though to be on the safe side the anchors would be driven somewhat deeper than this. According to The Engineering Toolbox, a 20 mm steel cable is adequate to lift 40 kN, implying an average steel fill fraction of 0.05% to anchor the roof, regardless of cable configuration. Span between anchors is determined by ground conditions and overall desired membrane tension, but I think 50 m is about the sweet spot. With a mass of 1.5 kg/m, the membrane could be flown as high as 6 km before the mass of the cable (to say nothing of its cost) was enough to hold down the membrane without an anchor.


So when one thinks of a city on Mars, don’t think of some quaint potato patch in a dome that’s too small for a game of tennis. Think instead of a gently puckered transparent plastic sky that stretches over the horizon in all directions, supported by a sparse forest of steel cables with endless open space between. A new savanna.

122 thoughts on “Domes are over-rated

      1. Fascinating idea! And if the membrane idea is as engineering solid as it seems, why not a test model here on earth? In a desert area for example, pressurized to one earth atmosphere, or something lower if that was the projected mars standard, say 10psi?
        This may already nearly exist since the writer states that pressure stabilization is already used in membrane structures. Just design for it and crank a test model up to 10 or whatever psi is chosen and let’s see how it works, how long it lasts, and how well air loss around the perimeter can actually be mitigated.
        If I was in the membrane structure business I’d be building a test model right now and shooting for publicity and funding to evolve this design.

        Liked by 1 person

  1. Would more familiar, hermetically-sealed skyscraper-like structures be any good? Seems like then you could bulkhead off lots of sections to protect against leaks and such. Maybe excavate some big underground cavities? I love the amount of thought put into thinking about these Mars colonies outside of the usual imagery, thanks for writing!
    Of course, just in case you hadn’t seen it yet—gotta plug my favorite web resource for hard science fiction/real designs for serious manned space travel:
    Has some sections on planetary bases and other kinds of habitats. Super interesting! Don’t start reading it if you have something important to do later! It’s a serious wormhole.

    Liked by 2 people

    1. Towers, bridges, and other structures are also possible with tensile membranes, or hybrid structures. But I don’t see any reason for highrise when land is infinitely available.


  2. I’ve been thinking about this. I can’t imagine people wanting to live somewhere that doesn’t even have an open area big enough for a soccer field. Knowing how a baby will develop in such a low g environment is a completely virgin area. Would muscular dystrophy provide some pieces to the model? The “fake” exercise on the ISS seems doesn’t appear to be a livable option for a whole city

    Liked by 1 person

    1. You can use a combination of this and circular tunnels. The author The article dismisses the boring machine, prehaps too quickly. One solution need not be used for all things. Circular tunnels (don’t recall the author of this idea) allow moving modules to be installed underground. these can spin on maglev (this will cost some power but will simulate any gravity that is required (let us say 1.05g so that exercise is not really required to mitigate bone and muscle atriphication. If deep enough, these tunnels will be effectively impervious to radiation and small meteors

      Assuming work and sleep time is spent in these areas, one can then use the low G surface habitat for vistas and to alleviate any claustrophobic tenancies.

      Note that we Already spend most of our days in enclosed office buildings, so there would be minimal change from normal earth life for one’s working day (assuming one is an office worker). You can use OLED panels to simulate windows to “open up” spaces.

      One can move between rings by a module that is ejected, moves down a shaft to the next ring and inserts into that spinning ring – much like an elevator ride between floors.

      Note that the boring machine creates tunnel liners made from the excavated rock as it goes. (The rock is processed into tunnel segments elsewhere). This process can be automated to a large degree to cut down on costs.


  3. This is great, but what about evacuation if there’s a leak? If you have large open spaces, then getting to a pressurized sector from a leaking one might take a lot of time.

    Liked by 1 person

  4. A great blog you run here Casey! Thanks.

    When I saw your tensioned grid membrane above I thought the tension cells could also be pressurized mesocosms (often used in marine and lake research), maybe growing high-nutrition crops (also handy refuges).

    Following on from Linus’s question it may become possible to achieve a level of seal on more porous bases using mycorrhiza – there are quite a few advances in this field at the moment with people considering sealing broken concrete, capping landfills and creating subsurface membranes without excavation, all with natural organic textiles that weave themselves.

    Liked by 1 person

    1. I think moisture will migrate downwards and outwards along with residual leaks until it hits the frost line. Provided the outside remains super cold leaks will self heal with permafrost.


  5. Agree about domes
    But Mars or Luna are terrible places to go
    There is no point in going back down a hole when everything you need is out there
    Cruithne would be a much much better place to put a colony – 100 Billion tons of material
    24/7 sun power
    And no hole to climb down
    Use tethers and spin for gravity


    1. I agree… much rather occupy the orbitals… but Casey makes a good point:

      | recycling of all materials with either huge reserve stocks or perfect efficiency

      That’s hard. Each person needs about 2.8 kg/day of oxygen, so an open system with a perfect way of pulling the CO2 out (which we don’t have) needs to produce that much a day. 80 tons a lifetime. A million people will breath away all the volatiles of that rock pretty quickly.

      This is not a surprise, you’ve gotta recycle the oxygen, and your food crops will help you do that. Open systems become closed systems over time – or as closed as we care to make them.


    2. Dreams pay the bills, and the first thing a space colony can sell on Earth is tickets to go live there.

      I think living at the bottom of a hole is a bad idea unless you happen to have started there. But I’m not the market for tickets. Most people think of planets when they imagine people going to space, and education is fundamentally more difficult than engineering.


    3. Cruithne is not a good target because of delta-v and travel time problems. It lacks adequate gravity for an Oberth effect assist, and there are no Hohmann-like transfer orbits to visit it due to the 1:1 orbit.

      In contrast, Deimos or Phobos require relatively little delta-v to/from Earth C3, with reasonably frequent transfer windows (every two years). The benefit of being close to Mars is that you get a good Oberth effect, as well as some help from aerobraking.

      Liked by 1 person

  6. I think I’ll do a before and after on this. Here’s what I think before reading the post.

    I’ve never taken domes seriously. They seem like just something that people draw to make the picture look nice. But now that I think about it, I like them. I think the Martian city should have some. Probably three.

    There’s only one thing that a new Mars colony can sell on Earth: tickets to Mars.

    Setting up a Mars colony is hard. It will be insanely expensive. So the founders will have to do almost everything they can to maximize the revenue from those tickets. That includes monumental architecture. More people will want to go to a shiny city in a sparkling diamond done, than would pay a gazillion bucks to go spend the rest of their lives in something that looks like a utilitarian cluster of industrial facilities.

    And a dome seems as though it would be a good way of doing monumental architecture on Mars.

    The Martian atmosphere, as far as I know, contains only one thing that’s deadly poison: carbon monoxide. If a building has a catastrophic failure of its pressure seals, I think people could survive with just emergency oxygen masks if it were just CO2 and the non-CO trace gases. But carbon monoxide binds hemoglobin with such high affinity that the amount you would be getting around a mask would be toxic rather quickly. So a secondary pressurized vessel with carbon-monoxide-scrubbed Martian air would provide useful protection.

    Another use for a dome is as a visible example of something built of Martian materials. Ordinary rock contains decent amounts of silica and aluminum, so glass and an aluminum framework should be fairly easy to make.

    Three seems like a good number, so that the Great Dome can have something to be bigger than, to help it fulfill its function of looking good in the pictures.


  7. “The Earth’s south pole in the middle of winter is closer to a beach in Hawaii than the nicest place on Mars on the nicest day of the year.”

    According to NASA’s website, highs can reach 20 degrees Celsius. Basic point still stands, though. Buildings have to be pressurized and insulated.

    “Nor can we operate efficient factories in space suits. ”

    Robots are vastly cheaper than humans to support on Mars. The cheapest thing to send across interplanetary space, in fact the *only* cheap thing to send across interplanetary space, is information. Work on Mars should be done by remotely operated semi-autonomous machines, 99% of them operated by people on Earth. Whole systems will be designed to function with a 24-minute lag. Because expensive as it is to redesign an entire production system, having people on Mars is even more expensive. When a machine encounters a situation that doesn’t fit any of the scenarios it’s prepared for, it will seek real-time operation from one of the relatively few people there.

    Martian autarky is a dream. That’s fine. On Mars, dreams are the only thing that pays the bills, and the push for autarky pays more than its share. But if it’s going to coexist with reality, it will have to give a little.

    Re your alternative —

    When you’re talking about forests and similarly profligate uses of pressurized space, I don’t want to do it with steel sent from Earth. Compressive structures can be built of locally quarried rock. It’s a lot of rock to move, but the cost of sending stuff to Mars is always going to be high unless we have a propellant source that’s not at the bottom of a 5+ km/s gravity well. A better option than compressive structures would be to produce tensile material locally. My favorite is rock glass fiber. Melt some rock, extrude it under the right conditions, and you have a decent approximation to glass fiber. If it has the right chemistry, it’s stronger.


      1. Oops. Saw that after I posted the comment. I should have known you wouldn’t have suggested using steel from Earth beyond the point where it makes sense.

        I’m still fond of basalt fiber. Good tensile strength, and it seems as though you could make it with less mass sent from Earth per newton of load.


  8. Yes. You can buy a bag of it on Amazon as chopped fiber for reinforced concrete. It’s also sold as a roll of fabric, or as tape for wrapping motorcycle exhaust pipes. For bobbins of fiber, you apparently have to contact the manufacturer. It’s a low-volume product, because its main advantage (from our point of view) isn’t particularly relevant on Earth.


  9. This concept doesn’t take into consideration radiation shielding. You’d want to have perhaps 5 tons per square meter of regolith on top, to emulate the long term radiation shielding Earth’s atmosphere provides. This would be relatively inexpensive regolith and/or dry ice, of course, but it’s still going to make it desirable to go higher rather than just flat and across.

    So here’s the thing. This level of radiation shielding still only weighs a fraction of the air pressure pushing up on the membrane. One obvious solution to this problem is to layer on more weight, so you don’t need any steel cables.

    But let’s suppose you have steel cables. To me, it seems more desirable to hang tall buildings using them than to drive them into the ground. Unlike tall buildings here on Earth, there is no wind to worry about. Hanging from above also eliminates a lot of engineering issues with compression structures. Extremely little of the building’s volume is consumed by the support tethers.

    I think that it makes sense to have sides that slope upward – the slope is gentle enough to easily layer regolith and/or dry ice on top, but it still gets higher so you get multiple stories for use. So, the habitat looks like a buried worm, expanding with new segments at both ends.

    The point is – multiple floors gives you a lot more bang for your buck.


      1. Do you mean that the radiation shielding layer of regolith/dry ice spoils the view?

        Well … I suppose an alternative could be liquid water. Liquid water is clear enough that you could still have a decent view of the sky through 5m of it. If you site your habitat near a pole, you could have access to large amounts of water ice.

        Using liquid water, your pressure membrane would actually be the outer membrane. You wouldn’t need sloping sides to make it easier to cover with regolith/dry ice. Instead, the sides could be vertical.

        So, in this case the colony is a half pipe shape, growing by adding segments to the ends. As before, the weight of the water is still only a fraction of air pressure. Thus, you still have plenty of weight bearing potential for hanging stuff.

        But the vertical sides let you directly support floors via the sides, rather than using hanging cables.

        Liked by 1 person

  10. Domes are indeed over rated.
    However, I do have a few comments regarding the tenting design, perhaps a few tweeks. One, I would imagine that the anchoring cables would be placed in a close packed hexagonal pattern, not the orthogonal arrangement shown in the conceptual illustration. Further, the cables might be brought together mid length, in a “Y” fan to a single cable to the ground to reduce obstructions at ground level.
    A couple commenters mentioned the atmosphere underneath the tent, one even suggesting using the unmodified, but pressurized native atmosphere, but mentioned the danger of CO. While CO is indeed poisonous, I would be FAR more concerned about leakage of CO2 into the mask, as it too is poisonous and is the dominant gas of the native Martian atmosphere. If this gas perfuses into the breathing mask, it will give rise to hypercapnia… a dangerous condition that is unpleasant and gives rise to feelings of suffication and panic, and eventually death.
    While the unmodified atmosphere may be pressured during construction and testing of a newly tented area, it would be best to fractionate the atmosphere, removing the vast bulk of the CO2, leaving the N2 & Ar, supplimenting the O2 with electrolytically cracked H20 as the source.
    Dr. Handmer suggested using a lower than standard atm. pressure but with a larger percentage of O2 to increase its partial pressure. I would be very cautious with such. The ratio of O2 to non-reactive gasses is critical to fire safety. Non-reactive gasses cool flames. Too little of the non-reactive gas can cause hot, fast moving flames. This is especially concerning when using ETFE, which when it combusts, releases HF gas which is corrosive, especially to human lungs. Also, in areas with parkland / forests under the tents, a high percentage of O2 presents a very high danger of fast spreading fires. I’ve heard it said that the danger point is found with any O2 percentage approaching 30%.


    1. Oxygen is probably going to be a lot more abundant than water, so hydrolyzing water doesn’t make sense as a source of O2. Ordinary rock has formulas like Mg2SiO4, with more atoms of oxygen than everything else combined. We’ll be making structural materials, such as plastic and steel. Plastic means getting the C from CO2, and steel means getting the Fe from stuff like MgFeSiO4. In each case, oxygen is a byproduct. Of course, plastic also means getting the H from H2O, so we’ll be getting some O2 from water, but as a byproduct.

      My understanding of CO2 vs CO is that CO2 makes you feel an urgent need to breathe, so you would adjust your mask if it leaks some CO2. But CO just accumulates in your blood until it makes you get sleepy and die, and CO is deadly at a concentration orders of magnitude lower than what it takes with CO2.

      For fire safety, what about having a moderately lower partial pressure of oxygen, comparable to what people deal with when they acclimate to living at high altitude?


      1. I think the water scarcity problem will need to be solved, and it’s much easier to get oxygen from water than slag. But I’m not precious about any particular approach.

        Re ppO2, Codyslab has a great video where he shows that combustion scales like ppO2 and nothing else. It surprised me but 0.2 bar pure O2 burned like regular air. Some reduction of oxygen availability is possible but we want to keep the occupants at peak performance.

        Liked by 1 person

      2. As I imagine it, O2 wouldn’t come from slag, except insofar as we need silicon. Rock can be thought of as a combination of oxides: instead of writing formulas MgFeSiO4 (which appropriately describe how the atoms are grouped in the crystal structures), we can write it as MgO+FeO+SiO2. If we want that Fe atom, we need to remove both it and the O that goes with it.

        On Earth, we start with ore that’s mostly FeO+Fe2O3, and we get slag that’s got lots of SiO2 but not much MgO or Al2O3. Most of the separation has been done for us by millions of years of
        geology: weathering, erosion, precipitation of dissolved material from seawater, partial melting of rock followed by slow gravitational separation, and so on. On Mars, we’ll have to do all of that ourselves, starting with the kinds of rock that exist there.

        I don’t know the best way to extract useful materials (Al, Fe, SiO2 pure enough for window panes, etc.) from ordinary basalt, because it’s not something that makes sense to do on Earth. We may have to cook crushed rock to separate the more soluble components (alkali metals, alkali earth metals, halides), before smelting the results to separate the transition metals from the silica. My guess is that we will want to do multiple stages of cooking at different temperature and pH. Then we’ll heat it with carbon to pull off enough oxygen that we get metal and slag phases, then we’ll blast it with excess oxygen to pull out the carbon. Then we’ll burn the mostly-iron to get mostly-iron oxides, then we’ll separate the oxides again by solubility. Then we’ll smelt them again to get iron pure enough to make decent steel. Then we’ll combine it with selected trace elements that we had to pull out of the solutions will ion-exchange resins, because Mars doesn’t have ores of those elements either. If I’m right, it’s a whole lot of work, which explains my fondness for just melting the whole rock and extruding it to make basalt fiber as our main tensile material. And it explains why I thought you might be talking about bringing substantial amounts of steel from Earth.

        Anyway, the point is that any time you make metals or carbon, you also make O2.

        Liked by 1 person

    2. You are correct WRT the dangers of CO2, but nevertheless I think filling the tent with pressurized native atmosphere would be the obvious initial choice from a simplicity and energy requirement perspective. It is true that inhabitants would then likely still have to wear full-body containment suits. This doesn’t quite bring you to a “shirtsleeve” environment, but it does solve the issue of having to wear balky, expensive and highly restrictive pressure suits.

      Overtime the atmospheric mix could be shifted to one that would be safe for breathing masks, and eventually complete habitability.

      A CO2 rich atmosphere may initially be preferable for farming as well, and the O2 produced would over time facilitate respiration. The limiting factor for complete habitability under the tent would thus be sourcing enough inert gas (N2 + noble gases) to complete an atmospheric mixture that is breathable and safe – i.e. high enough partial pressure O2, low enough partial pressure CO and CO2, and high enough total pressure.

      The area under the tent would likely also be further compartmentalized into areas of full and partial habitability. Settlements and industrial zones being constructed in fully habitable zones, with high partial pressures of inert gases, and agriculture being conducted in areas of low (or nonexistent) inert gas partial pressure. This would greatly reduce the requirements for scarce inert gases, while also likely maximizing crop yields.


      1. On a side note, it would be interesting to model the climate under the tent with various atmospheres. I’m guessing starting off with concentrated native atmosphere would yield a substantial greenhouse effect. Whilst heating area under the tent improves the habitability, if the greenhouse effect is too substantial it could present a thermal regulation problem.

        Also, as per the ideal gas law higher temperatures under the tent would result in higher pressures. This would imply that if you wanted to target a given atmospheric pressure (e.g. 0.329 atm, Everest Summit pressure), you could do so with less total atmospheric gasses than would be the case with the temperature held constant.

        Again, it would be interesting to model the climate of the tent with various initial mixtures.


  11. Mars has never had the full set of plate-tectonic processes that were necessary for the creation of high-quality ores on Earth. So a Martian city will need to process much larger amounts of rock for some of the materials it will need. It may make sense to use the waste as weight and as radiation shielding for core living spaces where people spend most of their time, while covering most of the city with tensile structures.


      1. As far as I’ve been able to find, the crust everywhere on Mars is almost all basalt/gabbro. There are a few patches of andesitic rock but nothing like the distinction between oceanic and continental crust that we have here: no ocean trenches, no subduction of entire slabs of crust, no subduction-zone arc volcanism, no way for water to move from the surface to the mantle and create chalcophile-concentrating ore.

        There are deltas, with evaporite deposits. There are shallow hydrothermal features. But I don’t see how those can produce anything like the range of ores that we have on Earth. We may be able to mine gypsum and halite, so that we can have a concentrated source of sodium, potassium, calcium, sulfur, and chlorine. Or we may not.

        The most important thing that’s missing for mining may be ocean-going shipping. Even on Earth, where we have a huge portfolio of mineral resources, we don’t have most combinations of them close enough together to be practical without putting them on a ship.

        I think we’re going to have to site the city according to either orbital parameters or water availability, and get everything else from air, from quarrying and processing ordinary rock, and from picking up a tiny quantity of meteorites — not from mining ore.


      2. You may be right. I am still hopeful we can find a site with water, sun, or a heinous vein of iron ore or uranium or something.

        On Mars there are probably plutonic intrusions, sedimentary sorting of minerals, breccia pipes, and skarns. Also all the good stuff won’t have been pilferred by Bronze age hordes.


  12. Maybe I am out of line but if we are thinking out of the box, how about using the asteroid belt for some your needs, metal, water. It may not be easy to obtain but neither is building on Mars.


      1. If you’re talking about a Mars city doing it from the ground on Mars, to the ground on Mars, then I would expect the same result: ground to orbit and orbit to ground are a big chunk of delta-v, even if not as big as on Earth. For bulk materials like steel or water, that you’re going to use on the ground, I don’t think it would be worth sending an empty Starship from ground to orbit to bring them down, even if the delta-v from Mars escape to the asteroid is zero.

        But how about a not-at-all-assembled space castle plus lots of propellant, for use in Mars capture orbit to receive stuff from Earth, with the stuff to get there and extract the propellant coming not from the ground on Mars, but from Earth as a substitute for some of the supplies that the Martian city would be receiving from Earth, and the propellant that would be bringing them? The delta-v is effectively negative, because you’re getting your propellant closer to the middle of your route than if you had gotten it from either Earth or Mars.

        Ordinary rock is full of oxygen (aka propellant) in the asteroids, same as it is on Mars. I’m assuming that you can pry some O2 loose from the rock entirely with remotely-operated equipment that works across a substantial light-speed lag, rather than needing people close enough to operate it in real time.

        I would also expect different profiles of elements at high concentration in asteroids and Mars dirt. So if it turns out that you have to move an extra million tons of rock to get each ounce of gallium on Mars (or whatever element is hardest to get locally), but it’s almost a byproduct of propellant production from a suitable asteroid, then my guess is that it would be well worth bringing down from orbit in the quantities you use a most-expensive element in. And I expect that it would be cheaper to do it from a near-Mars asteroid than to send the material from Earth and the propellant from the surface of Mars, where the initial cost of the material is negligible but transport costs are sky-high.

        The main downside I see is complexity. More things that can go wrong mean more cost in planning, engineering, and ground support.


      2. Aerobraking means only a few percent of the fuel load is required for the landing, as opposed to TMI.

        Rocks have lots of oxygen in them but it’s as hard to get out as, eg, the thermite reaction is hot. That energy has to come from somewhere, and Mars orbit is pretty low on infrastructure.

        Remember that large swaths of Mars’ surface are covered with pre-disassembled asteroids already!

        Liked by 1 person

      3. Yeah, it would be better to simply use raw rocks/regolith for propellant directly, rather than extract oxygen. Since asteroids are lacking in gravity well, the most suitable propulsion is electric thrusters anyway. Blasting a rock with a pulsed electron beam for thrust would be good.

        But … what even is the point? We have no expectation that the poor rocks available in asteroids would be any better than the poor rocks available on Mars.

        Liked by 1 person

      4. We do have reason to believe that rocky asteroids will be a better source of some elements than the surface of Mars. As a proto-planet differentiates, immiscible fluids percolate through the solid material with high melting temperature, and through each other. Some trace elements fit well in the crystal structures of the refractory minerals, so they remain in place. Some are more soluble in molten iron (or iron sulfide, I think, for proto-planets that formed farther from the sun). Those elements tend to wind up in the core. Some are more soluble in water/CO2 (they’re miscible). Those wind up in ore, when the fluid reaches a depth where the the temperature and pressure are low enough.

        Billions of years later, the stuff that headed to the core is inaccessible — unless the proto-planet got smashed to bits and is still floating around in many little pieces.

        The rock on the surface of Mars is basically all just basalt, or stuff extracted from basalt by a limited set of processes and left as evaporite or hydrothermal deposits. Some elements are not going to be easy to get from Mars rock.

        I don’t know how much asteroid material is available on the surface of Mars.


      5. The place that really makes sense as a source of scarce-on-Mars elements is Earth. Thermal tile is made of silica, but it could perfectly well be made of zircon or chromium oxide or something, whatever element is hard to purify from Martian basalt and useful for the Martians. Steel in the Starships can be reformulated to contain more of the scarce-on-Mars stuff than necessary. Even the small stuff may be worth reformulating, to increase it’s value as scrap on Mars.


  13. You’re concerned about building a habitable structure cheaply, but in terms of resources and labor, I would expect that the stuff that goes inside of the structure would be more expensive to produce. Maybe the structure itself is not an important part of the cost equation. For agriculture, maybe your structure makes sense, though.


  14. Just a forester that enjoys the blog. I’m also a father and as a father I’m used to cleaning. As a forester I’m used to cleaning and fixing equipment, rocky clay soils of the piedmont mid Atlantic states can be tough.

    So I have to ask…how the heck do you plan to clean dust off these huge inclosed spaces? How are you going to keep dust from abrading your fine manufactured worlds.

    Will there be a race to develop a iron meteorite collecting crawler that just trucks around the surface scooping up the iron meteorites that have the rovers keep spotting?


  15. I’ve thought of another possible reason for compressive structures on Mars: gravity as an energy source. The sun is more distant than it is from here, and it’s not clear to me whether photovoltaic panels can readily be made from Martian materials. I don’t think there is decent ore of any fissionable element, and public perception overstates the danger of launching it from Earth, which would risk turning popular opinion against the Mars colony.

    But the topography of Mars is impressive. There are landslide sites at the edges of Valles Marineris where the ground slopes for many many miles at the angle of repose that the debris had when it was unconsolidated. At the top of such a slope, a great abundance of regolith is probably available to be scooped up with a shovel. A motor and a generator are essentially the same thing, operating in opposite modes. It may be that a train going up empty and coming down loaded with regolith (or quarried rock) could generate power effectively.

    If so, then the loads could be dumped on top of a structure as easily as anywhere else, so that it could then be pressurized while being under compression rather than tension. It would need more structure than the tensile design, but it wouldn’t have the possibility of catastrophic failure if an anchor is damaged.

    I still don’t think domes are a good design for anything but a handful of pieces of monumental architecture, of course. I imagine columns and beams, same as how buildings are normally designed on Earth.


  16. Why is ice never discussed as a building material for the Moon and Mars? BOTH places are VERY COLD outside and NASA has shown that abundant water sources are available. Any base/colony will be located near these surface/underground water sources for numerous reasons. Imagine inflating a large bladder and then spraying it with water until the ice is as thick as you need. Then deflate the bladder and move it to another location and repeat. Cover the outside of the ice dome with sand/soil/etc for thermal radiation protection. Spray the underside with Aerogel to insulate the ice from the interior heating system. The fact that ice/water is one of the best barriers against cosmic and solar radiation is an added benefit.

    Liked by 1 person

    1. Wow. I don’t know why I hadn’t thought of water as a building material.

      Now that I think about it a little, I imagine it as a component of composite materials. Inflate a high-strength enclosure to let you have molten water instead of having it sublime. Heat it to a temperature where water will stay molten long enough to work with. Set up your rebar or basalt fiber, whichever turns out to be cheaper on Mars, along with molds for your structure. Then pour in a mixture of aggregate and molten water. Boom, you’re done with that section, go set up your molds and tensile elements for the next section. When you tear down that building, all you have to do to separate materials for reuse is heat them up inside an enclosed space. The water will sublime from your used construction materials and re-solidify on any cold surface.


  17. Today, I’m imagining barrel vaults.

    As discussed in my other comments above, I think there will be some elements that will be scarce on Mars, but useful enough to justify processing a lot of rock. Steel is very versatile, if you add the right trace elements. The waste material will be worth using somehow. It may be best to make it into some kind of fiberglass, or tempered glass bricks and beams, or to use it as aggregate in concrete. If so, the structures built with it will be more or less conventional buildings, with tensile pressure vessels to hold whatever amount of air pressure.

    My guess about air pressure is that it will be worth having a lot of area with shirtsleeves conditions, but an even larger amount with not-immediately-deadly conditions where you can go with an oxygen mask instead of a space suit. New factories are often nearly devoid of humans here on Earth, where labor is cheap and shirtsleeves conditions are free. When the air and the pressure vessel are a significant part of the cost of a building, I’m guessing that it will be better to have a lot of space that normally runs entirely on remotely-operated semi-autonomous machinery, and to have people deal with the inconvenience of oxygen masks on the rare occasions when they do need to do something by hand.

    But maybe the high-volume byproducts will make crummy glass, or maybe they’ll come from a process that doesn’t lend itself well to making glass as a byproduct, and not be worth reprocessing into glass. In that case, we’ll have a large volume of low-tensile-strength material — which lends itself to the construction of mostly-compressile structures. They’ll only have as much air pressure as their weight will hold (a bit less, after allowing a margin of safety). But because I think there will be lots of use for non-shirtsleeves space, I think it would be worth using the waste to use such things, if there’s no better use for it.

    However, I don’t think a dome will often be the best option, for purposes other than monumental architecture. If your design criteria are that there’s one spot that needs to have as much area as possible under a roof within some radius of it, under a compressile structure, then you want a dome. How often will that happen? My money is on never. A circular footprint isn’t all that convenient. But a barrel vault can enclose space under a compressile structure with a rectangular footprint. And if there’s a large amount of low-tensile-strength material that would otherwise just be waste, then it might as well.

    Liked by 1 person

  18. Hey, great blog, I’ve really been enjoying reading it! I couldn’t agree more that domes are very over rated – as you say, an engineering nightmare that doesn’t scale.

    I agree with you that for manufacturing (and heck, good living), you want a huge “shirt-sleeves” area where people can work/be without the hassles of being confined, airlocks, etc – indeed this will be essential. I think you underestimate the dangers of radiation at the surface of Mars considerably. But more important than even that for your idea of a “tented area” is the temperature control problem. The super thin Martian atmosphere means that lots of heat is lost as thermal radiation to deep space. q = σ T^4 A, so for a sq-km at say 0C (the upper parts of your trapped air and the thin ETFE and the martial atmosphere itself probably buy you 20C, so you are shirt-sleeves at ground level), the heat lost would be 314MW. Yes, megawatts. The incoming solar irradiance averages ~half of that (depending on a lot of details like latitude), but that still leaves you with a *gigantic* heating problem, and we haven’t covered conduction or convection yet. Cold floors suck!

    You could solve the heating problem with nuclear power plants. But I think a more realistic solution to the heating problem is the same one you need for the radiation problem anyway – cover your entire tent area with 5+m of regolith (ideally more, at low density, to increase the insulation value, and give more scattering distance for galactic cosmic rays). And insulate and seal the ground & walls too. Obviously this dramatically changes the appeal of working there, since now it actually feels like the giant warehouse/industrial space that it actually is. Your heating bill is likely to still be substantial, but you need big heaters to get it up to temperature on setup anyway, plus the (now required) lighting isn’t 100% efficient anyway… obviously the design of the whole structure should try to work out a “zero house” kind of policy; if you tile the top of the sq-km with solar panels, you could work forwards from there to your energy budget, and then use perhaps half of it for heating, getting you your R-value design target…

    All human-supporting projects on Mars need to consider this temperature control problem, because the average surface temperature is so cold: -55C, or somewhere around there, with lows below -100C. You need feet, not inches, of insulation around every human-temperature volume in order to not require very impractical amounts of energy for heating. It was this heating consideration, more than any of the others (radiation, pressure, deltaV/transit times, etc), that ultimatly made me think Venus was a better bet for colonization, despite the oddities of not being on the surface. Having approximately the right temperature and pressure is just a killer advantage if you’re going to support humans comfortably.



    1. Thanks for your nice words and thoughts.

      The thermal problem is a real one. I think much of the issue can be addressed with coatings on the ETFE membrane to drive a greenhouse effect. Then have plenty of water near the surface (pools, landscaping) to provide thermal mass and smooth out the diurnal swing.

      Not too worried about convection or conduction. So just have to cut down on thermal radiation from the outer membrane.


      1. A coating is absolutely requited, yes – but even with it, the top of the ETFE still radiates at the same blackbody temperature as the temperature of the material itself. My math already considered the ETFE coated, that’s why I said 0C as the temp (not the 20C of ground level). So yes, I agree, coating, but you haven’t actually made any progress on your thermal challenge…

        The only thing other than massive pile of insulation that I can see working would be to do multiple layers of the material, similar to the insulated glass/double glazing idea for windows, but focused on the radiation heat transfer. Typically double glazing has a vacuum (or heavy gas) between the glass sheets to cut down on conduction/convection – that’s not required here – instead we just need multiple layers so that we can basically ease-down the temperature deltas and thereby dramatically cut the losses due to radiant heat transfer – similar to how they do it in those crazy refrigerators for quantum computers. I haven’t done the math, but I think a 3 or 4 layers might be sufficient to cut losses to about 1/4 of my previous number, which would still leave it painful, but perhaps more manageable? The hard engineering problem then is: how are the layers being held apart from each other, how are they built, and what pressure of gas is between them, how it it held, etc? And what impact do the extra layers have on e.g. optical transparency? Perhaps only the bottom-most level is actually sealed, and the layers above it serve only for thermal management, that would presumably make it a lot easier to build, since they could literally just stack on the bottom most layer, no need to make the support wires go through / seal / etc. And then the conduction/convection in those upper layers is limited because they are at martian atmospheric pressure, which is near vacuum (probably much like existing insulated window, I doubt they are high vacuum either…). Though realistically, having at least one other layer seal also buys you some safety in terms of leaks, so might be a great nice-to-have in the design. Thinking about those top layers though, you also need a dusting plan of some sort, or no doubt soon enough you’ll have an opaque layer up there. So maybe the layer needs to support people going out there in suits to dust it off once a month? That sounds like it needs some thoughts/design work…

        And as I said, you also need substantial insulation on the ground and walls – it won’t be anything like the natural ground that you show in the renderings, but perhaps it could be made to conform to contours, so that at least there isn’t a large need to prepare the ground (leveling, etc) before you start building? Probably something thermally well engineered, six inches to a foot deep would give you enough to avoid the cold floor problem and prevent 90+% of heat loss there. If you really want it to look like natural ground, you could always put that in, and then bury it under some martial soil… or engineered real soil, so you could actually grow something (though you need to worry about drainage too then, which is another thing the insulation layer could be doing).

        Anyway, as you say, the thermal engineering challenges on Mars are real, and make your open spaces a lot more intensive to build – not one layer of plastic with some metal tension wires, but multiple layer of plastic, wires, and a thick insulated floor, plus whatever you need at the walls. And still megawatts of heating power, just not hundreds of megawatts 🙂


  19. It doesn’t seem plausible that moving 5m of regolith would be an efficient way of insulating. What I imagine is half a millimeter of plastic, a few microns of aluminum, a few centimeters of CO2, and repeat as needed. Either that or a layer of fiberglass insulation. You only have to melt ordinary rock once to extrude it into fiber, whereas purifying aluminum takes multiple separation steps in a world where there probably is no bauxite anywhere on the planet, in addition to the very energy-intensive final step.

    But I have trouble taking the idea seriously that the city will be hard to insulate. The square/cube ratio says big things don’t tend to need much insulation. A single-family house needs fiberglass in the attic and air spaces in the walls, to stay warm in the winter with its furnace running, while a big office building in the same climate needs air-conditioning year-round just from the heat realeased by lights and computers. It seems obvious that a whole city, full of all the heavy industry needed to replicate the world’s entire supply chain — an order of magnitude more energy-intensive than an office, and an order of magnitude bigger — would be more likely to need cooling towers than insulation, even on Mars. Of course, what seems obvious isn’t always true, especially when you just wave your hands at orders of magnitude. If the actual calculation says the Martians will need insulation, then they can make insulation.

    Liked by 1 person

    1. You wrote “The square/cube ratio says big things don’t tend to need much insulation.”

      Making things bigger only helps you if you expand in three dimensions. If you expand horizontally across the surface of Mars, then the surface area that needs to be insulated is proportional to volume.


      1. True. But the same concept applies to a large office building: it needs air conditioning even in the winter even though it extends mostly in one dimension, up. The order-of-magnitude-larger comparison is between the thinnest dimension of a Martian city and the thinnest dimension of a skyscraper.

        If insulation were a problem, building taller would be an option.

        But it won’t be. You can keep liquid nitrogen in an insulated tank at ambient pressure without using five meters of anything, and without using any unobtanium at all. A few millimeters of gas between shiny metal surfaces — a thermos — will do just fine. The nitrogen boils away eventually, but in the meantime you can hold the thermos in your bare hands without losing enough heat to make you uncomfortable. And liquid nitrogen at 1 atm is about 40 degrees colder than the surface of Mars on a winter night.


      2. You obviously don’t live in a cold city :-). All buildings need substantial heat here in Toronto in the winter, and our average temperature is only -5 or so. If it gets to -20 and stays there for a day or more (as it does perhaps once each winter) we all feel chilly inside because the insulation and heating are just not up to that kind of load. We bundle up and then we dread the power bill from running the furnace non-stop. And that’s with major heating systems and lots of insulation in the building code. To support -55C (average surface temperature of Mars) you’d need considerably more than the typical 6′ fiberglass insulation batting that we use here, and you’d need way better than the double glazed windows too. As you say, I think rather than go to thicker insulation, it might be cheaper to exploit the near-vaccuum which already exists on Mars, and use thermos (or dewar) like thermal engineering principles – even a double layer with vacuum between can be pretty effective against conduction and convection heat losses. That’s really NOT feasible here on Earth cause the vacuum is so hard to maintain, but it’s free on Mars, so might as well take advantage 🙂

        However, all that really ignores the MAIN thermal problem on Mars, which is the radiation heat transfer upwards. There you’re protecting not against -55C average surface temperature, but the -270C of deep space, with only a small effect from the thin CO2 atmosphere. That’s WHY the surface gets so cold so quickly at night, and only reaches -20C during the day, despite hundreds of watts/m^2 in solar irradiance. And that’s why you need the multiple thermos-like layers for your colony roofs, because fighting that kind of thermal delta is hard work. Like I said, I feel like if you had 3 or maybe 4 layers, with the near-vacuum atmosphere between, you might be able to cut your losses to <1/4 of single-layer loss, which might make the remaining heating requirements more manageable. Obviously you'd want to actually do the thermal modelling to figure out how many layers you should have, based on what kind of heating power you actually have available. If you want <25MW/km^2 of heating, probably you need more than 4 layers, but hey, maybe you have that kind of power; pretty clearly by the time you need a km^2 of space, you should also have some seriously large other requirements for power…

        All of which is to say – it's possible to protect the colony from that level of cold, but it's nothing even remotely like anything we do here on Earth. Even the Vostok Station just needs to fight the average surface temperature (apparently similar to Mars!), but doesn't have this additional radiation heat loss problem… and of course it's TINY, more similar to the long-proposed underground pressure chambers than anything like this shirt-sleeves working environment that we're talking about here.


  20. What may be obvious isn’t anything about where I live, but the fact that I’m not involved with HVAC for buildings that are large in all three dimensions, and aren’t mostly empty. I remember reading that such buildings require AC most of the time, even in winter, as an example of the effects of scale. If I’m wrong about that, I’m still not worried about radiative heat loss. The ISS somehow manages to stay warm inside, and it’s small and not full of heavy industry.


  21. I looked up US energy usage per capita, since that’s easy to find, and it converts to a bit over ten kW. To generate waste heat of 314 MW per square I’m, at that level of energy use, the Martians would be living at a population density of 31,400 per square km. For comparison, Toronto has a population density of about 4,000 per square km, and Manhattan has about 25,000. When every square meter of pressurized area needs structure capable of holding down most of an atmosphere of pressure, there’s already reason for people to live at a higher density than we do on Earth.

    But supporting human life is more energy intensive if you have to do everything on-site: manufacture the food, regenerate the air, and everything else that people on Earth benefit from that’s spread across all the wide open spaces. And the outside will necessarily be cooler than the inside, if heat is flowing out spontaneously. And the albedo won’t be zero. Put it all together, and it sounds as though the city is more likely to need cooling towers than to have insulation be a major problem.

    If the Martians want to pressurize lots of land for some reason, they probably won’t have it be a shirtsleeves environment. If they do, we can still fall back on the fact that an ordinary thermos can contain liquid nitrogen.


    1. That’s right. So, 10kW is per person is actually comparable to the ISS – about 100kW divided by 6 people is 17kW per person. And that’s without actually growing one’s own food.

      If we look at Kowloon City, it had a population density of over a million per square km … about 30x the 31,400 per square km you calculate.

      So basically, insulation isn’t a problem – keeping cool is the problem. Fortunately, this can be solved without large radiators. You have access to an endless supply of cold ambient atmosphere. It might not be very dense, but it’s still plenty dense enough to provide lots of cooling with fans and coolant pipes.


  22. Absolutely wonderful blog Casey, I love to read about your ideas. Regarding the concept of pressurized partitions, do you envision steel cables being needed for anchoring only? No frame or structure of any kind needed, steel cables or otherwise, to support / shape / brace the huge inflated membranes?


  23. This was a very interesting idea. One question though, is there a particular reason why you’d choose to make the cables out of steel instead of kevlar as well?


  24. Very interesting stuff that has given me food for thought. Two items that concern me: 1. GCR danger has been represented in some sources as being far more dangerous than you seem to indicate. Comment? 2. Using lower air pressure with enriched oxygen presents problems, according to NASA. The human body, it seems, doesn’t do very well without the full quota of nitrogen pressure. Have you seen any info on this? Anyway, loved reading this. I have a website that explores the interface between art and habitation of the Moon. Check it out if you like.


  25. Regarding the dome-construction methods you describe and their drawbacks: are you assuming ‘triangles’ and ‘joints’ of the geodesic variety? What about an inflatable bladder which is then coated to provide a dome? This seems very satisfactory to me because the construction is easy and the total cubic volume is maximized relative to materials, which seems attractive. Normal, rectangular structures can then be built inside the dome. The dome can also be covered with regolith to provide shielding from GCR.

    Of course, the inflatable need not be dome-shaped. It could be any shape we wanted, including a rectilinear one. And, theoretically, it could be any size we wanted. At this point, it becomes more like the system you describe. The cubic advantage would be lost but the expandability advantage would be greater.

    This would lose the strength advantage when it comes to covering the structure with shielding. So, for me I guess it all boils down to the following: if GCR really needs to be strongly shielded against, then the dome shape is best. If not, then a rectilinear inflatable, basically like you describe, would work well.


    1. Preloaded compressive vaults might also work but require much more material (10 T/m^2 vs maybe 10kg for tensile membrane). Even though much of the dome preload is rubble the sheer volume can’t be cheap.


  26. It has been a while since I researched the GCR issue. Even in finding the data again there are two problems:

    1. a preponderance of google returns than emphasize the cancer risk. But this is not the risk that concerns me (because everyone recognizes it). I am more concerned with physical tissue destruction, as happened to the Apollo astronauts (who had a 41% higher incidence of cardiovascular disease than their demographic baseline). This was the result of very high energy impact of GCR;

    2. a preponderance of google returns that emphasize discussions of relatively low energy GCR.

    My concern is NOT with solar radiation NOR is it with the lower energies. Those are well-known and well-discussed. It is with the higher energies that tear physical holes in the living tissue. This is what caused the cardiovascular problems with the Apollo astronauts. The lining of their blood vessels was ‘roughed’ up by this onslaught and they suffered for it…after a week or so in deep space! Imagine living there permanently.

    I will continue to look for these references. I bookmarked them but it was some years ago and it is hard to find them. More later.


    1. The Apollo astronauts are such a small sample size that I’m very wary of drawing conclusions. They almost all lived to advanced ages, and almost all had high risk factors for heart disease including tobacco use, jet fuel exposure, and incredible alcohol consumption. They were also physically very far from baseline due to selection requirements.


  27. As a side note, there have been papers released recently discussing crystalline material that can bend or ‘guide’ high energy particles around protected objects. These are similar to the ‘invisibility’ cloak that made the news some time ago. So, it may be possible to specify these materials to guide particles coming in at a certain angle of incidence and then simply layer them up to accommodate particles coming in from any angle.

    I envision a spacecraft or habitat covered in this layered material with the GCR then being guided completely around the inhabited volume within.

    I will look for references.


    1. I worked in that field some time ago. I would be very surprised if we found materials that can achieve big band gaps for a wide range of frequencies and incidence angles without causing copious secondaries.


      1. Yes, I envision a suite of strategies being used, including the brute force method of thick shielding. Not being an expert in this field, my imagination is necessarily going to be naive. But I imagine large, nuclear-powered space-craft, built from Moon materials and largely populated from the Moon, constructed in orbit around the Moon, and covered in processed regolith to a depth of 2-3 meters. The data says that this will stop GCR cold. I have seen the graph and will look for the reference. Now, that is one heavy ship! But so what? These could be almost permanent, mobile structures that, with nuclear power, could achieve significant speeds. As for the crystalline material: I see it as being like CPU’s, with refinement and sophistication coming over time. I also very much like a suspenders-and-belt approach to deep-space engineering. Do both!


  28. Crystal shield reference:

    Partial copy-and-paste of PDF:

    There is a low intensity, isotropic background of galactic cosmic radiation ions
    originating from supernovae[1,2] with the fluence peaking in the range of 100 to 1000
    MeV/nucleon, with energies extending beyond 1 TeV/nucleon as shown in Fig. 1a. The elemental
    distribution decreases with atomic number with an additional peak at 56Fe [3]. Such heavy, highenergy (HZE) ions have a large linear energy transfer (LET) of hundreds of keV/μm. Their
    fluence is much lower than that of protons but HZE ions cause comparable amounts of damage to
    biological tissue owing to their higher LET resulting in damage to DNA, elevated cancer risks
    and genetic mutations. Ref. [4] reviews the effects of HZE ions on biological tissue as relevant to
    manned missions to Mars. Such extended durations of many months outside Earth’s protective
    atmosphere and magnetic field mean that astronauts are exposed to the full cosmic and solar
    radiation backgrounds and suffer short and long-term exposure effects.

    The problem is serious because HZE ions are impossible to fully shield against owing to
    their very high energies. A thick ‘passive’ radiation shield of 20 g/cm2
    stops ions up to energies
    of ~200 MeV/nucleon but results in the production of secondary radiation such as heavy
    secondary ions and nuclear recoils, high-energy photons, secondary electrons and neutrons. Even
    2 g/cm2
    of shielding can result in the production of more secondary than primary ions, some of
    which have higher LETs. Secondary electrons form a low-LET background radiation which can
    also damage DNA and produce mutations [3]. The detailed effects of HZE ions on biological
    tissue and the adverse effects of shielding are under study [5,6]. Shields made of high Z elements
    tend to result in more secondary ion radiation from break-up of their large nuclei [5]. Shields
    made of low Z elements, such as liquid hydrogen or methane, are more effective in reducing the
    production of secondary radiation since they do not fragment into secondary particles [7], with
    proposals to shield spacecraft by enclosing them within liquid fuel tanks. Other forms of
    shielding spacecraft from HZE ions involve ‘active’ methods using electromagnetic deflection
    using high voltages or currents [8,9]. These approaches have difficulties sustaining the required
    voltages, high power and the unknown effects of subjecting astronauts to high magnetic fields
    over extended periods.
    This paper describes how spacecraft may be shielded from ions of all atomic number
    originating from a narrow angular range using channeling in bent crystals, where the lattice field
    can deflect charged particles through small radii equivalent to a magnetic field strength of >1000
    Teslas. Bent crystal channeling is normally used to deflect, extract and collimate high-energy
    charged particle beams in accelerators [10-15]. Protons with energies as low as 2 MeV [16] and
    as high as 900 to 980 GeV have been successfully deflected [17,18] with plans for use at 7 TeV
    [19,20]. Heavy ions such as 100 GeV/nucleon Au have also been deflected [21].

    End excerpt.


  29. Regarding the air pressure issue: the Moon, at least, should probably be built up at sea level pressures. Why? Because I envision a large amount of traffic between the Earth and the Moon. In round numbers, it’s only a 12-hour trip, same as Tokyo-NYC, so sea level seems appropriate.


      1. Actually, I was thinking of the Moon, but yes, there are differences of that nature. Anchoring and regolith on top. Perhaps a permanent concrete shell structure laid on the inflatable membrane. Hmmm….


  30. My mistake: the AD does have entry structures on the long side. They are so small (compared to the dome) that I missed them. Comment?


  31. That’s a lot of steel for the canopy cables, even with a fill factor of only 0.05% (unless I’m misunderstanding something about it). A canopy 10 kilometers across and 6 kilometers up would have an internal volume of about 314 billion cubic meters (very roughly speaking), of which 157 million cubic meters would be cable. Steel is about 7900 kilograms per cubic meter, so you’d be running around 1.2 billion tons of steel to anchor this. Global steel production in 2019 on Earth was around 1.86 billion tons in 2019 for comparison.

    You’d want to keep the canopy a lot lower to the ground. If it’s only 100 meters up, then you only need about 20.7 million metric tons of steel. That’s still a lot, but it’s a lot more doable for a nascent Mars steel industry.

    Or you do something like Kevlar cables, if you have the ability to produce large amounts of that.


  32. If the reason for the high ceiling is to use the atmosphere as radiation shielding, wouldn’t it be more practical to just add some radiation shielding? ETFE already seems like a good candidate for that since it’s a hydrogen heavy polymer.

    Steel could tether ~5000 times it’s own surface area at 1 atmosphere and it’s about 6.4 million times more dense then atmosphere so it’s staring down the bad side of a 1000-1 ratio. Lowering the pressure (with constant temperature) would reduce the steel needed but proportionally reduce the shielding so it’s a wash. So it seems to me like adding atmosphere for shielding is a bad tradeoff. You add 1 kg of steel and you get ~1 gram of shielding. Whereas if you add 1 kg of ETFE that’s 1 kg of shielding.

    Of course I totally see the desire for higher ceilings for aesthetic reasons. But that’s a luxury thing and unnecessary in farming areas for instance.


      1. I think the challenge of making the atmosphere is trivial compared to the challenge of providing the tethers. The tethers need to be a thousand times cheaper then the ETFE to justify using atmosphere as radiation shielding when I think the other way around is more likely.


  33. Sorry, I meant on Earth, because that is our baseline. Of course, on Mars we can envision many methods of stopping radiation.


    1. I’m pretty sure the Van Allen belts are helpful in blocking solar radiation. Otherwise our atmosphere itself would be eventually blown away in the solar wind.


      1. Sure, that’s true. Mars, in losing its magnetosphere, lost its atmosphere as well. That, at least, is one theory.

        I am always somewhat irritated/amused by two things;
        1. A persistent reluctance to take GCR, and its proven horrific cardiovascual effects, seriously enough.
        2. A persistent reluctance to embrace living underground.

        Of course, part of the reason for this reluctance is because people have a very limited idea of what “underground” means. I live in a concrete building with walls that are a foot thick. This is effectively “underground”, even though I am on the first floor and have a nice southern exposure surrounded by trees and grass.

        The point about being “underground” is that we MUST get a deep barrier between ourselves and GCR. That barrier can be above grade, but it must exist, for reasons similar to why we all live in houses to begin with: shelter from the elements.

        Shows such as The Expanse (which I love) are completely unrealistic in this respect. At least Star Trek has ‘shields’. There is no such protection specified in The Expanse (at least in the TV version) making that vision of the future utterly impossible, in my opinion.


      2. Very few astronauts have been exposed to them. Those few that have, did have a statistically greater incidence of cardiovascular problems than baseline. 41% greater, in fact.

        A small data set, to be sure, but we have what we have. And it does trend in ONE direction: GCR damages the cardiovascular system. There is no trend that is neutral or in the other direction.

        And I don’t believe it is a showstopper, either. But it must be addressed. I would never write fiction without doing so. I would never speculate factually without doing so.

        As I mentioned before, crystalline materials show great promise at ‘bending’ radiation around populated interiors.

        And a couple meters of regolith will stop anything. Secondary radiation as well as primary.


      3. A tiny dataset shows some variation that could be interpreted as GCR damage that has no measurable impact on their actual longevity or well-being. While all these men were exposed to jet fuel, alcohol and tobacco in terrifying quantities, and nearly all have lived to exceptionally ripe old ages


      4. Quote:

        “As multiple spacefaring nations contemplate extended manned missions to Mars and the Moon, health risks could be elevated as travel goes beyond the Earth’s protective magnetosphere into the more intense deep space radiation environment. The primary purpose of this study was to determine whether mortality rates due to cardiovascular disease (CVD), cancer, accidents and all other causes of death differ in (1) astronauts who never flew orbital missions in space, (2) astronauts who flew only in low Earth orbit (LEO) and (3) Apollo lunar astronauts, the only humans to have traveled beyond Earth’s magnetosphere. Results show there were no differences in CVD mortality rate between non-flight (9%) and LEO (11%) astronauts. However, the CVD mortality rate among Apollo lunar astronauts (43%) was 4–5 times higher than in non-flight and LEO astronauts. To test a possible mechanistic basis for these findings, a secondary purpose was to determine the long-term effects of simulated weightlessness and space-relevant total-body irradiation on vascular responsiveness in mice. The results demonstrate that space-relevant irradiation induces a sustained vascular endothelial cell dysfunction. Such impairment is known to lead to occlusive artery disease and may be an important risk factor for CVD among astronauts exposed to deep space radiation.”

        4-5 times higher!

        What is ‘endothelial cell dysfunction? Here: ” Endothelial cell dysfunction, in its broadest sense, encompasses a constellation of various nonadaptive alterations in functional phenotype, which have important implications for the regulation of hemostasis and thrombosis, local vascular tone and redox balance, and the orchestration of acute and chronic inflammatory reactions within the arterial wall.”

        This is what GCR does. Period. This is not an opinion.

        Not sure what “alcohol and tobacco” have to do with anything. Those are general facts, included in the population baseline.

        As for ‘jet-fuel’: pretty sure those boys had their helmets on when they went to the Moon.


      5. I did.

        But all science is “tentative”. Science never proves. Math proves. Science falsifies. I would love to have the indicated increased risk of cardiovascular disease be falsified. I hope that happens. But it ain’t happened yet.

        Until it does, I will take what data we have seriously and look for solutions.

        And, of course, there are other reasons for shielding underground. The incidence of micrometeorites is enormous, far higher than most people believe (100 times higher, in fact). But those studies may fail to convince some people as well, so I won’t bother trying. lol

        If even ‘Nature’ magazine isn’t authoritative enough, who could be? lol


  34. Re Cody’s lab and oxygen. That result with fire scaling to ppo2 rather than percentage surprised me and wasn’t what I was taught.

    He’s released an update where he says that was an experimental error, and he repeats the experiment, controlling better and gets a more conventional result. A birthday candle goes up like a flare under 0.1 bar (3 inches) of pure oxygen.


    1. And that’s in a partial pressure lower than what the Apollo astronauts had in their capsules (5 psi). It’s a misconception people have, that a pure oxygen atmosphere is okay as long as it’s just a partial pressure one. It was the other stuff they did after Apollo 1 to reduce flammability that made it a lot safer.

      Interestingly enough, NASA did a study where they attempted to model flammability under changes in gravity. Turns out lunar gravity is the worst in terms of flammability:


  35. Regarding shielding for radiation:

    According to Wikipedia, Mars gets 43% of solar radiation that Earth gets
    The Solar Constant (Gsc) = Solar radiation at Earth orbit
    On a clear day 75% of that energy reaches Earth surface (cloudy days get about 1/2 the energy)
    So the atmosphere and Van Allen belts stop about 25% of the energy. Under the proposed pressurized sheet there’s only 2% of Earth’s atmosphere (1% is the Mars atmosphere itself).

    Earth surface radiation = Gsc x (1 – 0.25) = 0.75 Gsc
    Mars surface radiation = 0.43 Gsc x (1 – 0.02) = 0.42 Gsc

    If I’ve done that right, Mars surface under the tent has only 56% of the solar radiation as Earth surface! Granted, there’s still cosmic radiation (1/2 of which is blocked by the ground underneath you), and not all frequencies are created equal, but that’s beyond the scope of this comment. Still, I agree with Casey, radiation is not the big monster we’ve been trained to think it is.


    1. Another quote from ‘Nature’:

      “During transit outside of LEO, every cell nucleus within an astronaut’s body would be traversed by a hydrogen ion or delta ray (a recoil electron caused by fragmentation after ion interactions) every few days, and by a heavier GCR ion (e.g., O, Si, Fe) every few months.12,13 Despite their infrequency, the heavy ions contribute a significant amount to the GCR dose that astronauts would incur outside of LEO. The energies of the heavier GCR ions are so penetrating that shielding can only partially reduce the intravehicular doses.13 Thicker shielding could provide protection, but is limited by mass and volume restrictions of exploration vehicles and dependent upon the capabilities of spacecraft launch systems.

      The high-LET radiation found in the GCR spectrum can produce excessive free radicals that instigate oxidative damage to cell structures. Chronic exposure to such oxidative stress contributes to the radiation-induced changes associated with premature aging, cardiovascular disease, and the formation of cataracts. The large ionization power of GCR ions makes them a potentially significant contributor to tissue damage and carcinogenesis, central nervous system (CNS) degeneration, and deleterious health outcomes.14,15 In addition, as GCR ions pass through a space vehicle, interaction with the spacecraft hull attenuates the energy of heavy-charged particles and frequently causes their fragmentation into numerous particles of reduced atomic weight, a process referred to as spallation.16,17 Spallation occurring as GCR particles collide with shielding materials can result in ‘cascade showers’ that produce progeny ions with much higher potential for biological destruction than the original particle.9,13,18,19 This process changes the makeup of the intravehicular radiation spectrum, adding to the complexity of the radiation environment unique to spaceflight.”

      Not that mere medical science will convince anyone. Everybody knows better than the experts lol


  36. Very interesting and detailed post, thank you Casey. Up until now I had usually thought of hyperbolic paraboloids when considering habitats with high pressure inside … but I should have just thought of the inflatable mattress!

    I find it unusual that you dismiss canyons in the same sentence as tunnels, given that the former are natural formations and therefore not labour intensive at all. In fact, if one of these membranes was stretched across a Martian canyon, would the walls not provide solid radiation shielding during most daylight hours? In addition to removing the need for almost all perimeter walls, this may also ease the steel requirements if the walls can provide some extra force at the edges.

    Like others, I’m also curious about the degree to which atmosphere would ‘leak’ out of the surrounding regolith … but agree that this would just impact the required replacement rate.

    Actually almost seems doable until you think about the thermodynamics!


  37. When this was first posted, I had taken for granted that domes were silly, just something that people imagine because they look cool in the artist’s-conception images.

    Today, though, I’m guessing that people will be risk-averse about radiation, so the parts of a Martian city that people will live in will be under a nice thick layer of regolith or mine tailings, thick enough to soothe all the irrational fears of radiation — which will also mean it’s thick enough to bear its weight by compression rather than being supported by gas pressure. Soothing irrational fears is likely to be cheaper than educating people. A Martian city will be dependent on support from Earth for a long time before achieving autarky, and if it’s perceived by the ignorant as being a death-trap for the Martians, that support will be harder to maintain.

    Part of the reason I think it’s likely to be cheaper is that I also think domes will be easier to make than I had been imagining. If you just hang a string between two poles, it forms a catenary. Spray it with epoxy or some such material, flip it over, and you’ve got an arch. Line up a bunch of such arches, put a layer of plastic over them, and you’ve got the shape for a vaulted roof. Have it vary in width, so that you can get some compressive support in the other horizontal dimension too, and you’ve basically got a dome. Use a machine resembling a snow-blower to fling regolith up onto it, mixing some epoxy or whatever into the stream if necessary, and you’ve got your permanent roof.

    I don’t think it will be as cheap as a pressure-supported tent, but I think it will be cheap enough. In particular, I think it will be cheaper than smelting metal to make beams to support regolith on the roof of a building. Earth has iron ore and coal, and no need for dirt on the roof, so domes are only good for making cathedrals and capitol buildings look cool (and for having relatively large covered spaces before anyone knew how to mass-produce good steel). I expect making steel from ordinary Mars rock to be perfectly feasible, and I expect it to be the cheapest option where the Martians relatively little steel, like the cables to hold down the roof of a pressure-supported tent. But where they would need more of it, like the beams for a building with dirt on the roof, I expect steel to be more expensive than just moving regolith and sticking it together with polymers made from Martian air and water.

    I still think that the vast majority of a Martian city will be covered by pressure-supported tents, not compressive domes, because people won’t live there. It will be pressurized and heated, but only enough so that you’ll be fine with just an oxygen mask long enough to get to shelter if your pressure suit fails, not to the point of being a shirtsleeves environment. Some people will work there, because the difference between 80% autonomous machinery and 100% autonomous will cost more than it’s worth. But most of the work will be done by mostly-autonomous machinery operated from the comfort of places that are fully pressurized.

    If I were imagining a Martian city to be set up ASAP by a hypothetical organization that can ignore other people’s irrational fears, I would probably still leave the dirt-on-the-roof living spaces out of it, on the grounds that the risk from radiation is low enough for motivated would-be Martians to accept. But I don’t believe in the feasibility of either getting started right away or ignoring others’ irrational fears.


    1. A tensile membrane only 100m high would be an adequate radiation shield against everything except cosmic rays. Sleeping areas can always have supplemental shielding but why bother?

      Material requirement for compressive masonry is about 1000x higher per hectare than tensile membranes, and would place quite stringent constraints on 2D layouts, CTE issues, logistics, traffic, and supplemental lighting.

      Key thing is to create a warm pressurized breathable area as large as possible, then do things inside it.


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