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In an imagined sci-fi universe where immense space habitats and constructs are common (however, nothing reaching nearly the size of a Niven Ring) one of the many forms are worldhouses: planets enclosed partially or entirely in immense domes or shells, 1 km in height. This concept has been around for a while now, but in my project I want to flesh it out a bit.

So, in this project, the worldhouse interior is lit by sunlight, which passes through the semi-transparent shell. Partial worldhouses are supported by single pillars at the centre, while planetary worldhouses use multiple pillars or “support mountains” built from rock.

What would be the best material with which to build the worldhouse roof? It needs to be able to withstand a variety of gravitational strengths, to reflect most forms of damaging radiation and stellar winds, and to be strong enough that meteor impacts don’t damage it.

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  • $\begingroup$ First of all it needs to support its own weight... $\endgroup$
    – AlexP
    Dec 29, 2022 at 21:35
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    $\begingroup$ VTC:Opinion-Based (brainstorming). Worse, no material today can produce what you're looking for. So any technobabble word would do (and should, there's a reason most scifi writers don't fully, or partially, explain all of the concepts in their books). $\endgroup$
    – JBH
    Dec 30, 2022 at 0:00
  • $\begingroup$ @JBH, not true. We could produce a complete shell over our planet just by enclosing it in mylar. We don't need super-duper structures, we just need to turn the planet into a big balloon. Technically, we already have a shell. It's called our atmosphere. $\endgroup$ Dec 31, 2022 at 6:42
  • $\begingroup$ @RobertRapplean You're correct about atmosphere, but that's a bit trivial. I don't believe mylar could do it. There's a constant rain of particles onto the planet that burn up in the atmosphere. I doubt Mylar would last long enough to be useful, and once it's unbalanced at all, it comes crashing down onto the atmosphere (that could make a good story...). It's the infrastructure that's the real problem - the solid stuff needed to hold it all together and against which thrusters apply force to keep it in place. $\endgroup$
    – JBH
    Jan 3 at 0:05
  • $\begingroup$ @JBH, I disagree. Yes, the barrier would suffer continuous holes in it, but those would be mostly meaningless since it isn't actually a pressure vessel, holding the atmosphere in. It would, in effect, be a solid layer of atmosphere. There wouldn't need to be structure holding it up, since the atmospheric pressure on the parts that don't hold it up would keep it away from the planet. It wouldn't be meteor-proof, as described, but making it meteor proof would be a matter of thickness, and how much you'd be willing to compress the atmosphere inside to hold it up. $\endgroup$ Jan 3 at 4:16

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Unless you are going to invent some magical material (Plasteel, Ceramite, Ferrocrete, Transparent Aluminium etc.) that has the properties you require, I don't think there is something that can even come close to what you need.

However, if you exclude Meteor resistance - some form of thin Carbon Weave material could potentially work (with a little futuristic materials magic), thin enough to allow some radiation (light) through and to reduce weight as much as possible (as this will be your biggest factor).

I would then have some form of alternate system to deal with Asteroids. Either actively (Laser Turret, missle system, railguns etc.) or a passive system (having a sacrificial satellite or super-dense object that attracts incoming meteors or some combination of the above.

Or you could simply allow the Meteors to pass through it and then have it automatically be repaired by a maintanence crew or nanobots or thing

Also, 1 km in height isn't very high, most commercial airliners operate at around 9-10 Km up.

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Air

This glass-weight calculator tells us that glass has a specific gravity in the range of 2 to3 grams per cubic centimeter. This means that 1 inch thick glass has a weight of about 0.11 pounds per square inch.

So you construct your roof out of at least two layers of glass with reinforcement rods between. And you pressurize the inside to something in the neighborhood of 0.5 pounds relative to the exterior. Air pressure holds it up.

Possibly you need to play some interesting games with relative thickness and weight so the dome forms correctly. The top of the dome is lighter. Or possibly you need some support struts. These are just beyond current construction.

The major challenge will be weather. Storm winds will produce huge pressures on the surface. A layer of snow could overwhelm air pressure. A heavy rain could depress a portion of the dome.

You might also manage an automatic self-sealing system based on sticky stuff in balloons. If a hole opens for any reason, the balloons burst releasing the glue which then is drawn by air flow to the hole. You could look up self-sealing tires and self-sealing fuel tanks as models.

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  • $\begingroup$ Best answer yet! I see an upvote in your future. Just one thing: if the worldhouse is high enough, (above what would be the cloud layer) most of the “weather” (air, water etc) will be contained within it. Does this help? $\endgroup$
    – user98816
    Dec 30, 2022 at 8:33
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    $\begingroup$ Fuller already suggested this: "Taking advantage of the fact that warmer air inside a dome provides lift, he proposed a gigantic geodesic covering all of Manhattan island. A dome over one-half mile in diameter would actually float with only a 1°F air temperature difference..." This would work for local domes only, not full-planet spheres. Also, it depends on the planet having an atmosphere. motherearthnews.com/sustainable-living/nature-and-environment/… $\endgroup$
    – Conrado
    Dec 30, 2022 at 17:40
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It needs to be able to withstand a variety of gravitational strengths

Use what seems appropriate for each situation. You want very different architectures for high and low gravity, the atmospheric composition and pressure above the roof, the local radiation and meteor hazard, etc etc. Under lunar gravity, you can make use of heavy but potentially common and cheap materials like steel as structural components, but with more Earthlike gravity you will need more exotic structures, alloys and fancy carbon-nanotube-based materials if you want huge domes with widely spaced supports. Transparent hydrocarbon polymers might work if the local star is distant or cool, but UV damage might be a problem on other worlds, etc.

Roofs which merely have to separate an external from an internal atmosphere with a similar ambient pressure have quite different design constraints than those which have to separate atmosphere from vacuum. The list of circumstances isn't quite endless, but there are a lot of different planetary environments you might be able to build these things in.

You'll also have to deal with local material availability. For example, carbon, silicon and oxygen are widely available in the universe, giving the possibility of diamond or quartz windows, but sodium is rather more rare so normal glass is less likely to be available (which is a shame, because it is nice and clear and is a good UV blocker).

to reflect most forms of damaging radiation and stellar winds

One does not reflect either with matter. For EM radiation (generally UV) you just need to stick enough mass in the way to prevent the contents of your worldhouse getting fried, and that depends on how much UV your star is emitting (very little for a red star, quite a lot for blue stars),l how far away the planet is from it, and how UV sensitive its occupants are.

Charged particle radiation, such as solar wind but also more hazardous things like solar proton events or galactic cosmic rays, can sometimes be deflected with a suitably clever arrangement of magnetic fields, but no matter how hard you try some of those particles are going to hit the ground. It may be possible to suspend a cloud of charged particles above the roof as a sort of plasma shield, but this is veering off towards quite theoretical kinds of engineering which I know too little about to talk about here.

Your best bet is mass, and as much of it as possible, though that can be rather hard to arrange on an airless world if you want a transparent roof. This problem was worked around in the Stanford Torus space station proposal found in appendix K of Space Settlements: A Design Study, using "chevron mirrors":

Figure 5-36

This shows the action of the chevron mirrors in allowing light through shielding, by bouncing it three times off reflective surfaces, but blocking direct transmission by any radiation that can't undergo specular reflection from the mirrors... that would include charged and neutral particle radiation, and short wavelength EM such as vacuum ultraviolet and x-rays and so on.

It would be tricky to keep this nice and lightweight for large expanses of unsupported roof, but with enough pillars and suspension structures it might be made to work. It might also spoil the view slightly, so no uninterrupted vast expanses of clear glass for you.

and to be strong enough that meteor impacts don’t damage it.

Problem is that on airless worlds, meteorites are going to be coming in at escape velocity. On the Moon that's 2.4 km/s, faster than supersonic antitank rounds, and on the Earth it is over 11 km/s and there's no armor in existence that can shrug off that sort of impact.

Your best bet is to make the roof out of multiple layers materials, widely spaced, somewhat like a Whipple shield. Accept that it is going to get damaged by small impacts, and be prepared to continuously repair it. To defend against larger impactors you need a serious "space guard" program with powerful and highly capable multispectral telescope and radar arrays and a range of interception equipment to deflect or vaporize problematic debris before it reaches the surface.

In any case, you'll almost certainly want to have undergound emergency shelters readily available throughout the worldhouse, and ideally have the ability to partition very large 'houses so that a big puncture doesn't strip the atmosphere from a country-sized region, but maybe only a city-sized one. You might also want to consider the threat of deliberate damage to the roof... sabotage of active support and repair systems, or use of weapons.

Partial worldhouses are supported by single pillars at the centre

I'd say that very much depends on the gravity and atmosphere of the world you're building on. Designs must adapt. There's no one-size-fits-all here!

Using air pressure to partially or wholly support the roof is great if you can do it, but this needs to be offset against the risk of puncture.

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