# For space colonies, how thick does the shielding need to be?

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I just had a bit of inspiration. I'm not certain how thick the shielding needs to be, but the problem with the mass of shielding - say it's a meter of lead - is that it enormously stresses the centrifuge wheel. You have to make cables and structural supports able to survive the apparent "weight" of a meter thick layer of lead all the way around. Terrible idea.

Instead, you build a gigantic can shape out of whatever the shielding material is. One logical thing would be to use glass->water->glass, with the water full of algae. The whole can doesn't spin. There's an inner can, and it has tracks inside it. Maglev tracks, specifically. The colony itself is a rotating disk that is much, much lighter - as light as possible - that spins, with the maglev tracks providing an occasional nudge. The linear motors in the maglev track are under computer control and you dump any angular momentum into the inner colony disk, keeping the outer can completely non rotating.

You'd actually use 2 colony disks, that counter rotate. That way you don't have to consume propellant to get the colony spinning and to despin. (entire colony has a net angular momentum of zero) . You would have to have some kind of maglev transfer car at the junction between the disks, where you get into the car at a boarding hatch, it decelerates and then spins the other direction inside a vacuum transit track located between the 2 disks.

So, how thick does the water layer need to be? I'm guessing it needs to be the same thickness as enough water to provides 1 atm of pressure in earth, as this means that there is as much shielding mass as earth's atmosphere. That means it has to be 32 feet thick. For this reason, you'd want to make the colony as big as possible in order to reduce the relative proportion of shielding mass.

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You can just use frozen ice that way, can't you? Skip the glass.

Or just hollow out an asteroid.

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The problem with the endless "hollow out out an asteroid" cliche is that all that rock could be melted down and used in a controlled manner. Why depend on a lumpy, random scattering of rocks for your shielding? Why leave all the rare elements in the asteroid unharvested? Doesn't make sense. Better to eat the asteroid and create exactly what you want.

My idea of using water instead of ice is that the algae in the water would be part of the colony's life support system, saving on needing farming mass elsewhere. It doesn't have to be "algae" either, we could probably genetically engineer small aquatic plants that taste like apples or steak or something.

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Glass is crap and heavy shielding, costing quite a bit and weigh a ton for the amount of shielding you get. Alge+water is better, but only because we are dealing with water. Instead, make the farming modules outside of the shielded areas, and use a thin layer of depleted uranium followed by a plastic of some sort ended with a nice thick sheet of lead. The lead only needs to be around a few centimeters thick as the brunt of the impact is absorbed by the depleted uranium which makes a far more efficient shielding material, though upon exposure to high energy cosmic rays would produce some neutrons and gamma rays. Another thing to note is the utility in machinery being twofold, one, it does stuff, but it also can be used as shielding. Thus you could place your algae farms, radiators, and everything that you can around the heavily shielded areas thus preventing them from being exposed to as much radiation while not adding any mass in the form of conventional shielding.

It should also be noted that the really high energy radiation usually has warning before it arrives, thus you should be able to get to a sheltered room in time.

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You just need mass. I believe it's 5 meters of steel per square meter...

There is a challenge with it, yes. But challenges are surmountable.

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I just had a bit of inspiration. I'm not certain how thick the shielding needs to be, but the problem with the mass of shielding - say it's a meter of lead - is that it enormously stresses the centrifuge wheel. You have to make cables and structural supports able to survive the apparent "weight" of a meter thick layer of lead all the way around. Terrible idea.

Instead, you build a gigantic can shape out of whatever the shielding material is. One logical thing would be to use glass->water->glass, with the water full of algae. The whole can doesn't spin. There's an inner can, and it has tracks inside it. Maglev tracks, specifically. The colony itself is a rotating disk that is much, much lighter - as light as possible - that spins, with the maglev tracks providing an occasional nudge. The linear motors in the maglev track are under computer control and you dump any angular momentum into the inner colony disk, keeping the outer can completely non rotating.

You'd actually use 2 colony disks, that counter rotate. That way you don't have to consume propellant to get the colony spinning and to despin. (entire colony has a net angular momentum of zero) . You would have to have some kind of maglev transfer car at the junction between the disks, where you get into the car at a boarding hatch, it decelerates and then spins the other direction inside a vacuum transit track located between the 2 disks.

So, how thick does the water layer need to be? I'm guessing it needs to be the same thickness as enough water to provides 1 atm of pressure in earth, as this means that there is as much shielding mass as earth's atmosphere. That means it has to be 32 feet thick. For this reason, you'd want to make the colony as big as possible in order to reduce the relative proportion of shielding mass.

This is pretty much the way the Kalpana One settlement handled radiation shielding. So, kudos for coming up with a plausible concept. The downside, of course, is much greater mechanical complexity, and quite the catastrophic failure point if the two nested cylinders mis-align.

And considering having extra structural mass means you have more radiation protection before you add the shielding... I'd say it might not be worth it. Just make your colony with a higher percentage of structural mass, it's cheap steel and aluminum after all, and it's a passive system. Most proposals use unprocessed regolith as shielding, BTW, or the slag form the colony construction. Water, while having some advantages (can be pumped, it's very useful), also has some disadvantages (moves on its own in weird ways in microgravity, corrodes stuff, could be used for more important things).

As to actual numbers on how much stuff you need to do proper radiation shielding, the best form of measure is tons/square meter (mT/m2), and anywhere form 10 to 40mT/m2 have been proposed as acceptable shielding levels, that I know of. Oh, and everything contributes more or less the same way: you are worried about GCR mainly, so the only thing that works is to put a lot of mass between you and them. So don't worry too much about the composition.

Rune. KISS.

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This is pretty much the way the Kalpana One settlement handled radiation shielding. So, kudos for coming up with a plausible concept. The downside, of course, is much greater mechanical complexity, and quite the catastrophic failure point if the two nested cylinders mis-align.

And considering having extra structural mass means you have more radiation protection before you add the shielding... I'd say it might not be worth it. Just make your colony with a higher percentage of structural mass, it's cheap steel and aluminum after all, and it's a passive system.

The way I'm describing doing it, each cylinder rides inside a series of ring tracks. If you use steel instead of water, it means the inner ring tracks are attached to several meters of metal. There is absolutely no chance they are going to fail. (well, magnet failures, but there are passive ways to do this where the magnets are just coils of wire that have to be intact to work)

The transition car - essentially a maglev car riding a set of rails in between the 2 colony disks - works by driving itself faster and faster (using a linear motor embedded in the track it is riding on probably) until it matches speed with one disk or the other. It then extends a gantry to the hatch on the colony disk in question. The gantry would be designed to break off in the event of it extending to the wrong disk. Passengers board, and sit in special chairs that rotate so the passengers are lying on their back with respect to the G forces.

Transit car transitions - amusingly, it could accelerate harder when the relative speed of the car is lower, keeping the gravity experienced by the passengers to a steady 1.5 G or so, and the seats would rotate in 2 dimensions. (they would be able to rotate 180 degrees, and also adjust their angle of tilt with respect to the floor).

Furthermore, you can actually use the track and the outer ring it is supporting as part of the structure. All the pieces of the colony would exert pressure down on the maglev track and this essentially becomes hoop stress on the outer, stationary ring. So the radiation shielding is also part of the structure, yet it is not adding to the structural strain on the structure as it is not spinning.

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i do like the double hull concept. you have an external pressure/shielding hull with an internal centrifuge hull on magnetic bearings. you could float around the internal hull unsuited, though the internal hull has its own emergency pressurization system in case the outer hull is breached. you can use multiple habitat rings for neutral torque or you can use a lighter disc at higher rpm to counter out the torque of the habitat. you could also have a number of rings with different gravity levels for different purposes, a low gravity havier industrial ring spinning at a lower rpm than a lighter habitat ring would not only balance out but also make heavy industrial operations require less effort due to reduced gravity. the whole solar exposed area of the station would be covered in solar panels or a solar-thermal plant (replaced by a nuclear fission or fusion reactor on orbits too far from the sun). so crops will require large scale hydroponic growing operations in artificial light which can be more finely tuned to specific crops. having a large pressurized outer hull is useful for large scale space construction, you can build space craft or work on strip mining a chunk of asteroid entirely in a shirt sleeve environment.

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Dandridge M. Cole did it best.

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The problem with the endless "hollow out out an asteroid" cliche is that all that rock could be melted down and used in a controlled manner. Why depend on a lumpy, random scattering of rocks for your shielding? Why leave all the rare elements in the asteroid unharvested? Doesn't make sense. Better to eat the asteroid and create exactly what you want.

Because excavating costs a lot less energy than melting an entire asteroid. Of course with the projects we're talking about the energy problem might have been solved but the same technology might bring us energy shields...

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If is all about mass, then to have the same shielding than our atmosphere, we need 10 meters of water between us and the particles. Not sure if this is right or not, it depends on if mass is the only factor.

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im thinking something as simple as building a 1km diameter spherical pressure hull out of 1/4" welded plate steel built on the inside of a steel exoskeleton of sorts, perhaps with a second layer outer hull. add a large airlock and a bunch of small ones. once the hull is in place you can pressurize it (probibly with an inert gas at first) and can begin constructing the internals. the hull would need a main structural backbone to support construction of various centrifuges over the life of the station. initially you could build up a temporary centrifuge for mining/construction crew with extra shielding that might be able to support 50 or so. you bring the asteroids in side for processing. as asteroids are mined for materials you can then collect mining debris on the surface and hold it in place with steel netting or some other mechanism, this becomes your shielding, essentially creates an artificial asteroid. processed materials go into further station construction. population would grow over time as more shielding is installed, more facilities added, bigger habbitats created. once the station is fully operational it becomes a thriving metropolis and hub of space comerce, facilities continue to process materials but for export. eventually you need to build a bigger one. i can see something like that orbiting ceres.

of course then you install a big ass raygun that can destroy an entire planet.

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