Author Topic: Rotating Habitats and Asteroids  (Read 14409 times)

Offline KelvinZero

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Re: Rotating Habitats and Asteroids
« Reply #40 on: 11/24/2018 09:56 am »
I would be worried about tunnelling subtly changing the rotational inertia of a rubble pile, and it suddenly reshaping itself in a series of avalanches that change the rotation again and trigger the next avalanche.

Maybe there is some robust mathematical way of becoming confident that this will not happen, eg triggering avalanches deliberately until it is in hydrostatic equilibrium. Even a sphere can precess though. A question for the engineers I suppose.

Offline Paul451

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Re: Rotating Habitats and Asteroids
« Reply #41 on: 11/25/2018 12:22 am »
I would be worried about tunnelling subtly changing the rotational inertia of a rubble pile, and it suddenly reshaping itself in a series of avalanches that change the rotation again and trigger the next avalanche.

That would be an issue for any asteroid mine, even if you were just operating on the surface, scooping out material. Just as there are similar structural issues on any Earth mine, tunnel or quarry. We'll slowly develop the knowledge and experience. It's not like the huge dedicated systems I've described are going to be the very first asteroid mining operation.

Also, in spite of the name, "rubble pile" asteroids are not just piles of gravel and smaller. They seem to have everything up to mountains. So the bulk internal structure and strength is where the largest components rest together. Picture a sack of potatoes. I suspect you can map those larger elements (via seismic surveys) and work out where the most self-supporting parts of the asteroid are. Similarly, basic physics will tell them the best places to dump the tailings/slag to avoid issues.

Offline mikelepage

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Re: Rotating Habitats and Asteroids
« Reply #42 on: 12/11/2018 04:08 am »
Because you'll gradually lose kinetic energy to friction/heat, you'll need to keep accelerating the train just to stay at the same speed/gravity-level.  This will, in turn, gradually de-spin the entire asteroid.

This is not quite correct, because of conservation of angular momentum. If the train is losing kinetic energy due to friction, it is not just being lost to heat, some of it is being converted back into kinetic energy of the asteroid, spinning it up again. So you are not going to gradually de-spin the asteroid.

So... you agree with me that some of the energy is being lost to heat? Yes, there is a portion of the energy due to friction that will be exerting a "spin up" effect on the asteroid, but the portion of the energy lost to heat will not.  Therefore the total angular momentum of the asteroid decreases, but conservation of energy is maintained.

Quote
There was a similar error in a short story you wrote, where you had people running to spin up a space station. Actually, as soon as they stop running again, their efforts will amount to nothing. Angular momentum is conserved. The real way for them to manually increase the spin of the station would be to lift mass towards the center.

Yeah I rewrote that section when I realised my error :P  In that case, the heat energy of the friction is being retained by the system (so it wouldn't work), while in the asteroid case above, I assumed it would be lost to space.  Not sure how big the effect would be though.

Offline ppnl

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Re: Rotating Habitats and Asteroids
« Reply #43 on: 12/11/2018 05:57 am »
Because you'll gradually lose kinetic energy to friction/heat, you'll need to keep accelerating the train just to stay at the same speed/gravity-level.  This will, in turn, gradually de-spin the entire asteroid.

This is not quite correct, because of conservation of angular momentum. If the train is losing kinetic energy due to friction, it is not just being lost to heat, some of it is being converted back into kinetic energy of the asteroid, spinning it up again. So you are not going to gradually de-spin the asteroid.

So... you agree with me that some of the energy is being lost to heat? Yes, there is a portion of the energy due to friction that will be exerting a "spin up" effect on the asteroid, but the portion of the energy lost to heat will not.  Therefore the total angular momentum of the asteroid decreases, but conservation of energy is maintained.

Quote
There was a similar error in a short story you wrote, where you had people running to spin up a space station. Actually, as soon as they stop running again, their efforts will amount to nothing. Angular momentum is conserved. The real way for them to manually increase the spin of the station would be to lift mass towards the center.

Yeah I rewrote that section when I realised my error :P  In that case, the heat energy of the friction is being retained by the system (so it wouldn't work), while in the asteroid case above, I assumed it would be lost to space.  Not sure how big the effect would be though.

Angular momentum is conserved separately from energy. If the asteroid is losing angular momentum then you have to be able to show where it went.  Basically, you have to show what is spinning that was not spinning before in order to account for the reduced spinning of the asteroid.

Offline Paul451

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Re: Rotating Habitats and Asteroids
« Reply #44 on: 12/11/2018 10:38 am »
Because you'll gradually lose kinetic energy to friction/heat, you'll need to keep accelerating the train just to stay at the same speed/gravity-level.  This will, in turn, gradually de-spin the entire asteroid.
This is not quite correct, because of conservation of angular momentum. If the train is losing kinetic energy due to friction, it is not just being lost to heat, some of it is being converted back into kinetic energy of the asteroid, spinning it up again. So you are not going to gradually de-spin the asteroid.
So... you agree with me that some of the energy is being lost to heat? Yes, there is a portion of the energy due to friction that will be exerting a "spin up" effect on the asteroid, but the portion of the energy lost to heat will not. Therefore the total angular momentum of the asteroid decreases, but conservation of energy is maintained.

The asteroid has 100 units of angular momentum, relative to us, the observers. The train is stationary WRT the asteroid and has an angular momentum of 10 units, relative to us, in the same direction as the asteroid. The total angular momentum of the system is therefore 110 units.

From stored energy in a battery, the wheels of the train are driven against the track and propel the train WRT the asteroid. The train is accelerated by 10 units of angular momentum, requiring 1 arbitrary unit of energy. The wheels push against the track which pushes against the asteroid in the opposite direction as the train (F = -F), slowing the asteroid's spin. The train now has 20 units of angular momentum, and the asteroid has 90. The total angular momentum of the system is 110 units. Total angular momentum conserved. One unit of energy has been converted from storage in the battery to kinetic energy in the train. Total energy conserved.

Friction converts the train's kinetic energy into heat, slowing it relative to the asteroid, until it comes to a stop WRT the asteroid, It has lost 10 units of rotational momentum and 1 unit of energy has been generated as heat. This friction also drags the asteroid in the direction of the train (the opposite direction as its deceleration, -F = F), accelerating it by 10 units of angular momentum, returning it to 100 units of angular momentum.

The components of the system have returned to their previous angular momentum. 100 units and 10 units respectively. As if nothing happened.

Except, what did happen was that 1 unit of stored energy in the battery was converted to 1 unit of energy of low grade heat on the rails and wheels and axles and bearings. It's as if the entire asteroid/train system was just a weirdly convoluted electric heater. Entropy smiled, but not a single unit of angular momentum was lost.

Yes you have to keep accelerating the train. No it won't despin the asteroid.
« Last Edit: 12/11/2018 10:44 am by Paul451 »

Offline mikelepage

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Re: Rotating Habitats and Asteroids
« Reply #45 on: 12/11/2018 10:57 am »
Well bugger. I stand corrected. Thanks for your explanations.

In any case, it still seems prudent to plan your “ring train tunnels” to have parallel axes to the spin axis of your asteroid.  No sense needlessly torquing the asteroid, and it still allows you to use the asteroid as reaction mass.

Offline alexterrell

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Re: Rotating Habitats and Asteroids
« Reply #46 on: 12/11/2018 11:21 am »
The shielding can also be in an outer shell that doesn't rotate. Need a lot of shielding but support structure required to hold it in place is very light as its 0g.

IMO, no large rotating settlement, such as an O'Neill, will ever be flown "naked". It'll either be built inside an asteroid, or surrounded by a non-rotating shell serving the same role.

Edit/Lar: Topic split... near term and inflating asteroids may or may not be useful in the same sentence...



Post-split edit:

I asked Lar to split this off the other thread. Thanks Lar.

For a rough topic description: How to use asteroids to build large (up to O'Neill scale) habitats and settlements in space. And how to use large habits and settlements to exploit asteroids.
I estimate that a 4km long, 1km diameter cylinder is going to need a 1 billion ton shield. This should probably be multilayer and hence about 200m thick (mostly vacuum) so even a nuclear blast won't penetrate.

I agree - that is not going to rotate.

A few ways come to mind to build this:
- Find a rubble pile astronaut and drill to the center (or far underground). Insert an airtight bag and inflate slowly.
- Find a small rubble pile. Build a thin metal frame around it (5km long, 1.2km diameter, 1cm thick). Positively charge the metal frame, and then use an electron gun on the rubble pile. Once the inner coating is covered so no more charge can flow inwards, repeat the process, to create multiple layers.
- Go to one of the poles of Phobos (or Deimos). Drill a ring (1km diameter) of small holes (5km deep) and fill with concrete. That stabilize the sides of the hole. Then remove the soil in the middle (probably by excavating from underneath, so it falls towards the center, before being brought to the surface elsewhere. The rotating structure would be fitted into the hole and rotate around the same axis as Phobos. 

Offline TrevorMonty

Re: Rotating Habitats and Asteroids
« Reply #47 on: 12/11/2018 03:05 pm »
It probably lot easier to build rotating habitat near to large asteriod and mine it for build materials. Shielding for pair of counter rotating Oneil cylinders is not very complicated. Just large outer shield  containing few metres of shielding regolith. Shield can be made up of two layers of iron frame work and chicken wire with thin plastic covering to contain regolith. Alternatively sinter regolith into large tiles and bolt them to frame work.

Offline KelvinZero

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Re: Rotating Habitats and Asteroids
« Reply #48 on: 12/11/2018 11:07 pm »
I like the way trains theoretically allow for practically any radius.

One concern I have with trains and even shielding independent of the spinning portion is how to maintain it without stopping everything. How do you confidently avoid a sort of cascade effect if anything goes wrong, with shrapnel continually gaining more destructive kinetic energy as it bounces back and forth between the two surfaces?

Being one piece (and outside the asteroid) often seems reasonable to me. Beyond a certain scale, a 10 meter water reservoir beneath your feet does not seem a big deal, and the lakes will have other uses too.

Offline mikelepage

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Re: Rotating Habitats and Asteroids
« Reply #49 on: 12/13/2018 07:12 am »
I like the way trains theoretically allow for practically any radius.

One concern I have with trains and even shielding independent of the spinning portion is how to maintain it without stopping everything. How do you confidently avoid a sort of cascade effect if anything goes wrong, with shrapnel continually gaining more destructive kinetic energy as it bounces back and forth between the two surfaces?

My thinking is that anytime humans arrive at an asteroid, there's already been a robot sent in advance to mine a circular (toroidal) tunnel.  Over time, multiple such tunnels would be excavated/mined.  That way if you ever need to shut down one tunnel for inspection/maintenance, you move your habitat module train cars into another, adjacent tunnel.  For that matter, adjacent tunnels would probably start as "pit lanes" to the first tunnel(s).

Quote
Being one piece (and outside the asteroid) often seems reasonable to me. Beyond a certain scale, a 10 meter water reservoir beneath your feet does not seem a big deal, and the lakes will have other uses too.

I think this will happen too, especially once in-space fabrication gets going.  Almost like two classes of spin gravity habits: additively manufactured (free spinning - with water-filled expandable modules for shielding), and subtractively manufactured (inside asteroids - using regolith for shielding).  The distinction will probably get fuzzy once space industry gets big enough.


Offline KelvinZero

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Re: Rotating Habitats and Asteroids
« Reply #50 on: 12/13/2018 11:11 am »
I think this will happen too, especially once in-space fabrication gets going.  Almost like two classes of spin gravity habits: additively manufactured (free spinning - with water-filled expandable modules for shielding), and subtractively manufactured (inside asteroids - using regolith for shielding).  The distinction will probably get fuzzy once space industry gets big enough.
btw I have a third class based on a pet idea, but it only applies to ice worlds. Im not sure how many asteroids it could apply to but one of course is Ceres, which is still referred to as an asteroid despite now also a dwarf planet.

My third "ceres-class" option is centrifugal habitats in oceans melted by waste heat, deep enough under the ice for earth pressure. For example a parabola-shaped spinning cup/diving bell where the direction of gravity is always aligned to the surface. You could swim into the diving bell from a minimal hole at the bottom, where there is no spin and only the 3% gravity of ceres, then swim up to the beach which is in low gravity with no doubt very interesting waves, and continue walking up to the beach-front properties and cafes in full gravity.

Unfortunately you cannot just have this diving bell spinning in water. I did a bit of investigation and apparently even with theoretical infinitely low friction you still have drag. I think the theory assumes at least an atom-width of water is carried along or something like that which still interacts with the rest of the water.

You could of course have something moving around in an evacuated chamber, like a hyperloop in a circle but I hope there might be a much better trick with a paint-thin counterrotating layer that gives you a stationary outer surface rather than a theoretical-frictionless one.. and ideally is self healing eg a magnetically suspended liquid layer. So the idea is practical but hard work, or easy but hand-wavy science fiction :)

Offline mikelepage

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Re: Rotating Habitats and Asteroids
« Reply #51 on: 12/14/2018 05:29 pm »
My third "ceres-class" option is centrifugal habitats in oceans melted by waste heat, deep enough under the ice for earth pressure. For example a parabola-shaped spinning cup/diving bell where the direction of gravity is always aligned to the surface. You could swim into the diving bell from a minimal hole at the bottom, where there is no spin and only the 3% gravity of ceres, then swim up to the beach which is in low gravity with no doubt very interesting waves, and continue walking up to the beach-front properties and cafes in full gravity.

Unfortunately you cannot just have this diving bell spinning in water. I did a bit of investigation and apparently even with theoretical infinitely low friction you still have drag. I think the theory assumes at least an atom-width of water is carried along or something like that which still interacts with the rest of the water.

You could of course have something moving around in an evacuated chamber, like a hyperloop in a circle but I hope there might be a much better trick with a paint-thin counterrotating layer that gives you a stationary outer surface rather than a theoretical-frictionless one.. and ideally is self healing eg a magnetically suspended liquid layer. So the idea is practical but hard work, or easy but hand-wavy science fiction :)

So the bell spins on its axis? With the wide part of the U bell/parabola facing "up" away from Ceres' centre, but because the bell is spinning, the habitat's "down" is nearly perpendicular relative to Ceres' surface?  But actually the top of the U is sealed to keep the air pressure in?  So you have to enter through the bottom... Dude :D

Pretty sure the friction issues make it a non-starter.  Assuming there is a liquid subsurface ocean, you'd basically be attempting to set up an ocean-wide whirlpool, with corresponding energy requirements.  Seems like you're introducing an insane amount of complexity so you can surf some cool waves  ;) 

Offline KelvinZero

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Re: Rotating Habitats and Asteroids
« Reply #52 on: 12/14/2018 10:37 pm »
Pretty sure the friction issues make it a non-starter.  Assuming there is a liquid subsurface ocean, you'd basically be attempting to set up an ocean-wide whirlpool, with corresponding energy requirements.  Seems like you're introducing an insane amount of complexity so you can surf some cool waves  ;)
Yes the friction would make that approach a total non-starter. Half of my text was speculating on about how to get around this.

It is obviously possible, for example it is not hard to imagine a hyperloop-like torus floating in the water.. I think we can do far better though.

Ideally you want a mystery layer that is
* stationary wrt to the water.
* frictionless wrt to the rotating surface.
* not a big investment like building an entire other wall. It is just a thin layer that gets its strength by pushing against the rotating surface somehow.
* ideally self-healing, like a fluid bubble or air bubble. You don't want to stop the whole thing for regular patching.

..but you can make the concept work with far less SF if you want.

Offline Generic Username

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Re: Rotating Habitats and Asteroids
« Reply #53 on: 12/18/2018 05:27 pm »
I'm familiar with the concept from Niven,
[/quote]

The concept precedes Niven. At least back to Dandrige Cole/Roy Scarfo in the very early 1960's.



One other major problem with this design, beyond the difficulty in actually making it: it's dynamically unstable. Unless either constantly actively controlled, or connected to another identical rotating habitat in the same way the O'Neill habs were meant to be, a cylinder like this won't rotate on its long axis. It might start off that way, but it'll soon end up tumbling end-over-end.
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Offline mikelepage

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Re: Rotating Habitats and Asteroids
« Reply #54 on: 12/19/2018 07:44 am »
Quote
I'm familiar with the concept from Niven,

The concept precedes Niven. At least back to Dandrige Cole/Roy Scarfo in the very early 1960's.



One other major problem with this design, beyond the difficulty in actually making it: it's dynamically unstable. Unless either constantly actively controlled, or connected to another identical rotating habitat in the same way the O'Neill habs were meant to be, a cylinder like this won't rotate on its long axis. It might start off that way, but it'll soon end up tumbling end-over-end.

I suspect these depictions of "wide open spaces" will come to be seen as a quaintly romantic (but ridiculous) aspect of 20th century science fiction.  If the number one nightmare of Earth bound civilisations is falling to one's death, I'd guess the number one nightmare of space faring civilisations is being sucked out of your ship/habitat in an explosive decompression.  It's not like there isn't an uncountable number of micrometeorites/debris flying about at speeds sufficient to create such a hole.  These events are a matter of when, not if.

In space, airlocks are your friends.  Air and water is life.  So compartmentalisation is a fundamental tenet of good design.  Also, even in full spin gravity, every single square cm of your habitat needs to be easily service-able with a silicone/putty "hole-sealer" glue gun.  Most bulkheads will probably have one of these at the ready (alongside a "break-glass" alarm panel and fire extinguisher).  So that's why I think it will be a long time before the internal diameter of space habitats ever gets so large that the ceiling can't be easily reached with a cherry picker (or similar) to perform such repairs.  I guess that limits us to something as tall/wide as a 4-6 storey building...?

Offline Paul451

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Re: Rotating Habitats and Asteroids
« Reply #55 on: 12/19/2018 08:23 am »
So that's why I think it will be a long time before the internal diameter of space habitats ever gets so large that the ceiling can't be easily reached with a cherry picker (or similar) to perform such repairs.  I guess that limits us to something as tall/wide as a 4-6 storey building...?

Not really relevant for a cylindrical rotating habitat, since your "ceiling" is the opposite floor, it can be as far away (as wide) as you want. No cherry-picker required. Ceiling-heights are only relevant for structures like wheel-stations, and if it's big enough for huge volumes to be an issue, it would be common sense to have a false-ceiling with accessways.

But the risk of puncture/breach gets us back to shielding. Having a non-rotating bulk shield that can deal with any impact that isn't on a structure-destroying scale. (Some other system would be required for larger asteroids.) I would expect the shielding to outmass everything else. Trying to put that shielding on the rotating structure itself drastically increases the structural loads.

Offline Generic Username

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Re: Rotating Habitats and Asteroids
« Reply #56 on: 12/19/2018 08:23 am »
It's not like there isn't an uncountable number of micrometeorites/debris flying about at speeds sufficient to create such a hole.  These events are a matter of when, not if.

When your habitat is measured in kilometers, the wall thickness - assuming materials such as steel - is measured in *meters.* An impactor dangerous enough to poke a hole through *that* would be detectable and interceptable. Even Bernal almost a century ago assumed electron beam directed energy weapons to zap impactors.

Realistically, if the habitat is well engineered the only conceivable, practical threats to them would be weaponry. And if you are ready to accept anti-hab warfare, then it doesn't matter how the hab is constructed; someone will make weapons to destroy it.
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Offline mikelepage

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Re: Rotating Habitats and Asteroids
« Reply #57 on: 12/19/2018 01:35 pm »
When your habitat is measured in kilometers, the wall thickness - assuming materials such as steel - is measured in *meters.* An impactor dangerous enough to poke a hole through *that* would be detectable and interceptable. Even Bernal almost a century ago assumed electron beam directed energy weapons to zap impactors.

Yes, but this is presumably much further in the future than the first asteroid settlements.   My question to you is, if you're capable of creating kilometer-scale habitats out of meters-thick steel, you're presumably also capable of creating dozens/hundreds of smaller habitats for the same price, so - aside from romantic notions of huge open spaces - what is the reasoning for making the single huge habitat?

But the risk of puncture/breach gets us back to shielding. Having a non-rotating bulk shield that can deal with any impact that isn't on a structure-destroying scale. (Some other system would be required for larger asteroids.) I would expect the shielding to outmass everything else. Trying to put that shielding on the rotating structure itself drastically increases the structural loads.

I'm curious how you see the asteroid mine working, such it's worth it (from an economic perspective) to have an "underground" mine going deep into the more compacted material, versus an "open cut" mine, skimming and processing loose material on the surface.  At least for PGM, they're just as likely to be on the surface as deep within the asteroid.  Is the argument that water and CO2 ices are likely to be embedded deeper in the asteroid? (and that's why your mining produces voids that can then be used for settlements).

Offline Generic Username

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Re: Rotating Habitats and Asteroids
« Reply #58 on: 12/19/2018 11:36 pm »
My question to you is, if you're capable of creating kilometer-scale habitats out of meters-thick steel, you're presumably also capable of creating dozens/hundreds of smaller habitats for the same price, so - aside from romantic notions of huge open spaces - what is the reasoning for making the single huge habitat?


The primary practical purpose of kilometer-scale is to reduce coriolis effects to negligible levels.  Secondarily: assuming the same internal pressure and same gravity levels and same materials, to first order a cylinder or sphere scales up directly. Whether its one hundred meters in diameter with a ten-centimeter wall, or one thousand meters diameter with a 100-cm wall, the math holds. But that thicker wall not only protects better from impactors but also from radiation.

A primary *psychological* reason for "bigger is better" is that habitats such as these will likely be where the vast majority of humanity will *eventually* live, and so you want them to be *good* places to live. Turning them into a vast armada of orbital Cabrini Greens and Pruitt Igoes? No thanks. Less "romanticism" than "how do we keep the humans from turning feral?"


But the risk of puncture/breach gets us back to shielding. Having a non-rotating bulk shield that can deal with any impact that isn't on a structure-destroying scale. (Some other system would be required for larger asteroids.) I would expect the shielding to outmass everything else. Trying to put that shielding on the rotating structure itself drastically increases the structural loads.

Sure. But a larger habitat gets a lot of the benefit of shielding without the bother of having to have dedicated shielding. Once you get to kilometer scales, the actual need for an external shell of rock likely drops close to zero.

If you break the large hab up into small habs, you will greatly increase the total surface area of the habs for the same total volume, which means you'll need to increase the area - and thus mass, since thickness remains the same - of the needed shielding.

Quote
I'm curious how you see the asteroid mine working, such it's worth it (from an economic perspective) to have an "underground" mine going deep into the more compacted material, versus an "open cut" mine, skimming and processing loose material on the surface.  .

Since the geological and hydrological forces that worked on Earth to concentrate desired ores into veins very likely won't have happened on asteroids, asteroid mining will probable involve a whole lot of "grind up everything and separate the elements on the molecular level." Maybe there will be veins of water or ammonia ice binding rubble piles together that will be worth following, but I suspect on the whole asteroid mining will just use giant-sized industrial shredders to turn rocks into power and furnaces to turn powder into vapor.
« Last Edit: 12/19/2018 11:50 pm by Generic Username »
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Offline mikelepage

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Re: Rotating Habitats and Asteroids
« Reply #59 on: 12/20/2018 05:01 am »
A primary *psychological* reason for "bigger is better" is that habitats such as these will likely be where the vast majority of humanity will *eventually* live, and so you want them to be *good* places to live. Turning them into a vast armada of orbital Cabrini Greens and Pruitt Igoes? No thanks. Less "romanticism" than "how do we keep the humans from turning feral?"

Not disagreeing with your larger point, but meant to say I was using "4-6 storeys" as a size indicator, not because I think population density will be that high. I actually think it will be - by necessity - far lower.

Al Globus in a recent email newsletter, sent out a paper indicating a maximum carrying capacity of 61 persons per hectare of cultivated area (obviously some caveats). If you assume people want at least that much (non-cultivated) space to themselves, you end up with maximum population density ~30 persons per hectare for any space habitat. 

Compare to population density of central Paris - most of which is 4-6 storey buildings, but still pleasant enough - of 250 persons per hectare.  Point being, I highly doubt any space settlement could ever become as dense as an Earth-bound city or housing project, no matter how big or small it is.
« Last Edit: 12/20/2018 05:02 am by mikelepage »

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