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Alright so I have this big colony ship/artificial world and I need to figure out what kind of population it can support. I’ll cut right to the chase and give you the details.

  • Length: About 1,000,000 miles long
  • Diameter: 100 miles
  • Circumference: 314 miles

Some more details, people live on the inside surface of the Spire, so if you were standing inside the Spire your feet would be pointing outward and your head would be pointed toward the center. The Spire is segmented into 500 mile long “units.” Each unit has 200 miles of food and water production, 100 miles of residential areas (which includes recreation), 100 miles of production, power generation, and administration, and 100 miles of waste processing, recycling, and a drone control center.


You can assume that there is a perfect cylinder that is 1,000,000 miles long by 100 miles in diameter of useable space.

So given this amount of space and food production, what is the maximum amount of people this Spire could technically support?

Edit: The people living here have technology that’s early industrial level at best, renaissance technology for some, and some splinter groups are still at medieval levels. However, the Spire itself is far future advanced. We’re talking AI, programmable matter, easy antimatter generation, exotic matter, pretty much anything you can think of. The Spire takes care of the atmospheric conditions and weather so they are perfect for growing food, but the people are responsible for planting and harvesting.

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    $\begingroup$ What level of technology is used for food production and life support? $\endgroup$ – Alexander Aug 21 at 18:06
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    $\begingroup$ They live inside the cylinder which produces and recycles the atmosphere. $\endgroup$ – Nick Aug 21 at 18:24
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    $\begingroup$ What you are describing is called a "O'Neill cylinder". $\endgroup$ – cowlinator Aug 21 at 18:26
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    $\begingroup$ Yeah assume the average inhabitants are human-sized. $\endgroup$ – Nick Aug 21 at 18:27
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    $\begingroup$ What crops do they grow? Do they have pests? Am I correct in understanding that the climate is fully controlled? What agricultural zone is simulated? Crop yields depend on crop type, agricultural technology, access to fertilisers, and zone. You need to specify these and the amount of arable land to be able to calculate how much food can be potentially produced. $\endgroup$ – Otkin Aug 21 at 18:46
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This answer is a combination of relatively simple math and assumptions.

The main limiting factor in population is assumed to be food supply. One agricultural section is 200 miles long and 314 miles wide, which gives us 62,800 square miles, or 162,651.25 square kilometers.

Assumption #1: agricultural land utilization is 90%. The rest is occupied by buildings, roads and water bodies.

Arable land per section: 56,520 sq mi; 146,386.13 sq km; 36,172,800 acres

Assumption #2: one acre of land can support from 2 people (non-intensive methods) to 4 people (intensive preindustrial methods).

Assumption #3: all sections of the Spire are well-managed, which means that diseases, wars and waste are not significantly affecting population.

Number of people per section: 72,345,600 to 144,691,200

A 1,000,000 miles long spire means that we have 2,000 sections.

Total population: 144,691,200,000 to 289,382,400,000

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  • $\begingroup$ The surface area is only 50% larger than the Earth. Where are you putting all the water? (The OP's assumptions for utilization are a bit off.) $\endgroup$ – JBH Aug 21 at 20:03
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    $\begingroup$ @JBH Nowhere. Surface area covered in water is not necessary. $\endgroup$ – Logan R. Kearsley Aug 21 at 20:35
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    $\begingroup$ Water storage could actually be helpfully located in a layer between the outer hull and the inner landscape. Water is an excellent shield against radiation. $\endgroup$ – Michael Richardson Aug 21 at 20:37
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    $\begingroup$ @JBH Water is necessary. Surface area covered by water is not. With a surface area 50% larger than Earth, that gives us 4.5 times as much land area to work with, and given that much of Earth's land is not habitable or not inhabited, a much larger multiplier on actual living area. $\endgroup$ – Logan R. Kearsley Aug 21 at 20:52
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    $\begingroup$ @LoganR.Kearsley I don't care which way it goes, I care that it's not accommodated for. It's going to take volume - a lot more than people think. A less-than-trivial amount of water is in Earth's air, rivers, lakes, (e.g., irrigation) and aquifers. The OP hasn't considered how thick that shell needs to be. $\endgroup$ – JBH Aug 21 at 22:40
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Without Serious Handwaving, Zero

Such a world is not structurally stable. The theoretical largest possible structure of this nature using carbon nanotubes (10,000 km) is shorter than the diameter of the earth (~12,750 km). A spire of your proposed length would collapse under its own gravity into a molten sphere of carbon nanotubes in a horrifying catastrophe not unlike a mole of moles. I find it extremely implausible that even hyper-advanced material science is able to produce a material over 300x stronger than carbon fiber that can somehow resist the cosmic tendency for very large things to collapse into spheres. Gravity always wins.

This structure is long enough that it will basically behave like a giant uncooked noodle (maybe even a cooked noodle, depending on the material's ability to bend). A large enough imbalance of mass, particularly at either end, would send this structure careening out of control (I assume that gravity is simulated via rotation) and cause it to shatter into pieces or, at the very least, cause frequent severe earthquakes throughout most of the structure.

If somehow physically possible, there are still other problems.

There's also the issue of gravitational pull toward the center of the spire. The extreme ends of the spire will feel like steep mountains and potentially not be inhabitable as a result. The spire needs to apply antigravity at the extremes for this setting to work.

Transporting goods and data is a huge logistic problem. A message sent from one end of the spire to the other via light will take approximately 6 seconds (1,000,000 miles / 186,000 miles/second) to arrive. This doesn't take into account signal loss. Keep in mind that this distance is a little bit more than 2 round trips to the moon.

If you had some sort of magnetic transport system like the Bullet Train, you could achieve speeds of at least 125 MPH, probably a lot more by avoiding air resistance, but even a gain of a factor of 10 is going to come at massively higher energy costs and thermal output. If you somehow achieved a practical 100x speed gain (for 12,500 MPH), it would still take 80 hours to transport a good from one end of the spire to the other. Realistically, you're going to be stuck with much lower speeds for everyday use, effectively isolating the extreme ends of the spire from each other.

Spire units will need to be fairly self-sufficient and can only realistically trade within a dozen units or so. If each unit has its own sovereign government, chances are high there will be progressive tariffs at each unit along the way, limiting trade even further.

As far as population goes, 1 million * pi hundred = pi hundred million square miles. At 10 people per square mile on average, that gives you a total population of about pi billion people. Higher population densities are not out of the question, putting a theoretical limit somewhere around 30 billion people, possibly more.


You wanted , so I'm giving you hard reality. I suggest removing that tag if you intend to pursue this setting seriously.

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    $\begingroup$ I'm not so sure about the gravity issue. A mole of moles is about a rather uncontrolled gravitational meat grinder, this is about engeneering. If the gravity is accounted for during construction, cleverly placing material reserves and slated floors might do it. I've no clue how to calculate something like that, but maybe placing mass deposits on both ends could help. Also, cylinder habitats are punny. Shellworlds, Topopolisses and Active-Support-Ringworlds reign supreme among the realistic megastructures. $\endgroup$ – TheDyingOfLight Aug 28 at 11:29
  • $\begingroup$ I beg to disagree. The whole structure, despite it's impressive length (1,000,000 miles) will have only a small fraction of Earth's mass, which, over such distances will transform into a minuscule gravity. The cited references do not state that gravity is a limiting factor here. Even if it is, supposedly nothing can stop us from rotating the entire spire over its middle (over 2nd/3rd principal axis) thus negating any possible gravity effects. $\endgroup$ – Alexander Aug 28 at 16:54
  • $\begingroup$ @Alexander you can't rotate around 2 axes at the same time. You also have to keep in mind that any rotation will require the structure to withstand bending and twisting or be able to perfectly synchronize millions of rocket boosters. Any force or torque happening on one end will take at least 6 seconds to propagate to the other end. Rigid bodies don't exist, especially not at that scale. $\endgroup$ – Beefster Aug 31 at 15:59
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    $\begingroup$ @Beefster No, an object can rotate around 2 axes: Two axes for rotational motion. Your second objection is valid if we have a requirement that the spire has to stay rigid and straight all the time - but I imagine this spire like a train of cars rather than one big solid rod. $\endgroup$ – Alexander Aug 31 at 16:29
  • $\begingroup$ @Alexander even still, that spells disaster for the stability of the structure and would likely cause severe earthquakes at minimum. $\endgroup$ – Beefster Aug 31 at 17:24
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We don't know exactly. But it's functionally unlimited.

The Earth is (roughly) a sphere whose radius is about 4K miles. Very roughly, we can compare the surface area of the Earth, to the surface area of this cylinder, for a workable estimate of how many people your vessel can support.

It won't be exact; people actually live in a volume, of course, but the two setups have an almost identical third dimension. Everyone living on the interior surface of the cylinder is very similar to everyone living on the exterior surface of the Earth. This is roughly a constant factor of (say) a third of a mile in height; it doesn't quite exactly cancel out, but close enough.

So the surface area of the Earth is ~ 197 million square miles. The surface area of your cylindrical vessel is A = 2πrh. (You can ignore the discs on each end; it works out to about a hundredth of a percent.) That end up being 314 million square miles.

Your vessel is about fifty percent bigger (by surface area) than the entire Earth. Interestingly, by volume, the Spire is a fortieth the Earth's size. This is because a sphere has optimal surface-area-to-volume ratio, and a cylinder gets worse as it gets more lopsided. The Spire is very lopsided.

We have not tested Earth's max supportable population limit

People have made grandiose predictions about the biosphere limit and so forth before. None of their dire prophesies of doom over an inability to grow enough food ever panned out. Certainly there must be some kind of limit imposed purely by how many people the Earth can support - but to date, societal pressures have been the constraining factor. If we wanted explosive population growth, we could restructure society in order to get it; we have other priorities.

TL;DR

The vessel is about 1.5x the size of Earth by surface area. A reasonable Fermi approximation for 'how many people can this vessel support' seems to be 'About half again as many as Earth can'.

But we don't actually know how many Earth can.

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    $\begingroup$ Why do you consider volume? The entire volume of the inside of the earth is mostly irrelevant to the number of people it can support. You need to look at the surface areas, not volume. $\endgroup$ – quarague Aug 22 at 19:10
  • $\begingroup$ Because it's an apples to apples comparison, instead of trying to compare the surface area of the Earth to the volume of the ship. My answer even brings that up. $\endgroup$ – Ton Day Aug 22 at 19:27
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    $\begingroup$ I think you need to compare the land surface of the Earth to the surface of the ship. People only live on the inside surface of the cylinder, not in the entire volume. $\endgroup$ – quarague Aug 24 at 9:31
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    $\begingroup$ Egads. You're right, my eyes glazed right over that highly significant little detail. It's a good thing someone around here is paying attention! I was under the impression the vessel's entire internal volume was usable interior structure. So I'm comparing apples to imaginary apples in the much-more-complex-than-necessary plane. I'll fix that. $\endgroup$ – Ton Day Aug 25 at 4:23
  • $\begingroup$ Add to that that the O'Neil cylinder would not contain vast oceans, of course. That would simply be waste of real estate. Farming would need to rely on water recycling and filtering anyways, because the constructors would go for the thinnest possible layer of dirt. (Dirt is heavy, and it does not add to the structural strength of the cylinder.) $\endgroup$ – cmaster - reinstate monica Aug 26 at 10:07
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Just compare with earth. Earth has around 57 million square miles of land area. Your cylinder has 314 million square kilometers, about 6 times as many. Earth population in 1600 was around 0.5 billion, in 1800 around 1 billion. This gives you an estimate of 3 to 6 billion.

This is massively less than Alexanders answer because he assume perfect climate and soil conditions everywhere, whereas my estimate just uses average earth climate.

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