Worldbuilding Stack Exchange is a question and answer site for writers/artists using science, geography and culture to construct imaginary worlds and settings. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

We live on this sphere of matter, and most people are content with that. However, it is really inefficient! A sphere is the shape with a minimal surface area compared to its volume, so assembling the same matter into another shape is going to give us more space.

I want a planet with:

  • Close to normal surface gravity (perfectly fine if it varies a bit, down to 0.5g is acceptable.)
  • No use of unobtainium

What I have found so far includes

  • Alderson discs, but they actually have the same ratio of surface-to-volume as a sphere at the same surface gravity.
  • Ringworlds, but they require unobtainium.

So, what I thought about was to strip off the outer layer of the Earth, and then reassemble it into multiple shells with a 100km of air in between them. That would leave us with multiple layers of surface area.

The question:

Is this going to require unobtainium, or can it actually be constructed? (please include the forces involved) If not, are there other ways to have more surface area using the same amount of mass?

share|improve this question
Slightly related: Have you read "Matter" from Iain M. Banks? He calls worlds like yours "shell-worlds" (funnily translated to something like "onion-worlds" in my native language but AFAIK this word is not mentioned in the original book), and he deals with lots of other problems like lighting and atmospheric pressure in that book. – mg30rg Feb 5 at 13:53
If you do, please be wary of stack overflow. – Thomas Jacobs Feb 5 at 14:41
ERROR loading shell 4294967296, consider to upgrade to 64-bit – Hohmannfan Feb 5 at 15:08
halving gravity is not 'close to surface gravity', just thought that this would be a point worth making. Maybe gravity capable of holding an atmosphere would be a better requirement? – Mourdos Feb 10 at 14:08
up vote 44 down vote accepted

Strange thing? We already stack the Earth. Not quite as grandiosely (is that really a word??) as you're asking for, but when you think about it modern cities, especially those with skyscrapers, are attempts to do exactly what you're doing.

So rather than getting the world engaged in a megaproject to build a series of 100km high shells, instead get the world to work towards covering the surface in high density urban environments. Even if your skyscrapers have each floor the height of ten, you can cram an awful lot of usable surface area into it. If you use the same amount of mass as you'd need for even one mega-shell purely to build cities you'll be able to eke out a lot more usable surface area.

This neatly sidesteps the issue of varying g and of needing unobtanium at the same time. Human superstructures, no matter how tall, won't take you high enough to get to 0.5g unless you use unobtanium. It does however raise a whole new slew of issues, but there are none that we haven't dealt with before:

First up: Food. Getting food into and out of the city is going to be impossible if you want to maximise the surface area, as you'll have a world-spanning Coruscantian conurbation. Instead: Grow the food in stacks. Vertical farming is beginning to take off as a science, with a combination of hydroponics, high quality nutrient feeds and solar-mimicking LED's, you can stack not only the earth, but also the fruits of the earth.

Secondly: Power. Power generation is going to be tricky, as people don't tend to like living directly on top of power stations, and you don't want the chance of a nuclear reactor melting down onto you. The solution? Cover the top of every building in eco-power solutions, and bury your nuclear reactors deep in geologically stable plates. Either that or build power generation districts where all the buildings are dedicated to generating power.

Thirdly: Water, sewage and transport. The plumbing will have to be IMMENSE. The easiest way to get round it is by localising the water systems to a certain radius and having municipal water processing buildings that take sewage in at the bottom, pump water out of the sides, water vapour out of the top and high-nutrient food blocks to the nearby farm towers.

The thing is that all these issues are logistical ones. OK, they're logistical nightmares, but so is the logistics for shifting a fraction of the Earth's mass into a free-floating shell!

share|improve this answer
This conversation has been moved to chat; please continue it there. Thanks. – Monica Cellio Feb 7 at 2:59

One thing to note is that along with Shell Theorem comes dealing with all your concentric shells and final (remaining) earth sphere. You would have to come up with a way to prevent them from colliding with each other (for whatever of many reasons) since air won't dampen the inner movement nearly enough. Such a collision could lead to mass destruction.

If you could somehow stabilize your world in some kind of pillar lattice, it would still be brittle (how large would your pillars have to be to hold planetary scale spheres and not crush themselves under their own weight, short of unobtainium).

IMO you'd be better off increasing surface area by creating structures which both rise in altitude and extend subterraneanly. Think something along the effect of a heat-sink on a computer's CPU. The heat-sink optimizes on surface area contact with air. Excavate large areas of land to create valleys and mountains of large incline. Essentially medium-scale terraform parts of your planet to increase surface area. It would look akin to a spiky ball or super fine-fringed honey dipper.

share|improve this answer
This would produce larger than 0.5g variations though possibly if the effect is significant – Nikola Radevic Feb 5 at 11:22

The biggest problem you have is heat.

If you take a sphere, like the Earth, and cover it with population as dense as Manhattan, and you give each citizen about as much energy to play with as a current US citizen does (via solar panels, nuclear reactors, or whatever else), your planet's temperature is basically the Earth's.

Black body radiation goes up with the 4th power of temperature, so 2x the population above the top number means 300 * 1.19 - 300 = a 57 degree Celsius increase in surface temperature to radiate twice as much energy.

You can do better by flattening your planet and radiating from both sides.

You can maintain local surface pseudo-gravity by having reasonably small radius rotating habitats (the bigger the habitat, the more insane the gravity), with core-axis connectors (where there isn't rotational motion, so travel is easy) to go between the sausages.

At which point, the hard part is making your habitat space-worthy without the blanket of protective air Earth has, and dealing with air/matter loss.

Another insane design is a single long sausage that rotates to maintain gravity. You only get one dimension of long-distance, but you have an awesome radiating surface. The radius of the habitat can be as large as your non-unobtanium technology can pull off safely. Leakage remains a problem.

Gravitational influcence falls off with the square of distance, so the integral of the pull from the ends towards the middle converges to a constant based off how big around the habitat is (and a small constant too: the near-infinite sausage has twice the "real" gravity as a a short one does at the end).

share|improve this answer
Wouldn't a rotating sausage push people OFF the surface, rather than maintain gravity? – Egor Feb 5 at 22:49
@egor the people are the sausage meat. – Yakk Feb 5 at 23:59
But that's just a regular Ringworld, OP already said he doesn't want those. – Egor Feb 6 at 2:37
If you are inside, then it is an O'Neill cylinder colony. With current technology, you can build something 8km in dimeter and 32 km long. Using carbon nanotubes or other super materials you make a McKendree Cylinder, up to a radius of 1,000 km and a length of 10,000 km. – Thucydides Feb 6 at 3:04
@thur with current technology, why are you limited in length? I mean engineering difficulties will occur as it gets longer, but calculating thst limit as 32 km? – Yakk Feb 6 at 11:26

You dismissed the Ringworld as requiring unobtainium but that's not completely true. Niven's design for a Ringworld certainly requires it (not only for the scrith, but the walls themselves.)

However, you can build one without it, although it will be much more massive--you'll need several solar systems worth of planets as raw material.

The basic problem is the Ringworld is moving far above orbital velocity and thus wants to depart into intergalactic space. To keep it in place he relies in a unobtainium band (I'd hate to think of the stretching force on that band!) but there's another approach: Don't have it moving above orbital velocity in the first place.

Wait a moment, though--doesn't that mean everyone falls off? No--because what matters is the average velocity of the ring. You can have part moving fast, part moving slow--a living surface moving at the 770km/sec of the Ringworld and a much heavier mass behind it that is moving at 0km/sec. Since the Earth is moving about 30km/sec this means the backing mass is roughly 25x the ring mass. No point experiences a force greater than it would sitting on the surface of the Earth. Obviously the ring must never touch the backing, you'll need a massive maglev setup. (And the Ringworld would have gone kablooie when the superconductor-eater got into that!)

Note that I said the walls also required unobtaininum--but we don't actually need walls. Instead of a surface extending vertically we simply put a small bend in the floor of the Ringworld and the backing mass. We can make the angle sufficiently small as to not cause issues, once the bent area extends high enough we have our atmosphere holder.

Now, the stability problem remains. We have no way of keeping the Ringworld in orbit with current tech.

share|improve this answer
You'd have to have a frictionless interface between the moving belt and the rest of the system or the thing burns up. – Oldcat Feb 8 at 20:05
@Oldcat Did you not see "maglev" in the 4th paragraph? – Loren Pechtel Feb 8 at 20:40

I'd say take a look at John Varley's Gaea. OK not her/it in particular but toroidal habitats in general. The idea of having them replicate themselves starting from an asteroid and a seed is a great idea if somewhat beyond current technology. Why spend time and risk lives building them when you could engineer the first seed and plant it in an asteroid belt and wait?

If you want to avoid that degree of future tech then your model is O'Neill colonies. You solve a lot of problems by not needing vast amounts of matter to create gravity at the surface. Instead you create pseudo gravity by living inside a rotating wheel. If it is large enough the coriolis effect is tolerably small.

In passing: if an AI ever tells you that it is getting senile, take it seriously.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.