I'm writing a story (part of a larger work) which features a generation ship. I don't want to get too deep into the science, because it's not the focus of the story, but I'd like it not to be so stupid as to annoy the scientifically literate. Crucially, this ship isn't going anywhere in particular- it's just going, a roaming tiny Utopia in space. So velocity, acceleration, deceleration, landers- none of that matters. It's small, with a fixed population of 150 (the way they keep the population fixed is the focus of the story.) I'm thinking a sphere 15-20km across, as they need plenty of space. Based on cursory reading I'm picturing some sort of balloon inflated inside an asteroid, to protect them from radiation.

I have two main issues. One, gravity. Could the habitat spin to create gravity? What would be the implications? Could it be a big open sphere inside, or would it need to be tunnels?

Two, energy, to provide them with heat and light, and power the AI that runs the whole show. As they have no destination, I'm looking for something ongoing, like a scoop system. If it happens to be destructive to any civilisations they might pass along the way, so much the better. But I'm assuming they'll be going so slowly they'll be spending thousand of years in between stars.

I'm prepared to make shameless use of handwavey nanobots to make the detail work, but I'd like the world as a whole to have some coherence. Any thoughts welcomed.

  • $\begingroup$ Welcome to Worldbuilding, TheSpiderMonkey, if you want centrifugal gravity by spinning a structure, then a sphere is the wrong shape. You need a cylindrical habitat. It can be 15-20 km long and a kilometre or two in diameter. There's no reason why your cylindrical habitat couldn't be installed inside an asteroid for radiation protection. There can be tunnels in the body of the asteroid. Going from the spinning habitat to enter the tunnels might be tricky, but I'm sure it's doable. Have fun! $\endgroup$ – a4android Nov 12 '19 at 2:25
  • $\begingroup$ Hi there. The format for questions here on Worldbuilding.SE is that you ask one specific question, which has one specific best answer (which should be apparent from how you ask the question). At present, you are asking a number of questions around a topic, which is likely to get your question closed. However, if you were to ask a series of specific individual questions on the topic of generation ships, you would likely get the answers you are looking for. $\endgroup$ – Arkenstein XII Nov 12 '19 at 2:27
  • $\begingroup$ @a4android Why not just spin the whole asteroid? $\endgroup$ – Arkenstein XII Nov 12 '19 at 2:29
  • $\begingroup$ Thanks both. Apologies for the formatting issue, I hadn't realised. I'll make sure to be more specific in future questions. $\endgroup$ – TheSpidermonkey Nov 12 '19 at 3:52
  • $\begingroup$ @ArkensteinXII Current knowledge about asteroids indicates they aren't solid objects. More like heaps of ruble clumped together by gravity. Spin them and they'd fly apart. An asteroid would need lots of structural reinforcement before you could spin it. Then you would have to ensure internal structures like tunnels and habitat areas would be oriented correctly to the centrifugal gravity. $\endgroup$ – a4android Nov 12 '19 at 4:17


You could provide this directly via gravity manipulation technology. Which when unpowered essentially turns off gravity. If you take this approach the ship's engine themselves could be grav drives, manipulating gravity directly for acceleration/deceleration.

You could also achieve gravity via rotation. In which case a series of rotating rings, or a rotating cylinder would win the day. If you go with a large cylinder/ring you will need baffles to prevent the winds within the tube from becoming too strong. Nothing ruins your day quite like having the atmosphere itself pierce the hull of your ship. This isn't as much of an issue in smaller rings, or a room layout as the furnishings/walls are natural baffles.


The longest lasting power-source known to man is a star. You could approach this in several ways.

Advanced fusion reactor cores fueled by a steady supply of deuterium/tritium. If they are really advanced this might even be miniaturised for use on smaller vehicles/ships.

An actual star, if you think about it our planet is a generation ship (sans engines) powered by a star. Of course our star is a little big for use on a ship designed for 150 people. Looking around the neighbourhood GRW +70 8247 has a rough radius of 3300km. It has a magnetic field, so with suitable magnetic/gravitic technologies (handwavium) could be harnessed and used directly.

Alternately something more out there: matter/Antimatter annihilation plants that use copious amounts of exotic mater. The exotic matter serves to catalyse lots of antimatter which is then annihilated with a steady stream of matter to produce lots of energy. Fusion converts a small fraction of the energy in mater to radiation (light/heat) that can be harnessed. Matter annihilation releases 100% of the energy, the only current stick point is that making antimatter is more energy intensive than its annihilation will produce. Hence exotic matter (and handwavium) to the rescue.

Food and Space

Hydroponics that fill a small 3*4*3 metre room are enough to grow the full caloric, vitamin, and mineral of a adult human sustainably. If plants are to accomplish the additional tasks of atmosphere scrubbing, water purification, and animal food production then the space requirements grow considerably. There are several research self-contained habitats capable of supporting 4+ humans. Although our current experiments have proven this difficult not due to the plants, but limitations in knowledge of our own construction technologies and social structures.

A human needs (as an approximate minimum) 26 cubic-metres of living space. A simple rotating ring 1 km across and say 10 metres wide and 4 metres high (120 000 cubic metres) is large enough for some 4600 humans to live in (albeit cramped, cranky, and not counting space for ship systems/work areas/food production/communal areas/corridors/etc...).


Most of the materials within the ship would need to be recycled. Some of these could be recycled more or less directly such as organic material becoming mulch, and metal objects being smeltered. Unfortunately a lot of the material will need a more advanced recycling, perhaps requiring atomisation and advanced nano-3d printing techniques. This would be energy intensive, requiring a big power source/frequent refueling.

Resource acquisition would perhaps be best done by autonomous, self-replicating, machines. They would be sent out in advance of the ship to nearby resources where they would replicate using the local materials, extract a laundry list of elements that are in demand, and launch those resources back to the generation ship.

If fleshy things existed there, some may be acquired for the curiosity of the generation ship. If resource acquisition was endangered the replicated machines would follow a sterilisation protocol. Some strategies: directed high energy gamma rays, deorbiting a nearby planet, stellar mass conversion (imploding the star/gas giant/planet to create a suitable replacement power source for the generation ship).

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  • $\begingroup$ Thanks for the detailed and knowledgeable response! $\endgroup$ – TheSpidermonkey Nov 12 '19 at 3:08

Dealing with the gravity issue first; sure you could spin your hollowed out asteroid to generate centrifugal force to approximate gravity, but that has two problems.

1) You want your ship to be as small in front profile as you can make it. Less chance of a strike against small rocks, dust particles etc. In Songs of a Distant Earth by Arthur C Clarke, he designed a sleeper ship that had a massive ice shield on the front that would absorb the damage dealt by dust and mote particles at close to relativistic speeds.

2) Spinning a globe like shape is going to cause a massive irregularity in the gravitational approximation depending on how close you are to the 'equator' according to the axis of spin. This is why most space launch sites being considered for future development on Earth are so close to the Equator; it actually requires less energy to launch a rocket into orbit from the equator than it does from closer to the poles.

The solution is actually tied to your method of propulsion. What you want, both to increase the speeds you can attain on your journey and for the comfort of your inhabitants, is a constant thrust engine.

Getting out of Earth's orbit needs a chemical rocket, pure and simple. Possibly one day we'll have space elevators but until that we need the kind of force that only a chemical rocket can produce. But for long range, especially interstellar travel, it's a very inefficient way to propel your ship. What you want is something similar1 to an ion or plasma drive, that can run for extended periods giving a small amount of constant acceleration. That acceleration would also simulate gravity, by pulling everything gently to the rear of the space ship. So, your generation ship is most likely an ion or plasma driven constant acceleration drive ship with a lot in common with a lighthouse, only bigger. It probably has a large ice shield on the front as well to deal with collisions of small particles at the speeds you can attain. The ship accelerates constantly to the half way point, then it turns and accelerates forward (IE decelerates) for the second half of the journey. In both cases, you feel the acceleration as a replacement for gravity pushing you to the floor, it's just that for the second half of the journey the floor is now facing in the opposite direction, not that you would notice the distinction.

As for energy; Hydrogen is the most abundant element in the universe by a long way, and there is even a theoretical design for a ram scoop engine called a Bussard RamJet2 proposed by scientists that has been used in a lot of science fiction already. Basically it's a drive that scoops up interstellar clouds of hydrogen and using it for fuel in a fusion reactor.

This would be sufficient to drive your constant acceleration drives, giving you your gravity at the same time.

1. It is noted from comments that ion drives and plasma drives in their current form are unable to provide sufficient acceleration to simulate gravity and that is accepted. You would need something similar in concept, but with a much larger specific impulse that can generate acceleration at a higher rate than the designs we have now.

2. The comment about the Bussard drive is also accepted, and if this was a hard-science tag I would never include it. If you're only looking for cursory nods to science, then this is definitely your engine but obviously the harder your science setting is going to be, the more you are going to have to modify such a drive to make it more believable. That said, some of the concept was good enough for Star Trek...

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    $\begingroup$ Unfortunately, all current ramscoop designs are expected to cause more drag than they do thrust. $\endgroup$ – Arkenstein XII Nov 12 '19 at 2:36
  • $\begingroup$ Awesome, thank you! $\endgroup$ – TheSpidermonkey Nov 12 '19 at 3:10
  • $\begingroup$ I second @ArkensteinXII's point about the non-functionality of Bussard ramjets. This has been known since around 1979. Forty years should be enough time for people to catch up. Plasma or ion drives will have extremely low rates of acceleration. If they are trying to push a habitat inside an asteroid, their acceleration will be negligible. The OP's generation ship isn't in a hurry to anywhere, so that may not matter. But internal gravity definitely not. Oh yes, good answer otherwise. $\endgroup$ – a4android Nov 12 '19 at 4:28
  • $\begingroup$ You don't want your internal gravity to come from acceleration of the ship in a forward direction anyway, you want it to come from spinning. Once it's spinning at the required rate, no more power required. In essence you're after a setup like Rama, from Arthur C. Clarke's novel Rendezvous with Rama. A giant hollow cylinder with people living on the inside. $\endgroup$ – SO failed us all... Bye... Nov 12 '19 at 8:40
  • $\begingroup$ Thanks everyone. Your comments are much appreciated, and I am chewing them over... $\endgroup$ – TheSpidermonkey Nov 12 '19 at 14:57

You don't need a rotating cylinder or wheel to create artificial gravity. You can just have two equal weights connected by cables, rotating around a common centre of gravity (which is where you would want to place your propulsion). To minimize impacts when travelling at near-light speed, you want to make the two weights elongated. Imagine two long, parrallel cylinders connected by a few dozen cables, with a smaller cylinder halfway between for propulsion. Or you could make it three cylinders connected to the engine, which I believe would provide greater stability. Cable elevators could connect the cylinders.

One or two of the main cylinders could house the people and arable land, while the remaining could house power plants and other hi-tech equipment, maintained by robots. All would have heavy shielding on the leading surfaces, and possibly carry lasers that scan the space ahead for objects that could impact the ship. If one is discovered, a projectile or powerful laser is fired to destroy the object. You get the advantage of travelling at relativistic speeds, meaning that a projectile fired at low speed from your ship will impact a slower-moving object with tremendous energy.

For power, there are several options. The most effective is carrying matter and antimatter for total matter-energy conversion. The reaction of 1 kg of antimatter with 1 kg of matter would produce 50,000,000 megawatt hours of energy. The average US household uses around 10 megawatt hours a year, so 1 kg of matter/antimatter could fuel 50 households (assuming an average of 3 people per household) for 100,000 years, even without improved energy efficiency. A few thousand tons more of matter/antimatter could provide propulsion, basically shooting high-energy photons backwards, or the energy could be used to accelerate normal-matter ions to near-light speeds.

Another option is fusion using Helium-3, which is a stable molecule. The matter-to-power ratio is far smaller than pure annihilation, at 163,000 megawatt hours per kilogram of helium-3, but storing and helium-3 is far less dangerous than storing and using antimatter. If your spaceship stops at solar systems during its travels, it could even harvest more helium-3, e.g. from asteroids or gas giants.

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    $\begingroup$ I was looking for the numbers in your answer :-) Essentially a fusion reactor will provide enough power for a long time, no need to go to any fancier technology. By your numbers, 150 people in the US today use around 500 MW hours per year. Give our future space farers a generous 10.000 MW hours a year and a single ton of Helium-3 will last 16.000 years. That should be good enough for a generation ship and 1 ton of fuel is nothing for the size of the ship you need. $\endgroup$ – quarague Nov 12 '19 at 12:45
  • $\begingroup$ Thanks both! I like the cylinders idea... as long as they would be sufficiently radiation shielded. This isn't a temporary trip, it's a permanently moving habitat. $\endgroup$ – TheSpidermonkey Nov 12 '19 at 14:55
  • $\begingroup$ Radiation will not be a huge problem away from stars. If you are travelling at relativistic speeds, radiation coming from the front may be blue.shifted into dangerous wavelengths - but you already have particle shielding there. $\endgroup$ – Klaus Æ. Mogensen Nov 12 '19 at 15:32
  • $\begingroup$ @KlausÆ.Mogensen Cosmic rays in interstellar space are predicted to be a major radiation hazard for generation ships. $\endgroup$ – Arkenstein XII Nov 13 '19 at 22:52
  • $\begingroup$ @ArkensteinXII: Cosmic rays mainly consist of high-energy protons and ions. That's fairly simple to shield unless you have very thin walls (which current spacecraft have due to weight considerations). A vehichle built to withstand one gravity for extended periods is likely to have fairly thick walls. $\endgroup$ – Klaus Æ. Mogensen Nov 15 '19 at 11:24

For spin gravity, you'd probably be better off with a capsule-like shape, instead of a sphere, if you are limited to a single body. The enes of the capsule should be the front / back of the ship, to minimize the area dealing with micrometeorite impacts.

For power: Fusion goes a long way. For an interstellar journey, you'll still need absurdly high amounts for propulsion, but 1kg of Fusion fuel can provide a human's energy needs for a rather long time. Additionally, it's possible to use lasers to send energy or particles over great distances, so if they have such a beam station behind them, or build them as they pass rogue planets or such[1], those could provide both energy and acceleration, if they have sails / collectors to catch the beams. From your description, though, I'm guessing this ship does not have access to that sort of infrastructure, unless it's building it as it goes.

[1] If your rogue planets are mostly icy bodies, ice-giants, or gas-giants, then they have enough Fusion fuel to power the whole trip. If you prefer to travel light, have your drones and/or early-check-out crew build beam stations on these objects, and use part of the local material to power the beam. You can also focus light from stars with mirror arrays, but at the distances we're talking about, that's not likely to be as great without the star in question being Dyson Swarmed. Alternatively, turn one of these planetoids into your ship.

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  • $\begingroup$ Thank you! The ship can have any infrastructure it needs that's plausible, there's no story reason why not... but it also needs to be totally self-sufficient. Although it could have drones, that would work. $\endgroup$ – TheSpidermonkey Nov 12 '19 at 14:54

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