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Thread: How big would a multi-generational spaceship really have to be

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    How big would a multi-generational spaceship really have to be

    If we can't travel faster than light speed (and probably not even close to it) it seems the only way for people to reach other stellar systems is with a multi-generational spaceship. I'm asking about the payload rather than the drive system. Let's say travel time exceeds 1,000 years. For a start I figure you'd need a breeding population with genetic diversity, so at least 50 people? Some will have to teach and others do health care. Food production might be mostly algae, insects, lab meat and recycling. You'd need to manage the population size over generations so presumably no-one would have more than two kids and there be some expectation the older folks would make noble sacrifices at a certain point. And maybe you need cinemas and restaurants and a police force to avoid Lord of the Flies - or otherwise just a holodeck and robot enforcers? I'm curious if anyone has thought this through and decided what the bare essentials would really be. Thanks!

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    By the time we are able to build multi-generational ships, we will probably be capable of storing DNA (or genetic material in general) in some way that would allow an arbitrary level of genetic diversity in a small population. I suspect we could even do this today, with frozen sperm and ova.

    A worse problem would be maintaining a diverse level of skills in a small ship. You could have teaching material that allows the colonists to learn new skills, but this would have its limits.

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    For living space, I'd estimate 20 m2 per person, with at least four times that for common space, including at simulated parkland, social spaces, restaurants, shops, etc, plus a lot of volume for the machinery to make the vessel habitable, storerooms, manufacturing, food production, etc. I think about 2,000 people would be a fair minimum. So about 200,000 2 floor space, at nearly one g. People also need some fairly long vistas, so I'd start with a long cylinder, with one level at about 55 m from the axis of rotation, and a second level about 65 m from the axis of rotation, where things like machinery and hydroponics would be placed. The centripetal acceleration on the outer level would be about 18% greater than on the inner one. With this layout, the cylinder would be about 250 m long. Propulsion machinery, external shielding, etc would probably require a like volume, plus reaction mass, so maybe another 250 m length and 20 m radius. This would be about 500 m long and 170 m diameter.

    Of course, the size is strongly influenced by the population, and I suspect 2,000 is too few for long-term social stability.
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    Quote Originally Posted by Cheap Astronomy View Post
    If we can't travel faster than light speed (and probably not even close to it) it seems the only way for people to reach other stellar systems is with a multi-generational spaceship. I'm asking about the payload rather than the drive system. Let's say travel time exceeds 1,000 years. For a start I figure you'd need a breeding population with genetic diversity, so at least 50 people? Some will have to teach and others do health care. Food production might be mostly algae, insects, lab meat and recycling. You'd need to manage the population size over generations so presumably no-one would have more than two kids and there be some expectation the older folks would make noble sacrifices at a certain point. And maybe you need cinemas and restaurants and a police force to avoid Lord of the Flies - or otherwise just a holodeck and robot enforcers? I'm curious if anyone has thought this through and decided what the bare essentials would really be. Thanks!
    The size of the earth. We have no evidence that anything smaller than earth, would be necessary for a multi-generational spaceship. Everything else is hypothetical.
    Last edited by Copernicus; 2020-Jan-12 at 03:41 PM.
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    Studies of stable small island communities might be helpful. As for base populations, the entire pre-Columbian Native American population was supposedly descended from a gene pool of less than 80 individuals.
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    If you want to let children freely choose what jobs they want to do, and try to have low risk that some generation won't have anyone in it with some required aptitude, you may need a lot more than 80.
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    I'd think swampyankee's thought of at least a couple thousand as a minimum is closer to the mark than 50 or 80.
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    Quote Originally Posted by Grey View Post
    I'd think swampyankee's thought of at least a couple thousand as a minimum is closer to the mark than 50 or 80.
    Assuming humans still do jobs by then.
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    A 1000 year genship would have to be a mobile civilization. It would at minimum require the same population as a long period fully self sustaining space habitat at the tech level of the time of launch. Since we don't currently have self sustaining space habitats, we don't and can't have any specific numbers. Experience will have to be our teacher someday.
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    50, 80, or a couple of thousand members for the original crew will have been thoroughly professionally and psychologically screened for the hardships of a thousand year trip. Future generations might not be as stable to carry on the voyage to a successful completion.

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    As I mentioned earlier, genetic diversity should not be a problem. Even if the generation ship contained only 30 people there could be enough DNA or enough frozen gametes on board to have a gene pool of thousands, or even millions.

    But could a population of 30 people maintain a generation ship for thousands of years? Would a population of 30 people be able to create a colony in a distant solar system?
    I think this would need a different form of technology- very smart artificially intelligent databases capable of guiding people who are not technical experts into making competent technical decisions. We are nowhere near that yet. Capable, competent robots would be really useful too. But if we could make such capable, competent robots, why would we need to send people at all?

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    I am concerned that there is no way to guarantee that the crew will not suddenly vote to change course, abort the mission, or something else. People are good at acting out in unpredictable ways over long periods (or even short periods) of time. Can't figure out a way around it.
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    Human egos would always be the weak point in any long space flight - that and trying to figure out how to get electronic equipment to work for a 1,000 years.

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    Multi-gen ships are not the only way to reach far-flung stars.

    Presumably suspended animation will see its heyday some time in the future. Keep a skeleton crew awake at all times. The crew rotate so the population all age at similar rates.

    This also goes a ways toward mitigating mutiny, since ideally, all crew can expect to live a long life on the target planet, rather than living out their lives in a giant tin can.

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    Quote Originally Posted by DaveC426913 View Post
    Multi-gen ships are not the only way to reach far-flung stars.

    Presumably suspended animation will see its heyday some time in the future. Keep a skeleton crew awake at all times. The crew rotate so the population all age at similar rates.

    This also goes a ways toward mitigating mutiny, since ideally, all crew can expect to live a long life on the target planet, rather than living out their lives in a giant tin can.
    Great, if that's ever achieved. It's like fusion... 30 years away for the last 60 years. But the OP was about generation ships.
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    Quote Originally Posted by Noclevername View Post
    ... the OP was about generation ships.
    The OP was actually about 'reaching other stellar systems', and in his analysis, drew his own conclusion that multi-gen ships seems to be the only way to do it. That conclusion was hasty.
    Quote Originally Posted by Cheap Astronomy View Post
    ... it seems the only way for people to reach other stellar systems is with a multi-generational spaceship....
    We can deduce from the OP's mention of FTL that he is at least open to exploring alternate methods for achieving the ultimate goal.
    Quote Originally Posted by Cheap Astronomy View Post
    If we can't travel faster than light speed...


    Quote Originally Posted by Noclevername View Post
    Great, if that's ever achieved. It's like fusion... 30 years away for the last 60 years.
    We cannot make a multi-gen ship with existing technology.
    eg. Hulls of 250m size that can hold air over generations are also x years away for the least 2x years. Same with fuel and power.

    Thus, the only way we will make one is with advances in technology.


    It is a matter of taste which the technolog(ies) will advance sufficiently to achieve the goal.

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    A while ago I made a list of the options open for interstellar travel, not counting FTL since that seems to be a pipe-dream. I'm quite pleased to see that Isaac Arthur has expanded my list somewhat for use in his videos, but I'll just give my version for now. The problem is any interstellar journey is likely to take more than a human lifetime; if you try to go faster, the amount of energy required reaches ridiculous levels and the ship (and occupants) simply melt.

    1/ Sleeper Ships. (Suspended animation). This requires the development of suspended animation technology as well as all the other technologies associated with interstellar flight, so that makes a successful sleeper ship less likely.
    2/ Methuselah Ships. ((Extended lifespans for space travellers) This requires the development of life-extension technology as well as all the other technologies associated with interstellar flight, so that makes a successful methuselah ship less likely.
    3/ Data ships (Autonomous unmanned ships). This requires the development of competent autonomous AI technology as well as all the other technologies associated with interstellar flight, so that makes a successful Data ship less likely. Within the concept of a Data ship I include the possibility that human minds might be emulated and stored as data, something that seems even more unlikely than the idea of an autonomous unmanned ship. But maybe this will be an option eventually.
    4/ Generation ships. A generation ship requires the development of viable closed ecological life-support systems, something we don't have at the moment.

    Most concepts for a generation ship envisage something much larger than any of the other types, in order to support a breeding population, but that may not be necessary if genetic material can also be stored. But a generation ship would have many advantages if it was built on a larger scale. Larger ships would need much more energy to accelerate (you start running into the melting problem again) or they can travel more slowly, taking more generations to reach their goal. If your generation ship takes thousands of years to arrive, instead of merely hundreds, this could result in considerable savings in the budget for propulsive energy. On the other hand a very slow generation ship would require more energy for life support. The energy required for acceleration and deceleration is so vast that you don't need to factor in life support energy consumption unless you ship is en-route for tens of thousands of years.

    My favoured options at the moment are the hybrid generation ship, where a small population maintains genetic diversity using stored genetic material, and relies upon competent AI to bootstrap a technological culture at the destination; and the data ship concept, where a competent AI with a vast database explores by itself, maybe building infrastructure for a later wave of human colonists that may never arrive.

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    Another option on my list is the Seed Ship, an idea that dates back at least to J.D. Bernal; a very small, lightweight ship that carries the seeds and spores of life to a nearby star in the hope of staring a biosphere on one or more of the planets. I've seen this idea revived in various forms recently - it does seem a bit reckless, since the end result need not resemble our own biosphere at all, but it is perhaps the easiest option.

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    The OP titled this thread and asked about multi-generational ships. For now let's keep the answers focused on that. If Cheap Astronomy is willing to expand the discussion, that can be done.
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    Quote Originally Posted by Roger E. Moore View Post
    I am concerned that there is no way to guarantee that the crew will not suddenly vote to change course, abort the mission, or something else. People are good at acting out in unpredictable ways over long periods (or even short periods) of time. Can't figure out a way around it.
    It depends a little on the technological assumptions but if the ship is not horrifically overbuild there is very little chance that they would have enough deltaV to go anywhere else but their target.

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    Quote Originally Posted by Roger E. Moore View Post
    I am concerned that there is no way to guarantee that the crew will not suddenly vote to change course, abort the mission, or something else. People are good at acting out in unpredictable ways over long periods (or even short periods) of time. Can't figure out a way around it.
    The goal is, presumably, to reach a destination and build a civilization there. So, in the end, if they can survive for 1000 years in the interstellar void with minimal losses and full regeneration, does it matter where they finally end up? Certainly planets would be unnecessary. Any system, a brown dwarf or non-stellar exoplanet system, even a dense nebula, would have far more resources available than the travelers had previously relied on.

    Maybe not let the passengers steer the ship? Put a reliably designed AI in control until the target is reached? It's almost as big a technical challenge as the propulsion.
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    My intention was to show that the multi-generation starship concept requires fewer innovations in technology than other possible methods, but that it also has some drawbacks and assumptions of its own. For instance (as I have repeatedly pointed out) a multigeneration starship does not need to maintain a breeding population, since we can simulate a breeding population with technology we already have. But a multigeneration spacecraft would need to have a large pool of skilled people in order to maintain the technical knowhow necessary to reboot their culture at the destination.

    This series of papers assumes a complement of 500 people. The authors, Marin and Beluffi, still seem to assume that inbreeding is a problem, but I have discounted that. However 500 people is probably a good estimate of the number required to maintain a viable civilisation, especially if we assume suitable advanced educational resources.
    https://arxiv.org/ftp/arxiv/papers/1806/1806.03856.pdf
    https://arxiv.org/pdf/1901.09542.pdf
    According to those links, the ship would have (for a simple cylinder) a radius of 224 m and a length of 320 m, giving a surface area available to grow crops of 0.45 square kilometres. You could make the ship smaller in length by placing several concentric cylinders inside each other; if the floors were 10 metres apart you could get twenty floors or so, with decreasing gravity on each level, which would cut the length down to about 60 metres. Note that this would not affect the total mass of the ship very much. Could crops be successfully grown in the low gravity conditions at the centre of such a cylinder? I don't know, but by the time we are ready to launch a generation ship, I think we would have worked that out.

    You could make such a cylinder out of 2 inch steel, which would weigh about 77000 tonnes. The weight of the crops and biosphere would be extra, as well as the weight of the humans and their colonisation equipment. Not to mention the weight of the propulsive system.
    Last edited by eburacum45; 2020-Jan-17 at 09:32 AM.

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    The propulsion for such a heavy ship would be the real problem. In my earlier post (which was not intended to be an attempt to go off topic, but I apologise if it seemed that way) I tried to show that the generation ship is the largest and most massive of the interstellar options. Such a large mass would require stupendous amounts of energy to boost to a decent interstellar cruising speed; far more than the entire energy budget of the Earth, for example.

    So a generation ship would almost certainly go slower than all then other options. Remember that if you half the cruising velocity of an interstellar ship, you reduce the energy required to roughly the square root of the original figure. So the slower the better. A generation ship to our nearest neighbour would probably take more than four hundred years.

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    The interstellar Daedalus project assumed a fuel/payload ratio of about 100/1 for a flyby mission. By cutting the cruising speed of a generation ship to about 1/10th of the Daedalus option, I figure you could cut the fuel/payload ratio to about 5/1, and arrive at orbital speeds. This would make the total mass of the generation ship about 400,000 tonnes, mostly fusion fuel. It might be useful to shed mass on the approach to the destination star, so you don't have to decelerate the whole ship; but this means you arrive in a smaller, stripped-down version of your interstellar ark.

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    Would the be a permanent living environment of would it be a vacation place

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    I'm not sure what you are asking here, but the interior of the cylinder would probably resemble the inside of a greenhouse or polytunnel; not much room for a picnic.

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    I mean would people live there full time or just visit

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    Quote Originally Posted by Sinbad View Post
    I mean would people live there full time or just visit
    The inhabited section would be a ring or cylinder that spins like a centrifuge, to simulate gravity. The human living spaces would be towards the outer areas, to have the closest to Earthlike gravity, for health reasons. The food growing areas would be inward towards the center, and probably farmed remotely to avoid contamination. No visitors. But for human beings, some green space might be psychologically necessary, so there may be small areas in the living quarters where gardens grow.

    A resilient design would have several such compartments/towns; in case something happens to one of them, the rest could survive the trip. But the ship would have to be even more enormous to accommodate that. Better yet, send a fleet if you have the resources (the Solar System has enough for redundancy, Earth alone probably can't afford one).
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