I'm always fascinated in the assumptions underlying some of these proposals, and how little thought they've given to the problem. For example, the complete disregard for the lives or autonomy of the "replacement crew", who are effectively treated as a consumable resource starting from childhood.

Science fiction set on generation ships tends to be extremely gloomy for good reason.

The issue of crew as consumable resource is interesting.

It's particularly interesting I think because now that you've brought it up, it seems to me there is a similar issue with much Earthly colonization or frontier expansion, and I've never seen it mentioned.

For example, anyone who left England to start or join a New World colony early on, when settlements were far apart and it was a struggle for each to survive, was entering an environment where their children would have a much harder, much more constrained life than they could have had back in England.

Were there debates back then about the morality of moving somewhere where you descendants, possibly for generations, would have harder lives than if you stayed where you are?

17th century American settlers were fleeing an environment where the child mortality rate was at least 1/3rd even for the upper classes, nearly all land and wealth was controlled by inbred gangsters, and the penalties for petty theft or incorrect religious worship included public hanging. From that perspective, the risks associated with colonization may have seemed much more attractive than the best-case scenario of spending several more decades of poverty in the same village. At least once you were in America, voting with your feet and moving west was always an option, something that was already recognized by 19th-century European socialists as a unique factor in the development of the US economy.

Of course I'm also assuming that the majority of the colonists were not only dirt poor, but also ignorant and uneducated and shamelessly lied to by the promoters of colonization.

And the early colonies had their problems too. IIRC at least one colony enacted capital punishment for anyone caught abandoning the colony to join the native American tribes.

I hadn't heard that one, but the freedom-loving Pilgrims executed Quakers for proselytizing in Massachusetts. Not really an environment founded on respect for individual autonomy or other ethical considerations.

Aren't all humans consumable resources for today's society?

Seems a bit silly, you can buy a glass sphere with small closed ecosystem that can last years with indirect sunlight. Surely we can manage something that recycles astronaut poop/pee into something vaguely edible with the help of bacteria, fungus, plants, and algae. Sure some genetic editing (of humans, bacteria, fungus, plants, and algae) might be necessary. Even just some minor tweak like happened in the last 1000s of years to make humans more compatible with cows milk.

That is taking energy from the sun, and it is relying on the Earth to maintain certain temperature parameters. It won't last 10k years anyway. If nothing else, the glass will probably degrade over a long enough period of time. There's also a constant chance of a freak mutation destroying the equilibrium of the system.

In outer space, cosmic rays will make this process faster. Also, in outer space, it will need a constant source of light for biological reactions, and also just to keep from slowly cooling down to 0k through heat emission.

> a sphere with a radius of 24.4 meters

A more human-sized comparison is "100 average single-family homes in the US, all filled to the brim with rice". Or 60 ISSes, all filled completely with rice. Per person. And this is volume, not taking into account the weight of rice.

Minimum sustainable population size seems to be about 100 people, so consider how large a ship you'd need just to fit all that rice storage.

It sounds ridiculously large when you put it as 100 average single family homes filled to the brim (I get about 50, BTW, using 2623 sq ft with 9 ft ceilings for the average single family home).

On the other hand, it is under two Mount Palomar observatory domes full.

I'm assuming anyone building a generation ship is going to build it in space, not build it on a planet's surface and then try to launch it, so size is not really going to be an issue.

The big engineering issue will be propulsion. We don't have anything now that could drive such a ship. My guess is that this is either something we'll have within the next couple hundred years or we'll never have it.

I'm assuming that by the time you want to build this kind of ship, you've gotten raising people from frozen embryos or frozen eggs and sperm perfected, and that is where most replacement crew would come from. That should greatly reduce the crew size needed.

I think we'll be able to handle the biological side of this within a hundred years, with a good chance of it being quite a bit sooner than that.

I am referring to all of the machinery on the ship in general - whether it's life support systems, engines, monitoring/navigation systems etc. Even if you could take a ship to outer space, freeze it to ~absolute 0, and launch it on its trajectory, you still need a computer to allow it to warm back up and land at its destination.

Keeping a computer running in outer space for a few thousand or hundred thousand years is a gigantic engineering task, far beyond anything we could achieve today.

And of course, in reality its unlikely you could rely on a single computer system and on 0 propulsion. It's far more likely that you'll need life support, engines, complex medical equipment, cooling systems, lights, all sorts of mechanical parts that will need power, replacements, and that degrade in time, especially at such huge scales.