Kim Stanley Robinson:
A crossing to even the closest stars will require a multiple generation effort, and the spaceship will need to be a kind of ark, carrying all the other animals and plants the humans will carry with them to their new world. This suggests a very large and complicated machine, which would have to function in the interstellar medium for two centuries or more, with no possibility of resupply, and limited possibilities for repair. The spaceship would also have to contain within it a closed biological life support system, in which all the flows of energy and matter would have to recycle as close to perfectly as possible, minimizing catches or clogs of any kind.
Here is where the biological and ecological problems come to the fore, but sticking for now to purely physical problems, the starship would be exposed to far more radiation than we are on Earth, where the atmosphere and magnetosphere protect us to an extent. Effects of that extra radiation are not fully known, but they won’t be good. Cladding would help, but would add to the weight of the ship; the fuel carried for deceleration might serve as cladding en route, but that fuel will get burned as the starship slows down, increasing the starfarers’ exposure, already higher than it would have been on Earth.
A starship would be something like an island, but an island far more isolated than any island on Earth. Processes identified by island biogeography would apply inside a starship, and many of these processes would be accentuated by the radical isolation. As generations of people, plants and animals passed, reproductive and evolutionary success would be harmed by genetic bottlenecks, also disease, limits on resources, and so on. The super-islanding effect might cause more species than usual to become smaller, and to mutate in other ways, as one sees on ordinary islands. And because bacteria tend to evolve at faster rates than mammals, complete isolation may lead to the development of a suite of bacteria quite different from what the spaceship was sent off with. All mammals include huge numbers of bacteria living inside them, either symbiotically, parasitically, or without significant interaction, so this more rapid genetic shift in the bacterial community could become a big problem to all the larger creatures.
Cycles of oxygen and carbon dioxide, nitrogen, phosphorus, and many other chemicals and elements, would have to occur without major fluxes and without catch-points along the way where the element is getting clogged in the system. Earth experiences large ecological fluxes over time, with build-ups of certain elements (oxygen in the atmosphere, carbon in sedimentary rocks) that force evolutionary processes: whatever is alive has to adapt to the new conditions or go extinct. Both often happen.
These fluxes and build-ups would happen inside a starship too, but as the starfarers would be interested in keeping themselves from going extinct, they would have to manage or finesse all the flows to keep from being harmed by them. This would require supporting almost every other living component of the system, except the diseases they would inevitably carry with them; and if chemicals like phosphorus were bonding to substrates as they cycled in the water cycle, which is something they tend to do, this would be bad for the system as a whole.
In short, a perfectly recycling ecological system is impossible; Earth is not one, and an isolated system a trillion times smaller than Earth would exacerbate the effects of the losses, build-ups, metabolic rifts, balance swings, clogging, and other actions and reactions.
The people inside the starship will constitute a small and isolated community compared to the population of Earth. And they will be trapped inside their spaceship, and will have to keep the spaceship functioning in order to survive. So whatever their political organization, whether it be military or anarchic, hierarchical or democratic, the situation itself can be called totalitarian.
By this I mean that their situation will demand certain behaviors to ensure their survival. They will have to tightly control their population; both maximum and minimum human numbers will be necessary, and whatever system they devise to achieve this stability, it will not include individual unconstrained choice. Also, there will be quite a few jobs that will simply have to be filled in order for their life support systems to be maintained. Again, however they manage this issue, people will not be free to do what they want, or to do nothing. So in these areas of reproduction and work, generally regarded as basic to human meaning and political freedom, the society in the starship will have to rigidly control themselves. No matter their methods for achieving this control, they will end up living in some version of a totalitarian state. The spaceship will be their state, and to keep the spaceship functioning, the state will rule.
But say all these problems get solved somehow. Say the starship reaches its target star system, and goes into orbit around the planet the starfarers hope to inhabit. What happens then?
The planet or moon they hope to inhabit will be either alive or dead. It will either harbor indigenous life, or it won’t. Both possibilities represent terrible problems for the settlers.
There is a third possibility, of course, which is that they won’t be able to tell if the planet or moon is alive or not, just as we can’t tell now whether Mars is alive or dead. In that case they would still have a problem, they just wouldn’t know what kind of problem they had. Finding out could be hard.
When we consider how we should behave now, we should keep in mind that the idea that if we wreck Earth we will have somewhere else to go, is simply false. That needs to be kept in mind, to set a proper value on our one and only planet, so that a moral hazard is not created that allows us to get sloppy with our caretaking of it.There is no Planet B!http://boingboing.net/2015/11/16/our-generation-ships-will-sink.html