Why do you limit a reasonable amount of time to a human lifetime? If we're going to colonize interstellar space with anything like our current technology we're probably going to have to ditch that requirement.
There's no reason you can't have a multi-generational ship that takes a thousand years to arrive. You don't have to send thousands of people either, it's enough to send a hundred or so, have them populate on the way, and maybe send some frozen sperm & eggs with them to increase genetic diversity.
Another possibility that'll probably become viable within 50-200 years is to not send any humans at all, but to only send a fast & small ship with human sperm & eggs that can start hatching humans once it gets there. Raising the infants could be done with robots and other infrastructure manufactured once we get there.
Read 'Orphans of the Sky' by Robert Heinlein to get an idea of the many problems with this approach. Human society is not culturally stable enough to utilize a generation-ship method of space exploration.
Heinlein's pessimistic vision of a the population of a generation ship turning into savage illiterates is hardly realistic, or one of the biggest problem with the approach.
If you have an ark ship that can handle on the order of hundreds of years of space travel, you'll have an ark ship that can handle essentially indefinite space travel.
The reason you're headed for another solar system is to get access to more matter and energy. You can't find that in interstellar space, so eventually your supplies will run out.
In the traditional colony-ship scenario you'll have had to solve the self-sufficiency problem. So additional matter and energy will only be used and run out inasmuch as you're developing and creating new things beyond the originally-conceived equilibrium point of your ship.
As that's likely something an inventive race will always do, I'll grant matter and energy exhaustion even though it wouldn't be in the form of 'running low on food/fuel'.
But even then, matter and energy are far more safely and easily found, harvested and retrieved if you never enter any planet's gravity well.
If you're thinking more along the lines of a hyper-space scenario, where FTL technology is achieved before self-sufficiency is solved, sure, you'd have more-traditional 'supplies' and need to land and colonize Earth-like planets.
We've been referring to different things. Since you referred to "indefinite space travel" I thought you were talking about not going to another solar system at all, i.e. that "land" meant orbiting another sun.
But that was obviously a misreading on my part. Anyway, I completely agree with you that once your ark ship arrives in a new solar system landing on a planet like Gliese 581g would be counterproductive.
Instead of having to deal with a gravity well it would be better to make a home in something like the asteroid belt. You only need energy & resources for manufacturing capacity, a gravity well can only complicate that process.
People might still want to live on planets as a novelty, or for their resources. But I think any society sufficiently advanced to develop a generational ship that can traverse interstellar space would resemble The Culture (i.e. a swarm of migrating ships & habitats) more than they would resemble The Federation (i.e. people mostly living on planets with ships mainly for transport).
A thousand years, yes fine (this is ~ 10 x human lifetime) But to get anywhere in that time you're still going to have to get to some non-infinitesimal fraction of light-speed. Anything less than this you're looking at 10k, 100k, a million years to get to even the closest stars.
I could imagine the feasibility of a multi-generational ship lasting 100 - 1000 years, but not longer. Just think of mechanical and materials fatigue. I doubt anything could be built which would last any longer (if that long).
The length of time is almost beside the point. If one (long-term) goal is to put our eggs (so to speak) in a basket other than that of our sun, then there aren't a lot of alternatives. Other stars are simply very far away.
Ideally we'd be trying a mixture of probes and generational ships, while also experimenting with Moon and Mars bases.
If you're able to bring machines that can construct and repair other machines from raw materials the limitation becomes how much material and energy you can bring with you, not the amount of time you're out there.
Comments
Why do you limit a reasonable amount of time to a human lifetime? If we're going to colonize interstellar space with anything like our current technology we're probably going to have to ditch that requirement.
There's no reason you can't have a multi-generational ship that takes a thousand years to arrive. You don't have to send thousands of people either, it's enough to send a hundred or so, have them populate on the way, and maybe send some frozen sperm & eggs with them to increase genetic diversity.
Another possibility that'll probably become viable within 50-200 years is to not send any humans at all, but to only send a fast & small ship with human sperm & eggs that can start hatching humans once it gets there. Raising the infants could be done with robots and other infrastructure manufactured once we get there.
Read 'Orphans of the Sky' by Robert Heinlein to get an idea of the many problems with this approach. Human society is not culturally stable enough to utilize a generation-ship method of space exploration.
Heinlein's pessimistic vision of a the population of a generation ship turning into savage illiterates is hardly realistic, or one of the biggest problem with the approach.
Most human societies aren't stable enough for that. But visit most presently isolated communities, and you'll find that they're quite stable.
If you can visit them, they're not isolated.
If you have an ark ship that can handle on the order of hundreds of years of space travel, you'll have an ark ship that can handle essentially indefinite space travel.
At that point: why land?
The reason you're headed for another solar system is to get access to more matter and energy. You can't find that in interstellar space, so eventually your supplies will run out.
In the traditional colony-ship scenario you'll have had to solve the self-sufficiency problem. So additional matter and energy will only be used and run out inasmuch as you're developing and creating new things beyond the originally-conceived equilibrium point of your ship.
As that's likely something an inventive race will always do, I'll grant matter and energy exhaustion even though it wouldn't be in the form of 'running low on food/fuel'.
But even then, matter and energy are far more safely and easily found, harvested and retrieved if you never enter any planet's gravity well.
If you're thinking more along the lines of a hyper-space scenario, where FTL technology is achieved before self-sufficiency is solved, sure, you'd have more-traditional 'supplies' and need to land and colonize Earth-like planets.
We've been referring to different things. Since you referred to "indefinite space travel" I thought you were talking about not going to another solar system at all, i.e. that "land" meant orbiting another sun.
But that was obviously a misreading on my part. Anyway, I completely agree with you that once your ark ship arrives in a new solar system landing on a planet like Gliese 581g would be counterproductive.
Instead of having to deal with a gravity well it would be better to make a home in something like the asteroid belt. You only need energy & resources for manufacturing capacity, a gravity well can only complicate that process.
People might still want to live on planets as a novelty, or for their resources. But I think any society sufficiently advanced to develop a generational ship that can traverse interstellar space would resemble The Culture (i.e. a swarm of migrating ships & habitats) more than they would resemble The Federation (i.e. people mostly living on planets with ships mainly for transport).
A thousand years, yes fine (this is ~ 10 x human lifetime) But to get anywhere in that time you're still going to have to get to some non-infinitesimal fraction of light-speed. Anything less than this you're looking at 10k, 100k, a million years to get to even the closest stars.
I could imagine the feasibility of a multi-generational ship lasting 100 - 1000 years, but not longer. Just think of mechanical and materials fatigue. I doubt anything could be built which would last any longer (if that long).
The length of time is almost beside the point. If one (long-term) goal is to put our eggs (so to speak) in a basket other than that of our sun, then there aren't a lot of alternatives. Other stars are simply very far away.
Ideally we'd be trying a mixture of probes and generational ships, while also experimenting with Moon and Mars bases.
> Just think of mechanical and materials fatigue.
If you're able to bring machines that can construct and repair other machines from raw materials the limitation becomes how much material and energy you can bring with you, not the amount of time you're out there.