In the prior essay, I complained about the inadequate thinking behind the execrable plot of the movie Avatar. However, I think my objections need to be more fleshed out, and so - apologies in advance - I'm going to have to go geek on you.
Living in space has been covered by so many scientists and science fiction authors, that I wonder if it is really necessary to expound upon the problems. But the fact is I haven't covered it, so here goes.
Every portrayal we have had of the future of people in space is terribly flawed in this respect: it's always viewed as ultimately planet dependent, the same way surface going ships on Earth are land dependent. But the fact of the matter is to live in space for even a brief period of time requires that you plan on living there permanently. In other words, the only viable form of space travel is going to require that you can exist out there all on your lonesome, with the tools and techniques and resources to pretty much recreate everything you are ever going to need to live out there.
More to the point, every bad science fantasy movie involving aliens invading the Earth to gain a resource such as fresh water is stupid from the start. If you can travel from star to star (or planet to planet) to invade a world, you can darn well extract what you need from asteroids, or comets, or uninhabited worlds, or even the gas and dust and solar winds around you. You don't need to bother fighting some pesky monkey people (or big blue cat people) for that resource you want. In fact, if you can't get what you need from the vast resources floating out in space, your space journey is pretty much fucked from the get-go.
If you wish to call yourself any kind of spacefaring civilization, you pretty much have to make yourself planet independent. You have to take care of the Three Ps: Power, Propulsion, and uh, Semi-Self-Contained Sustainable Habitat (Place). Power, duh, because you need energy to run shit. Propulsion, duh, because you need to move around. But you can't do that in some sealed coffee can with some grow lights and a hydroponic rack of pond scum and sea monkeys to recycle your bad breath and poo. You need a biosphere. (I'm assuming you need one even if you have a magical faster than light star drive that can poot you around to pretty much anywhere like in your standard space fantasy scenario).
You need a biosphere. An artificial ecology. And a means to replenish it (because even the Earth is not self-contained). You need an extraction technology that can resupply just about anything within your biosphere. There's just no way around it. If humans ever head out, we are going to be like hermit crabs - hauling all the shit (ALL the shit) we need around with us to survive. And hauling all the shit we need (think about it, mining, smelting, refining, manufacturing, recycling, disposal, repurposing, basically everything that a civilization does) to keep things going.
And not only don't we know what we need, we don't even know what we need to need. The one time we tried to create an artificial environment (Biosphere II), it turned into a badly planned, badly mismanaged, awful, dystopian cult-fetish of a joke - you'd have thought the whole thing had been conceived and run by libertarians, it was that fucked up.
Hey, it's probably just as well we didn't just blithely send 'em all off to Mars and then wonder what went wrong, huh?
More importantly, the Biosphere II project was not realistic in making the demand that it be self-contained. Of course you're going to forget something from your grocery list. Of course you are going to run out of some things. Of course your tin can is going to leak. Of course extra air, water, soil, metals, volatiles, you name it, can't be carried along. You are going to need to be able to get that shit from your surrounding environment somehow. It's not cheating if you have to go outside to get what you need. It's common sense to have the ability to do that.
So, space faring civilizations, it's not going to be a romantic 19th century Star Trek vision of isolated sailing ships zooming between the stars. You're going to have to take your whole fucking civilization around with you.