Obviously, terraforming and Dyson spheres are amazing and all of that but, in terms of learning to crawl before you learn how to invent the self-walking mecha power suit, we probably need to start with some smaller concepts in mind.
The first thing is to note that the key component to colonizing space is the financial viability of it.
Right now, for example, outside of some scientists, a few die-hard explorers, and some penguins there aren’t a lot of people in Antarctica. But, on the day that they discover that there is a boat-load of unobtanium under the ice and a very efficient means of extracting it, you’re suddenly going to see some real-sized towns forming down there and people figuring out the technology for heated sidewalks.
Space needs to be profitable. All other initiatives of any size are dead on arrival until that’s solved.
In theory, there is a fair amount of unobtanium in various forms available in space. But that brings us to our second point: Gravity sucks and is expensive to overcome.
If you’re mining ore, having to haul it off the surface of Mars or the Moon or anywhere else is like trying to mine something that weighs ten tons per ounce, haul it a few thousand miles, and expect that you’re somehow going to hit a reasonable price point on the market after paying for all the logistics of hauling something that weighs ten tons per ounce a few thousand miles.
The key to space expansion, if there is one, is in mining near-Earth asteroids. In theory, that allows us to create a continuously operating “factory” in outer space that’s constantly mining ore and chucking it down to Earth to be picked up. You can amortize the “fighting gravity” part of building the factory over the long-term of having gravity somewhat on your side, once you’re in production. You don’t have to fight gravity for every single shipment (though, you do have to contend with your orbital velocity - but only enough to break orbit).
And note that this is still, in all practical senses, a large and nigh-impossible task with our current technology and commitment levels if we’re talking about trying to develop something that’s profitable over any reasonable period (e.g. 20-30 years). Colonizing a planet is massively beyond that. We can’t even colonize a chunk of ocean, 20 feet deep.
But, thirdly, asteroid mining is almost certainly a necessary step towards colonization because, again, you have to contend with gravity and economics.
Engineering is most successful when you can focus on a very specific task and eliminate the amount of variety you’re contending with.
If you have to build a Colony Ship[sup]TM[/sup] that can take off from Earth and which already contains everything you need to fly across the galaxy, restaurants, theaters, dance halls, food, farms, sleeping quarters, schools, hospitals, etc. - that’s going to be a damn-heavy thing. And not just heavy, massive as well. Almost certainly, too massive to be structurally sound enough to withstand the forces necessary for launch.
Versus, if you just have a ship that simply gets humans off Earth far enough to get largely out of our gravitational pull, with no food, no accommodations, it’s just a single-purpose box - that’s a far more achievable task in terms of finding a solution that’s not too insanely expensive (rail launchers, space elevators, laser propulsion rockets, etc.) And that’s what you want because, outside of humans and maybe some seeds, you don’t want to use anything from Earth for space travel.
Aluminum is fungible. If we mined it out of an asteroid, it works just as well as aluminum which was mined from the surface of the Earth. But, to get your Earth aluminum into outer space costs way more than using aluminum which is already out there.
Minus Star Trek’s transporter technology, humans aren’t elementally fungible. We can’t vaporize the ones here and reconstruct them at the other end out of raw elements. Us and other complex life are the only things that have to be brought out of Earth’s gravity well and, from an economic standpoint, that’s the only thing worth bringing out of the gravity well.
So you need mines and factories floating around in outer space as a prerequisite to building your space “shipyard” so that you can build your Colony Ship[sup]TM[/sup].
We’re probably a hundred or more years away from getting to that point.
By the time we do, we might realize that planetary colonization is a waste of time.
Because, we have to consider point 4: Planets are horrible.
Right now, if you wanted to live on Mars, it’s sort of like deciding to live under the ocean, in a tin can, where (for reasons unspecified) you have to live upside down and walk around with magnet boots. Should we, through magic, achieve some amount of terraforming of the planet, then it will be like you’re now living at the South Pole, still upside down, walking around with magnet boots. Woo-HOO!
You have a significant difference in the level of available light, vastly different gravity, different day lengths, etc. The human body is simply not made for Mars. Being able to grow plants on it and breath the air doesn’t suddenly make it a pleasant place to live. You’re not actually going to be much more comfortable down there than you would be in outer space. It’s all uncomfortable. But it’s probably easier, in actuality, to make space comfortable than a planetary surface.
A space station is smaller, it’s all man-made and modifiable, and the technologies will have a longer development history so they’ll all be ahead of terraforming technologies. Forcing an entire planet to become like Earth is a far larger task than forcing a small metal box to become so.
And there’s no real benefit to going down to a planet, in many ways. Once you go down, you have to bring everything back up again. Why add that cost? If you’ve developed society enough for space to be a self-sustaining market, then planets are just asteroids too large to mine and too expensive to mine, because of their gravity well.
The only case where another planet becomes a genuine source of attraction is, realistically, if we genetically engineer ourselves at the same time as we terraform, so that we can get a fully satisfying experience that’s better than living in a space station.
But, by that token, if we can do that then we can genetically engineer ourselves to be perfectly adapted to living in outer space so that it’s the most comfortable place for us, period.
And all of that is assuming that we don’t all start living in Second Life or some similar virtual reality in the next 50 years - with infinite exploration and endless possibilities - and simply stop having so many babies so that we don’t have to worry about expanding our physical production capabilities.
If we can genetically engineer ourselves, all fear of nuclear war goes away because we can engineer the killer aspect of our nature out. No more need to colonize the galaxy, to ensure our continued survival.
Should we get to the point where we start trying to colonize space, though, step 1 will be to colonize “space”, not another gravity well.