Living in space is a uniquely difficult problem, currently the best space habitat available is the International Space Station (ISS) and most astronauts are limited to only a couple of months, while spending upwards of 40% of their time exercising to prevent catastrophic muscle and bone decay. Clearly this is not a viable strategy for long term habitation in space. Thankfully there are several ideas about technologically feasible ways, primarily generating “artificial” gravity, for humans to actually live in space long term. The best known concepts would be the O’Neill cylinder concept from Gerard K. O’Neill, an American physicist, and the Stanford Torus from a NASA study in the mid 70s.
Featured in movies like Interstellar and books like Rendezvous with Rama (a classic from Arthur C. Clarke), O’Neill cylinders are perhaps the most efficient means of space habitation on large scales. A lot of this has to do with simply geometry, cylinders, especially long thin cylinders, have more interior surface area. Meaning more space for living, per unit volume, than almost any other regular shape. Plus being round, cylinders are more comfortable and aesthetically pleasing to the natural senses as opposed to, say, a box.
Finally, and the really important thing, it can be constructed entirely with modern technology, albeit without the giant windows. Yup no unobtanium, Star Trek technology, or uninvented physics, the human race could build one of these right now if there was sufficient will. (Although most likely you would want two of them to counteract gyroscopic effects)
Now whether or not it would be economically viable with our current technology is another matter entirely, as you can see below. Selling it as ultra luxury “living space”, cashing on the futuristic space land aspect, would most likely be the only method to even come close to economically justify the investment in building one of these.
Back of the envelop calculations for the total amount of material required:
Length: 10 km
Thickness of shell: 10m (it would most likely be thicker but there would also be empty volumes for access ways and machinery, 10m is a rough approximation)
Total material: 100 million cubic meters, assuming light alloy steel at 7.8 g/cm^3, that would be 780 million tons of material, not including the plants, buildings, and other things you would want to add on both internally and externally.
A total estimate would be closer to a billion tons of material. Needless to say, getting this from earth would be impossibly expensive, you would need 208 million Falcon 9 launches, or 137 million Delta IV Heavy launches. to get this much mass to geosynchronous orbit.
So mining the moon or mining the asteroid belt would essentially be the only source of material for a project this large. Now this isn’t as big of a hurdle as you might think, no aspect of asteroid or moon mining is impossibly from a technological or physical science standpoint. There just isn’t much need for either at the moment because it would cost more, a lot more, to get bulk resources from outside of Earth’s gravity well down to the surface then to simply mine it on Earth, even compared to the harshest terrestrial conditions. This all changes however if we require large amounts of bulk material outside of Earth’s gravity well.
As our material science progresses, building O’Neill cylinder and other space habitats, will get progressively cheaper, so much so that eventually it will make sense to simply build it to create new land and housing for more people, than to live on other planets or to try and terraform other planetary bodies. And even if major advances are made in the next century regarding terraforming, the amount of time it would take would make it a multi-generational project. And when the first humans are free of earthly shackles in the first space habitats, I predict that the paradigm of space exploration will change into space colonization. Where explosive and exponential growth will take place in all space related industries.