Comment by brendoelfrendo

I always appreciated the Mobile Suit Gundam approach (U.C. timeline, to be specific): humans in space largely live in O'Neill Cylinder-style space colonies arranged in constellations at Earth-Moon Lagrange points, allowing them to a) be built from materials gathered from space and b) once built, manufacture things from those materials without having to land them on Earth first. There are large settlements on the Moon, and while they are important manufacturing and research centers, they're not the primary population centers in space. Mars is, to the best of my knowledge, uninhabited. As far as the outer solar system goes, only Jupiter has a permanent human presence as the primary source of humanity's helium-3 (Gundam predates the proposal of mining the Moon for helium-3 instead).

I like this approach. It's plausible based on the assumptions made by the story, lets people in space have the benefits of mostly-normal gravity and radiation exposure (as compared to, say Mars), and keeping things local to the Earth means you don't need to be too concerned with the distances involved. Where they really lose the plot, though, is with population; Gundam claims that, in less than 100 years, billions of people--in fact, the large majority of humanity--have moved to space. I can't even begin to fathom what kind of effort would be needed to build that many space colonies, and then shuttle the people up there to populate them.