How a lifeless rock could hold a trillion humans.
People have lived for weeks in slowly rotating reference frames. (Rooms or cars in large centrifuges.)
Animals raised in hypergravity for lifetimes & generations, are stronger, more healthy and live longer than controls, and we know that a marathon runner who strives against gravity will be expected to live longer and be more healthy than normal, who will be expected to live longer than a couch potato.
The S.F. trope of living in low G, and it even being beneficial, seems to have a lot of contra-indications.
It's been known, from the earlier work on space colonies that in just the inner Solar system small bodies, there are materials for effectively hundreds of times the Earths useful land area. In the main Belt, thousands, and by varying the sizes of mirrors, the "habitable zone" around the Sun goes out into the Oort cloud and beyond.
Meanwhile, terraforming of Mars, Venus, the Moon, Titan, Europa, Callisto, etc yields only maybe 2x the Earth's land area, in inferior conditions, in low G which is a complete unknown.
I've seen even in discussions about living in space and terraforming, people who object that the lead-time and cost to building O'Neill habitats is too much. You're not going to terraform a planet for less cost, less infrastructure, in less time than it'll take to get large returns from mining asteroids and building space habs.
Forget about the long cylinders with huge areas of windows. They're unworkable for a number of reasons. See the "Kalpana" space habitat design for drum or short cylinders. The "Stanford Torus" is the baseline model. 1.8km diameter for 1RPM which NASA Ames thought safe for practically anybody to move in without problems.
It would have been done by ≈ 2010, long with all the ground, launch, and in-space infrastructure to reproduce it. The projected cost was like ≈4x the Apollo program, or ≈4 of our CVNs and their air-wings and escorts and the logistics infrastructure to deploy them to fight for oil. Cost ≈ to many other large infrastructure or industrial developments down here.
Building it would mean mining NEAs, and building Solar power sats, which ends the relevance of the scarcity model for energy, raw materials, previously rare or "precious" or "strategic" metals. Ends oil wars or budget crunches, forever.
According to the NASA Ames / Stanford studies, the largest pressure vessel which we cold build to spin for 1G and hold everything inside was ≈30km diameter, either drum or torus. Concrete and steel, not fanciful sentient nanotech self-assemblers or graphene. With titanium (which isn't rare out there) even larger.
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