"A likely candidate for an orbital is waht's called an O'Neil Cylinder: 3-4 miles in diameter, 10-20 miles long, producing Earthlike gravity on the interior by spinning. Smaller than this and it needs to spin too fast. Larger than this and you need stronger materials to stop it ripping itself apart from centrifugal forces. Stainless steel is sufficiently strong to build an O'Neil Cylinder;"
It's unclear what exactly you mean by "too fast", but assuming you're referring to human tolerances: human tolerances from NASA + Soviet studies put unambiguous, continuous tolerance without needing medication or training or anything else at 2rpm, which equates to a diameter of 450m. That is a lot smaller than an O'Neill cylinder and a lot more feasible to build sometime soon. IMO the best option is to build a 100m diameter testbed now from Earth materials, as the successor to the ISS. Then take the lessons learned there and build a 450m diameter prototype, which we can use space materials for if space mining has developed enough. We could technically throw enough material into orbit for a 450m diameter cylinder but it would be a lot of material. Any of the larger sizes and we'd need real-deal asteroid mining to make that happen.
Basically, build a small testbed now to conduct actual experiments on human health at different gravity levels + RPMs, and also start trying to figure out asteroid mining. Build a bigger prototype habitat once we can get materials for it, either from massive launch cost reductions or asteroid mining. After that point we really do need asteroid mining.
It's unclear what exactly you mean by "too fast", but assuming you're referring to human tolerances: human tolerances from NASA + Soviet studies put unambiguous, continuous tolerance without needing medication or training or anything else at 2rpm, which equates to a diameter of 450m. That is a lot smaller than an O'Neill cylinder and a lot more feasible to build sometime soon. IMO the best option is to build a 100m diameter testbed now from Earth materials, as the successor to the ISS. Then take the lessons learned there and build a 450m diameter prototype, which we can use space materials for if space mining has developed enough. We could technically throw enough material into orbit for a 450m diameter cylinder but it would be a lot of material. Any of the larger sizes and we'd need real-deal asteroid mining to make that happen.
Basically, build a small testbed now to conduct actual experiments on human health at different gravity levels + RPMs, and also start trying to figure out asteroid mining. Build a bigger prototype habitat once we can get materials for it, either from massive launch cost reductions or asteroid mining. After that point we really do need asteroid mining.