I don't understand why NASA has never built a rotating space habitat to solve this problem. Because it's difficult to do right. There is a maximum usable RPM b

I don't understand why NASA has never built a rotating space habitat to solve this problem. Because it's difficult to do right. There is a maximum usable RPM beyond which humans cannot function due to dizziness/disorientation, and a somewhat lower max beyond which excessive coriolis effects would make working in the habitat incredibly annoying; generally, you don't want to go about 2rpm, though you could push it by screening your astronauts for spin-adaptability. This puts a lower limit on how large the habitat must be to achieve any particular level of spin gravity; for anything approaching 1g, it's pretty large, and no one knows just how little gravity is required to prevent the physiological effects of microgravity. To get even half a g at 2rpm, you'd need a structure 112 meters in radius, which is pretty dang big for a spacecraft. You could probably still manage that with current technology if you just have two small spacecraft modules at the ends of a 224m tether (or a somewhat shorter tether if you have some unmanned equipment that's heavier than the crew module and can serve as a counterbalance rotating with a smaller radius), but then you've got a bunch of other problems to deal with. How do you dock with a rotating structure? Safely approaching a rapidly spinning tether in orbit is not an easy task. Then you have to match rotation, unless there's a non-rotating hub, which would result in enormous complications to the structure. How do you move it / do stationkeeping? All thruster burns now have to account for the structure's angular momentum. And stationkeeping will be more of an issue that with 'normal' spacecraft because tidal effects will make a tether in orbit want to wobble. Additionally, there may be electromagnetic drag to deal with, and the associated voltage buildup across the structure that you'll need to find a way to safely discharge. And then there's space debris. If the cable is damaged, you either get flung off into an unrecoverable orbit, or you hit the atmosphere and burn up. So, all in all, hardly surprising that NASA hasn't bothered yet.