I would rather treat Orbitals, sort of like in The Culture but sans doubletalk magic. I once calculated a ring (IIRC) a half-million km in radius, orbiting the

I would rather treat Orbitals, sort of like in The Culture but sans doubletalk magic. I once calculated a ring (IIRC) a half-million km in radius, orbiting the sun in place of Earth (or opposite it, if you like), could spin at 1G in 24 hours, but would also need to be made of scrith. The interesting result was that the tension would be the same as supporting a quarter-length at the target acceleration, hanging from one end. So, for mine you need a material that can support a straight length cable 0.5M*pi/2 km long hanging in a uniform 1G field, or (equivalently) while accelerating it at 1G pulling on that end. Of course the cross-sectional area cancels out, so you can just talk about how strong the material is. Steel can support quite a few km. Kevlar can support hundreds of km. Idealized carbon fiber might support, what, ten thousand km? A ten-thousand km orbital would need to spin too fast to give you a 24-hour day, and could not keep the air in without a lid, but ought to be interesting enough to work on, in-principle buildable. Someday.