Background: Larry Niven's Ringworld was declared to be unstable because it rotates fast enough to create artificial gravity on the inner surface (never mind the structural material issues) and is therefore not technically in orbit around the central star. So I've been reading up on the topopolis megastructure, which is essentially a torus that stretches all the way around the central star. It generates artificial gravity by rotating around the minor radius, like a very long O'neill cylinder that connects to itself. Several sites have commented that the structure would have an unstable orbit, like a ringworld. But the topopolis can orbit at orbital velocity, so why would the orbit be less stable than, for instance, a bunch of O'neill cylinders in the same orbit?