There are faint, faint wisps of Earth's atmosphere a fair distance out, which is the big part of an orbit's challenge. Perturbations from other bodies nearby (n

There are faint, faint wisps of Earth's atmosphere a fair distance out, which is the big part of an orbit's challenge. Perturbations from other bodies nearby (notably the Moon), deformities of Earth's gravity, and even solar weather and tiniest fractions of gases are other contributors on a long-enough time scale. Micrometeorites would probably be the big risk over time for our orbital monument to humanity, and I would expect Kessler syndrome, as dramatized in Gravity, to actually happen over a long enough period without human existence. (Hell, even with us around it's a possibility, but not as depicted in the film. It'd be slower. Some say it's already underway.) Distant orbits might decay away from Earth given enough time and certain circumstances, oddly enough. A return to Earth is not guaranteed.