Larger habitats can orbit lower (was : A simple question)

--- In spacesettlers@yahoogroups.com, Al Globus wrote:
> The minimum is close to 1000 km up, to avoid deorbiting any time
soon.

Drag is less of an issue with a larger, more-massive habitat. A larger habitat also enjoys greater economies of scale and therefore can more-easily afford things like ion thrusters to continuously overcome drag. A low-altitude habitat is more attractive to traffic from Earth and so therefore should encourage earlier growth to larger sizes -- which, again, are more aerodynamically efficient.