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

On Jan 28, 2012, at 1:49 AM, hitssquad wrote:

> --- 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.
>

That's fine if you assume everything works well and no one ever screws
up. Unfortunately, that hypothesis has been decisively shown to be
false. I'm not sure exactly what the requirement should be, but
given no inputs I would not like to see a habitat with an orbital
lifetime of under a century or two.