On Tue, 23 Sep 2003 14:29:42 -0400, Chris Jones wrote:

First, there is no simple answer to how low can you orbit, although 100
miles (and note, every time I say miles I mean statute miles, which is
what you meant too.

Right. Some of the early missions had perigees as low as 154 km (e.g.
Sigma 7), which is ~95.7 statute miles.

It turns out
that a 100 mile high circular orbit will last about a day, give or take
hours (this number varies because the density of the atmosphere varies,
mostly due to solar activity, and I might be off about a day, but it's
the right order of magnitude).

So a 95 statute mile circular orbit may last a few hours? At 80-90
miles would you be able to make one orbit? (Assume average
atmospheric conditions and a Mercury or Gemini spacecraft.)

Some earlier US spy satellites would lower themselves from an orbit
about 400 miles circular to one of about 75x400 miles for an orbit or
two

That must be just about the limit, since I've read that at 400,000
feet (75 miles) you start to get noticeable drag.