QUESTION:
Just before the de-orbit burn, HP = HA = about 200 nautical miles. i.e.
circular orbit. So, just after the de-orbit burn, what is the low point of
the orbit? Is it e.g. just equal to sea level? Is it below sea level? Is it
above sea level?  And, how high would that low point have to be before the
shuttle would not "de-orbit"? 100 nm? 30 nm?

ANSWER from Eric Hammer on May 6, 1997:
The perigee (Hp) following the deorbit burn is a function of many things. 
Primarily, the orbit prior to the burn dictates the post burn perigee.

For example, the STS-82 pre-burn orbit was 333x312 nmi and the post-burn
orbit was 333x28 nmi.  The post burn perigee can be below the Earth's
surface, at the surface, or above the surface!  The difference between the
targeted (optimum) perigee and the minimum pergiee acceptable for landing is
again variable (maybe 5 nautical miles in one case and 10 nautical miles in
another).  It has to be low enough such that the flight path angle at which
the orbiter enters the atmosphere (~65 nmi) provides an acceptable drag
profile through the atmosphere!