How low can you orbit?

In article ,
Jan Philips wrote:
Some of the early US manned orbital flights orbited only a little more
than 100 miles up. How low can you orbit? What is the relationship
between height and the maximum number of orbits?

There isn't a simple relationship, because it depends on things like the
density of the satellite and the state of the (highly variable) upper
atmosphere. Generally speaking...

Orbits at 200km and below are now considered suitable only as short-term
(a few hours) parking orbits; the decay in altitude is quite noticeable
and the orbital lifetime is a few days at most. About the lowest orbit
that has seen practical use is the 160km parking orbit used by the later
Apollos, which were leaving orbit (one way or another) within hours.

LDEF was retrieved by the shuttle when its orbit had decayed to about
240km, and that was considered a last-minute save -- the mission would not
have been feasible had it gotten much lower. There have been only one or
two other shuttle flights that low.
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