In article ,
says...
On or about Tue, 23 Sep 2003 17:29:08 GMT, Henry Spencer
made the sensational claim that:
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...

Because you're Henry...What about a lunar orbit?

LOL -- I'm not Henry, but sometimes I play him on TV... well, OK, on my
video camera...

Obviously, lunar orbits are not affected in the same way by atmospheric
drag. The Apollo landings temporarily doubled (or more) the Moon's
atmosphere with the gas from the LM engine exhausts. So drag is not an
issue. You can remain in orbit with no drag effects at an altitude of as
low as 10 miles.

However, lunar orbits deteriorate quickly because of the Moon's uneven
gravitational field. The maria deposited into impact basins are made of
basaltic rock that is denser than the surrounding terrain, so there are
positive gravitic anomalies over most of the maria. That means that
spacecraft "fall" faster as they pass over these mass concentrations
(mascons) and "slow down" as they pass away from them. This disturbs
lunar orbits, causing them to become more and more misshapen over
relatively short timeframes.

The subsatellite carried by Apollo 16 was released into an orbit that was
roughly 70 statute miles high, circular. (It was actually a slightly
lopsided orbit, but close enough to 70 miles circular to make no
difference in the current discussion.) That satellite was drawn into an
orbit that intersected the surface after only six weeks.

My impression is that lunar orbital spacecraft aren't "dragged down" by
having energy removed from their trajectories, as upper atmospheric drag
does to earth orbital vehicles. It's more that the *shape* of their
orbits are changed by the mascons until the trajectory intersects the
surface.

--

Do not meddle in the affairs of dragons, for | Doug Van Dorn
thou art crunchy and taste good with ketchup |