In article ,
Doug... wrote:
...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 effect of a single mascon is relatively simple to analyze to a first
approximation: it changes the *direction* of the orbital motion slightly
without changing the speed. This translates into (skipping some details)
a semi-random change in the eccentricity of the orbit: the overall size
of the orbit doesn't change, but how elliptical it is does. The change
can be for better or for worse, i.e. less or more elliptical.

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.

Correct. As the value of the eccentricity wanders around randomly, the
orbit gets less or more elliptical. If it ever gets elliptical enough
that its lowest point is at or below the surface, it's game over. For a
low lunar orbit, mascon effects are strong enough that this tends to
happen fairly quickly.

(It is possible that there are stable low orbits around the Moon, where
the mascon effects cancel out, at least for a while. We don't know enough
to predict where they might be, because we don't have good gravity maps of
the lunar farside. All gravity mapping to date has been based on tracking
from Earth, which is impossible over most of the farside. You can get a
little bit of information by looking at how the orbit has changed when the
spacecraft comes back into view, but not very much. There have been many
proposals to do farside gravity mapping, using a pair of satellites and an
intersatellite radio-tracking link, but so far it hasn't been done.)
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MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |