In article , says...
In article ,
Doug... wrote:

snip

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.

Consider that the Apollo 15 CSM/LM stack was placed into the descent
orbit on LOI day, at the end of the second rev. The crew went to bed as
per the flight plan, the next day being PDI day. They were awoken a bit
early because, in roughly 12 hours after the DOI burn, the pericynthion
that had started at 50,000 feet had degraded to about 35,000 feet -- the
height at which airliners fly over the earth. And that was after only 12
hours. (They had to do a quick RCS bail-out burn to get back into the
proper orbit for CSM circ and LM PDI later that day.)

Of course, Apollo 15 was the first Apollo mission to directly overfly the
large mascons of Serenitatis and Imbrium, so there wasn't a lot of
experience in how those mascons would affect a descent orbit. However, I
will point out that Apollo 17, which overflew the same mascons, was put
into a descent orbit with an 80,000-foot pericynthion. This was because
the landing site was farther east, and if there had been an overburn on
the DOI-1 burn, there might not have been time to track and establish the
correct bail-out burn parameters before impact. But I think a small part
of the reason for the 80,000-foot PC was that the mission planners
remembered the degradation of 15's descent orbit and wanted to give the
stack a little more leeway, just in case it happened again.

--

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