On Sat, 10 Nov 2007 03:00:58 GMT, Brian Thorn
wrote:

Lunar orbits are unstable over long-ish durations. They crashed, some
deliberately... to provide seismic data.

Thanks to the wikipedia links 2 others provided, it appears Apollo 11
and 16 LM were left in lunar orbit and eventually crashed into the
moon. My questions:
1. forgetting mascons for the moment, wouldn't the effects of the
earth and sun on a lunar orbit tend, over time, to pull the LM into a
higher orbit around the moon?
2. considering mascons as well as outside gravity effects, wouldn't
the effect of these be only to alter the orbit, but not cause a crash
onto the moon? My limited background suggest to me that the LM would
have to lose a lot of energy in order to lower it's orbit enough to
crash. What would be the process that causes this?
Thanks to all for the help. Stan