In article om,
John Halpenny wrote:
Beware, however, that all such work is based on current maps of the Moon's
gravity... and there are sizable uncertainties in the farside parts...

Would it be possible to map the farside gravity with something like
the GRACE mission? - two satellites in formation tracking each other.

Yes. In fact, one of the short-list candidates for the next Discovery
mission (the final selection is due soon, I think) is GRAIL, which is more
or less GRACE in lunar orbit.

Mind you, if picked, GRAIL is going to cost nearly half a billion. On a
much smaller scale, both physically and financially :-), there's Lunette:
a 6-kg nanosatellite with an estimated cost under $5M that would ride to
the Moon on a larger orbiter, and then separate and fly formation with it.
Lunette wouldn't be as good as GRAIL -- it would basically just give
reliable global maps with about the same precision as the best current
nearside maps (from Lunar Prospector) -- but it would still eliminate the
farside gap, and for two orders of magnitude less money. Now, if only we
could find two orders of magnitude less money. :-( We're trying...

And Japan's SELENE, which is supposed to fly... well, sometime... (it's
already several years behind schedule) will try to do some farside gravity
mapping, but it's a multi-instrument orbiter and neither the spacecraft
nor the mission plan are ideal for gravity work, and they'll have a really
horrendous data-analysis problem. They do, at least, have money.

It sounds like this is information that is sorely needed.

Yeah, it would be really useful, both for mission planning and for lunar
geoscience. There have been *proposals* to do something about it for a
long time. (Lunette's Principal Investigator, in his younger days, was
part of a proposal to deploy a pair of small satellites from Apollo 18,
for farside gravity mapping...) But nothing has ever actually flown.
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