I was always under the impression that gravity on all objects in space was consistent. Now it turns out that the gravity pull on the moon varies
depending on where you stand.
 The source of the gravitational quirkiness is a number of huge mascons (short for "mass concentrations") buried under the surfaces of lunar
maria or "seas." Formed by colossal asteroid impacts billions of years ago, mascons make the moon the most gravitationally lumpy major body in the
solar system. The anomaly is so great—half a percent—that it actually would be measurable to astronauts on the lunar surface. A plumb bob held at
the edge of a mascon would hang about a third of a degree off vertical, pointing toward the central mass. Moreover, an astronaut in full spacesuit and
life-support gear whose lunar weight was exactly 50 pounds at the edge of the mascon would weigh 50 pounds and 4 ounces when standing in the mascon's
center.
NASA are now mapping the moons gravity anomilies with GRAIL -Gravity Recovery and Interior Laboratory.
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Mascons exist on Earth as well as on the Moon, and probably on all the other rocky planets too. I'm not sure about gas giants.
Back in the Sixties, the possibilty of lunar mascons perturbing the circumlunar orbits of the Apollo missions was discussed. I don't think there was
any problem arising from them in the end, though.
[edit on 30-5-2008 by Astyanax]
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Actually, Astyanax, even over a period of months low orbits to the Moon can be drastically altered by the mascons if these alterations go
uncorrected. The only reason the mascons posed no problem to the Apollo programs is due to NASA's Lunar Orbiter series of probes that were sent out
to test the navigation systems planned for use in Apollo (as well as map the Lunar surface to a resolution of 60 meters or better.)
When the Lunar Orbiter probes were testing the predictions for landing they found margins of error of upwards of 100 times what was predicted. Because
of NASA's expedient calculations and teams of scientists working on this they came up with a navigational fix.
As for the article in hand, GRAIL is definitely going to be an immense asset for any future manned missions to the Moon. It'll allow future teams to
land on the Lunar surface with greater precision and safety.
If I recall correctly, and I've been out of the game for awhile here, GRAIL is based of of GRACE, which mapped out mascons here on Earth.
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