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Historical tracking data from the Lunar Orbiter and Apollo missions in the mid-1960s to early 1970s, as well as more recent data from the 1994 Clementine mission, yielded a generally low-resolution (before) representation of lunar gravity fields. Lunar Prospector's Doppler Gravity Experiment (DGE) has provided the first polar low altitude measurement of the lunar gravity field. This provided the spacecraft with the first truly operational gravity map of the Moon and immediately improved orbit and fuel efficiency. Improved gravity information will not only help scientists build better models of the role of impact processes on the history and evolution of the Moon, but will also help in estimating the lunar core size and metallic iron content. A more practical benefit of the new lunar gravity data provided by Prospector's DGE experiment is that a more precise gravity map of the Moon will inevitably aid future mission planners in planning fuel-efficient journeys to the Moon, and may even help identify potential resources.

Shown (in color) are contours of the lunar gravity field, represented by changes in acceleration. Negative values do not indicate anti-gravity, rather decreases in what's called the "central term" (average surface gravity) of 160,000 mgal, or what the lunar gravity field would be if the Moon were perfectly spherical with no density changes across its surface.

Lunar gravity data graphic

Recent measurements have revealed three new mascons ("mass concentrations") on the near side of the Moon coincident with the large impact basins Mare Humboltianum, Mendel-Ryber, and Schiller-Zucchius. Furthermore, although there is no direct measurement of the lunar farside gravity, LP data indicate four additional new mascons in the large farside basins of Hertzsprung, Coulomb-Sarton, Freundlich-Sharonov, and Mare Moscoviense, and clearly show a central area of increased gravity in these basin centers.



The newest gravity map based on Lunar Prospector data



This image shows the gravity field at the Mare Serenitatis (Sea of Tranquility). The upper segment represents the topography- a fairly flat low region-and the lower segment shows the corresponding strong gravity field. This is a mascon.



This diagram shows the three new mascons identified on the near side of the moon. The top segment for each location shows topography. The segment beneath shows the gravity field which, surprisingly, is particularly strong in the center of each area where the topography is low. The Mare Humboldtianum location includes a third segment below the gravity field as identified by Lunar Prospector. This third segment shows the gravity field as measured before Prospector.



Lunar Prospector's high quality gravity data, improved by roughly a factor of five over previous estimates, indicate the existence of a lunar core, probably iron, with a radius of more than 300 km.

Such improvements to the lunar gravity field also offer the practical benefits of modeling long-term spacecraft orbits about the Moon, which allows more accurate planning of future mission fuel needs and enables the development of fuel efficient orbital maintenance strategies. LP engineers are currently relying on the improved lunar gravity model in devising strategies for maintaining LP's extremely low orbit during the extended mission phase.

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