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The Gravity Recovery and Interior Laboratory (GRAIL) mission comprises a pair of satellites launched in September, 2011 and placed in orbit around the Moon in January, 2012. The two satellites, named Ebb and Flow, used radio signals to precisely measure their separation as they flew in formation, one following the other in the same nearly circular polar orbit. These measurements allowed mission scientists to build up an accurate and detailed gravity map of the Moon.GRAIL ends its successful mission by impacting the Moon on December 17, 2012 at approximately 5:27 p.m. EST (22:27 UT). The two spacecraft were placed in an orbit that takes them within a kilometer of the surface, so low that they will hit the side of an unnamed mountain that lies between Mouchez and Philolaus craters, near the north pole at 7511 s last orbits across the night side of a waxing crescent Moon. On their penultimate orbit, Ebb and Flow just miss the mountain they will hit on their next pass. A more distant view of the impact site, just seconds before the impact of Ebb.

The Gravity Recovery and Interior Laboratory (GRAIL) mission comprises a pair of satellites launched in September, 2011 and placed in orbit around the Moon in January, 2012. The two satellites, named Ebb and Flow, used radio signals to precisely measure their separation as they flew in formation, one following the other in the same nearly circular polar orbit. These measurements allowed mission scientists to build up an accurate and detailed gravity map of the Moon.If the Moon were a perfectly smooth sphere of uniform density, the gravity experienced by the spacecraft would be exactly the same everywhere. But like other rocky bodies in the solar system, including the Earth, the Moon has both a bumpy surface and a lumpy interior. As the spacecraft fly in their orbits, they experience slight variations in gravity caused by both of these irregularities, variations which show up as small changes in the separation of the two spacecraft.The free-air gravity map shows these variations directly. (Free-air is a historical term; there is, of course, no air on the Moon.) The Bouguer gravity map subtracts the effect of the bumpy surface to show the lumpiness underneath. The elevation maps from the laser altimeter on Lunar Reconnaissance Orbiter (LRO) were used to create a model of what the gravity would be if the Moon were bumpy but not lumpy. This model was then subtracted from the free-air map to produce the Bouguer map. (Note: The Bouguer map shown here was filtered to emphasize smaller features; harmonic degrees 1 to 6 were excluded.)The crustal thickness map is inferred from the Bouguer map: If the density of the crust is assumed to be uniform, then the gravity anomalies visible in the Bouguer gravity map can be explained by variations in the thickness of the crust. Highs in gravity indicate places where the denser mantle is closer to the surface, and hence where the crust is thinner.While aiding navigation for future lunar missions, GRAIL s gravity measurements reveal information about the internal structure of the Moon, improving our understanding of the origin and development of not just the Moon, but also the Earth and the rest of the inner solar system. Side-by-side rotating Moon globes showing LOLA elevation and GRAIL free-air gravity.This video is also available on our YouTube channel. Side-by-side rotating Moon globes showing Bouguer gravity and crustal thickness. The Bouguer map is filtered to emphasize smaller features. Rotating Moon globe showing free-air gravity as measured by GRAIL. Rotating Moon globe showing Bouguer gravity. The data is high-pass filtered to emphasize smaller features. Rotating Moon globe showing crustal thickness. Rotating Moon globe showing topography as measured by LOLA on LRO. Print-res still of the free-air lunar gravity globe, centered on 120 degrees west longitude. Includes an alpha mask. Print-res still of the Bouguer lunary gravity globe, centered on 120 degrees west longitude. The data is high-pass filtered to emphasize smaller features. Includes an alpha mask. Print-res still of the lunar crustal thickness globe, centered on 120 degrees west longitude. Includes an alpha mask. Print-res still of the LOLA lunar topography globe, centered on 120 degrees west longitude. Includes an alpha mask. Moon free-air gravity maps in a cylindrical projection suitable for spherical texture mapping. Centered at 0 longitude. Includes shaded relief of surface features. Versions of the map are provided which either include or omit lower harmonic degrees corresponding to large features in the gravity field. Moon Bouguer gravity maps in a cylindrical projection suitable for spherical texture mapping. Centered at 0 longitude. Includes shaded relief of surface features. Versions of the map are provided which either include or omit lower harmonic degrees corresponding to large features in the gravity field. Moon LOLA elevation map in a cylindrical projection suitable for spherical texture mapping. Centered at 0 longitude. Includes shaded relief of surface features. Moon crustal thickness map in a cylindrical projection suitable for spherical texture mapping. Centered at 0 longitude. Includes shaded relief of surface features. For More InformationSee [http://www.nasa.gov/home/hqnews/2012/dec/HQ_12-417_GRAIL.html](http://www.nasa.gov/home/hqnews/2012/dec/HQ_12-417_GRAIL.html)