The Gravity Recovery and Interior Laboratory (GRAIL) was an American lunar science mission in NASA's Discovery Program which used high-quality gravitational field mapping of the Moon to determine its interior structure. The two small spacecraft GRAIL A (Ebb) and GRAIL B (Flow)^[5][6] were launched on 10 September 2011 aboard a single launch vehicle: the most-powerful configuration of a Delta II, the 7920H-10.^[1][7][8] GRAIL A separated from the rocket about nine minutes after launch, GRAIL B followed about eight minutes later. They arrived at their orbits around the Moon 25 hours apart.^[9][10] The first probe entered orbit on 31 December 2011 and the second followed on 1 January 2012.^[11] The two spacecraft impacted the Lunar surface on December 17, 2012.^[12]

Maria Zuber of the Massachusetts Institute of Technology was GRAIL's principal investigator. NASA's Jet Propulsion Laboratory managed the project. NASA budgeted US$496 million for the program to include spacecraft and instrument development, launch, mission operations, and science support.^[13] Upon launch the spacecraft were named GRAIL A and GRAIL B and a contest was opened to school children to select names. Nearly 900 classrooms from 45 states, Puerto Rico and the District of Columbia, participated in the contest. The winning names, Ebb and Flow, were suggested by 4th grade students at Emily Dickinson Elementary School in Bozeman, Montana.^[6]

Each spacecraft transmitted and received telemetry from the other spacecraft and Earth-based facilities. By measuring the change in distance between the two spacecraft, the gravity field and geological structure of the Moon was obtained. The two spacecraft were able to detect very small changes in the distance between one another. Changes in distance as small as one micrometre were detectable and measurable.^[14][15] The gravitational field of the Moon was mapped in unprecedented detail.^{[4][16][17][18][19]}

• Map the structure of the lunar crust and lithosphere
• Understand the asymmetric thermal evolution of the Moon
• Determine the subsurface structure of impact basins and the origin of lunar mascons
• Ascertain the temporal evolution of crustal brecciation and magmatism
• Constrain the deep interior structure of the Moon
• Place limits on the size of the Moon's inner core

The data collection phase of the mission lasted from 7 March 2012 to 29 May 2012, for a total of 88 days. A second phase, at a lower altitude, of data collection began 31 August 2012,^[20] and was followed by 12 months of data analysis.^[4] On 5 December 2012 NASA released a gravity map of the Moon made from GRAIL data.^[21] The knowledge acquired will aid understanding of the evolutionary history of the terrestrial planets and computations of lunar orbits.^[22]

Gravity passes through matter. In addition to surface mass, a high-resolution gravity field gives a blurred, but useful, look below the surface. Analyses of the GRAIL data have produced a series of scientific results for the Moon.

• The resolution of the gravity field has improved by a large amount over pre-GRAIL results. Early analyses gave the Gravitation of the Moon with fields of degree and order 420 and 660.^[43][16][17] Subsequent analyses have resulted in higher degree and order fields.^[18][19] Maps of the gravity field were made.
• The crustal density and porosity were determined.^[44] The crust was fragmented by large ancient impacts.
• Long narrow linear features were found that are interpreted to be ancient tabular intrusions or dikes formed by magma.^[45]
• Combining gravity and Lunar Laser Ranging data gives the 3 principal moments of inertia.^[46] The moments indicate that a dense core is small.
• Combining gravity and lunar Topography, 74 circular impact basins were identified.^[47] Strong increases in gravity that are associated with circular impact basins are mascons discovered by Muller and Sjogren.^[48] The strongest gravity anomalies are from basins filled with dense mare material, but the strong gravity also requires that the boundary between the crust and denser mantle be warped upward. Where the crust is thicker, there may be no mare fill but the crust-mantle boundary is still warped upward.
• The radius, density, and rigidity of interior layers is inferred.^[49]
• The Orientale basin is the youngest and best-preserved impact basin on the Moon.^[50] The gravity field of this 3-ring basin was mapped at high resolution.

• List of artificial objects on the Moon
• Satellite gravimetry
• Selenoid