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The Gravity Recovery and Interior Laboratory (GRAIL) was a lunar gravity mapping mission led by the Massachusetts Institute of Technology (MIT) and managed by the National Aeronautics and Space Administration (NASA) Jet Propulsion Laboratory (JPL). GRAIL was the eleventh mission of NASA’s Solar System Exploration Discovery Program. The GRAIL mission flew high-precision instrumentation onboard twin spacecraft, dubbed Ebb and Flow, which orbited the Moon in tandem to measure and map variations in the lunar gravitational field. The data collected by the GRAIL spacecraft were used to construct high-resolution lunar gravitational maps, the highest resolution of any celestial body, including Earth. The high precision and accuracy of the gravitational measurements allowed the GRAIL mission to make fundamental contributions to understanding the internal structure and thermal evolution of the Moon.
The GRAIL spacecraft were operational at the Moon between December 31, 2011, and December 17, 2012. The GRAIL mission, inclusive of science data analysis, concluded on September 30, 2016.
Selection and Budget
The initial proposal for the GRAIL mission was submitted in response to the request for Step 1 proposals in the NASA Discovery Program Announcement of Opportunity issued on January 3, 2006. NASA selected GRAIL and two additional proposals out of the 25 total submitted proposals to continue mission concept development. On December 21, 2007, GRAIL was selected and approved as the 11th mission of NASA’s Solar System Exploration Discovery Program. The GRAIL mission was completed on schedule and under budget, with a total mission cost of under $500M (GAO 2012).
Scientific Objectives and Investigations
Primary science data acquisition for the GRAIL mission was completed on May 29, 2012 (Zuber et al. 2013a). Science data acquisition for the NASA-approved extended mission (XM) occurred between August 30, 2012, and December 14, 2012.
Map the structure of the crust and lithosphere.
Understand the Moon’s asymmetric thermal evolution.
Determine the subsurface structure of impact basins and the origin of mascons (mass concentrations) associated with impact basins.
Ascertain the temporal evolution of the crustal brecciation and magmatism.
Constrain the deep interior structure from tides.
Place limits on the size of a possible solid inner core.
Structure of impact craters;
Mechanics and timing of deformation;
Causes(s) of crustal magnetization;
Estimation of upper-crustal density;
Mass bounds on polar volatiles.
Key parameters and values for GRAIL spacecraft (GRAIL-A and GRAIL-B) (NASA 2011)
Orbiter wet mass (incl. fuel)
Orbiter dry mass
Orbiter fuel mass
Solar power (at 1 AU)
Orbiter size (main structure)
1.09 m × 0.95 m × 0.76 m
Lunar Gravity and Ranging System (LGRS)
The LGRS instrument onboard each orbiter emitted Ka-band (~32 GHz), X-band (8.4 GHz), and S-band signals (~2.3 GHz) (Asmar et al. 2013; Klipstein et al. 2013). In spite of the similar concept between the dual-orbiter GRAIL and Gravity Recovery and Climate Experiment (GRACE) missions, the LGRS instrument required significant departures from the ranging system on the Earth-orbiting GRACE mission that relied on Earth’s global positioning system (GPS) for time synchronization, which was not available at the Moon. To permit for time synchronization without GPS, onboard each orbiter LGRS, a one-way beacon to Earth (X-band) and a second one-way inter-spacecraft ranging system (S-band) were added to measure the time offset between each LGRS (Klipstein et al. 2013). The Ka-band inter-spacecraft ranging was used to measure the relative motion between each spacecraft. The combination of the three instrument signals allowed for the determination of the position, velocity, and acceleration of each orbiter (Asmar et al. 2013). After correcting for spacecraft accelerations due to nongravitational forces, the gravitational field of the Moon was determined (Asmar et al. 2013).
Moon Knowledge Acquired by Middle-School Students (MoonKAM)
Each GRAIL orbiter was equipped with a MoonKAM instrument comprised of a digital video controller and four camera assemblies provided by Ecliptic Enterprises Corporation (NASA 2011). The MoonKAM instrument was capable of taking images or videos of the lunar surface at up to 30 frames per second (NASA 2011). The MoonKAM instruments were dedicated exclusively to education and public outreach (Zuber et al. 2013a).
Launch and Lunar Transit
Science Phase and Termination
Data acquisition for the GRAIL primary science phase began on March 1, 2012, and concluded on May 29, 2012. During the 82-day primary science mission, three mapping cycles of the Moon were conducted in 27.3-day cycles (Zuber et al. 2013a). The average altitude of the orbiters during the primary science phase was 55 km. Science data collection was not possible between June 2012 and August 2012 due to low solar illumination on the solar panels of the spacecraft while in science formation. The science data acquisition for the extended mission occurred between August 30, 2012, and December 14, 2012, at an average altitude of 23 km. The GRAIL mission collected 99.99% of the total available science data acquired during the primary and extended missions.
The GRAIL spacecraft were terminated with controlled impacts into the lunar surface on December 17, 2012.
Science Data Products
Education and Public Outreach
Education and public outreach of the GRAIL mission were primarily achieved via its GRAIL MoonKAM program led by Sally Ride Science. The program was directed toward middle-school and undergraduate students. Students sent image requests for target areas of the lunar surface to an undergraduate-run GRAIL MoonKAM mission operations center. At the mission operations center, the MoonKAM instrument onboard Ebb and Flow were used to capture target images that were then sent to the requesting student. The captured target images were used by students to study the lunar surface. More than 2,000 schools participated in the program.
Additionally, public outreach occurred in other aspects of the mission, including through the GRAIL spacecraft naming contest allowing students to submit essays proposing names for the GRAIL-A and GRAIL-B spacecraft. The contest was won by fourth grade students in Bozeman, Montana, resulting in the GRAIL-A and GRAIL-B spacecraft being named Ebb and Flow, respectively.
The author thanks Maria T. Zuber for providing GRAIL mission information and data that were used in the preparation of this document.
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- Zuber MT et al (2013b) Gravity field of the Moon from the gravity recovery and interior laboratory (GRAIL) mission. Science 339:668–671. doi: 10.1126/science.1231507