Encyclopedia of Lunar Science

Living Edition
| Editors: Brian Cudnik

GRAIL Mission

  • Alexander J. EvansEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-319-05546-6_120-1


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.

The two primary science objectives of GRAIL were to (1) determine the structure of the lunar interior, from crust to core, and (2) advance understanding of the thermal evolution of the Moon (Zuber et al. 2013a). These primary science objectives led to the following primary science investigations of GRAIL, to be conducted with the GRAIL-derived gravitational data:
  1. 1.

    Map the structure of the crust and lithosphere.

  2. 2.

    Understand the Moon’s asymmetric thermal evolution.

  3. 3.

    Determine the subsurface structure of impact basins and the origin of mascons (mass concentrations) associated with impact basins.

  4. 4.

    Ascertain the temporal evolution of the crustal brecciation and magmatism.

  5. 5.

    Constrain the deep interior structure from tides.

  6. 6.

    Place limits on the size of a possible solid inner core.

Completion of the above investigations required that GRAIL characterize the gravitational field of the Moon at various spatial scales, ranging from local to global. The above investigations were used to define the accuracy required of the GRAIL-derived gravitational field (Zuber et al. 2013a).
For the extended mission of GRAIL, the scientific objective was to determine the structure of the lunar highland crust and basaltic plains (maria), by addressing the impact, magmatic, tectonic, and volatile processes that shaped the lunar surface (Zuber et al. 2013a). The scientific objective of the extended mission led to the following scientific investigations focusing on:
  1. 1.

    Structure of impact craters;

  2. 2.

    Near-surface magmatism;

  3. 3.

    Mechanics and timing of deformation;

  4. 4.

    Causes(s) of crustal magnetization;

  5. 5.

    Estimation of upper-crustal density;

  6. 6.

    Mass bounds on polar volatiles.



As a result of the designed operational lifetime (less than 1 year) of GRAIL, the two orbiters associated with the GRAIL mission, GRAIL-A (Ebb) and GRAIL-B (Flow), were designed to be nearly identical and single string, with few exceptions, to reduce complexity and cost (Zuber et al. 2013a). High heritage for the GRAIL mission was achieved through employment of the Experimental Small Satellite-11 (XSS-11) spacecraft structure provided via contract by Lockheed Martin Space Systems Corporation (LMSSC) as well as a flight system derived from the Mars Reconnaissance Orbiter (MRO) (Taylor et al. 2013). Each spacecraft was equipped with one scientific payload, the Lunar Gravity and Ranging System (LGRS), and one education/public outreach payload, the Moon Knowledge Acquired by Middle-School Students (MoonKAM) (NASA 2011). The GRAIL orbiters generated power via a non-articulated set of four panels of silicon solar cells mounted on the top and side exterior surfaces of the spacecraft. Each GRAIL orbiter was also equipped with a single Li-ion battery (30 amp-hours) to be utilized when insufficient power was available from the solar panels (Zuber et al. 2013a). Parameters for the mass, size, and power generation capability of each orbiter are listed in Table 1.
Table 1

Key parameters and values for GRAIL spacecraft (GRAIL-A and GRAIL-B) (NASA 2011)



Orbiter wet mass (incl. fuel)

307 kg

Orbiter dry mass

201 kg

Orbiter fuel mass

106 kg

Solar power (at 1 AU)

763 W

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

On September 10, 2011, the twin spacecraft Ebb and Flow were launched from Cape Canaveral, FL, USA, onboard the United Launch Alliance Delta II 7920H-10C launch vehicle. A photograph of the launch is shown in Fig. 1. A 3.5-month, low-energy transit from Earth to the Moon was chosen to maximize the number of possible launch windows, minimize launch mass, and allow ample time for spacecraft outgassing and performance testing prior to the science mission commencement (Zuber et al. 2013a). Ebb and Flow achieved lunar orbit on December 31, 2011, and January 1, 2012, respectively (Chung et al. 2010; Roncoli and Fujii 2010; Hatch et al. 2010; Zuber et al. 2013a). After lunar orbit insertion, the orbits of each spacecraft were adjusted to establish the altitude and separation between the spacecraft required for data acquisition (Roncoli and Fujii 2010; Hatch et al. 2010). An illustration of the GRAIL spacecraft in lunar orbit is shown in Fig. 2.
Fig. 1

Illustration of GRAIL spacecraft in formation at the Moon. LGRS and telemetry signals are identified. Image Credit: NASA

Fig. 2

Image of the GRAIL launch on board the United Launch Alliance Delta II 7920H-10C. Image credit: NASA

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

The derivation of the gravitational field from LGRS data was conducted independently at NASA Goddard Space Flight Center (GSFC) and NASA JPL (e.g., Lemoine et al. 2013; Konopliv et al. 2014). Derivations by both organizations were made available by the GRAIL mission on the Planetary Data System (PDS) Geosciences node (http://pds-geosciences.wustl.edu/missions/grail). The GRAIL mission produced lunar gravitational maps with spatial blocksizes as low as 3.6 km. The two main derived data products that were produced by the GRAIL mission were the free-air gravity anomaly and Bouguer anomaly maps. Examples of the maps are shown in Fig. 3. The lunar free-air gravity anomaly map shows the variations of the lunar gravitational field about the average lunar gravitational acceleration, as measured on the surface of the Moon. The Bouguer anomaly maps incorporate a correction for topography, such that the variations in the lunar gravitational field due to subsurface mass perturbations are highlighted.
Fig. 3

Maps of the Bouguer anomaly (upper panel) and free-air gravity anomaly (bottom panel) in units of mGal (1 Gal = 10−2 ms−2)

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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Copyright information

© Springer International Publishing AG 2017

Authors and Affiliations

  1. 1.Lunar and Planetary LaboratoryUniversity of ArizonaTucsonUSA