Revealing the Moon’s Structure and Thermal History
Gravity Recovery and Interior Laboratory (GRAIL) was a NASA Discovery-class mission that worked to unlock the mysteries of the Moon. By globally mapping the lunar gravitational field to unprecedented accuracy and resolution, the mission peered deep inside the Moon to reveal its internal structure and thermal history.
- Determine the structure of the lunar interior from crust to core
- Advance understanding of thermal evolution of the Moon
- Extend knowledge gained from the Moon to other terrestrial planets
GRAIL placed twin spacecraft, Ebb and Flow, in a low-altitude (~34 miles/55 km), near-circular, polar lunar orbit to perform high-precision range-rate measurements between them using a Ka-band instrument. Subsequent analysis of the spacecraft-to-spacecraft range-rate data provided a direct measure of the lunar gravity.
GRAIL is managed for NASA by the Jet Propulsion Laboratory. The Principal Investigator is Dr. Maria Zuber of Massachusetts Institute of Technology.
The GRAIL flight system included two nearly-identical Lockheed Martin-built spacecraft whose designs were partially based on two successful missions. The spacecraft bus structure and propulsion system have heritage to the Experimental Small Satellite-11 (XSS-11). The orbiter’s flight avionics and software are based on the Mars Reconnaissance Orbiter (MRO), but uses a single-string fault protection variation.
The bus was a rectangular composite structure and included two fixed solar arrays. The propulsion system was a warm gas system that creates a precise, lower-energy thrust when used for maneuvers.
During the cruise to the Moon, science phases, and the lunar impact, the GRAIL twins were flown from Lockheed Martin’s Mission Support Area near Denver, Colo. Even thought the orbiters orbited in a precise formation, each spacecraft was operated as a complete separate system.
The primary payload of the GRAIL mission was the Lunar Gravity Ranging System (LGRS). LGRS was responsible for sending and receiving the signals needed to accurately and precisely measure the micron-level changes in distance between the two orbiters. The LGRS was developed by the Jet Propulsion Laboratory and was derived from the highly-successful GRACE mission.
The primary mission yielded the highest-resolution gravity field map of any celestial body. Future gravity field models developed from data collected during the extended mission will be of even higher resolution. The map will provide a better understanding of how the moon, Earth and other terrestrial planets in the solar system formed and evolved.
GRAIL’s two spacecraft launched side by side on a Delta II Heavy rocket on Sept. 10 2011, from Cape Canaveral, Fla. The two spacecraft took a four month, low-energy trajectory to the Moon and entered orbit on Dec. 31, 2011 and Jan. 1, 2012.
Following a successful primary and secondary science mission of mapping the gravity of the moon, the washing machine-sized spacecraft were nearly out of fuel. On Dec. 17, 2012, JPL and Lockheed Martin worked together to send both spacecraft to the surface in a controlled manner at a known location.
Education and Public Outreach
MoonKAM was a four-camera digital video imaging system that was used as part of the education and public outreach activities for the mission. The MoonKAM system was operated by undergraduate students at the University of California at San Diego in coordination with Sally Ride Science. NASA named the site where spacecraft impacted the moon in honor of the late astronaut Sally Ride, who was America's first woman in space and a member of the probes' mission team.