Revealing the Moon’s Structure and Thermal History
Overview
Gravity Recovery and Interior Laboratory (GRAIL) is a NASA Discovery-class mission that will unlock the mysteries of the Moon. By globally mapping the lunar gravitational field to unprecedented accuracy and resolution, the mission will peer deep inside the Moon to reveal its internal structure and thermal history.
Objectives
- 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 will place twin spacecraft (GRAIL-A and GRAIL-B) 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 provides 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.
Spacecraft
The GRAIL flight system includes two nearly-identical Lockheed Martin-built spacecraft whose designs are 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 is a rectangular composite structure and includes two fixed solar arrays. The propulsion system is a warm gas system that creates a precise, lower-energy thrust when used for maneuvers.
During the short four month cruise to the Moon and the three month science phase, the GRAIL twins will be flown from Lockheed Martin’s Mission Support Area near Denver, Colo. Even thought the orbiters fly in a precise formation, each spacecraft is operated as a complete separate system.
Science Instruments
The primary payload of the GRAIL mission is the Lunar Gravity Ranging System (LGRS). LGRS is 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 is being developed by the Jet Propulsion Laboratory and is derived from the highly-successful GRACE mission.
Timeline
GRAIL’s two spacecraft will launch side by side on a Delta II Heavy rocket in September 2011, from Cape Canaveral, Fla. The two spacecraft will take a four month, low-energy trajectory to the Moon and enter orbit on Jan. 1, 2012. During the 82-day science phase, the instruments will make their precise measurements as the Moon rotates three complete times beneath the GRAIL orbit.
Education and Public Outreach
MoonKAM is a four-camera digital video imaging system that is used as part of the education and public outreach activities for the mission. The MoonKAM system is operated by undergraduate students at the University of California at San Diego in coordination with Sally Ride Science.
