The Gravity Recovery And Interior Laboratory (GRAIL) will unlock the mysteries of the Moon. By mapping the lunar gravitational field globally not just on the nearside to unprecedented accuracy and resolution, GRAIL will, in essence, peer deep inside the Moon (see right) to reveal its internal structure and thermal history.
Knowledge acquired about the Moon from GRAIL will be extended to understand the broader evolutionary histories of the other rocky planets in the inner solar system: Earth, Venus, Mars, and Mercury. Indeed, the Moon is a lynchpin for understanding how the terrestrial planets evolved.
GRAIL is the lunar analog of the very successful Gravity Recovery and Climate Experiment (GRACE) twin-spacecraft terrestrial gravity recovery mission that was launched in 2002 and continues to operate. GRAIL will be implemented with a science payload simplified from GRACE and a spacecraft derived from the Lockheed Martin Experimental Small Satellite-11 (XSS-11) launched in 2005.
GRAIL will place two spacecraft (represented as GRAIL-A and GRAIL-B to the left) in a low-altitude (50 km), near-circular, polar lunar orbit to perform high-precision range-rate measurements between them using a Ka-band payload. Subsequent data analysis of the spacecraft-to-spacecraft range-rate data provides a direct measure of the lunar gravity.
The payload, flight system and mission design ensure that all error sources
that perturb the gravity measurements are contained at levels well below
those necessary to meet science requirements. The figure below illustrates
performance margin between the science requirements (red and green), the
allocated performance (black), and Current Best Estimate (CBE) performance
(gold). These margins enable GRAIL's low-risk implementation.