BACKGROUND

GRAIL is the lunar counterpart of the very successful Gravity Recovery and Climate Experiment, or GRACE, twin satellites that launched in 2002 to make detailed measurements of Earth’s gravity field. Major breakthroughs from GRACE include establishing the rapid melting of ice mass in recent years in Greenland and Antarctica and major changes in water storage in China's Yangtze River and other water sources—sometimes as a result of human use.

The knowledge from GRACE should lead to a better understanding of the forces that drive El Niño and La Niña, more accurate seasonal forecasts of Earth's weather patterns, an ability to track the changing distribution of water resources in critically important land aquifers, and improved forecasting of natural hazards.

MISSION OBJECTIVES

GRAIL has the following two primary science objectives:

• Determine the structure of the lunar interior, from crust to core
• Advance understanding of the thermal evolution of the Moon

Additional mission objectives include:

• Extend knowledge gained on the internal structure and thermal evolution of the Moon to other terrestrial planets
• Reduce risk to future lunar robotic or human science and exploration missions by providing a high resolution, global gravity field that will eliminate gravity uncertainties for precision lunar navigation and landings

Knowledge acquired about the Moon from GRAIL will be extended to understand the broader evolutionary histories of the rocky planets in the inner solar system: Earth, Venus, Mars, and Mercury. The Moon is key to understanding how the terrestrial planets evolved because it is the most accessible planetary body that preserves a surface record spanning most of solar system history.

A view of the Moon taken by the Galileo spacecraft in 1992.

Reconstructing planetary evolution also requires an understanding of the structure of the planetary interior. Despite mapping from numerous orbital platforms, it has not been possible to develop a holistic understanding of the interior structure of the Moon due to the lack of reliable far side gravity data.

APPROACH

GRAIL will carry a science payload derived from GRACE and a spacecraft derived from the Lockheed Martin Experimental Small Satellite-11 (XSS-11) launched in 2005. GRAIL will use JPL’s Lunar Gravity Ranging System instrument to map lunar gravity variations by measuring the change in distance between the spacecraft as they orbit the moon.

Planned for launch in 2011, the twin GRAIL spacecraft will fly in a low-altitude, near-circular, polar lunar orbit to perform high-precision range-rate measurements between them using a Ka-band payload. Analysis of the spacecraft-to-spacecraft range-rate data will provide a direct measure of the lunar gravity.

MISSION MANAGEMENT

The GRAIL mission is managed for NASA by the Jet Propulsion Laboratory, Pasadena, CA. The Principal Investigator is Dr. Maria T. Zuber of the Massachusetts Institute of Technology, Cambridge, MA. The two GRAIL spacecraft will be provided by Lockheed Martin Space Systems, Denver, CO.

For more complete information on the mission, visit the GRAIL website.