The GRAIL Mission: A Fact Sheet

	GRAIL stands for Gravity Recovery and Interior Laboratory. Mission planners will send twin satellites into orbit around the Moon to map its quirky gravitational field in unprecedented detail. By revealing the Moon's inner structure, GRAIL will shed light on how the Moon - and, by extension, the Earth and other rocky planets - formed. photo: NASA

GRAIL is a NASA mission. The leading scientific investigator is Maria Zuber, a geophysics professor at the Massachusetts Institute of Technology (MIT). NASA's Jet Propulsion Laboratory (JPL) in Pasadena will manage the mission and develop the scientific instruments, and Lockheed Martin Space Systems of Denver will build and operate the spacecraft. Educational and public outreach for the mission is led by Dr. Sally Ride, the former astronaut, through Sally Ride Science, her science content company. Maria Zuber of MIT is the leading scientific investigator on the GRAIL mission. Bill Ingalls / NASA

The two satellites, called GRAIL-A and GRAIL-B, are scheduled to be launched aboard a robotic Delta II rocket in September 2011. The rocket will follow an indirect route that will allow it to save fuel by taking advantage of the Earth's and Moon's gravity. When the rocket nears the Moon in January of 2012, the two satellites will be boosted into orbit. They will send back information during a three-month "science phase" of the mission from March through May of 2012. After their work is done, the satellites eventually will crash into the Moon's surface. (For a satellite diagram, visit the GRAIL web site, http://moon.mit.edu/spacecraft.html) The GRAIL satellites will be carried to the Moon by a Delta II rocket. NASA

The twin satellites will fly in tandem orbit, one following the other, around the Moon. As they travel, they will exchange microwave beams to measure the precise distance between the orbiters. As one satellite passes through an area of greater or weaker gravity, the distance between the two will expand or shrink slightly, and these measurements will be beamed to Earth. Researchers will use the fluctuations to map the Moon's gravitational field with a degree of precision never before achieved. The twin satellites will exchange microwave beams to measure the distance between them. NASA / JPL

The satellites will orbit at an altitude of 50 kilometers, or about 30 miles. The distance between them will fluctuate from 175 to 225 kilometers, or about 110 to 140 miles.

Yes, GRAIL is a lunar version of GRACE, an ongoing mission in which twin satellites orbiting the Earth send back data that allow researchers to monitor the planet's changing gravitational field. Some of the scientists involved in GRACE, which began in 2002, are taking a role in GRAIL. GRAIL will chart the Moon's gravity at a higher resolution than GRACE can achieve on Earth. That's because the Moon's lack of an atmosphere allows the GRAIL satellites to orbit at a much lower altitude. The two GRACE satellites continually map Earth's gravity field. University of Texas Center for Space Research and NASA

Scientists know the Moon has an extremely uneven field of gravity. In fact, the Moon is the most gravitationally "lumpy" major body in the solar system. This unevenness is caused by huge "mascons" (mass concentrations) under the lunar surface that are thought to have been created by asteroid impacts long ago. The mascons tug on spacecraft, so that unless course corrections are made, orbiters eventually crash into the Moon. In this map created by the Lunar Prospector mission, mascons, the areas of high density that give the Moon its "lumpy" gravitational field, are shown in red. NASA

By studying GRAIL's gravitational maps, which reveal areas of greater or lesser density, researchers can decipher the structure of the Moon's interior. Insights into the lunar structure will help scientists figure out how the Moon formed and provide clues about the beginnings of the Earth and other rocky planets, which are thought to have formed in a similar way. An artist's rendering shows debris orbiting a distant star before coalescing into a rocky, Earth-like planet. NASA / JPL-Caltech / C. Lisse, Johns Hopkins University Applied Physics Laboratory

Students at middle schools across the country will be able to get in on the GRAIL mission's spirit of discovery. The satellites will carry special cameras, and during the science phase of the mission, students can send in requests for the cameras to take photos of specific areas on the lunar surface. The images will be posted on the Internet, and students can refer to them as they study highlands, maria, and other features of the Moon's topography.

GRAIL's mapping, along with high-resolution pictures of the Moon's surface sent back by the Lunar Reconnaissance Orbiter (LRO), represents an important step for future Moon probes. The gravity maps will help mission planners calculate orbits that minimize the interference of mascons. Of particular importance will be GRAIL's discoveries about mysterious areas such as the Moon's far side and polar regions. An artist's conception shows a future crew exploration vehicle docked to a lunar lander. GRAIL's gravity maps will make it easier to plan safe moon landings. NASA / John Frassanito and Associates