Using a precision formation-flying technique, the twin GRAIL spacecraft maps the moon's gravity field, as depicted in this artist's rendering. Image Credit: NASA/JPL-Caltech

Launch Date Sept. 10, 2011 | 13:08:52 UT
Launch Site Cape Canaveral, Florida, USA
Destination Earth’s Moon
Type Orbiter
Status Successful
Nation United States
Alternate Names GRAIL-A, GRAIL-B, 2011-046A, 37801, 2011-046B, 37802


GRAIL flew twin spacecraft -- named Ebb and Flow -- in tandem around the moon to precisely measure and map variations in the moon's gravitational field. The goal was to reveal differences in density of the Moon's crust and mantle and will help answer fundamental questions about the Moon's internal structure, thermal evolution and history of collisions with asteroids.


The twin GRAIL probes orbiting Earth's moon generated the highest resolution gravity field map of any celestial body. The gravity field map revealed an abundance of features never before seen in detail, such as tectonic structures, volcanic landforms, basin rings, crater central peaks and numerous simple, bowl-shaped craters. Data also show the moon's gravity field is unlike that of any terrestrial planet in our solar system. The map will provide a better understanding of how Earth and other rocky planets in the solar system formed and evolved.

At the end of an extended mission, Ebb and Flow were sent purposely into the lunar surface because their low orbit and low fuel levels preclude further scientific operations. They impacted at mountain near Goldschmidt crater on the lunar near side.

Key Dates

Sept. 10, 2011 | 13:08:52 UT: Launch

Dec. 31, 2011: GRAIL A Orbit Insertion

Jan. 1, 2012: GRAIL B Orbit Insertion

Dec. 17, 2012: GRAIL-B Lunar Impact

Dec. 17, 2012: GRAIL-A Lunar Impact

In Depth

Because of GRAIL's findings, spacecraft on missions to other celestial bodies can navigate with greater precision in the future.

GRAIL's twin spacecraft studied the internal structure and composition of the moon in unprecedented detail for nine months. They pinpointed the locations of large, dense regions called mass concentrations, or mascons, which are characterized by strong gravitational pull. Mascons lurk beneath the lunar surface and cannot be seen by normal optical cameras. GRAIL scientists found the mascons by combining the gravity data from GRAIL with sophisticated computer models of large asteroid impacts and known detail about the geologic evolution of the impact craters.

"GRAIL data confirm that lunar mascons were generated when large asteroids or comets impacted the ancient moon, when its interior was much hotter than it is now," said Jay Melosh, a GRAIL co-investigator at Purdue University. "We believe the data from GRAIL show how the moon's light crust and dense mantle combined with the shock of a large impact to create the distinctive pattern of density anomalies that we recognize as mascons."

The origin of lunar mascons has been a mystery in planetary science since their discovery in 1968 by a team at NASA's Jet Propulsion Laboratory in Pasadena, Calif. Researchers generally agree mascons resulted from ancient impacts billions of years ago. It was not clear until now how much of the unseen excess mass resulted from lava filling the crater or iron-rich mantle upwelling to the crust.

On a map of the moon's gravity field, a mascon appears in a target pattern. The bulls-eye has a gravity surplus. It is surrounded by a ring with a gravity deficit. A ring with a gravity surplus surrounds the bulls-eye and the inner ring. This pattern arises as a natural consequence of crater excavation, collapse and cooling following an impact. The increase in density and gravitational pull at a mascon's bulls-eye is caused by lunar material melted from the heat of a long-ago asteroid impact.

"Knowing about mascons means we finally are beginning to understand the geologic consequences of large impacts," Melosh said. "Our planet suffered similar impacts in its distant past, and understanding mascons may teach us more about the ancient Earth, perhaps about how plate tectonics got started and what created the first ore deposits."

This new understanding of lunar mascons also is expected to influence knowledge of planetary geology well beyond that of Earth and our nearest celestial neighbor.

"Mascons also have been identified in association with impact basins on Mars and Mercury," said GRAIL principal investigator Maria Zuber of the Massachusetts Institute of Technology. "Understanding them on the moon tells us how the largest impacts modified early planetary crusts."

Launched as GRAIL A and GRAIL B in September 2011, the probes, renamed Ebb and Flow, operated in a nearly circular orbit near the poles of the moon at an altitude of about 34 miles (55 kilometers) until their mission ended in December 2012. The distance between the twin probes changed slightly as they flew over areas of greater and lesser gravity caused by visible features, such as mountains and craters, and by masses hidden beneath the lunar surface.


Launch Vehicle: Delta II

Spacecraft Mass: 132.6 kilograms

Spacecraft Instruments:

The payload on each orbiter consists of a Lunar Gravity Ranging System and an education and public outreach MoonKAM System.

Additional Resources

National Space Science Data Center Master Catalog: GRAIL-A (Ebb)

National Space Science Data Center Master Catalog: GRAIL-B (Flow)

Related News