WASHINGTON - NASA's Gravity Recovery And Interior Laboratory (GRAIL) is in final launch preparations for a scheduled September 8 launch from Cape Canaveral Air Force Station in Florida, the space agency announced Thursday.

GRAIL's twin spacecraft are tasked for a nine-month mission to explore Earth's nearest neighbor in unprecedented detail. They will determine the structure of the lunar interior from crust to core and advance our understanding of the thermal evolution of the moon.

The spacecraft twins, GRAIL A and B, will fly a circuitous route to lunar orbit taking 3.5 months and covering approximately 2.6 million miles (4.2 million kilometers) for GRAIL-A, and 2.7 million miles (4.3 million kilometers) for GRAIL-B.

In lunar orbit, the spacecraft will transmit radio signals precisely defining the distance between them. Regional gravitational differences on the moon are expected to expand and contract that distance. GRAIL scientists will use these accurate measurements to define the moon's gravity field. The data will allow mission scientists to understand what goes on below the surface of our natural satellite.

"GRAIL will unlock lunar mysteries and help us understand how the moon, Earth and other rocky planets evolved as well," said Maria Zuber, GRAIL principal investigator from the Massachusetts Institute of Technology.

GRAIL's launch period opens September 8 and extends through October 19. On each day, there are two separate launch opportunities separated by approximately 39 minutes. On September 8, the first launch opportunity is 8:37 am EDT (1237 GMT); the second is 9:16 am (1316 GMT).