Two small spacecraft flew to the Kennedy Space Center in the belly of an Air Force cargo plane Friday, ready to start final preparations for launch to the moon in September.

Technicians prepare to hoist the two GRAIL spacecraft out of a vacuum chamber in April. Credit: NASA/JPL-Caltech/Lockheed Martin
 
The Gravity Recovery and Interior Laboratory, or GRAIL, mission is scheduled to blast off Sept. 8 on a Delta 2 rocket from Cape Canaveral, Fla.

GRAIL's two nearly identical spacecraft flew on a U.S. Air Force cargo plane from Lockheed Martin Corp. in Denver to Kennedy Space Center's space shuttle landing strip Friday, then the probes were moved to a nearby clean room facility.

Workers will complete assembly of the GRAIL probes, fuel the spacecraft and place the satellites on top of the Delta 2 launch vehicle over the next three months.

Each GRAIL spacecraft is about the size of a washing machine and will weigh about 440 pounds at liftoff.

The mission's Delta 2 rocket is fully stacked on Complex 17B at Cape Canaveral Air Force Station. The rocket will fly in the 7920-Heavy configuration with nine 46-inch-diameter solid rocket boosters.

Liftoff is expected Sept. 8 at 8:37 a.m. EDT (1237 GMT). After receiving a boost into space, the GRAIL spacecraft will be on a low-energy trajectory to the L1 Lagrange point, the location in deep space where gravity from the sun and Earth exert equal influence.

The Delta 2 rocket's first stage and solid rocket boosters were stacked on the launch pad in April.. Credit: NASA/KSC
 
The probes will approach the moon under the south pole, each executing a one-hour rocket burn to enter lunar orbit Dec. 31 and Jan. 1. More engine firings will place the spacecraft in a 30-mile-high orbit as the satellites drift more than 100 miles apart during a three-month science mission.

GRAIL's mission design is based on the similar GRACE project, which continues flying two satellites in formation in Earth orbit to precisely measure gravity.

A Ka-band ranging system will keep track of the distance between the GRAIL satellites, detecting slight changes in their orbits to study variations in the lunar gravity field.

GRAIL's discoveries will help researchers determine the structure of the lunar interior and advance the understanding of the thermal evolution of the moon, according to scientists. GRAIL will also search for evidence of a solid core inside the moon's liquid core.

Data from the moon will give scientists a clearer picture of how Earth and other rocky planets formed in the early solar system, according to NASA.

Led by principal investigator Maria Zuber of the Massachusetts Institute of Technology, NASA says the GRAIL mission will improve knowledge of the moon's near-side gravity by 100 times and of far-side gravity by 1,000 times.

GRAIL also carries a camera suite called MoonKAM to give middle school students the chance to take their own pictures of the moon from lunar orbit.

Once its data-collecting phase is complete in the summer of 2012, NASA will command the GRAIL probes to impact the moon.