NASA is planning to return to the Moon with the launch of twin lunar orbiters that will map the moon's gravity in unparalleled detail.

The two Grail Probes will fly to the moon on Thursday, September 8 from Florida's Cape Canaveral Air Force Station. The two probes will reach the moon on New Year's Day where they will begin to probe the moon's composition from the crust down to the core. The data will be used to better understand the moon's evolution and formation, NASA scientists said.

Grail-A and Grail-B or Gravity Recovery and Interior Laboratory, will travel independently to the moon despite being launched together. It will take three to four months to reach the moon because of the smaller Delta II rocket used to boost the probes.

Researchers will also use the twin probes to pinpoint the best landing sites for future explorations.

Despite having 109 missions to the moon and having men walk on it, Earth's only satellite still has many secrets scientists want to discover.

"Nearly every human who's every lived has looked up at the moon and admired it," said Massachusetts Institute of Technology planetary scientist Maria Zuber, Grail's principal investigator. "The moon has played a really central role in the human imagination and the human psyche."

The moon's formation still baffles scientists and its far side is still largely unexplored.

"You would think having sent many missions to the moon we would understand the difference between the near side and the far side, but in fact we don't," she said.

The twin Grail probes will orbit the lunar poles and will end up circling the moon's surface. The two spacecraft will chase each other around the moon, in formation, for three months. Scientists will then be able to measure any variation between the two Grail probes every single second. Any changes will indicate shifting masses below or at the moon's surface like mountains or craters.

"We measure the velocity change between the two spacecraft to a couple of fractions of a tenth of a micron per second. It is an extremely accurate measurement that has to be made," Zuber said.

The two Grail probes will crash into the moon after its mission. This is the second robotic mission NASA launched since it ended its shuttle program in July.