The twin Grail lunar exploration spacecraft that NASA launched in September will begin their gravity mapping mission on New Year’s Day. The Grail probes, standing for Gravity Recovery and Interior Laboratory, will both orbit the moon in tandem at 35 miles above the surface, and an average separation of 124 miles. The probes will speed up or slow down dependent on the lunar gravity field, which will change the distance between the craft. The measurement of this distance gap provides the map of the underlying gravity field.
This is the first mission to focus solely on gravity mapping, as other missions have had partial success with this mapping as an ancillary mission. This dedicated mission is expected to return results that are 100 to 1,000 times more accurate than the current measurements.
With the gravity data, scientists will be able to determine what lies beneath the lunar surface. The data will help scientists understand how rocky planets are formed, and the precise measurements of gravity will improve mission navigation and exploration targets.
In addition to the gravity data, the spacecraft each carry four cameras that are being used for educational purposes. A nonprofit founded by Sally Ride, are using the camera for outreach to middle school students that can choose lunar targets to image during the mission. The Grail MoonKam even includes an application in the Apple App Store for viewing this imagery.
The mapping mission will take two months, with data collection from the satellites beginning in March.
This eight-minute NASA JPL video details the mission.