The so-called GRAIL mission spacecraft (for Gravity Recovery And Interior Laboratory) were designed with the goal of studying the gravitational forces of the Earth’s closest celestial neighbor.
Information on the project’s Web site explains the mission thusly:
As the two spacecraft fly over areas of greater and lesser gravity, caused both by visible features such as mountains and craters and by masses hidden beneath the lunar surface, they will move slightly toward and away from each other. An instrument aboard each spacecraft will measure the changes in their relative velocity very precisely, and scientists will translate this information into a high-resolution map of the Moon's gravitational field.
This gravity-measuring technique is essentially the same as that of the Gravity Recovery And Climate Experiment (GRACE), which has been mapping Earth's gravity since 2002.
Stu Spath, GRAIL program manager at Lockheed Martin Space Systems Company, was thrilled by the successful orbit. “We used the moon's gravity to capture the two GRAIL spacecraft into orbit,” Spath said, “and now the science team is going to analyze that same gravitational field to an extraordinary level.”
Scientists plan to shorten the orbit of the spacecraft to just two hours, holding at a distance of 35 miles above the moon’s surface over the next month or so. Once the orbit has been achieved, the researchers will begin the experiments to map the gravitational field of the moon in extreme detail. Beginning in early March, the process is expected to take up to three months.
According to NASA, the data will be used to determine the structure of the lunar interior from crust to core and advance scientists’ understanding of the thermal evolution of the moon and other terrestrial planets.