NASA's twin spacecrafts are preparing to leave to the Moon to map the gravitational field and explore the interior of our companion.

The liftoff of the two GRAIL (Gravity Recovery and Interior Laboratory) space probes is currently scheduled to 8^th September. After leaving the Earth, they will slowly reach the Moon around New Year's Eve, on a low-energy trajectory. The main scientific instruments are two special radio transmitters: the spacecrafts will measure the distances between them with very stable wavelength microwave signals and time codes. By measuring the slight wobbles and nudges caused by the varying gravitational field of the Moon, they will be able the determine the mass distribution as well. They will also carry 5-5 miniature cameras called MoonKams that will allow high school students to collect imagery from the Moon's surface.

Much is known already about the gravity of the Moon: above some of the mare regions (the black “seas”) of the nearside for example the gravitational force can be stronger by as much as half a percent compared to other places. That is partially attributed to the heavier, denser basalt filling these basins, but the true origin of such mascons (mass concentration) is not fully understood yet. Their effects have to be considered nevertheless during planning orbits or landing on the Moon. GRAIL is expected to collect at least a hundred times more accurate data from the nearside and even 1000 times better from the far side compared to current measurements.

The other main goal of the mission is to explore the inner composition of the Moon, from core to crust. There are unanswered questions here too: what is the size and the composition of the core and the mantle, why is the crust thicker on the far side of the Moon, how did the mare regions and the mascons from precisely. GRAIL will map the entire Moon three times during its mission and the probes will impact the surface 40-50 days later. But the real work, the interpretation of the data will have just started by then.