GRAIL space probes

Traditionally, the Moon was a major focus for the man. In 2012, scientists interest in new, to the Moon to establish a permanent base in the near future. Although this is the best known aster in our solar system after Earth question here with the mission GRAIL (Gravity Recovery And Interior Laboratory), to measure the gravity field of the Moon, already measured but with many imperfections. The GRAIL project, 375 million dollars, was launched in 1997. In 2012, the two probes GRAIL (GRAIL GRAIL-A and-B) of NASA, launched in September 2011, were placed on the same lunar orbit, a few hours apart, after four months of travel. They revolve around the Moon at an altitude low enough, about 55 km to better probe the internal layers of the Moon. The two probes are close to each other, between 175 and 225 km, the distance is measured finely, with a precision of one micrometer. Both probes transmit a radio signal to Earth to make permanent their respective positions. Thanks to the variation of the distance between them, the scientists collected the slight local variations, due to the gravity of the Moon.

The Moon is not perfectly homogeneous sphere, the distance between the two probes will be disturbed by small variations in gravity due to the density of the subsoil and the distribution of lunar masses. The various mineral layers of the Moon including the nucleus can be analyzed using the mapping of geological structure. Know perfectly the structure of the Moon and in particular its nucleus will validate the hypothesis of its formation, the most accepted by scientists that a giant impact.
Its origin is due to a massive collision with the Earth, of a planet the size of Mars, which is called "Theia".

Image: The two probes GRAIL (GRAIL-A and-B), launched in September 2011, scan the Moon at an altitude of 20-55 km, to learn about the layers of its subsoil.