The Gravity Recovery and Interior Laboratory, which will neither be recovering gravity or being inside the moon but “GRAIL” apparently sounds good, is coming into Lunar Orbit as I write this.
As you know from watching Apollo 13 the travel distance to or back from the moon is a matter of several days … Apollo crews got there in about three Earth Days. But the GRAIL mission has taken months to get there, because of the kind of orbit they wanted to achieve. Also, the long duration allows for other things to happen, like temperatures to stabilize within the machinery.
GRAIL is two space ships flying together and will achieve a more or less polar orbit around the moon, going around the moon ever 11.5 hours. Over time, the period of the orbit will be reduced to about 2 hours. Then, GRAIL will start mapping the Moon’s mass.
When science collection begins, the spacecraft will transmit radio signals precisely defining the distance between them as they orbit the moon. As they 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. The data will allow mission scientists to understand what goes on below the surface. This information will increase our knowledge of how Earth and its rocky neighbors in the inner solar system developed into the diverse worlds we see today.