Remember the GRAIL mission? At the beginning of the year, two satellites, named Ebb and Flow, arrived at the Moon and fell into a parallel orbit. There is an instrument on board that very precisely determines the distance between the two space craft, said to be about the size of a typical washing machine. As the craft circle the Moon in an orbit that takes them all over the place, the exact distance between them changes as a result of differential gravitational forces that are in turn caused by the details of the shape of the Moon below them. Thus, the precise measurements of distance between Ebb and Flow can be converted into a very good gravitational map of the Moon.
Ultimately, this map will be used to test hypotheses about the Moon structure and, indirectly, origin. It is an interesting fact that the nature of the Moon's origin is an unsettled scientific question.
The GRAIL (which stans for "Gravity Recovery And Interior Laboratory") space craft completed a survey of the Moon's gravity from an altitude of about 55 kilometers in the Spring, and has now lowered in altitude to do more survey at 23 kilometers, which will provide a different view of the Moon's gravity. This process of data collection starts now.
Ultimately the mission hopes to map the structure of the Moon's solid rock zone, better understand variation in the Moon's crust in relation to the planetoid's cooling down from a molten phase, figure out certain long known of anomalies in the Moon's gravitational field, figure out the history of the gravity, magnetics, and other aspects of the geology of younger (less than 3.2 billion years old) formations, better understand Earth-tides on the Moon, and figure out the size of the liquid core.
The GRAIL Mission home page is here.
I am guessing the "parallel orbit" thingy came from the MIT ppls.
Not if you are talking about Gravity Probe B. Here, I mean that there are two washing machines flying side by side.
Back In the mid 1950's I took a course in geophysics. We where told of a gravity meter which could detect differences in gravity over a distance as small as two centimeters. I thought that rather impressive.