The Grail twins will make their measurements by executing a carefully calibrated pursuit in orbit.
The latest spacecraft to go to the Moon have launched from Florida.
Nasa is sending twin probes called Grail to map tiny variations in the pull of gravity around the lunar body.
The information should give scientists fresh insight into the internal structure of Earth's satellite.
This will help explain many mysteries, such as why the farside of the Moon looks so different from that of the nearside with its great swathe of dark volcanic plains, or maria.
The data also will be an invaluable navigation tool for future exploration, enabling other spacecraft to make more precise landings.
A Delta rocket sent the Grail twins on their way. Its launch from the Cape Canaveral Air Force Station occurred at 09:08 EDT (13:08 GMT; 14:08 BST).
The journey to the Moon is a slow cruise, however. The duo is not expected to enter into orbit until the turn of the year.
'Super-lens' on gravity
Scientists already have some lunar gravity maps. Much of this information was built up by tracking small perturbations in the orbits of previous spacecraft as they circled the Moon. But the maps are very coarse, especially for the farside.
http://www.bbc.co.uk/news/science-environment-14825641
Note: 10 September 2011 Last updated at 13:09 GMT