Lumpy Moon 
When spacecraft began orbiting the Moon in the 1960s, flight controllers quickly noticed something odd. It didn’t take long for the craft to wander off course. Within weeks or even days, both their direction around the Moon and their altitude above it were way off. And many of those craft slammed into the Moon far sooner than expected.
The source of the interference was the Moon itself. It turns out that the Moon’s gravitational field is lumpy. So as a craft orbits the Moon, it sometimes gets a stronger tug toward certain parts of the lunar surface — a tug that quickly pulls the craft out of position.
Those lumps are known as mass concentrations — mascons.
By carefully measuring the motions of all the craft that have orbited the Moon, scientists have mapped where the mascons are. Most of them are associated with the lunar plains — “seas” of volcanic rock. If you look at the Moon tonight, you can easily pick them out, because they form the dark regions on the lunar surface.
The volcanic rock is denser than the rocks around it, and it forms deep basins in the lunar crust. So these formations have a slightly stronger gravitational pull than other regions.
Scientists would like even better maps of the mascons, though, to help them learn more about how the mascons formed, and about the structure of the entire lunar crust. And a new mission will provide such maps. More about that tomorrow.
Script by Damond Benningfield, Copyright 2011