Article from Washington Post - By Brian Palmer, Published: August 22
Many space enthusiasts are mourning the end of the shuttle program, but NASA is still going to the moon. The space agency plans to launch two unmanned probes on Sept. 8 that will orbit our nearest celestial neighbor simultaneously. The mission is intended to help answer questions about how the moon and the Earth came into being, the temperature conditions at various points in the history of the solar system, and the composition of the moon, from crust to core. (I’m trying hard not to mention green cheese right now. Shoot. I just did it.)
Like much of NASA’s work, the GRAIL mission — named for the Gravity Recovery and Interior Laboratory probes — is all about gravity. Here’s how it’s supposed to work: The spacecraft will leave Earth together on a 125-foot-tall Delta 2 rocket. Once they’re carried away from the pull of Earth, the two GRAILs will be released. The craft each weigh about 500 pounds, and NASA describes them as about the size of a washing machine.
The GRAIL twins will take the long way to the moon — about three or four months, compared with the four days it took Neil Armstrong et al. in 1969 on the Apollo 11 mission. The longer, more circuitous route saves energy and also enables the GRAIL craft to maintain a reasonably constant speed by passing through a so-called Lagrangian point, where the gravitational forces of the Earth and moon interact in a way that eases the craft’s transition between them.
GRAIL A will enter lunar orbit on New Year’s Eve, with GRAIL B doing the same on New Year’s Day. Over two months, the spacecraft will shift from an initial 11.5-hour elliptical orbit to a two-hour orbit that is nearly circular, with GRAIL B constantly chasing GRAIL A around the moon.
After that, the real science will begin. The key to the mission is maintaining the distance between the two spacecraft. As GRAIL A and B whip around the moon over the following 82 days, small variations in the moon’s gravitational field will change the speed of each craft, causing them to drift slightly closer together or farther apart.
The slight changes in the moon’s gravity at different points indicate what’s going on inside the moon itself.
You may remember your high school physics professor telling you over and over that, for purposes of simplicity, you could ignore certain complicated aspects of the real world. Friction, for example, was regularly dropped, even though it’s crucial to nearly every aspect of physics. Another small fib in your early calculations was treating objects as if their mass was just a single point located at the object’s geometric center. That would work for many calculations involving perfect spheres — there’s a reason that teachers often refer to billiard balls when talking about Newton’s laws — but the moon isn’t a perfect sphere, and its mass isn’t evenly distributed around its geometric center.
These eccentricities inside and on the surface of the moon mean that the gravitational field — that is, the attractive forces caused by the mass of the rock itself — isn’t spherical. It’s rather lumpy, in fact