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.)
So how can two hunks of metal floating far above a celestial body tell what’s inside 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.
A NASA effort similar to GRAIL — named GRACE, for Gravity Recovery and Climate Experiment — has been mapping the Earth’s gravitational field since 2002. It shows a bizarrely shaped field, with bumps and ridges caused by the uneven distribution of landmass, ocean currents, ground water runoff and ice sheets. The moon’s gravitational field has similar lumps and bumps.
Of course, irregularities in the moon’s gravitational field reveal only so much. Changes in density can mean differences in composition, the presence of liquid and temperature shifts. Scientists will rely on work from previous trips to the moon to make inferences about what’s under the moon’s crust.
“We have excellent measurements of the composition of the surface taken from the Lunar Prospector spacecraft,” says Maria Zuber, principal investigator for the GRAIL mission, referring to a NASA mission that probed the moon in 1998 and 1999. “Based on the composition of the top meter of the surface, petrologists are pretty good at guessing what source rock it came from and at what depth.”
The GRAIL craft will be able to measure incredibly small changes in their speed relative to each other, down to two-tenths of a micron — that’s about the diameter of a human blood cell — per second. Such precision will improve mapping of the moon’s gravitational field by orders of magnitude. The calculations are complicated, though. Scientists have had to consider not only the moon’s gravity but also the pull from all eight planets — plus Pluto — and even the shifting tectonic plates on which the Earth-based sensors rest.
The scientific payoff of the GRAIL mission could be huge, according to Zuber.
“We are testing the giant-impact theory of the moon — the idea that the moon formed very early in Earth history due to impact with a Mars-sized body at a strafing angle,” says Zuber. “The impact threw off material that coalesced to form the moon.” The mapping of the moon’s interior will help reveal both the internal composition and the temperature changes the moon experienced over time.
Zuber, a professor of geophysics at MIT, also notes that the mission might confirm an exciting new corollary theory that Earth once had two moons, which collided to form one. It will also teach us what Earth looked like when life began taking root. Depending on what it finds, the mission could also represent a big step toward creating a mining operation on the moon that enables us to shuttle rare and valuable minerals back to Earth.