No space mission is easy, but gravity measurement poses special challenges. "In essence, the spacecraft itself is the science measurement," said Project System Engineer Neil Dahya. "We are measuring the effects of gravity on the entire spacecraft." But lots of other forces are acting on the spacecraft as well, and all of them have to be identified and subtracted from the total measurements in order to accurately measure the very subtle gravitational changes that the GRACE missions are after.
"We spend a lot of time understanding how solar pressure (the pressure of sunlight) pushes the spacecraft so we can back that out," Dahya said. "We also want to understand how the moisture in the spacecraft leaves over time, and how the spacecraft will warp and move" with changes in temperature. All of these things, along with fuel consumption, will change the location of the spacecraft's center of mass and threaten to skew the measurements.
"That's why you have the ultra-precise accelerometer onboard," said Deputy Project Manager Sammy Kayali. "That's why you have the magnetometer onboard. And that's why we have the overall balance of the two spacecraft so precisely calibrated, so we know exactly where the center of mass is for the spacecraft."
When the center of mass moves from its desired location, the spacecraft goes into action to restore it. "We have these giant slugs of tungsten on spindles and worm gears that we can precisely move," Dahya said. "So as the center of mass moves, we'll measure that and then we'll move those masses to bring the center of mass back to the point where we're doing the measurement of the accelerations."
In addition to non-gravitational forces acting on the spacecraft, GRACE scientists need to account for the gravitational pull of the Sun and Moon, both on the spacecraft and on the Earth. The Moon, in particular, not only raises tides in the oceans, but also deforms the solid Earth in ways that vary with the location of faults and the density of rock. Modeling these complex effects is essential to the mission. "One of the reasons that the GRACE products have been getting better and better," said Charley Dunn of the Instrument Advisory Group, "is that people are learning how to do that better."