Earth is by no means a perfectly smooth sphere, but a rather bumpy, oblate ball of mass. The nonuniform mass distribution gives rise to variations in Earth’s gravitational pull. Although these variations have been measured over many years, the data have come from diverse sources having uneven quality and incomplete geographical coverage. Today’s geophysicists want to know more accurately not only what the gravity field is at each point on Earth, but how it is changing with time. Such changes may reflect, for example, charge and depletion of continental aquifers, melting of land-based glaciers, or changes in ocean currents.
The Gravity Recovery and Climate Experiment (GRACE) is designed to track those changes over its 5-year lifetime. Richard Peltier of the University of Toronto calls GRACE “the most important satellite experiment in geophysics in the past two decades.” Launched last March, the two GRACE satellites are completing the commissioning phase. Already,...
