© Courtesy of The University of Texas Center for Space Research (NASA/DLR Gravity Recovery and Climate Experiment).
In 2002, NASA launched a pair of satellites, the Gravity Recovery and Climate Experiment (GRACE) mission, to measure precisely the gravitational field at the surface of the Earth. As the two satellites orbit the Earth, their precise velocities and the distance between them are constantly monitored. By comparing the difference between the orbits of the two satellites, GRACE mission scientists can infer how local gravity varies around the globe. The image shows the regions of strong (red, raised) and weak (blue, depressed) gravitational acceleration measured by the GRACE mission. (Unit: 3)