Gravity Recovery and Climate Experiment

The Gravity Recovery And Climate Experiment (GRACE), a joint mission of NASA and the German Space Agency, has been making detailed measurements of Earth's gravity field since its launch in March 2002.

Gravity is determined by mass. By measuring gravity, GRACE shows how mass is distributed around the planet and how it varies over time. GRACE data are important tools for studying Earth's ocean, geology, and climate.

GRACE is a collaborative endeavor involving the Center for Space Research at the University of Texas, Austin; NASA's Jet Propulsion Laboratory, Pasadena, Calif.; the German Space Agency and Germany's National Research Center for Geosciences, Potsdam.^[1] The Jet Propulsion Laboratory is responsible for the overall mission management under the NASA ESSP program.

The principal investigator is Dr. Byron Tapley of the University of Texas Center for Space Research, and the co-principal investigator is Dr. Christoph Reigber of the GeoForschungsZentrum (GFZ) Potsdam.^[2]

The GRACE satellites were launched from Plesetsk Cosmodrome, Russia on a Rockot (SS-19 + Breeze upper stage) launch vehicle, on March 17, 2002.

The monthly gravity maps generated by Grace are up to 1,000 times more accurate than previous maps, substantially improving the accuracy of many techniques used by oceanographers, hydrologists, glaciologists, geologists and other scientists to study phenomena that influence climate.^[3]

From the thinning of ice sheets to the flow of water through aquifers and the slow currents of magma inside Earth, measurements of the amount of mass involved provided by GRACE help scientists better understand these important natural processes.

Among the first important applications for GRACE data was to improve the understanding of global ocean circulation. The hills and valleys in the ocean's surface are due to currents and variations in Earth's gravity field. GRACE enables separation of those two effects to better measure ocean currents and their effect on climate. GRACE data are also critical in helping to determine the cause of sea level rise, whether it is the result of mass being added to the ocean, from melting glaciers, for example, or from thermal expansion of warming water or changes in salinity.^[4]

The data so far obtained by GRACE are the most precise gravimetric data yet recorded: they have been used to re-analyse data obtained from the LAGEOS experiment to try to measure the relativistic frame- dragging effect. In 2006, a team of researchers led by Ralph von Frese and Laramie Potts used GRACE data to discover the 480-kilometer (300-mile) wide Wilkes Land crater in Antarctica, which probably formed about 250 million years ago.^[5] GRACE has been used to map the hydrologic cycle in the Amazon River basin and the location and magnitude of post-glacial rebound from changes in the free air gravity anomaly. GRACE data have also been used to analyze the shifts in the Earth's crust caused by the earthquake that created the 2004 Indian Ocean tsunami.^[6] Scientists have recently developed a new way to calculate ocean bottom pressure—as important to oceanographers as atmospheric pressure is to meteorologists—using GRACE data.^[7]