The Gravity Recovery and Climate Experiment (GRACE) was a joint mission of NASA and the German Aerospace Center (DLR). Twin satellites took detailed measurements of Earth's gravity field anomalies from its launch in March 2002 to the end of its science mission in October 2017. The two satellites were sometimes called Tom and Jerry, a nod to the famous cartoon. The GRACE Follow-On (GRACE-FO) is a continuation of the mission on near-identical hardware, launched in May 2018. On March 19, 2024, NASA announced that the successor to GRACE-FO would be GRACE-Continuity (GRACE-C), to be launched in December 2028. By measuring gravity anomalies, GRACE showed how mass is distributed around the planet and how it varies over time. Data from the GRACE satellites is an important tool for studying Earth's ocean, geology, and climate. GRACE was a collaborative endeavor involving the Center for Space Research at the University of Texas at Austin, NASA's Jet Propulsion Laboratory, the German Aerospace Center and Germany's National Research Center for Geosciences, Potsdam. The Jet Propulsion Laboratory was responsible for the overall mission management under the NASA ESSP (Earth System Science Pathfinder) program. The principal investigator was Byron Tapley of the University of Texas Center for Space Research, and the co-principal investigator is Christoph Reigber of the GeoForschungsZentrum (GFZ) Potsdam. The two GRACE satellites, GRACE-1 and GRACE-2, were launched from Plesetsk Cosmodrome, Russia, on a Rockot (SS-19 + Briz upper stage) launch vehicle on 17 March 2002. The spacecraft were launched to an initial altitude of approximately 500 km at a near-polar inclination of 89°. During normal operations, the satellites were separated by 220 km along their orbit track. This system was able to gather global coverage every 30 days. GRACE far exceeded its 5-year design lifespan, operating for 15 years until the decommissioning of GRACE-2 on 27 October 2017. Its successor, GRACE-FO, was successfully launched on 22 May 2018. In 2019, a glacier in West Antarctica was named after the GRACE mission.
Discoveries and applications
Measurements provided by GRACE have improved understanding of phenomena including the thinning of ice sheets, the flow of water through aquifers, and shifts in the Earth's crust.
Oceanography, hydrology, and ice sheets GRACE chiefly detected changes in the distribution of water across the Earth. Scientists use GRACE data to estimate ocean bottom pressure (the combined weight of the ocean waters and atmosphere), which is as important to oceanographers as atmospheric pressure is to meteorologists. For example, measuring ocean pressure gradients allows scientists to estimate monthly changes in deep ocean currents. The limited resolution of GRACE is acceptable in this research because large ocean currents can also be estimated and verified by an ocean buoy network. Scientists have also detailed improved methods for using GRACE data to describe Earth's gravity field. GRACE data help 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. High-resolution static gravity fields estimated from GRACE data have helped improve the understanding of global ocean circulation. The hills and valleys in the ocean's surface (ocean surface topography) 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 have provided a record of mass loss within the ice sheets of Greenland and Antarctica. Greenland has been found to lose 280±58 Gt of ice per year between 2003 and 2013, while Antarctica has lost 67±44 Gt per year in the same period. These equate to a total of 0.9 mm/yr of sea level rise. Increases in ocean heat content resulting from Earth's Energy Imbalance of about 0.8 W/m2 were similarly found spanning 2002 thru 2019. GRACE data have also provided insights into regional hydrology inaccessible to other forms of remote sensing: for example, groundwater depletion in India and California. The annual hydrology of the Amazon basin provides a strong signal when viewed by GRACE. A University of California, Irvine-led study published in Water Resources Research on 16 June 2015 used GRACE data between 2003 and 2013 to conclude that 21 of the world's 37 largest aquifers "have exceeded sustainability tipping points and are being depleted" and thirteen of them are "considered significantly distressed." The most over-stressed is the Arabian Aquifer System, upon which more than 60 million people depend for water.
Geophysics
GRACE also detects changes in the gravity field due to geophysical processes. Glacial isostatic adjustment—the slow rise of land masses once depressed by the weight of ice sheets from the last ice age—is among these signals. GIA signals appear as secular trends in gravity field measurements and must be removed to accurately estimate changes in water and ice mass in a region. GRACE is also sensitive to permanent changes in the gravity field due to earthquakes. For instance, GRACE data have been used to analyze the shifts in the Earth's crust caused by the earthquake that created the 2004 Indian Ocean tsunami. In 2006, a team of researchers led by Ralph von Frese and Laramie Potts used GRACE data to discover the 480-kilometer-wide (300 mi) Wilkes Land crater in Antarctica, which was probably formed about 250 million years ago.
Geodesy Data from GRACE has improved the current Earth gravitational field model, leading to improvements in the field of geodesy. This improved model has allowed for corrections in the equipotential surface from which land elevations are referenced. This more accurate reference surface allows for more accurate coordinates of latitude and longitude and for less error in the calculation of geodetic satellite orbits.
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