Two satellites with more than 15 productive years in orbit have led to paradigm-shifting insights into the interactions of our planet’s oceans, atmosphere and the human accelerant to global climate change.

The Gravity Recovery and Climate Experiment (GRACE), a German/U.S. partner mission, launched in 2002 to track the continuous movement of water, ice and the Earth’s surface. End of life battery issues prompted the decommission. The mission was originally only scheduled for five years to study the gravity field, according to University of Texas at Austin principal investigator Byron Tapley.

But the satellites revealed how water, ice and solid Earth mass move on or near Earth’s surface due to Earth’s changing seasons, weather and climate processes, earthquakes and even human activities, such as from the depletion of large aquifers. It did this by sensing minute changes in the gravitational pull caused by local changes in Earth’s mass, which are due mostly to changes in how water is constantly being redistributed around our planet.

“GRACE has provided paradigm-shifting insights into the interactions of our planet’s ocean, atmosphere and solid Earth components,” Tapley said. “It has advanced our understanding of the contribution of polar ice melt to global sea level rise and the amount of atmospheric heat absorbed by the ocean. Recent applications include monitoring and managing global water resources used for consumption, agriculture and industry; and assessing flood and earthquake hazards.”

The planet’s gravity field was calculated monthly using a microwave ranging system to measure the change in distance of each satellite to within a fraction of the diameter of a human hair at over a distance of 137 miles (220 km) apart. The range data was combined with global positioning system (GPS) tracking for timing; star trackers for altitude information and an accelerometer to account for solar radiation and atmospheric drag.

“GRACE was an excellent example of a research satellite mission that advanced science and also provided near-term societal benefits,” said Michael Freilich, director of NASA’s Earth Science Division at the agency’s headquarters in Washington. “Using cutting-edge technology to make exquisitely precise distance measurements, GRACE improved our scientific understanding of our complex home planet, while at the same time providing information — such as measurements related to ground water, drought and aquifer water storage changes worldwide — that was used in the U.S. and internationally to improve the accuracy of environmental monitoring and forecasts.”

Sadly, the satellites watched the loss of ice mace from Earth’s ice sheets, but also provided a better understanding of the processes responsible for sea level rise and ocean circulation. The observations also provided insight into where global groundwater resources were depleting or expanding and where dry soils contributed to drought.

Users from over 100 countries routinely download GRACE data for analyses.

“GRACE was a pioneering mission that advanced our understanding across the Earth system — land, ocean and ice,” said Mike Watkins, director of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, and the mission’s original project scientist. “The entire mission team was creative and successful in its truly heroic efforts over the last few years, extending the science return of the mission to help minimize the gap between GRACE and its successor mission, GRACE Follow-On, scheduled to launch in early 2018.”

GRACE is a joint NASA/Deutsches Zentrum für Luft- und Raumfahrt (DLR, the German Aerospace Center) mission led by Tapley and Co-principal Investigator Frank Flechtner at GFZ. GRACE ground segment operations are co-funded by GFZ, DLR and the European Space Agency. JPL manages GRACE for NASA’s Science Mission Directorate at the agency’s headquarters in Washington. GRACE was the first mission launched under NASA’s Earth System Science Pathfinder program, designed to develop new measurement technologies for studying the Earth system.grace