October 27th, 2017
The Gravity Recovery and Climate Experiment (GRACE) mission has come to an end after more than 15 years in Earth orbit. The twin satellites chronicled the changes of the Earth’s water, ice, and land since the spacecraft were launched from the Plesetsk Cosmodrome on March 17, 2002, on a mission that was originally only slated to last some five years. More than a decade after that, GRACE was still beaming data back to Earth when a technical issued forced mission planners to close out the program.
Similar in some aspects to other missions launched, GRACE made precise measurements via the two spacecraft – GRACE-1 and GRACE-2 – that comprised the mission. For GRACE’s overall scientific objectives to be achieved the two satellites both had to be fully functional. However, this past September (2017), GRACE-2 encountered a battery issue that made it clear by mid-October that the battery would not allow scientists to operate its science instruments and telemetry transmitter. It was decided to decommission GRACE-2 and, in so doing, end GRACE’s scientific mission.
GRACE‘s Principal Investigator Byron Tapley hails from the University of Texas at Austin and was in charge of GRACE’s efforts to precisely map Earth’s gravity field.
GRACE helped detail how our home world’s changing seasons move water, ice, and even land (as a result of surface water mass changes) across the planet’s surface, providing researchers with a better understanding of what drives the motion of these substances. Earth’s climate, earthquakes, and our own activities all play their part in shaping the face of our world and GRACE provided insights into the dynamics of this change.
GRACE was able to detect changes in Earth’s gravitational field that is related to our planet’s mass which is, perhaps unsurprisingly, impacted by the redistribution of water across the globe.
The spacecraft judged the distance between its two components using a microwave ranging system which, according to NASA, had the ability to judge that distance “…within a fraction of the diameter of a human hair over 137 miles (220 kilometers).”
That distance variation was then combined with GPS data (which was used for timing) in addition to altitude information provided via star trackers and an accelerometer that was used for non-gravitational effects (atmospheric drag and solar radiation). Mission managers used the combined data to determine Earth’s gravity field on a monthly basis. This, in turn, allowed any changes to the field to be monitored over time.
“GRACE has provided paradigm-shifting insights into the interactions of our planet’s ocean, atmosphere, and solid Earth components,” said Tapley. “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 mission has also helped to improve our understanding of ocean circulation and the processes that impact sea levels and could also help us manage groundwater resources. At present, NASA has estimated that users in 100 countries use data acquired from the GRACE mission.
“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 groundwater, 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.”
“GRACE was a pioneering mission that advanced our understanding across the Earth system – land, ocean, and ice,” said Michael Watkins, director of NASA’s Jet Propulsion Laboratory 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.”
So what now for the pair of GRACE satellites? GRACE-1 is expected to continue operating until the end of this year (2017).
“GRACE-1’s remaining fuel will be used to complete previously planned maneuvers to calibrate and characterize its accelerometer to improve the final scientific return and insights from the 15-year GRACE record,” said GRACE Project Scientist Carmen Boening of JPL.
In terms of her sister, GRACE-2 has depleted its fuel and is predicted to slowly deorbit, with the satellite expected to make atmospheric reentry at the end of this year or early at the start of next year (2018). GRACE-1 should be decommissioned and deorbited around the same time or soon thereafter.
NASA and Helmholtz Centre Potsdam German Research Centre for Geosciences (GFZ), seeking to build on the success of GRACE are preparing the Grace Follow-On mission for flight. If everything goes as it is presently planned, GRACE Follow-On will further the U.S./German partnership via a new laser-ranging interferometer that the duo developed.
While it was led by Tapley along with Co-principal Investigator Frank Flechtner at GFZ, GRACE was a joint mission managed by NASA and the Deutsches Zentrum für Luft- und Raumfahrt (DLR, the German Aerospace Center). The ground elements of the mission were co-funded by GFZ, DLR and the European Space Agency with the NASA portion of GRACE being handled by the Jet Propulsion Laboratory (JPL) near Pasadena, California. GRACE was the first mission to be sent aloft under the U.S. space agency’s Earth System Science Pathfinder program which was flown, in part, to develop new measurement technologies for studying our home world.
Video courtesy of NASA
Jason Rhian spent several years honing his skills with internships at NASA, the National Space Society and other organizations. He has provided content for outlets such as: Aviation Week & Space Technology, Space.com, The Mars Society and Universe Today.