Having lasted three times longer than expected, the German/American Gravity Recovery and Climate Experiment (GRACE) space mission is finally coming to a close. Launched in March 2002, the twin GRACE satellites have spent the last 15 years using the Earth's gravitational field as a tool to gain insights into the movements of water, ice and the Earth's crust.
The GRACE mission was originally scheduled to last five years, but until this year the two spacecraft were in such good condition that the timetable kept getting extended. In September 2017, GRACE-2 developed an age-related malfunction in one of its batteries and in mid-October it became clear that its other battery was not up to the task of running both the satellite's instruments and its telemetry transmitter. It was therefore decided that rather than run the mission to the point of failure, the science phase would be discontinued and efforts concentrated in sending GRACE into a controlled reentry next year.
GRACE is based upon the principle of gravitational anomalies. That is, the way in which gravity on the Earth varies from place to place and even month to month. Though gravity may seem the most stable thing there is, that's only because the variations are so small that on a human scale, they aren't detectable. But they are there.
This is because the Earth is not a homogeneous mass. Some spots on its surface are marked by concentrations of dense, heavy materials while others are lighter. Equally, important, these anomalies can change over time as glaciers accumulate and flow, subterranean aquifers fill and empty, and even due to the weather or changes in the climate over time. If it were possible to detect and map these anomalies, they would act as a type of X-ray or ultrasound to allow scientists to gain a better understand of the Earth and the processes that form it.
Mapping these anomalies is what GRACE was designed to accomplish. GRACE-1 and GRACE-2 were set in a formation orbit around the Earth at a distance of 137 miles (220 km) from one another. Using a microwave ranging system, the satellites could measure the distance between them to a few microns – a fraction of the width of a human hair. When this measurement was combined with GPS tracking data, attitude data from star trackers, and an accelerometer, it was possible for scientists to eliminate such external factors as atmospheric drag, orbital decay, and solar pressure to provide a precise record of the pair's velocity and separation distance.
This is very important because as the GRACE orbiters passed over a denser region, they speed up as they approached, then slowed down as they moved away. By measuring how their distance changed during an orbit, GRACE provided scientists with a monthly map of how these anomalies are distributed around the Earth and how they change over time.
These maps allowed researchers to monitor the loss or growth of mass in ice sheets, the processes behind the rise of sea levels and ocean circulation, how groundwater sources change, and the effects of earthquakes.
Though GRACE-2 will go offline, GRACE-1 will continue to operate until the end of the year, using its remaining propellant to calibrate and characterize its accelerometer to help determine the validity of the collected data. GRACE-2 will make a controlled entry into the Earth's atmosphere in December or January while GRACE-1 will follow in early 2018.
After the success of the GRACE Mission, the international partners plan to continue with the GRACE Follow-On mission.
"GRACE was a pioneering mission that advanced our understanding across the Earth system – land, ocean and ice," says Michael Watkins, director of the 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."
The video below discusses the GRACE mission.