Published : Tuesday, September 19, 2017 | 7:27 PM
A twin satellite mission that has been helping scientists map Earth’s ever-changing gravity field in unprecedented detail for over 15 years will soon start its final science operations phase, soon be running out of fuel and shortly thereafter go dark.
A Mission Status Report from the Jet Propulsion Laboratory in Pasadena said the U.S.-German Gravity Recovery and Climate Experiment – or GRACE – mission, launched in March 2002, is now making plans for a final science collection.
One of the satellites, GRACE-2, is almost out of fuel, the JPL status report said.
“On September 3, one of 20 battery cells aboard the GRACE-2 satellite stopped operating due to an age-related issue,” JPL said. “It was the eighth battery cell loss on GRACE-2 since the twin satellites that compose the GRACE mission launched in March 2002 on a mission designed to last five years. The following day, contact was lost with GRACE-2.”
Communication with the spacecraft was restored on September 8 after the GRACE mission operations team, working from JPL and from two locations in Germany – DLR (Deutsches Zentrum für Luft- und Raumfahrt), the German Aerospace Center in Oberpfaffenhofen, and the Helmholtz Centre Potsdam German Research Centre for Geosciences (GFZ) in Potsdam – sent commands to bypass the satellite’s flight software system, the report from JPL said
The report added that later analysis showed that the battery cell lost on September 3 had recovered its full voltage and that GRACE-2 had essentially hibernated during the period of lost contact, consuming no fuel. Following an assessment of the satellite’s overall health, the team has determined that GRACE’s dual satellite science mission can continue, at least until its next science operations phase, which runs from mid-October to early November.
With GRACE-2 now in a passive state that will allow it to maintain its current level of fuel, operational procedures have begun that will extend the GRACE mission to its next science operations phase, the report said. As directed by the mission’s Joint Steering Group, final decommissioning for both GRACE-1 and GRACE-2 will begin once the dual satellite science phase concludes.
In early 2018, NASA will launch GRACE’s successor mission, GRACE-Follow-On, according to the JPL report.
Since it was launched in 2002, the report said GRACE has tracked the movement of water around Earth caused by the changing seasons, weather and climate processes, and human activities. The mission has mapped Earth’s ever-changing gravity field in unprecedented detail, showing how water, ice and solid Earth material move on or near Earth’s surface.
GRACE operates by sensing minute changes in gravitational pull caused by local changes in Earth’s mass. To observe these changes, GRACE uses a microwave ranging system that measures micron-scale variations in the 137-mile distance between the twin spacecraft, along with GPS tracking, star trackers for attitude information and an accelerometer to account for non-gravitational effects such as atmospheric drag, the report said. From these data collected over Earth’s surface, scientists can infer Earth’s gravity field.
The report also mentioned that GRACE’s monthly maps of regional variations in gravity had given scientists new insights into Earth system processes. It added that GRACE had been used to monitor the loss of ice from Earth’s ice sheets, improved understanding of the processes responsible for sea level rise and ocean circulation, provide insights into where aquifers may be shrinking or where dry soils are contributing to drought and monitored changes in the solid Earth.
The report said GRACE is a joint NASA/DLR mission led by the principal investigator at the University of Texas at Austin and co-principal investigator at GFZ. GRACE ground segment operations are co-funded by GFZ, DLR and the European Space Agency.
The JPL in Pasadena manages GRACE for NASA’s Science Mission Directorate in Washington.