Two U.S.-German satellites launched more than 15 years ago to measure Earth’s changing gravity field will stop collecting data in November, a few months before a pair of replacement craft will launch to resume gravity measurements, NASA officials said.
The Gravity Recovery and Climate Experiment, or GRACE, mission has measured Earth’s ever-changing gravity field, initially from an orbital perch around 300 miles (485 kilometers) above Earth.
The GRACE satellites circle the planet in formation 137 miles (220 kilometers) apart, using a microwave ranging instrument to measure the exact distance between the two spacecraft with extreme precision. Once scientists cancel out affects such as atmospheric drag, the measurements yield data on the strength and variations in Earth’s gravity field.
Launched together from Russia in March 2002 aboard a Rockot booster made from decommissioned military missile stages, the GRACE satellites were designed to collect scientific data for five years.
NASA provided the K-band ranging system for the GRACE satellites, and the Jet Propulsion Laboratory in California is responsible for managing the project. The satellites were manufactured in Europe by Astrium, now Airbus Defense and Space.
Ground controllers temporarily lost contact with the GRACE-2 satellite Sept. 4, a day after the spacecraft lost one of its 20 battery cells, NASA’s Jet Propulsion Laboratory said in a press release. It was the eighth battery cell loss on the GRACE-2 satellite since its launch more than 15 years ago.
Engineers restored communications with the GRACE-2 satellite Sept. 8 after numerous attempts by uplinking commands to bypass the craft’s flight software system, JPL said in a statement.
The lost battery cell had recovered its full voltage, officials said, and the satellite had “essentially hibernated during the period of lost contact, consuming no fuel.”
GRACE-2 was already running low on fuel, and ground controllers have placed the satellite into a passive configuration to save maneuvering propellant until mid-October, when the next science collection phase of the mission is due to begin.
Flying around Earth in a near-polar orbit, the GRACE-2 satellite will be in full sunlight from mid-October through early November, allowing its solar panels to produce enough electricity to operate the spacecraft and its instruments without relying on batteries.
“The team expects the October/November science data collection to be the mission’s last before GRACE-2 runs out of fuel,” JPL said in a statement. “The additional monthly gravity map produced will help further extend GRACE’s data record closer to the launch of GRACE’s successor mission, GRACE Follow-On, scheduled for early 2018.”
Scientists hoped the GRACE Follow-On mission, which will use the same gravity measurement principle with two formation-flying satellites, would launch before the end of GRACE’s mission, allowing scientists to calibriate the new spacecraft’s instruments by comparing their results to GRACE.
Engineers have struggled with battery woes on the GRACE-2 satellite for several years, and attempted to adjust the spacecraft’s operating parameters to be more energy-efficient. Waning fuel supplies and decaying altitude caused by atmospheric drag have also limited the mission’s operating lifetime.
The GRACE mission has produced more than 150 global gravity maps since launch. The results show how material on and inside Earth, such as water, ice and rock, move around over time, revealing insights into climate change, sea level rise, ocean circulation and droughts.
Once the final data collection phase is complete, controllers will maneuver one of the satellites, if necessary, to ensure there is no possibility of a collision with the other GRACE spacecraft, according to Alan Buis, a spokesperson at JPL.
Buis said engineers will empty the satellites’ fuel tanks before ending active control of the spacecraft.
“Under recently agreed to plans, the GRACE mission operations team will expend most of the remaining fuel aboard the satellites during the final science phase in October and early November,” Buis wrote in an email to Spaceflight Now. “There will be a small amount of fuel left after the science phase and some mission-end calibration activities are completed.
“As the fuel is exhausted, there will be a few final satellite settings to be made in terminating control of the satellites and then the team will let the mission end as the satellite altitude decreases, in an uncontrolled manner, due to the effects of atmospheric drag,” Buis wrote. “The team expects this to occur during the early part of 2018, but the actual date will be dependent on solar activity, which will influence the atmosphere density in a way that can’t be predicted.”
The satellites will largely burn up in the atmosphere during re-entry in early 2018. The actual date of re-entry can’t be predicted with certainty, Buis said.
“A few small pieces are expected to survive reentry and reach the ground, but the risk they pose is very small and is within NASA requirements for satellite re-entry,” Buis wrote. “The satellites will be tracked passively by NASA and the German Space Operations Center as they de-orbit and re-enter the atmosphere.”
The GRACE Follow-On satellites are scheduled for launch in the first quarter of 2018 on a SpaceX Falcon 9 rocket from Vandenberg Air Force Base in California. Built by Airbus Defense and Space, the twin satellites will launch in tandem with five Iridium communications payloads into orbit several hundred miles above Earth.
Iridium, NASA and the Helmholtz Centre-Potsdam German Research Centre for Geosciences, or GFZ, agreed to the shared launch earlier this year after the Russian government suspended launches of the Russian-Ukrainian Dnepr rocket, the GRACE Follow-On mission’s original choice.
The satellites were supposed to launch on a Dnepr rocket this year, allowing at least several months of carryover between GRACE and GRACE Follow-On.
Officials have not announced a scheduled date for the GRACE Follow-On launch, but Iridium chief executive Matt Desch said earlier this month that the shared rocket flight will be the sixth in the deployment sequence for Iridium’s next-generation voice and data relay constellation.
That puts the target launch date some time in February or March 2018.
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