NASA prepares to retire GRACE Earth science satellites

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WASHINGTON — An aging German-American Earth science mission will come to an end this fall, months before the launch of a next-generation satellite pair.

In a statement Sept. 14, NASA’s Jet Propulsion Laboratory announced it expected the Gravity Recovery and Climate Experiment (GRACE) satellite duo to make its last science observations in November, more than 15 years after their launch.

That assessment comes after recent problems with one of the twin spacecraft, designated GRACE-2. Controllers lost contact with the spacecraft Sept. 4, a day after the eighth of 20 battery cells on the spacecraft failed. They restored communications with GRACE-2 on Sept. 8, found that the failed battery cell was back to full voltage and concluded that the spacecraft could continue operating.

The two GRACE spacecraft, developed as a joint project by NASA and the German Aerospace Center (DLR), operate in low Earth orbit, separated from each other by 220 kilometers. Scientists use measurements of very small changes in that separation to calculate local changes in gravitational field caused by mass variations in the Earth. Those measurements can, in turn, be used to track motions of water around the Earth caused by seasonal patterns and climate processes.

The two GRACE spacecraft launched in March 2002 on a Russian Rockot vehicle under a contract arranged by DLR. The spacecraft have long surpassed their original five-year lifetime.

That mission, though, is coming to an end because of the recent issues with GRACE-2, coupled with it running out of fuel. In the Sept. 14 statement, JPL said the two spacecraft would be decommissioned after one final science collection phase scheduled for mid-October through early November.

The decommissioning, JPL spokesman Alan Buis said Sept. 20, will include maneuvering one of the satellites to eliminate any chance it could collide with the other, followed by other steps to render the spacecraft inert. The spacecraft will make an uncontrolled reentry some time in early 2018, with the exact time dependent on solar activity and its effects on the Earth’s atmosphere.

Buis said that most of each spacecraft will burn up upon reentry. “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 reentry,” he said.

That schedule will leave a data gap of potentially several months before the launch of a replacement pair of satellites known as GRACE Follow On, or GRACE-FO. The satellites, built again as a joint German-American project, are similar to the original GRACE spacecraft, but with the addition of a laser interferometer for more accurate measurements of their separation.

The two GRACE-FO satellites were originally planned to launch on a Dnepr rocket from the Baikonur Cosmodrome in Kazakhstan under a contract the German Research Centre for Geosciences (GFZ) arranged with Kosmotras. However, the Dnepr has not launched since March 2015 because of difficulties winning Russian government approval for additional launches of the Ukrainian-built rocket.

In January, Iridium and GFZ announced they were partnering on the purchase of a SpaceX Falcon 9 launch that will carry the two GRACE-FO satellites as well as five Iridium Next communications satellites. That launch is in addition to seven dedicated Falcon 9 launches Iridium previously purchased, each carrying 10 Iridium Next satellites.

A launch date for the joint Iridium Next/GRACE-FO mission has not been set, JPL’s Buis said, other than it is expected to occur in early 2018. NASA’s fiscal year 2018 budget proposal, published in May, projected a February 2018 launch of GRACE-FO.

Jordan Hassin, a spokesman for Iridium, said Sept. 19 that the joint mission would likely be the fifth or sixth in the overall sequence of Iridium Next launches, and would not be the last. The third Falcon 9 Iridium launch is currently scheduled for Oct. 4, Iridium confirmed in a Sept. 20 release.