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GRACE follow-on satellites peer beneath the Earth’s surface

GRACE follow-on satellites peer beneath the Earth’s surface

Image: Twin GRACE-FO satellites follow each other in orbit around the Earth. Credit: NASA

Featuring in the 2018-19 edition of the ARC’s Making a Difference publication, May 2018 saw the launch of the twin Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) satellites on board a SpaceX Falcon 9 rocket. The satellites were carrying laser technology adapted from space-based gravitational wave detectors, developed by researchers at the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).

The core technology at the heart of NASA's GRACE-FO mission draws on techniques developed for an upcoming space-based gravitational wave mission planned by the European Space Agency, the Laser Interferometer Space Antenna (LISA). GRACE-FO, like LISA, uses a laser-based technique known as heterodyne interferometry to detect tiny changes in the distance between the orbiting satellites. GRACE-FO will also test other techniques that LISA will rely on, such as Time-Delay Interferometry and Arm Locking, that are challenging to fully test in the lab.

The launch was particularly exciting and nerve-wracking for OzGrav Chief Investigator Professor Daniel Shaddock, based at The Australian National University (ANU), as 15 years of his work was on board the rocket. "It was a little bit surreal," Professor Shaddock says. "So many years of your life working on something—it was hard to believe it was actually happening and finally launching. And the most exciting part is still yet to come."

Professor Shaddock developed a retroreflector that uses lasers to measure the world's water reserves from space with unprecedented accuracy. 

“It measures something that's really important; the presence of water—whether that's frozen form or liquid form—across the entire globe at once. And that's something you can only do from space. 

“Any large body of water will generate gravity and that gravity can be picked up by GRACE,” Professor Shaddock explains.

For the device to work, two laser beams from two separate satellites—each travelling at thousands of kilometres per hour—need to link with each other from a distance of over two hundred kilometres. The link acquisition system was initially designed and tested by researchers at the ANU in collaboration with international colleagues.

In the case of the Laser Ranging Interferometer, the technology can pick up changes in the separation of the spacecraft by ten nanometres. That's ten billionths of a metre—about the diameter of a virus.

This sensitivity gives scientists the extraordinary ability to peer beneath the Earth's surface to weigh groundwater reserves where a third of all freshwater lies. In Australia, the first GRACE mission has already shown groundwater levels in the Murray-Darling Basin still have not recovered from the Millennium Drought, which ended in 2011. The mission also revealed that freshwater is disappearing from Greenland and West Antarctica faster than any other place on Earth as ice caps melt. 

Such data gives scientists the solid numbers to state with increased confidence how much ice is disappearing, how much is ending up in the oceans, and also how it is changing our water cycle and our water resources.

Now that Australia has a Space Agency, OzGrav researchers look forward to making even greater contributions to NASA’s next GRACE mission, planned for the middle of next decade, as well as to a space-based gravitational wave detector

Image: Twin GRACE-FO satellites follow each other in orbit around the Earth. Credit: NASA

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