Weight of world, by GRACE of space

GRACE-FO launch: Falcon 9, Vandenberg (NASA/Bill Ingalls)

23 May 2018

A joint US/German two-satellite mission to monitor the movement of water and other changes to the Earth’s mass – a key measure of the impact of climate change – began its five-year mission on Tuesday. The pair of Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) spacecraft shared the ride into space from California’s Vandenberg air force base on a SpaceX Falcon 9 rocket with five Iridium NEXT communication satellites.

GRACE-FO, a partnership between NASA and the German Research Centre for Geosciences (GeoForschungsZentrum, or GFZ), will continue the work of the original GRACE mission, which operated from 2002 through 2017, measuring changes to Earth’s gravitational pull. As NASA explains, more than 99% of Earth’s gravitational pull doesn’t change from one month to the next, because it represents the mass of the solid Earth itself. However, a tiny fraction is constantly on the move, and it is mostly water: rain is falling, ocean currents are flowing and ice is melting.

GRACE-FO’s maps of regional variations in gravity will show where that small fraction of overall planetary mass is moving every month. This is possible because of the twin satellite nature of the mission. Gravity changes cause the distance between the two satellites to vary slightly. Flying tandem in a near-circular polar orbit of almost 300 miles (490km) altitude, the pair will be separated by 137 miles (220 kilometers) – onboard instruments are capable of continuously measuring the distance between them within the width of a human red blood cell.

As well as providing scientific benefits, it is hoped GRACE-FO data “will be used throughout the world to improve peoples lives lives – from better predictions of drought impacts to higher-quality information on use and management of water from underground aquifers,” says Thomas Zurbuchen, associate administrator at NASA’s Science Mission Directorate.

GRACE was the first mission to measure the amount of ice being lost from the Greenland and Antarctic ice sheets. It has also improved understanding of the processes responsible for sea level rise and ocean circulation and provided insights into where global groundwater resources are shrinking or growing. According to NASA, a third of the world’s underground aquifers are being drained faster than they can be replenished. Melting ice sheets and dwindling aquifers are contributing to Earth’s rotational wobbles. And Australia seesaws up and down by two or three millimetres each year because of changes to Earth’s centre of mass that are caused by the movement of water.

The GRACE-FO satellites will spend their first few days in space moving to the separation distance needed to perform their mission. When they reach this distance, the mission will begin an 85-day, in-orbit “checkout” phase. Then the satellites – which at 300 miles altitude take about 90 minutes to orbit – will begin gathering and processing science data. The first science data are expected to be released in about seven months.

The spacecraft were built by Airbus Defence and Space in Friedrichshafen, Germany, under subcontract to NASA’s Jet Propulsion Laboratory, which is managing the mission.

Changes to distance between twin GRACE-FO satellites indicates variations in Earth’s gravitational pull (artist’s impression; NASA)