US-German mission to renew study of icy regions

Flying at 500km, one GRACE-FO spacecraft will trail the other in polar orbit by just 220km (NASA)

31 March 2018

2018 is set to be a big year for study of Earth’s ice sheets, glaciers, sea ice, snow cover, and permafrost – collectively known as the “cryosphere”. As soon as 10 May comes the launch of GRACE-FO, a follow-on mission to the Gravity Recovery and Climate Experiment, run jointly by NASA’s Jet Propulsion Laboratory and the Postdam-based German Research Centre for Geosciences (GFZ). Then, in the autumn, NASA will launch the Ice, Cloud, and land Elevation Satellite-2 (ICESat-2), equipped with an advanced laser instrument to measure the changes in ice sheet elevation over the course of a year to within 4mm.

Like its predecessor, GRACE-FO, pictured, is a tandem satellite mission to measure fluctuations in Earth’s gravity field and so allow scientists to track Earth’s mass redistribution, and monitor changes in underground water storage, ice sheets, glaciers and sea level. The two identical satellites will orbit at about 500km, one trailing the other by about 220km. Variations in the distance between them indicate variations in local gravitational pull – caused by local changes in mass, which can be affected by weather patterns, seasonal change, climate change and even large earthquakes.

Like GRACE, GRACE-FO works by using a microwave ranging system to measure the distance between a pair of spacecraft to within a fraction of the width of a human hair. Combined with GPS tracking for timing, star trackers to measure attitude and accelerometers to account for non-gravitational effects like atmospheric drag or solar radiation, those distance measurements feed into a monthly map of Earth’s gravity field.

Built in Germany by Airbus Defence & Space, the two FO spacecraft are now awaiting a Falcon-9 launch from Vandenburg Air Force Base in California.

The original GRACE mission – also a NASA-GZW partnership – launched in 2002 and took its final science measurements in October 2017, far exceeding its 5-year design life and ending only with the age-related battery failure of one of the spacecraft.

According to JPL’s GRACE-FO programme manager Frank Webb: “GRACE data have revolutionised our understanding of Earth’s water cycle and how water and ice are distributed on the planet. From it, we can see clear trends in the ice-mass loss in the Arctic and the Antarctic, and clear trends in droughts in South America, Australia and Asia. These are key indicators of how the planet is responding to changes in our climate.”

GRACE, he adds, has been instrumental in documenting the loss of groundwater in California and around the world.

Bryon Tapley of the University of Texas at Austin, who shares GRACE principal investigator duties with GFZ’s Frank Flechtner, describes the mission as having “provided paradigm-shifting insights into the interactions of our planet’s ocean, atmosphere and solid Earth components.”

Recent applications, he adds, “include monitoring and managing global water resources used for consumption, agriculture and industry, and assessing flood and earthquake hazards.”

These images, created with GRACE data, show changes in Antarctic ice mass since 2002. In general, areas near the center of Antarctica experienced small amounts of positive or negative change, while the West Antarctic Ice Sheet experienced a significant ice mass loss (dark red) over the fourteen-year period (NASA)