Keeping an eye on Earth's water cycle, ice sheets and crust from space

A pair of spacecraft that will observe our planet's ever-changing water cycle, ice sheets and crust are in final preparations for launch from California no earlier than May 19. The Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission, a partnership between NASA and the German Research Centre for Geosciences, will take over where the first GRACE mission left off when it completed its 15-year mission in 2017.

GRACE-FO will continue monitoring monthly changes in the distribution of mass within and among Earth's atmosphere, oceans, land and ice sheets, as well as within the solid Earth itself. These data will provide unique insights into Earth's changing climate, Earth system processes and even the impacts of some human activities, and will have far-reaching benefits to society, such as improving water resource management.

"Water is critical to every aspect of life on Earth -- for health, for agriculture, for maintaining our way of living," said Michael Watkins, GRACE-FO science lead and director of NASA's Jet Propulsion Laboratory in Pasadena, California. "You can't manage it well until you can measure it. GRACE-FO provides a unique way to measure water in many of its phases, allowing us to manage water resources more effectively."

Like GRACE, the first mission, GRACE-FO will use an innovative technique to observe something that can't be seen directly from space. It uses the weight of water to measure its movement -- even water hidden far below Earth's surface. GRACE-FO will do this by very precisely measuring the changes in the shape of Earth's gravity field caused by the movement of massive amounts of water, ice and solid Earth.

"When water is underground, it's impossible to directly observe from space. There's no picture you can take or radar you can bounce off the surface to measure changes in that deep water," said Watkins. "But it has mass, and GRACE-FO is almost the only way we have of observing it on large scales. Similarly, tracking changes in the total mass of the polar ice sheets is also very difficult, but GRACE-FO essentially puts a 'scale' under them to track their changes over time."

Like its predecessors, the two identical GRACE-FO satellites will function as a single instrument. The satellites will orbit Earth about 137 miles (220 kilometers) apart, at an initial altitude of about 305 miles (490 kilometers). Each satellite continually sends microwave signals to the other to accurately measure changes in the distance between them. As they fly over a massive Earth feature, such as a mountain range or underground aquifer, the gravitational pull of that feature tugs on the satellites, changing the distance separating them. By tracking changes in their separation distance with incredible accuracy -- to less than the thickness of a human hair -- the satellites are able to map these regional gravity changes.

GRACE-FO will be launched into orbit with five Iridium NEXT communications satellites on a commercially procured SpaceX Falcon 9 rocket from Vandenberg Air Force Base in California.