Many of today's most pressing climate science challenges hinge on knowing how and where water is moving on Earth. But keeping track of Earth’s evolving water cycle is a formidable task. Water can be in plain sight in a lake or hidden underground. It can evaporate in moments when sunlight warms Earth's surface or be stored for centuries as ice in a glacier. It can be almost anywhere on, above and below Earth's surface, from your kitchen sink to the South Pole.

Regardless of whether water is solid, liquid or vapor, visible or invisible, it has one attribute that does not change: its mass, which exerts a gravitational pull. By tracking the changing pull of gravity very precisely around Earth, the U.S./German Gravity Recovery and Climate Experiment, or GRACE, mission observed the movement of water around our planet from 2002 to 2017 -- from the top of the Himalayas to the depths of the ocean to deep underground. GRACE Follow-On, set to launch in spring of 2018, will continue GRACE’s critical mission of tracking the evolution of Earth’s water cycle by monitoring changes in the distribution of mass on Earth. It will also continue the successful partnership between NASA and Germany that began on the original GRACE mission, via NASA’s GRACE-FO mission partner, the German Research Centre for Geosciences (GeoForschungsZentrum -- GFZ).

Maintaining a consistent, continuous climate data record of water and mass transport in the Earth system over decades is essential to understanding short-term climate variability and long-term climate change. Because some climate patterns take several decades to unfold, the only way to determine whether a multi-year trend is representative of a long-term change is to extend the length of the observational record.

This visualization of a gravity model was created with data from NASA's Gravity Recovery and Climate Experiment (GRACE) and shows variations in Earth’s gravity field.

GRACE-FO data will improve our understanding of Earth system processes, and the accuracy of environmental monitoring and forecasts, by extending GRACE's legacy of scientific achievements, which includes several thousand scientific publications.

GRACE Observations of Antarctic Ice Mass Change (map and graph)

GRACE Observations of Antarctic Ice Mass Change
Image credit: NASA/GSFC/SVS

These include:

  • Tracking mass changes of Earth's polar ice sheets.
  • Estimating global groundwater storage changes.
  • Measuring mass changes caused by large earthquakes.
  • Inferring changes in deep ocean currents, a driving force in climate.

Monitoring changes in ice sheets and glaciers, underground water storage, the amount of water in large lakes and rivers, and changes in sea level provides a unique view of Earth’s evolving climate and its water and energy cycles, with far-reaching societal benefits.

U.S. map, groundwater drought indicator

U.S. map, groundwater drought indicator
Image credit: NDMC

The importance of mass change measurements from space -- first provided by GRACE and now to be continued by GRACE-FO -- to understanding the Earth system was recognized by the National Academy of Sciences in its 2017-2027 Decadal Survey, Thriving on Our Changing Planet, released in December 2017. In this community report, mass change measurements were identified as one of the five highest-priority Earth observation needs that are considered essential to NASA’s overall Earth science program, addressing the “most” and “very important” objectives for studies of climate, hydrology and the solid Earth. The measurements were also identified as contributors to the water and energy-cycle Earth System Science themes identified in the report. The survey recommended that mass change measurements be continued on future missions to maintain continuity with the GRACE and GRACE-FO data records.

GRACE Observations of Greenland Ice Mass Changes

GRACE Observations of Greenland Ice Mass Changes
Image credit: NASA/GSFC/SVS

While similar in design to GRACE, GRACE-FO incorporates lessons learned from 15 years of GRACE operations. The changes made will improve the new mission’s satellite performance and reliability, as well as mission operations. GRACE-FO will also fly a technology demonstration of a new, more precise inter-satellite laser ranging interferometer, developed by a German/U.S. instrument team, for use in future generations of GRACE-like missions.

Artist Concept of the GRACE-FO satellites orbiting Earth.

Illustration of GRACE-FO orbiting Earth
Image credit: NASA/JPL-Caltech

Launch services for GRACE-FO are contributed by Germany, which procured a rideshare for the two GRACE-FO satellites from Iridium Communications Inc. The GRACE-FO satellites and five Iridium NEXT communications satellites will be launched into orbit together on a SpaceX Falcon 9 rocket from Vandenberg Air Force Base in California. This unique rideshare launch will first deploy the GRACE-FO spacecraft, then the Falcon 9 second stage will continue onward to the deployment orbit for the Iridium NEXT satellites.