Spaceflight Insider

GRACE mission reaches 15-year mark

Artist's concept of the twin GRACE satellites. Image Credit: NASA

Artist’s concept of the twin GRACE satellites. Image Credit: NASA

The twin satellites of the Gravity Recovery and Climate Experiment (GRACE) mission have been collecting data about how water moves and is stored around Earth for 15 years – three times longer than originally planned. GRACE, a joint effort between NASA and two German agencies, was launched on March 17, 2002. 

“With GRACE, we effectively created a new field of spaceborne remote sensing: tracking the movement of water via its mass,” said Michael Watkins, the original GRACE project scientist and now director of NASA’s Jet Propulsion Laboratory, Pasadena, California.

The greater an object’s mass is, the greater its gravitational pull. Massive mountain ranges exert more gravitational pull than flat plains. While this small difference isn’t noticeable to humans, it can be detected by satellites. When they orbit the Earth, satellites speed up slightly as they approach a massive object and slow down as they move away from it. Measuring changes in mass has been crucial to discovering how water and the solid Earth are changing in places that can’t be seen directly.

“The completely new idea about GRACE was the perception that measuring and tracking mass gives you a way to probe the Earth system,” said Principal Investigator Byron Tapley, University of Texas Center for Space Research (UTCSR) at Austin.

GRACE measures changes in mass through their effects on the two satellites orbiting Earth one behind the other about 137 miles (220 kilometers) apart. Small changes in the speed of the spacecraft caused by the mass below them alter the distance between them very slightly, by a few microns (a fraction of the diameter of a human hair). The spacecraft constantly beam microwave pulses at each other to measure the constantly fluctuating distance between them.

Onboard GPS keeps track of where the satellites are in relation to Earth’s surface, and onboard accelerators record forces on the spacecraft other than gravity – such as solar radiation and atmospheric drag. Scientists process these data to produce monthly maps of regional variations in global gravity which show how water on or near Earth’s surface has moved each month.

GRACE is a collaboration between NASA and two German partners: the Deutsches Zentrum für Luft- und Raumfahrt (DLR, the German Aerospace agency) and the Helmholtz Centre Potsdam German Research Centre for Geosciences (GFZ). The twin satellites were built in Germany at Airbus Defence and Space and DLR procured a Russian “Rockot” as the launch vehicle.

Researchers worldwide have used the data collected by GRACE during its 15 years of operation. GRACE has contributed greatly to the measurement of water stored in soil and underground aquifers and data from the mission are used in preparing weekly maps of U.S. drought risk.

“I can’t think of another set of measurements that have so revolutionized the science,” said JPL Senior Water Scientist Jay Famiglietti.

Scientists studying ice sheets and glaciers have used the GRACE data set to measure the rate of ice loss in Antarctica and Greenland, two of the world’s most difficult places to collect data. Since GRACE launched, its measurements show Greenland has been losing about 280 gigatons of ice per year on average – a bit less than twice the weight of Mt. Everest – and Antarctica has lost slightly under 120 gigatons a year. GRACE data can also be used to study changes in sea levels and ocean temperatures.

While GRACE project managers have been able to extend the mission for three times as long as originally planned, the spacecraft will probably run out of fuel this summer. NASA and GFZ have been working since 2012 on a new mission called GRACE Follow-On (GRACE-FO). Germany will again procure the launch vehicle and the twin satellites built at Airbus in Germany.

“With GRACE, we have gained new insight into how global and regional water resources are evolving,” said Frank Webb, the GRACE-FO project scientist. “Through GRACE-FO, we will extend into the next decade our capacity to gain an accurate picture of the global water cycle.”

GRACE-FO is scheduled to launch sometime between December 2017 and February 2018. The new satellites will use similar hardware to GRACE and will carry a technology demonstrator with a new laser ranging instrument to track the distance between the two satellites. The laser instrument has the potential to produce an even more accurate measurement.

“GRACE-FO allows us to continue the revolutionary legacy of GRACE,” said JPL’s Watkins. “There are sure to be more unexpected and innovative findings ahead.”

Video courtesy of NASA



Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise. While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004. Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.

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