For 20 years researchers at The University of Texas at Austin’s Center for Space Research have been Weighing the Earth from Outer Space with the GRACE missions. The GRACE mission, which started as a gravity measuring mission, has evolved into a tool for climate study in which the mass movement in and between the Earth system components has been recognized as a critical element. Estimates of variations in the total land water storage from GRACE have illuminated patterns in the water cycle that arise from natural climate variability and from human water consumption. A wide variety of hydrological processes continue to be observed globally from the tropics to the polar latitudes. Recent advances in the methods for assimilating GRACE data into hydrological models have also facilitated the overall GRACE contributions to hydrometeorology.
The GRACE Mission data provides new insights into the evolution of the Earth’s climate system by providing global mass change measurements at spatial resolution scales from 300 km to global and time scales from 30 days through the interannual and secular. The GRACE measurements are recognized as a key component of NASA’s climate measurement capability.
Dr. Byron Tapley, PI of the initial GRACE mission and former Director of UT’s Center for Space Research noted:
“The GRACE data products have made paradigm shifting contributions to the diverse areas of Earth System research including hydrology, oceanography, cryology, and solid earth science. It has provided a significant stimulus for interdisciplinary Earth Science investigations.”
GRAVITY Measurements as KEY for Climate change Measurements
The GRACE Mission
In 1997, the Gravity Recovery and Climate Experiment (GRACE) mission was selected by NASA as the first mission to be implemented under a new Office of Mission to Planet Earth program called the Earth System Science Pathfinder (ESSP) where the Mission PI at an academic institution was given responsibility for mission implementation. The objective of the GRACE mission was to characterize the spatial and temporal variations in the Earth’s mass transport, through accurate measurements of its gravity field. The science goals of the GRACE mission focused on enabling a better understanding of the mass change within and between the Earth System components (for applications in Oceanography, Hydrology, Cryology, Solid Earth Science, and Geodesy). The mission focused on using accurate global measurements of gravity change as an indicator of mass change. The variations of the Earth’s gravity field manifest variations in the orbits of the two co-orbiting Earth satellites. The GRACE science objectives were achieved globally and to a high accuracy by using micron level measurements of the gravity induced variations in the inter-satellite range between the two coplanar Earth satellites in low altitude, near-polar orbits.
In a collaboration between NASA, the German Aerospace Center (DLR), and the GeoForschungsZentrum (GFZ) in Potsdam, the two GRACE Mission satellites were developed and launched on March 17, 2002. The implementing team consisted of the University of Texas Center for Space Research, the Cal Tech Jet Propulsion laboratory, the GeoForschungsZentrum (GFZ) in Potsdam and the German Space Operations Center(GSOC).
The nominal mission was five years in length, but the importance of the paradigm shifting mass change measurements collected during the nominal mission led to strong support for continuing the mission. Through a sequence of bi-annual mission reviews, the mission was extended for an additional 10 years-over until propellant was exhausted in October 2017
The Grace time-varying gravity measurements have made paradigm shifting contributions in the diverse areas of Earth System research including hydrology, oceanography, cryology, and solid earth science.
The mission goals for GRACE were to produce, with unprecedented accuracy, global models for the time-variable gravity field of the Earth with an approximately 30-day sampling interval. The primary science objectives of the mission were realized by collecting the gravity induced changes at satellite altitudes over all points on the Earth’s surface.
The mission is characterized by:
- the unique GRACE ability to observe coupled surface and sub-surface processes within the Earth’s land, ocean, atmosphere, and cryosphere sub-systems;
- the consequent multidisciplinary viewpoint of the users of GRACE data products; and
- a cost-effective mission implementation achieved through the international cooperation enabled by NASA and DLR.
The GRACE measurements are recognized as a key component of NASA’s climate measurement capability. The temporal sequence of 163 approximate monthly estimates of the gravity field established a very accurate mission mean as well as a time history of its temporal variability. Examples where the GRACE measurements have had a significant impact include areas where the GRACE Measurements are unique in their contribution.
Tapley went on to say, “The GRACE measurements span multiple Earth Science Focus Areas. Examples of measurements uniquely determined or enabled by GRACE include:
- Mass change in the polar ice sheets, continental glaciers and the permafrost
- The quantity and location of land surface mass contribution to sea-level rise,
- Ocean mass change to enable separation of ocean thermal expansion (heat-content) from mass changes in the sea-level height change measurement, and the estimation of deep (> 2000 m) ocean heat content
- Global and regional measurements of the hydrological cycle: Seasonal and interannual river basin water storage changes; human influences on regional water storage changes; large-scale evapotranspiration; land-ocean mass exchange; and regional sub-surface aquifer change
- Change in the deep ocean currents and mass and energy transport; inter ocean-basin mass variations; and regional oceanic processes
- Large-scale regional geophysical isostatic adjustment and the associated post-glacial rebound
- Episodic mass displacements and mantle flow associated with large earthquakes.
The ability of GRACE to measure the mass-transport associated with water and ice distribution is essential for answering these questions.”
4.7 trillion tons of ice melted in Greenland
The Greenland ice sheet is melting – and much faster than expected. Every year, this ice sheet loses 277 gigatons of mass. One gigaton corresponds to a cube of one kilometer edge length, on the floor area of which a small town with about 2200 inhabitants would have space. “In the last twenty years, the Greenland ice sheet has lost around 4.7 trillion tons of ice. Danish researchers have gained these insights from the long GRACE data series, which are also regularly incorporated into the reports of the Intergovernmental Panel on Climate Change (IPCC). In the latest report, GRACE is the third most cited satellite mission. This underlines the importance of this mission for climate research,” emphasizes Walther Pelzer. But not only the melting of the ice is an important climate indicator. Several months of dry periods in the Amazon and drastic decreases in the groundwater level in northern India could be observed just as reliably with the GRACE data as the rise in sea level over two decades. In addition, the so-called Potsdam heavy potato was created from the GRACE data. This “gravitational globe” shows that the gravitational field is not evenly distributed. Thanks to this three-dimensional geoid, the deviations of the Earth’s shape are clearly visible. The influence of gravity is particularly strong over the Himalayas and the North Atlantic, but rather weakly over the Indian Ocean and the Lesser Antilles.
Water masses and continents are weighed from space
But how exactly do these satellites actually measure these mass transports? The trick is that masses in GRACE and GRACE-FO are recorded solely on the basis of their gravity effect. The two satellites each flew or fly at an average distance of only about 220 kilometers in a row. The relative distance and speed of the two satellites are constantly measured exactly with the help of microwaves and a laser. The accuracy of one to two micrometers corresponds to about one hundredth of the thickness of a sheet of printer paper. “Rock and water – whether in solid or liquid form – exert a weight force with their masses. If it is stronger, the satellite flying ahead is attracted to it during the overflight. As a result, it becomes faster and moves away from the other satellite. This minimal change in mutual distance is measured continuously over each orbit around the Earth. Smaller masses accelerate the advancing satellite less and cause an approach again. In a figurative sense, we can use the satellites to weigh like ice sheets and also the continents decrease or increase from month to month,” explains Peter Schaadt, GRACE-FO Program Manager at the German Space Agency at DLR. However, weighing does not take place in space, but on the basis of complicated calculation methods on the ground, whereby the minimal movements of the satellites in Earth orbit are translated into gravity field values.
In addition, the GRACE monthly gravity field products have become an important factor in applications such as drought monitoring in the U.S and global hazard assessment resulting from floods and earthquakes. and the GPS occultation data contributions to the daily monitoring of the international weather.
The Continuation of the GRACE Mission
The importance of the GRACE measurements for both climate change studies and operational issues related to assessing and managing water availability emphasized the need for continuing the measurements. In 2018 the GRACE FO Mission was launched in an extension of the highly successful collaboration between NASA, DLR and GFZ. The future continuation of the measurement is under consideration in a collaborative study by NASA, DLR and GFZ working together with the Max Planck Institutes to prepare a next mission to continue the observations following GRACE-FO. “The cooperation with NASA in Earth observation is a great sign of the common goals that the USA and Germany want to pursue in climate policy,” emphasizes Walther Pelzer, member of the DLR Executive Board and Head of the German Space Agency at DLR.
GRACE and GRACE-FO – Two successful missions for the Mass Change Measurement
GRACE, a joint mission of NASA and the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), operated until 2017 and thus operated three times longer than originally planned. The overall responsibility to NASA for the mission, along with scientific data analysis, resided with the University of Texas at Austin, implementation of the mission was delegated to the Jet Propulsion Laboratory, the GeoForschungsZentrum Potsdam (GFZ) was responsible for the German elements of the mission and the science data analysis. The mission operation was the responsibility of the German Space Operations Center at DLR in Oberpfaffenhofen. JPL managed the mission on behalf of NASA’s Science Mission Directorate in Washington. The GRACE satellites were built by Airbus (formerly Astrium) in Friedrichshafen under contract to NASA. The satellites for GRACE FO, the NASA-funded successor to the GRACE mission which has continued to carry out gravitational measurements since their launch on May 22, 2018, were also provided by Airbus.
Thanks to DLR for their contributions to this article.