With the German-American satellite mission GRACE-FO ("Gravity Recovery and Climate Experiment Follow-On"), the measurement of the Earth's gravity field and its temporal change will enter “extra time”. Between the years 2002 and 2017, the predecessor mission GRACE measured the Earth’s gravity field extremely precisely and was a full success. The data collected hereby help, for example, to document the loss of ice mass or an overuse of groundwater resources. This highly valuable series of measurements will now be continued thanks to the GRACE-FO mission, a joint project of the US Space Agency NASA and the Helmholtz Center Potsdam, GFZ German Research Center for Geosciences, which heads the German contributions. The launch of the two satellites, constructed by Airbus, is scheduled for Saturday, 19 May 2018, on board a SpaceX Falcon 9 rocket from the Vandenberg Air Force Base in California. "Thanks to improved hardware and software, the data collected will be even more accurate," says Reinhard Hüttl, Scientific Executive Director of the GFZ. "They will allow for a better characterization and understanding of the dynamic processes in the Earth system."
The basis of research is the fact that the mass within the Earth and on its surface is not evenly distributed all over the planet. Within the Earth’s molten interior, rock shifts around, water masses flow in the oceans as well as on the continents, and air masses are also in constant motion. Since the gravity force of a body depends on its mass, the irregular mass distribution on our planet leads to an inhomogeneous gravity field.
The satellites of the GRACE-FO mission, flying one behind the other at a distance of about 220 km are, thus, affected at staggered intervals by sometimes stronger and sometimes weaker gravitational forces depending on how much mass is below them. This leads to a small change in the distance between the satellites which, thanks to a precise microwave technology, can be determined to a few thousandths of a millimeter. In comparison, this would represent a detection of a variation in length on route from Potsdam to Hanover corresponding to one tenth of the diameter of a human hair. As a result, even small mass differences in the Earth system can be detected. Since the two satellites orbit continuously around the Earth, both spatial and temporal changes in the gravitational field can be documented.
"The primary goal of this mission is to create global monthly gravity maps. With the help of these data, various changes in the Earth system can be reconstructed," explains Frank Flechtner, GFZ Project Manager of the mission. The predecessor mission GRACE revealed that the ice mass loss in Greenland was around 270 billion tons per year between 2002 and 2016, a good 20 billion tons more than previously assumed. Flechtner: "GRACE-FO will continue to track developments in Greenland, but also in the Antarctic and other ice regions, and will provide up-to-date data."
In addition, the mission can detect changes in groundwater levels in large basins - without on-site measurements. These include losses such as recently observed in California or the Middle East, as well as increasing underground water levels. This case is equally exciting for scientists as a well-filled aquifer means that less water seeps away after heavy rains and this, in turn, increases the risk of flooding. GRACE-FO should help to recognize such a threat at an early stage.
The measurement data are also important for the marine sector as they serve, for example, to investigate the sea-level rise. Data on the gravity field can be used to determine the actual proportion of additional water - for example from melting glaciers – contributing to the rising water levels and what share is attributable to the thermal expansion of existing seawater. Furthermore, the data from GRACE-FO are also used to study ocean currents.
A further objective is the measurement of the status parameters of the atmosphere with the help of GPS radio occultation. This method is based on the fact that the radio signals received by GRACE-FO from the GPS satellites vanishing behind the planet are subject to varying degrees of refraction due to temperature and humidity-related density changes in the atmosphere. These changes can be reconstructed from the GPS signals recorded on board the satellites. The atmospheric measurements obtained via GRACE-FO are made available by the GFZ to various international meteorological centers within approximately two hours after recording where they are used to improve their daily forecasts.
In addition, the satellites carry a new laser-based system for distance measurement (Laser Ranging Interferometer, LRI), which was developed at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute Hannover) together with NASA Jet Propulsion Laboratory as a technology demonstrator. This can determine the distance between the two spacecraft even more precisely than before: up to 80 nanometers, which corresponds approximately to the diameter of a hepatitis B virus. A significantly improved distance measurement between the two satellites means that the collected heavy field data are even more accurate.
"GRACE Follow-on will build on the success of GRACE both in scientific terms, where we expect a wealth of new insights into climate-related changes on Earth, but also in terms of the close cooperation between Germany and the US, which has already proven to be most profitable during the preparation phase," says Michael Watkins, science director of the mission and Director of NASA's Jet Propulsion Laboratory in Pasadena, California.
As with GRACE, the two satellites were built by Airbus in Friedrichshafen. Operation of the mission is carried out by the space control center of the German Aerospace Center (DLR / GSOC) on behalf of the GFZ. After a successful launch, the spacecraft will gradually be put into operation while in orbit and, thus, prepared for permanent use. For the summer, the participating researchers expect the first scientific data. The mission is planned for an initial duration of five years, an extension is possible.
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