GRACE Follow-On successfully started

The tandem satellites document global change.

The German-American satellite mission, GRACE-FO (Gravity Recovery and Climate Experiment Follow-On), was successfully launched. On 22 May at 21:47 hours (CEST) the two satellites aboard a SpaceX Falcon 9 rocket lifted off from the Vandenberg Air Force Base (California) and were set in polar orbit. Now, controlled from Oberpfaffenhofen, they will then settle into their final position during the course of the coming months.

The GRACE Follow-On mission will measure the Earth's gravity field and its spatial and temporal variations very accurately. It, thus, provides for precise information on global change, in particular on variations in the hydrological balance such as ice mass loss, ocean currents and changes in groundwater levels.

"We are very pleased that now with GRACE Follow-On the highly successful measurements performed by its predecessor mission, GRACE, which was active from 2002 to 2017, can now be continued. This will allow not only for documentation of current changes in System Earth, but also for the identification of long-term trends that often appear after several years or even decades," says Reinhard Hüttl, Scientific Executive Director of the Helmholtz Centre Potsdam – GFZ German Research Centre for Geosciences, which heads the German contributions to the mission.

GRACE Follow-On is a joint mission of GFZ and the US Space Agency NASA. Here, the close cooperation between the two countries which had already started with the previous GRACE mission will continue. For example, the two satellites were again built by Airbus in Friedrichshafen, contracted by the Jet Propulsion Laboratory of NASA in Pasadena (California). In addition, there are further collaborations in science and technology development, largely supported by funds from the Federal Ministry of Education and Research (BMBF) and the Federal Ministry for Economic Affairs and Energy (BMWi).

As the NASA mission partner, the GFZ German Research Centre for Geosciences takes the lead in the collection and analysis of the recovered data. More than 5000 users worldwide are eagerly awaiting the new results. The Max Planck Institute for Gravitational Physics (Albert Einstein Institute) Hannover is largely involved in the development of a new laser process, which will significantly increase the measurement accuracy of GRACE Follow-On and which will also be used in future space missions such as the satellite-based gravitational wave observatory LISA.

According to Federal Research Minister, Anja Karliczek, "The GRACE Follow-On mission with its long-term measurement of the changes in the Earth's gravity field will make a critical contribution to understanding climate change on our planet, including its causes and the influence of human activity. This mission is an impressive example of how research in Germany contributes to the success of major international projects."

"For the Helmholtz Association, the successful launch of the GRACE Follow-On mission is an important milestone. The geosciences will, thus, gain valuable insights for a better understanding of global change and its impact on humanity," says Otmar D. Wiestler, President of the Helmholtz Association. GRACE Follow-On is a prime example of the systemic approach of our research: This mission is made possible through large infrastructures that are used by many scientists from a wide range of disciplines, both nationally and internationally, and that are operated on a long-term basis. This is how we contribute to solving major societal challenges.

In this specific case, Germany has contributed 77.7 million Euro to the mission. The GFZ's share amounts to around 20 million euros. NASA is providing funding of approx. 430 million dollars (about 360 million Euros).

The basis of research within this mission is the fact that mass distribution is not constant within the interior and on the surface of our planet. Molten rock shifts around within the Earth’s interior, 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 tandem 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 process, 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, leading scientist of the mission at the GFZ. The predecessor mission, GRACE, revealed that the ice mass loss in Greenland was around 270 billion tons per year between 2002 and 2016. Flechtner: “GRACE-FO will continue to track developments in Greenland but also in the Antarctic and other ice regions and will deliver 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 in 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 will be made available by the GFZ to various international meteorological centers within approximately two hours after recording where they shall be used to improve 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 the NASA Jet Propulsion Laboratory as a technology demonstrator. This can determine the distance between the two spacecrafts 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 gravity field data are yet again 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, Scientific Director of the Mission and Director of NASA's Jet Propulsion Laboratory in Pasadena, California.

As with GRACE, the two satellites were again constructed by Airbus in Friedrichshafen, contracted by the Jet Propulsion Laboratory of NASA. The launch was a 'rideshare' with five communication satellites from the US company Iridium Communications Inc. Operation of the mission is carried out by the German Space Operation Center in 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.

Josef Zens
Deutsches GeoForschungsZentrum GFZ
Medien und Kommunikation
14473 Potsdam
Tel.: +49 (0)331/288- 1040
Fax: +49 (0)331/288- 1044

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