31.01.2017: Since 2002 the satellite mission GRACE (Gravity Recovery and Climate Experiment) is in orbit to measure the Earth’s gravity field. Unfortunately, around summer this year the two satellites will run out of fuel, but a successor is already in preparation. The twin GRACE-FO (Follow On) satellites, which will operate in tandem as well, will be launched on a Falcon 9 of SpaceX as a rideshare with satellites of Iridium Communications Inc. The rideshare is anticipated to launch out of Vandenberg Air Force Base in California by early 2018 as stated in a joint announcement of Iridium, NASA and the GFZ.

“GRACE-FO will continue GRACE’s legacy of tracking changes in the distribution of Earth’s mass over time by creating monthly maps of Earth’s gravity field”, says Frank Flechtner, project manager of the mission at the GFZ. The movements of masses of water, ice, air, and the solid Earth are driven by processes such as precipitation, droughts, floods, the melting of snow and ice, ground water usage and storage, and even tectonic events such as large earthquakes. “GRACE is improving our understanding and knowledge of a variety of important Earth system processes such as the terrestrial water cycle and changes in ice sheets, glaciers and sea level or surface and deep-ocean currents. These climate change related measurements provide a unique view of the Earth system and have far-reaching benefits to society.”

In order to measure the Earth’s gravity field two identically constructed satellites will circle the planet within a distance of 220 kilometers on the same low polar orbit in a height of about 500 kilometers. The distance between the two satellites will be constantly measured by a microwave instrument with a precision of 0.002 millimeters. Any distance changes are subject to gravitation and allow for a precise modelling of the Earth’s gravity field. Throughout the entire mission of at least five years the measurements will provide an updated model of the field every 30 days.

For the first time, the satellite tandem is equipped with a Laser Ranging Interferometer (LRI). The LRI has significant contributions from Germany and allows measurements of the distance between the satellites about 30 times more precisely than via the microwave instrument. This technology demonstration serves as a preparation of future GRACE-like satellite missions.

GRACE-FO is a joint mission with NASA. Within NASA the JPL is responsible for the realization of the GRACE FO project. The German contributions are managed by the GFZ and jointly funded by the Federal Ministry of Education and Research, the Federal Ministry of Economics and Energy, the Helmholtz Association, the German Aerospace Center, and the GFZ.

The German contribution to the mission is € 77.7 million accounting for about a quarter of the total costs. The GFZ’s share is € 13 million, amongst others for mission control at the German Space Operations Center (GSOC) of the DLR in Oberpfaffenhofen.

The twin satellites were built by Airbus Defence and Space. At the moment they will be tested at the IABG in Ottobrunn for operation in space environment. The mission life time is planned for at least five years. (rn)

>> Joint press release of Iridium Communications Inc., NASA und GFZ