20 years ago: Launch of the GRACE geodesy mission

Anniversary event at the GFZ German Research Centre for Geosciences looks back and towards the future

March 25, 2022

March 17 marked the 20th anniversary of the launch of GRACE into its Earth orbit. The occasion was celebrated with an event at the GFZ German Research Center for Geosciences in Potsdam. Members of the Albert Einstein Institute (AEI) in Hannover were among the guests. Until 2015, GRACE monitored the Earth's gravitational field, the global water balance and indicators of global warming from its Earth orbit. Since 2018, the GRACE Follow-On mission has continued the valuable measurement series. For the first time, a novel and highly precise German-US laser instrument, on the German side developed under the leadership of AEI Hannover is being used. It has proven to be reliable and will be the standard for similar missions in the future. GRACE-I is the planned successor of GRACE Follow-On. In addition to observing the Earth’s gravitational field, ICARUS – a project observing animal migrations from space – and a novel technology demonstrator are to be on board.

How does GRACE Follow-On observe ice and water on Earth?

The GRACE Follow-On satellite pair circles the Earth 490 kilometers above its surface. The satellites follow one another in a distance of 220 kilometers in a 90-minute orbit, which takes the satellites above the Earth’s poles. The inter-satellite distance varies over some hundreds of meters each orbit because of the flattening of the Earth. On top of these large changes are much smaller variations in the micrometer and nanometer range, caused by the local structure of Earth’s gravity field from mountain ranges, ice masses, bodies of ground water, and others.

Observing how these minuscule variations change over the course of months allows researchers to precisely detect the melting of ice sheets in Greenland and the Antarctic, rising sea levels, changing groundwater levels, droughts and floods, and to define the geoid, which is at the base of global height measurements. The novel Laser Ranging Interferometer technology could significantly increase the precision on future GRACE Follow-On like missions to help those missions provide more detailed measurements of Earth’s gravity field and its changes over time.

Why is GRACE Follow-On an important milestone on the path to LISA?

The GRACE Follow-On Laser Ranging Interferometer is the second space-borne laser interferometer with significant contributions from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) and Leibniz Universität in Hannover, Germany. The first such instrument was a 40-centimeter instrument on board the LISA Pathfinder mission, which showed the key technologies for LISA, the future gravitational-wave observatory in space.

After its launch in 2034 LISA will use technologies very similar to those of GRACE Follow-On to measure tiny length changes over a distance of 2.5 million kilometers. LISA will detect low-frequency gravitational waves from millions of binary stars in our Milky Way, merging supermassive black holes in the entire Universe, and from other exotic objects.

Who is involved in GRACE Follow-On?

GRACE Follow-On is a joint project of NASA and German partners led by the GFZ German Research Centre for Geosciences. It is the improved successor to the successful GRACE mission which operated from 2002 to 2017. The project is funded by the Federal Ministry of Education and Research under the code 03F0654B.

Who is involved in the Laser Ranging Interferometer?

The LRI is a cooperation between NASA and German partners, with the German contribution led by the AEI. In Germany, the LRI concept, its prototypes and technical specifications were done at the AEI. AEI researchers have been heavily involved in developing and testing the flight hardware. The development of the LRI is based on a longstanding partnership between AEI and NASA’s Jet Propulsion Laboratory. Future missions can rely solely on the more precise laser interferometers instead of microwave interferometers.

The German contributions to the LRI include the entire optical system, consisting of a steering mirror, built by Hensoldt (previously Zeiss) in Oberkochen and the optical bench from SpaceTech GmbH in Immenstaad, opto-electronics from the German Aerospace Center (DLR) in Berlin-Adlershof, and electronic systems from Apcon AeroSpace & Defence in Neubiberg near Munich. SpaceTech GmbH is responsible for the industrial part of the entire German LRI contribution. The DLR Institute of Space Systems in Bremen developed and delivered calibration and testing instruments. Both satellites were built on behalf of NASA by Airbus in Friedrichshafen. Mission control for GRACE Follow-On is provided by the German Space Operations Centre (GSOC) in Oberpfaffenhofen near Munich under subcontract of GFZ.

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