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The two Grace (Gravity Recovery and Climate Experiment) satellites were developed to further advance research in this field. Grace, a system of two identical satellites, is a joint NASA–German Aerospace Centre (DLR) project, and the second mission under NASA’s Earth System Science Pathfinder programme.
The two Astrium-built satellites, each weighing just 480kg, were launched on 17 March 2002 by a Rockot launcher for a five-year mission. They are orbiting in tandem, flying at about 220km apart, but the exact distance between them is altered by the influence of the Earth’s gravity. A microwave connection between the two satellites enables this fluctuating distance between them to be constantly determined to within a few thousandths of a millimetre, thereby ‘mapping’ the Earth’s gravitational field with unprecedented accuracy. It has been possible to detect minor gravitational changes, which result from revolving magma in the interior of the earth or from melting glaciers or shifting ocean currents, for the first time.
The Grace satellites are the follow-on to the Champ mission, which was launched in 2000. They are based on Astrium’s Flexbus concept, enabling the satellites to be built very quickly at extremely low cost.
Astrium has been commissioned to build two new Grace Follow-On research satellites for NASA’s Jet Propulsion Laboratory (JPL). The agreement was signed in November 2012. For a minimum of five years from August 2017, the Gravity Recovery and Climate Experiment Follow-On (Grace FO) mission will continue the extremely accurate measurement data collection of the first twin Grace satellites.
