The double satellite GRACE (Gravity Recovery And Climate Experiment) is a project for exact determination of earth's gravitation field in a low orbit. On account of their correlated orbit, they got the nickname "Tom" and "Jerry". The two EADS 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 millimeter, 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 EADS Astrium's Flexbus concept, enabling the satellites to be built very quickly at extremely low cost.
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