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,