A joint US–German project to measure the Earth's gravitational field very precisely using two satellites in a polar orbit and flying approximately 220 kilometres apart. Slight variations in the strength of the Earth's gravitational field are detected by measuring changes as small as one micrometre in distance between the two satellites as they orbit Earth. Such changes are caused not just by variations of density within the Earth but also by movements of water and ice on the surface of the Earth. The twin GRACE satellites were launched on 17 March 2002 for an intended 5-year mission.
Subjects: Astronomy and Astrophysics.