Gravity Recovery and Climate Experiment

The Gravity Recovery and Climate Experiment, GRACE, was a joint mission of NASA and the German Aerospace Center that used twin satellites, nicknamed Tom and Jerry, to map variations in the Earth's gravity field. The two spacecraft were launched together from Plesetsk on a Rockot rocket on 17 March 2002, flew about 220 kilometres apart in a near-polar orbit and measured tiny changes in the distance between them with a microwave ranging link. They were manufactured by Astrium of Germany, now part of Airbus Defence and Space, on its Flexbus platform. GRACE far exceeded its five year design life, working for 15 years until GRACE-2 was decommissioned in October 2017, and its data tracked ice sheet loss, groundwater change and ocean mass. A near-identical follow-on mission, GRACE-FO, was launched in 2018.