September 25, 2015


Since 2002, two satellites nicknamed Tom and Jerry have been trailing one another in the same 480-km orbit, where changes in the distance between them reveal the fluctuations in Earth’s gravity field.

GRACE (Gravity Recovery And Climate Experiment) is a joint mission of the U.S. and German space agencies, NASA and DLR. It is based on two twin satellites, GRACE-A and GRACE-B, nicknamed Tom and Jerry, permanently connected by a microwave link that enables the distance between them to be determined with micrometre accuracy—roughly the thickness of a human hair. This separation distance (150 to 300 km), which can be adjusted by manoeuvring the satellites, varies by a few tens of metres every time they pass over fluctuations in Earth’s gravity field.

By measuring these changes in distance between the two satellites continuously, scientists are able to compile detailed maps of Earth’s gravity at a spatial resolution of 200 km. Monthly or 10-day maps of the geoid highlight temporal variations in masses of water, snow and ice, and even ground displacements due to major earthquakes. Such data are crucial for studying Earth’s oceans, geology and climate, notably with a view to better understanding the mechanisms driving global warming.

GRACE complements the European GOCE mission (Gravity field and steady-state Ocean Circulation Explorer), which came to an end in 2013 having mapped Earth’s geoid at a finer spatial resolution of 100 km. Many French research laboratories, including CNES’s space geodesy team, are working with GRACE data and this collaboration is set to continue as NASA and DLR have decided to launch two more twin satellites in 2017 for the GRACE Follow-On mission (GRACE-FO).