The U.S. and German space agencies have agreed to a further two-year extension, to 2015, of the GRACE satellite mission to measure Earth’s gravity field, the two agencies announced June 10.

Launched in 2002, the two identical Gravity Recovery and Climate Experiment (GRACE) satellites operate in identical 500-kilometer low Earth orbits separated by some 220 kilometers. They measure small changes in the gravity field due to changes in mass in different areas of the Earth.

NASA and the German Aerospace Center, DLR, signed their initial bilateral accord on GRACE in 1998. The satellites originally were expected to remain in service for five years but are now expected to continue to function through 2015.

The two-year renewal signed at the Berlin air show, ILA 2010, by NASA Deputy Administrator Lori Garver and DLR Chairman Johann-Dietrich Woerner extends operations to the end of the satellites’ functional lives. DLR officials said the DLR share of GRACE operating costs, including data download and distribution, is around 1.75 million euros ($2.1 million) per year.

Peter B. de Selding was the Paris bureau chief for SpaceNews.