NASA is moving forward with plans to build and launch two new satellites to replace the aging GRACE gravity-mapping mission, and the space agency has commissioned EADS Astrium to build the spacecraft for the joint U.S.-German project.

Artist's concept of the GRACE Follow-On satellites. Credit: EADS Astrium
 
But the replacement satellites, which will generally resemble the current spacecraft, may be launched well after the GRACE mission succumbs to aging hardware, according to scientists.

The Gravity Recovery and Climate Experiment, or GRACE mission, has spent the last 10 years mapping Earth's gravity field and using the data to track changes in soil moisture, polar ice, and the planet's climate.

Scientists have used GRACE's monthly gravity maps to chart ice melt, sea level rise, and droughts.

The twin satellites fly in orbit 137 miles apart, and GPS and microwave ranging systems constantly measure the exact distance between the spacecraft. When one satellite feels the pull from a mass of rock, ice, or water, gravity's influence registers in the range measurements between the spacecraft.

The method produces gravity maps more than 100 times more sensitive than previous models, according to NASA. GRACE can resolve reservoirs of groundwater and water vapor in the atmosphere.

But the GRACE mission may come to an end soon.

The GRACE satellites, which were launched in 2002 and designed for a five-year lifetime, are now contending with aging batteries. The satellites are unable to collect data on the night side of each orbit, according to scientists.

NASA has begun developing a pair of new GRACE satellites, also in partnership with Germany.

Scientists expect the GRACE Follow-On mission to launch in 2017, and EADS Astrium announced on Thursday it was selected by NASA to build the satellites in Germany.

The GRACE Follow-On satellites will each weigh about 1,300 pounds and will continue the GRACE gravity measurements for at least five more years.

The new satellites will function much like the current spacecraft, but with an added laser ranging system for further precision, according to Astrium.