The GRACE mission was selected as the second mission under the NASA Earth System Science Pathfinder (ESSP) Program in May 1997. The Gravity Recovery and Climate Experiment (GRACE) was launched on March 17, 2002 from the Russian Plesetsk Cosmodrome. The project, a joint effort between NASA and the German Center for Air and Space Flight, aims to produce 60 gravity maps of the Earth. Designed for a nominal mission lifespan of five years, GRACE is currently operating in an extended mission phase, which is expected to continue through at least 2015. The gravity mapping information, which will chart the evolution of gravity over time, has many applications ranging from oceanography to hydrology and glaciology.
GRACE consists of two identical spacecraft that fly about 220 kilometers (137 miles) apart in a polar orbit 500 kilometers (310 miles) above Earth. GRACE maps Earth’s gravity field by making accurate measurements of the distance between the two satellites, using GPS and a microwave ranging system. It is providing scientists from all over the world with an efficient and cost-effective way to map Earth’s gravity field with unprecedented accuracy. The results from this mission are yielding crucial information about the distribution and flow of mass within Earth and its surroundings. These discoveries could have far-reaching benefits to society and the world’s population.
Ocean currents transport mass and heat between different regions of the Earth. Knowledge of these currents is therefore vitally important for Earth climate sciences. Some of the more familiar currents include the Gulf Stream off the eastern seaboard of the USA, the Kuroshio Current in the western Pacific off the coast of Japan; the Antarctic Circumpolar Current (ACC), which is the only current to travel through all major ocean basins; and the Equatorial Currents.
Historically, knowledge of these currents has come from measurements from very limited number of current meters or drifter buoys (which measure the current directly at selected points in the oceans), or indirectly from the knowledge of slopes of the dynamic ocean topography. The dynamic ocean topography, and thus the currents, can be computed in two ways: 1) from measurements through the ocean depth of temperature and salinity, using instruments dropped from ships or from moored buoys, or 2) the difference between sea surface height measured by satellite altimeters and a geoid model from GRACE. This independent knowledge of absolute surface currents from altimetry and GRACE can now be used in combination with the temperature and salintiy profiles to extract the currents as a function of depth. This in turn will improve our knowledge of mass and heat transported by these current systems.
The gravity variations studied by GRACE include: changes due to surface and deep currents in the ocean; runoff and ground water storage on land masses; exchanges between ice sheets or glaciers and the ocean; and variations of mass within Earth. Another goal of the mission is to create a better profile of Earth’s atmosphere. GRACE results are making a huge contribution to the goals of NASA’s Science Mission Directorate, Earth Observation System (EOS) and global climate change studies.