David E. Steitz, Alan Buis

NASA and the German Space Agency are preparing to launch

the Gravity Recovery and Climate Experiment (GRACE), a

scientific pathfinder mission that will test a novel approach

to tracking how water is transported and stored within the

Earth’s environment. The mission will precisely measure the

planet’s shifting water masses and map their effects on

Earth’s gravity field, yielding new information on effects of

global climate change.

The twin GRACE satellites are set to launch March 16, 2002,

from Russia on a five-year mission that will revolutionize

understanding of changes in the Earth’s gravity field over

time and space. The mission will provide measurements of the

gravity field that are far more accurate and sensitive than

any that can be obtained by ground-based observations or

single remote-sensing spacecraft.

“GRACE marks the first launch of NASA’s Earth System Science

Pathfinder program, designed to develop new measurement

technologies for studying our Earth system,” said Dr. Ghassem

Asrar, associate administrator for NASA’s Earth Science

Enterprise, NASA Headquarters, Washington. “Through NASA’s

continuing investment in technology development, we’ve been

able to create an innovative mission at a fraction of the

cost of missions formulated just a decade ago. GRACE will

provide us with a new view of our home planet and help us to

better understand climate change and its global impacts such

as changes in sea level and the availability of water

resources,” Asrar said.

A more precise gravity map of Earth is expected to increase

the accuracy of many techniques used by scientists who study

Earth with space-based instruments. These techniques —

ranging from satellite altimetry and radar interferometry to

digital terrain models covering large land and ice areas —

provide critical input to many scientific models used in

oceanography, hydrology, glaciology, geology and related


As they race around the globe 16 times a day, the satellites

will sense minute variations in the Earth’s surface mass

below and corresponding variations in the Earth’s

gravitational pull. Regions of slightly stronger gravity will

affect the lead satellite first, pulling it slightly away

from the trailing satellite. By measuring the constantly

changing distance between the two satellites and combining

that data with precise positioning measurements from Global

Positioning System instruments, scientists will be able to

construct a precise Earth gravity map.

GRACE is the first Earth-monitoring mission in the history of

space flight whose key measurement is not derived from

electromagnetic waves bounced off the Earth’s surface.

Instead, the mission will use a microwave ranging system to

accurately measure changes in the speed and distance between

two identical spacecraft flying in a polar orbit about 220

kilometers (137 miles) apart, 500 kilometers (311 miles)

above Earth. The ranging system is so sensitive it can detect

separation changes as small as 10 microns — about one-tenth

the width of a human hair over a distance of 220 kilometers.

An additional instrument aboard the satellites called an

atmospheric limb sounder will measure the amount by which the

Global Positioning System satellite signals are distorted by

Earth’s atmosphere. Scientists will use these data to improve

the accuracy of key atmospheric observations, which serve as

input for weather forecast models.

GRACE is a joint partnership between NASA and the German

Center for Air and Space Flight (Deutsches Zentrum fur Luft

und Rumfahrt, or DLR). The U.S. portion of the project is

managed for NASA’s Office of Earth Science, Washington, by

NASA’s Jet Propulsion Laboratory (JPL), Pasadena, Calif.

Science data processing, distribution, archiving and product

verification are managed under a cooperative arrangement

between JPL and the University of Texas’ Austin-based Center

for Space Research in the United States and Germany’s Earth

Research Center (or GeoForschungsZentrum).

More information about the GRACE program is available on the

GRACE web site at:

Information on NASA’s Earth System Science Pathfinder Program

may be found at: