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.
Î 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: