Launch Date: March 17, 2002

The Earth's gravity varies across space and time, but it has never been accurately measured on a global scale, until GRACE - the Gravity Recovery And Climate Experiment.

GRACE uses two identical satellites, placed in a polar orbit and flying approximately 220 kilometers apart, to measure Earth's gravity field and record its changes. It does this by detecting subtle movements in the distance that separates the satellites with an accuracy of one micrometer (about one-half the diameter of a human red blood cell). These movements are caused by variations in the strength of the gravity they encounter.

GRACE is being built and managed with the cooperation of people all over the world. The Principal Investigator, Dr. Byron Tapley, is at the University of Texas. The Co-Principal Investigator is Dr. Christoph Reigber of the GeoForschungsZentrum (GFZ Potsdam) in Germany. Expertise is drawn from several international organizations and industry partners.

The twin GRACE satellites were launched on March 17, 2002 from the Eurockot dedicated launch facilities at Plesetsk in Northern Russia on a Rokot launch vehicle. Rokot payload capacity of 1,900 kg is ideal for launching these two medium-sized satellites into Low Earth Orbit.

This 5-year mission will yield fundamental information about gravity and the distribution and variations of Earth's mass. GRACE will provide a new global map of Earth's gravity every 30 days.

The gravity variations that GRACE will study include:

• Changes due to surface and deep ocean currents
• Runoff and ground water storage on land masses
• Exchanges between ice sheets or glaciers and the oceans
• Densities of mass within the Earth

Results from GRACE will make a significant contribution to the goals of NASA's Earth Science Enterprise, Earth Observation System (EOS) and other global climate-change studies. Outreach materials and Education Programs are available through the University of Texas Center for Space Research (UTCSR).

The Jet Propulsion Laboratory, located in Pasadena, California, has contributed to the GRACE mission with a broad range of Project Management and Systems Engineering Activities. JPL also provides instruments and computer expertise, including:

• K-Band Ranging Instrument System - which determines the distance between the satellites.
• Star cameras - which are used to orient the satellite in space.
• Blackjack Global Positioning System Receiver integration - to determine exact satellite locations.
• Instrument Processing Unit - that combines information from the GPS receiver, K-Band ranging system and star cameras - for transmission to Earth.
• Science Data System - the computer system on the ground that extracts the relevant science information from the combination of data send by the Instrument Processing Unit.

For more comprehensive information, visit the main Grace Website, at the University of Texas, Austin.