NASA's Gravity Probe B mission, also known as GP-B, will use four ultra-precise gyroscopes, orbiting the Earth in a unique satellite, to experimentally test two extraordinary predictions of Einstein's 1916 theory that space and time are distorted by the presence of massive objects. The two effects being tested are: The geodetic effect, the amount by which the Earth warps local spacetime in which it resides, and the frame-dragging effect, the amount by which the Earth drags local spacetime around with it as it rotates.

"Gravity Probe-B has the potential to uncover fundamental properties of the invisible universe, a universe which seems very bizarre and alien to our everyday perceptions yet one that Einstein tried to show us almost a century ago," said Dr. Anne Kinney, director of the Astronomy and Physics Division in NASA's Office of Space Science, Washington. "Testing the key aspects of Einstein's theory, such as GP-B will do, will provide crucial information to science just as it has already helped America by pushing technological progress in developing the tools needed for these ultra- precise measurements," she added

Once placed in its polar orbit of 640 kilometers (400 miles) above Earth, GP-B will circle the globe every 97.5 minutes, crossing over both poles. In-orbit checkout and calibration is scheduled to last 40-60 days, followed by a 13-month science-data acquisition period and a two-month post-science period for calibrations.

To test the general theory of relativity, GP-B will monitor any drift in the gyroscopes' spin axis alignment in relation to its guide star, IM Pegasi (HR 8703). Over the course of a year, the anticipated spin axis drift for the geodetic effect is a minuscule angle of 6,614.4 milliarcseconds, and the anticipated spin axis drift for the frame-dragging effect is even smaller, only 40.9 milliarcseconds. To illustrate the size of the angles, if you climbed a slope of 40.9 milliarcseconds for 100 miles, you would rise only one inch in altitude.

During the mission, data from GP-B will be received a minimum of two times each day. Earth-based ground stations or NASA's data relay satellites can receive the information. Controllers will be able to communicate with GP-B from the Mission Operations Center at Stanford University.

Data will include space vehicle and instrument performance, as well as the very precise measurements of the gyroscopes' spin-axis orientation. By 2005 the GP-B mission will be complete, and a one-year period is planned for scientific analysis of the data.

"Developing GP-B has been a supreme challenge requiring the skillful integration of an extraordinary range of new technologies," said Professor Francis Everitt of Stanford University, and the GP-B principal investigator. "It is hard to see how it could have been done without the kind of unique long-term collaboration that we have had between Stanford, Lockheed Martin, and NASA. It is wonderful to be ready for launch," he said.

NASA's Marshall Space Flight Center, Huntsville, Ala., manages the GP-B program. NASA's prime contractor for the mission, Stanford University, conceived the experiment and is responsible for the design and integration of the science instrument, as well as for mission operations and data analysis. Lockheed Martin, a major subcontractor, designed, integrated and tested the spacecraft and some of its major payload components. NASA's Kennedy Space Center, Fla., and Boeing Expendable Launch Systems, Huntington Beach, Calif., are responsible for the countdown and launch of the Delta II.

The launch from Vandenberg will be broadcast live on NASA Television on the AMC-9 satellite, transponder 9C, located at 85 degrees West longitude, vertical polarization, frequency 3880.megahertz, audio 6.8 megahertz. Information about launch events and video will be carried on a NASA website called the Virtual Launch Control Center at:

http://www.ksc.nasa.gov/elvnew/gpb/vlcc.htm

For information about the GP-B mission on the Internet, visit:

http://einstein.stanford.edu/and http://www.gravityprobeb.com