A NASA spacecraft designed to test two important
predictions of Albert Einstein’s general theory of relativity
is set to launch from Vandenberg Air Force Base, Calif., at
1:09 p.m. EDT, April 17.

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