During its 16-month mission, GP-B will attempt to verify two subtle physical effects predicted by Albert Einstein's General Theory of Relativity, which provides the foundations for understanding the large-scale structure of the Universe.

"We're enormously pleased that Gravity Probe B is safely in orbit and operating as expected. It is because of close collaboration with our Stanford and NASA colleagues that we have reached this important milestone," said Jim Crocker, vice president, civil space, Lockheed Martin Space Systems Company. "We look forward to the mission ahead and the data that will increase our understanding of the fundamental structure of the universe."

The GP-B space vehicle comprises the spacecraft, built by Lockheed Martin, and its payload. The payload is made up of the dewar, the key structural component around which the GP-B space vehicle was built, and the flight probe, a nine-foot-long cigar-shaped vacuum chamber. Both elements were built at the Lockheed Martin Advanced Technology Center, Palo Alto. Inside the flight probe is the Science Instrument Assembly, built by Stanford University.

The GP-B experiment will be conducted in an extremely stable environment, free from all outside forces. Lockheed Martin designed the enclosure within the spacecraft in which the Science Instrument Assembly will operate at a temperature near absolute zero.

When the Gravity Probe B experiment begins, the instrument apparatus will measure minute changes in spin axis orientation of four ultra-precise gyroscopes contained within. The gyros will be so free of disturbances that they will provide a nearly perfect space-time reference system. They will measure two predicted effects of Einstein's theory: whether and how space and time are warped by the presence of Earth, and whether and how the rotating Earth drags space-time around with it. This will be by far the most accurate test of any of the predicted effects of Einstein's theory.

Small as the two effects measured by Gravity Probe B are, their measurement will provide an extremely important advance by testing previously unproven predictions of Einstein's theory. They may provide critical clues to modern attempts to unify the four fundamental forces observed in Nature: electromagnetism, gravity, and the so-called strong and weak interactions that govern the behavior of atomic nuclei.

"Gravity Probe B is one of the few space missions NASA has conducted with relevance to fundamental physics," stated a review of GP-B undertaken in 1995 by the Space Studies Board of the National Research Council. "If successful, it would assuredly join the ranks of the classical experiments of physics. By the same token, a confirmed result in disagreement with General Relativity would be revolutionary."

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