bing forces, always point in the same direction in inertial space. But this principle is where Dr. Einstein and Sir Isaac Newton differ. In Newton's physics, a perfect gyroscope pointed at a star should stay aligned forever. In Einstein's physics, the direction of the spin axis of the gyroscope would gradually change due to the mass and rotation of the Earth by an amount that could be exactly predicted. The gyroscopes measured 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.
Relativists call the first of the two Einstein effects the ‘geodetic effect.' The second is called the ‘frame-dragging effect.' Small as the two effects measured by Gravity Probe B are, their measurement has provided 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.
The GP-B space vehicle comprises the spacecraft–built by Lockheed Martin Space Systems' Sensing & Exploration Systems organization–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 (ATC) in Palo Alto. Inside the flight probe is the very precise Science Instrument Assembly, built by Stanford University.
The GP-B requirements for stability and freedom from outside forces were extremely demanding. The ATC team provided Stanford an enclosure within which the scientific instrument could operate at a temperature near absolute zero. The magnetic field at the Science Instrument Assembly was less than one millionth of the Earth's magnetic field and the science gyroscopes inside the probe operated in a very quiet, low acceleration environment.
The Science Instrument Assembly was simple in its concept: A block of fused quartz 21-inches long, with a bonded quartz telescope on one end, held four gyroscopes. Very sensitive magnetometers provided the gyroscope readouts. The gyro-telescope instrument was held in the flight probe, which was inserted into the dewar, an extremely complicated Thermos-like bottle. The dewar held 613 gallons of superfluid, supercold helium that kept the instrument chilled to about 2 Kelvin (-455 degrees Fahrenheit) for the duration of the mission.
When Gravity Probe B was launched into a 400-mile-high polar orbit, its science instrument measured minute changes in the directions of spin of the four ultra-precise gyroscopes contained within. The gyros were so free of disturbances that they provided a nearly perfect space-time reference system. They were referenced to the science telescope that sighted on a far-field highly stable reference star.
"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."
The ATC is the research and development organization of Lockheed Martin Space Systems Company (LMSSC). LMSSC, a major operating unit of Lockheed Martin Corporation, designs and develops, tests, manufactures and operates a full spectrum of advanced-technology systems for national security and military, civil government and commercial customers. Chief products include human space flight systems; a full range of remote sensing, navigation, meteorological and communications satellites and instruments; space observatories and interplanetary spacecraft; laser radar; ballistic missiles; missile defense systems; and nanotechnolog