Gravity Probe B (GPB)
Full Name: Gravity Probe B
Phase: Past
Launch Date: April 20, 2004
Mission Project Home Page: http://einstein.stanford.edu/
NASA’s Marshall Space Flight Center and Stanford University have developed a sophisticated experiment, Gravity Probe B (GP-B), to test Einstein’s general theory of relativity. It will measure how space and time are warped by the presence of the Earth, and, more profoundly, how the Earth's rotation drags space-time around with it. Einstein’s theory predicts that space and time are distorted by the presence of massive objects. Launched on April 20, 2004, the GP-B mission is one of NASA’s first to address a question of fundamental physics in the new millennium.
The GP-B experiment contains the world’s most precise gyroscopes. The gyroscopes have been specifically developed to measure two distinct effects of general relativity.The first of these, the geodetic effect, should cause the spin axis orientation of a gyroscope, circling the Earth in a polar orbit, to change by a tiny angle of 6.6 arcseconds (0.0018 degrees) in a year, relative to a distant guide star. The second effect, known as frame-dragging, predicts that massive celestial bodies, such as Earth drag their local spacetime around with them—ever so slightly—as they rotate.
The major components of the science instrument (four gyroscopes, the optical telescope, the mounting block) are all made of fused quartz. Quartz is very stable over wide temperature ranges—expanding and contracting very little and uniformly.
The science instrument’s optical telescope has an aperture of 14.0 centimeters (5.5 inches). Throughout the experiment, it is pointed at the center of GP-B’s guide star IM Pegasi (HR 8703), which provides the experiment’s frame-of-reference in space. Technicians at the Marshall Space Flight Center originally built the highly advanced polishing equipment needed to manufacture the gyroscope rotors. Engineers at Stanford developed the thin-film technology for placing a superconductive metal coating of Niobium on the gyroscope rotors.
The gyroscope rotors are perhaps the most spherical objects ever made. If the gyroscope rotors were enlarged to the size of the Earth, the tallest mountain or the deepest ocean ravine would be only 2.4 meters (8.0 feet) in height. These gyroscopes are sufficient to achieve the specified 0.5 milliarcsecond (1.4x10-7 degrees) per year accuracy in their drift rate measurements.