GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

This chapter provides an overview of the Gravity Probe B experiment, the spacecraft and its associatedtechnologies, a description of the mission and mission phases, including a summary and discussion of theactivities, issues, and accomplishments that took place during the Initialization and Orbit Checkout (IOC),Science (data collection), and Instrument Calibration phases of the mission.2.1 Gravity Probe B in a NutshellThe Gravity Probe B (GP-B) mission is a physics experiment that uses four spherical gyroscopes and a telescope,housed in a satellite orbiting 642 km (400 mi) above the Earth, to measure, with unprecedented accuracy, twoextraordinary effects predicted by Albert Einstein’s general theory of relativity—our present theory ofgravitation. The first measurement is the geodetic effect—the amount by which the Earth warps the localspacetime in which it resides. The other effect, called frame-dragging, is the amount by which the rotating Earthdrags local spacetime around with it. The GP-B experiment tests these two effects by precisely measuring thedrift angles of the spin axes of the four gyros over the course of a year and comparing these experimental resultswith predictions from Einstein’s theory.2.1.1 A Brief History of GP-BThe idea of testing general relativity by means of orbiting gyroscopes was suggested independently by twophysicists, George Pugh and Leonard Schiff, in late 1959-early 1960. Pugh’s Proposal for a Satellite Test of the CoriolisPrediction of General Relativity appeared in an unusual location: the U.S. Department of Defense Weapons SystemsEvaluation Group (WSEG) Memo #11 (November, 12, 1959). Schiff ’s Possible New Experimental Test of GeneralRelativity Theory was published in the March 1, 1960 issue of Physical Review Letters. For an historical review andcomparison, see C. W. F. Everitt: The Stanford Relativity Gyroscope Experiment (A): History and Overview, in Near Zero:New Frontiers of Physics, ed. J. D. Fairbank et. al., New York, W. H. Freeman, 1988.Schiff, then chairman of the Stanford University Physics Department, discussed his developing ideas with twophysics department colleagues, Professors William Little and William Fairbank, both low-temperaturephysicists. Fairbank, in turn, had an exchange with Professor Robert Cannon, who like Fairbank had justarrived at Stanford. Cannon had considerable experience with state-of-the-art gyroscopes, and he had come toStanford to create a Guidance and Control Laboratory in the Department of Aeronautics & Astronautics. Therefollowed an amusing and much publicized three-way meeting between Fairbank, Cannon and Schiff at the nowno-longer-existingswimming pool in the Stanford Men’s Gymnasium. Thus began the essential collaborationbetween the Physics and Aero-Astro departments which has been fundamental to the success of GP-B.Schiff ’s more extended paper, entitled Motion of a Gyroscope According to Einstein's Theory of Gravitation was thenpublished in the June 15, 1960 issue of the Proceedings of the National Academy of Sciences (Volume 46, Number6, pp. 871-882). Pugh’s too-little-known article was finally published in Nonlinear Gravitodynamics: The Lense-Thirring Effect, ed. R. Ruffini and C. Sigsmondi, Singapore, World Scientific, 2003. An important feature ofPugh’s paper was that it contained the first proposal and analysis of the concept of a drag-free satellite.During the startup period of GP-B, at the 1961 meeting of the International Union of Theoretical and AppliedMechanics, Cannon presented the first engineering paper describing the planned orbital experiment:Requirements & Design for a Special Gyro for Measuring General Relativity Effects from an Astronomical Satellite, published inKreiselprobleme Gyrodynamics, Symposium Celerina, August, 1962, ed. H. Ziegler, Berlin, Springer-Verlag.Copies of all the papers mentioned in this section, plus other historical GP-B papers, are available in AdobeAcrobat PDF format on the Web at: http://einstein.stanford.edu/content/sci_papers/origins.html.It is worth noting that earlier theoretical discussions of gyroscope tests of general relativity were given in the1920’s by J. A. Schouten and A. S. Eddington. In the 1930’s, P. M. S. Blackett investigated the possibility of aground-based experiment, concluding correctly that it was beyond the reach of technologies known at that time.Gravity Probe B — Post Flight Analysis • Final Report March 2007 25