Building a practically perfect gyroscope

Mac Keiser

Mac Keiser was the chief scientist of the Gravity Probe B program. He is now a Senior Research Scientist, Emeritus, at Stanford.

Spherical, electrostatically-supported gyroscopes have been used for navigation for more than 40 years but their accuracy is far from what was needed to measure the effects predicted by general relativity. Fortunately, the accuracy could be significantly improved by reducing the forces required to support the gyroscope, improving the sphericity of the rotor, and increasing the gyroscope spin speed. Operating in a satellite reduces the support forces by a factor of 107, and using one of the gyroscopes as a drag-free sensor brings a further reduction of nearly 104. Improving the sphericity of the rotor further reduces the support-dependent torques but leads to the additional questions about how to spin-up and measure the spin-axis orientation of the rotor. Using a superconducting readout that Continue reading

Gravity Probe B, the simplest data analysis ever

John Conklin

John W. Conklin is assistant professor in the University of Florida’s Department of Mechanical and Aerospace Engineering. Photo: Aurora M Agüero

What could be simpler than fitting a straight line to a set of data? The slope of the line is the science result for NASA’s Gravity Probe B experiment, a landmark satellite test of two predictions of general relativity. Prior to launch, the data analysis for GP-B was thought to be straight forward. After precise calibration of the measured data (not as easy as fitting a straight line), the resulting signal should change at a constant rate over time. Our goal was to measure this rate of change, hopefully with an accuracy of 1%.

In the fall of 2005, as the year-long science mission was concluding, the GP-B science team began producing plots of the science data collected from the Earth-orbiting probe.  The figure shows one such plot. The slope of the best-fit straight line to these data is the magnitude of the elusive frame dragging effect, the main science goal of Gravity Probe B. Fear began to set it in. Where was the straight line? Continue reading