Gravity Probe B Mission Update September 2007

Note: In response to a number of recent inquiries, the fact that I am sending out/posting this update almost a month later than previously anticipated is simply the result of the growing number of tasks on my plate as we approach the conclusion of GP-B. -BK

PROGRAM STATUS

On 14 April 2007, at the American Physical Society (APS) meeting in Jacksonville, FL, we announced preliminary results of the GP-B experiment. In our June 2007 mission update, we provided a comprehensive overview of these results, including an in-depth discussion of two surprises in the data:

1) A time variation in the polhode motion of the gyroscopes, which creates complications in the gyro scale factor calibrations (conversion of electrical signals to angles).

2) Much larger than expected classical misalignment torques on the gyroscopes, attributable to “patch effect” (contact potential difference) interactions between the gyro rotors and their housings.

We also reported on the two methods being used to process the data–“geometric” and “algebraic,” including a refinement of the former, known as the “integral geometric” method–and we emphasized the process of mapping the profiles of trapped magnetic flux in the gyro rotors as a means for improving scale factor determination. In the last three and one half months, we have made significant progress in understanding and addressing both surprises, as well as combining the insights generated by both the geometric and algebraic methodologies towards a final result.

For example, the trapped flux mapping has now been completed for all four gyroscopes and results processed for each throughout the year. One very remarkable result is that we have succeeded in improving by a factor of ~500 the determination of the polhode phase and angle for each gyroscope throughout the duration of the experiment. In particular, the polhode motion for each gyro is now “phase-locked” and known to 0.1 radian (6 degrees) over the course of the experimental year.

As a result, a previous discrepancy between the scale factor calibration computed by the geometric and algebraic methods has been completely eliminated, and these two scale factor determinations are now in full agreement, within the current limits of measurement.

Furthermore, the improved knowledge of the polhode phase has laid the foundation for greatly improved determinations of the patch-effect torques. One of the intriguing puzzles at an earlier stage of the analysis was that the clocking of the gyroscope housings with respect to the rolling spacecraft frame appeared to vary with time, and from gyro to gyro, in a manifestly unphysical way. In other words, the rolling spacecraft is like a flywheel, and the gyro readout planes are very stably locked to it, so there cannot be any physical variation