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

Near Zeroes.) The exception that we could not eliminate as a source of error was the seventh extrinsic designconstraint that the electric dipole moment of the gyro rotors be “near zero.” In other words, the gyro rotorsmust be highly electrically spherical as well as mechanically spherical.The electrical out-of-roundness is due to larger than anticipated electrostatic patches (regions of electricalcharge) on the rotor’s surface. These patches interact with similar ones on the housing’s inner surface to give riseto small “misalignment torques” that occur when the spin axes of the gyros are not aligned with the roll axis ofthe spacecraft. These patches also provide a subtle damping mechanism that extracts small amounts of energyfrom the spinning rotor, and this causes the polhode path to change. The potential existence of these patches wasknown prior to launch, but was believed to be negligible, based on careful physical analysis and laboratorymeasurements.Nature is full of surprises, and it is common for physics experiments, particularly a ground-breaking experimentsuch as GP-B, to uncover unexpected phenomena. Because we anticipated this situation and were activelylooking for surprises in the data, we were prepared to deal with them. Consequently, the team has been able tomodel precisely the polhode motion of all four gyroscopes, as well as the classical torques imparted by patcheffects on the rotors and housings. This painstaking work, which is still in progress, has lengthened the dataanalysis phase of the mission by more than a year—such is the nature of scientific inquiry. Ultimately, the timespent isolating and removing these confounding effects from the final results will reduce the current margin ofexperimental error to the desired level.Our goal in GP-B has always been to perform the best possible experiment and to use great care and redundancyto ensure the validity of the results. Every experiment has its share of unknowns or challenges that no one couldhave anticipated. But once such challenges surface, it is incumbent on the investigators to spend time addressingthem and accounting for them in the process of determining the final outcome.15.4.1 Time-Varying Motion in the Gyro RotorsThe gyroscope’s polhode motion is akin to the common “wobble” seen on a poorly thrown (American) football,though it shows up in a much different form for the ultra-spherical GP-B gyroscopes. While it was expected thatthis wobble would exhibit a constant pattern over the mission, it was found to slowly change due to minuteenergy dissipation in the spinning gyro rotors, caused by interactions of electrostatic patches on the rotor'ssurfaces and patches on the metallic surfaces inside the housings. For example, Figure 15-6 below shows“snapshots” of the changing polhode path of gyro #1 at various times during the mission.Figure 15-6. Plots from different viewing angles showing the changing polhode path of gyor #1 over the life ofthe experiment.The polhode wobble complicates the measurement of the relativity effects by putting a time-varying wobblesignal into the data.430 March 2007 Chapter 15 — Preliminary Results