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

Figure 15-5. A slide from the GP-B plenary talk that Francis Everitt will deliver at the APS meeting on April 14,2007. This slide shows the misalignment torques discovered during a series of instrument calibration testsperformed during the last six weeks of the mission, prior to depletion of helium in the dewar.However, the team was prepared for such an eventuality. In fact, the GP-B experiment was carefully designed toenable us to discover such unexpected sources of error. As Francis Everitt notes: “When trying to do anexperiment that goes seven orders of magnitude beyond where anyone has gone before, some extraordinaryplanning is necessary. First, you design the experiment to make all the errors as close to zero as possible. Next,you average everything you can—for example, you roll the spacecraft throughout the experiment. Finally, youpurposely look for things that were not expected.”During one of many NASA program reviews prior to launch, experienced investigators from other NASAmissions asked us what processes we had put in place to discover systematic sources of error that might crop upin the data. Our answer was that we would follow a principle set forth by the 18th century scientist, HenryCavendish—namely, if you are worried that something in an experiment might be a potential source of error,you take the step of deliberately exaggerating the error sources to determine how bad they can get. This processenables you to quantify the magnitude of the problem and determine a way to deal with it. (For a thoroughdiscussion of the “Cavendish” principle, see the 1977 paper by Francis Everitt entitled Gravitation, Relativity andPrecise Experimentation, which is available in Adobe Acrobat PDF format on our GP-B Web site athttp://einstein.stanford.edu/content/sci_papers/papers/Everitt_Gravitation-Precise-Expt-1977.pdf.From mid-August through September 2005, after collecting 50 weeks of science data and before the liquidhelium was exhausted from the dewar, we followed Cavendish's principle by performing a barrage of calibrationtests on all parts of our science instrument. These tests, along with other tests that we performed throughout themission, enabled us to confirm that all of the known and expected sources of error—with one exception—were,in fact, negligible as expected. As William Bencze put it: “Without the extensive calibration tests, we would havebeen looking for a needle in a haystack; instead, we only had to look for a needle on the floor.”Specifically, these tests confirmed that six of the seven extrinsic “near zero” design specifications of theexperiment had been met or exceeded, and we were able to eliminate each of these factors as sources of error inthe final results. (For an explanation of GP-B’s seven near-zero specifications, see Section 3.1.1, GP-B’s SevenGravity Probe B — Post Flight Analysis • Final Report March 2007 429