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

We have received inquiries about a recent letter to Nature by IgnazioCiufolini and Erricos Pavlis claiming to have verified the Lense-Thirring frame-dragging effect of general relativity through laserranging data observations of the LAGEOS I & II spacecraft. In theirmeasurement, the frame-dragging effect needs to be separated by anextremely elaborate modeling process from Newtonian effects morethan 10,000,000 times larger than the effect to be measured. The letterdoes not provide enough detail of the methods of verification andvalidation to allow a critical evaluation. We and other members of therelativity community look forward to a more complete account. Ifverified, their result will be of considerable interest.This past Wednesday evening, for the first time since 1918, whenAustrian physicists, Josef Lense and Hans Thirring published theirimportant paper on the gravitomagnetic or “frame-dragging” effect,the Boston Red Sox swept the World Series. Some say the heavensresponded with a glorious total eclipse of the moon—or perhaps, itwas the eclipsed moon that enabled the Red Sox to achieve thisstunning victory. Whatever the case, these cosmic events hadabsolutely no effect on the GP-B spacecraft.The GP-B science instrument and spacecraft were specificallydesigned to create a pristine environment to perform directmeasurements of both the frame-dragging and geodetic effects ofgeneral relativity with all Newtonian disturbances several orders ofmagnitude smaller than the effects to be measured. Theoretically, onlyone gyroscope is needed to make GP-B’s measurements, but we usefour gyroscopes to give highly accurate independent checks of the twoeffects. As a further validation, throughout the whole GP-B mission weconduct a continuing series of verification/calibration tests. Inparticular, during our final month-long instrument re-calibrationfollowing data collection, we will perform a series of tests in whichcertain classes of potential disturbances are deliberately increased inorder to uncover any previously unknown effects. Tests of this kind,where possible disturbances are enhanced in order to calibrate andremove them, are a vital part of good experimental physics practice.29 OCTOBER 2004—GRAVITY PROBE B MISSIONUPDATE: Day 192In its 28th week in orbit, GP-B is continuing to perform well. We havecompleted two months of data collection, and the quality of the datareceived thus far is excellent. The spacecraft is maintaining a constantroll rate of 0.7742 rpm (77.5 seconds per revolution) and flying dragfreearound gyro #3. The dewar temperature remains stable at 1.82kelvin, and the flow of helium from the dewar through the microthrusters is nominal. All other spacecraft subsystems are continuing toperform well.However, in preparation for solar events that could have an effect onour spacecraft, yesterday, the GP-B Mission Operations teamperformed an all-day simulation of the recovery process we will needto go through in the unlikely event that proton radiation from the suntemporarily disables our main, A-side computer, triggering anautomatic switch-over to our backup, B-side computer. Chances ofsuch a switch-over increase slightly whenever our spacecraft passesthrough the region over the South Atlantic Ocean, called the “SouthAtlantic Anomaly,” where the fluxes of particles trapped in the earth’sgeomagnetic field are greater than anywhere else.In fact, this may have been the cause of the B-Side switch-over weexperienced during the second week of the mission. At that time, wewere just beginning to adjust various parameters on the spacecraft,and switching back to the A-side computer went smoothly andquickly. Now that we are well into the science phase of the mission,flight-control parameters have changed, so it is prudent to rehearse therecovery procedures in order to minimize down time and loss of datain the event of a switch-over. To this end, we were pleased with theteam’s excellent performance during yesterday’s simulation.Again, this past week, oscillations in the drag-free control force thatwe have reported on in previous updates remained insignificant. Weare continuing to perform analysis and adjustments on the drag-freesuspension parameters to ensure that no harmonic coupling occursbetween the drag-free control system and helium sloshing in thedewar, and we continue to monitor this situation.490 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission