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

On Mission Day 528, the Gravity Probe B vehicle and payload are ingood health, with all subsystems performing nominally. The dewar isnow depleted of liquid helium, and this has affected varioussubsystems, as summarized below. Drag-free mode has been turnedoff.week's Mission News section when a colleague stopped by and said, “Itappears that we've run out of helium.” Suddenly, my dilemma of whatto write about this week was a non-issue.Yesterday at 1:55 pm Pacific Daylight Time, the last of the liquidhelium in our dewar transitioned to the gas phase, and the scienceinstrument began to warm. The team correctly assessed the status ofthe dewar, based on a set of pre-approved indicators, and they beganrunning our planned Helium Depletion Procedure.As of 9:30 am PDT this morning, the temperature in the dewar hadrisen from 1.8 to 5.6 kelvin, and it is slowly continuing to rise.Likewise, the temperature in SQUID Readout Electronics (SRE) hadrisen to 7 kelvin, but as of our 10:00 am PDT status meeting thismorning, the dewar was still cold enough for the SRE to providereadouts of the gyros' spin axes. As the SRE temperature continues torise, we will gradually lose these readouts.I grabbed my camera and dashed off to our Mission OperationsCenter (MOC). In recent weeks, our MOC has been relatively quiet,and typically there have only been a handful of people there at anygiven time. But, yesterday afternoon, the MOC was teeming withactivity, reminiscent of the days when we spun up the gyros in thesummer of 2004.The evidence was clearly visible on the dewar monitor screen. Thepressure display had suddenly taken a nose-dive. Elsewhere in theMOC, GP-B scientists and engineers were clustered around the statusmonitors representing their respective areas of expertise, engaged inanimated conversations. Meanwhile, our mission operations teammembers, headphones and microphones in place, werecommunicating with a NASA ground station, arranging for extratelemetry time.This past Monday, 26 September 2005, we made a third attempt toswitch from backup drag-free mode to primary drag-free mode ongyro #1. This latest attempt also failed, but for a different reason.Configuration issues were determined to be the root cause in each ofthe first two failures, and these issues were addressed prior to makingthis 3rd attempt on Monday. Analysis later this week showed thatMonday's failed attempt was consistent with similar failures we hadseen during the Initialization and Orbit Checkout (IOC) phase of themission in July and August, 2004, and it was likely due to un-modeledforces between the gyro rotor and its housing.As was the case during IOC, these issues could have been resolved withfurther experimentation and fine tuning the Attitude and TranslationControl system (ATC). However, given the imminent depletion of thedewar's helium supply, we decided to return the spacecraft to its stablescience configuration (backup drag-free mode, locked on the guidestar) and remain in this configuration until the helium ran out—whichas it turned out, happened three days later.Our science team is interested in comparing the data collected inscience configuration during this past week with the science data theycollected prior to beginning gyro torque calibrations in mid August.GP-B MISSION NEWS—THE DAY THE DEWAR DIEDAs we have been anticipating for some time now, the moment finallyarrived yesterday when the liquid helium in the dewar ran out. I wassitting in my office, thinking about what I was going to write in thisThere were some tense moments when all were patiently awaiting thedownload of more data that would confirm, unequivocally, that theliquid helium was depleted. We were also attempting to turn on theExperiment Control Unit (ECU) on the spacecraft so that we couldreceive some important data, such as the current temperature in thedewar. It took several hours to arrange all the necessary telemetrypasses, send the required commands to the spacecraft, and downloadGravity Probe B — Post Flight Analysis • Final Report March 2007 519