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

On Wednesday, we visited the star HR Pegasi (HD 216672), which isabout 0.4 degrees west of IM Pegsi. We had previously visited this starduring the Initialization and Orbit Checkout (IOC) phase of themission in June 2004, as a test to confirm that the star we were using asthe guide star was, in fact, IM Pegasi. (You can read about ourprevious visit to HR Pegasi in our 11 June 2004 update.Thus far during these final calibration tests, our visits to HD 216635and to Zeta Pegasi were predominately in a north-south plane, relativeto IM Pegasi. Our visit to HR Pegasi this past Wednesday was our firstcalibration test with a star located in the east-west plane, with respectto IM Pegasi. Continuing this line of exploration, yesterday and todaywe completed several visits to locations less than one degree west, andless than one degree east of IM Pegasi. No stars visible to the on-boardtelescope exist in these recently visited locations, so we used data fromour star trackers and our standard navigational gyroscopes, which areaccurate to within 20 arcseconds, to determine the positions of these“virtual stars.” Since the purpose of these calibrations is to evaluate theeffects of telescope mis-alignments on the science gyros, greaterpointing accuracy is not required for these excursions.We have postponed our previous plans to slow down the spacecraft'sroll rate until after the helium in the dewar is fully depleted. Over thisweekend, we will continue making calibration tests similar to those wehave been performing, and if the helium lasts into next week, we willperform other types of calibration tests. Once the helium is depleted,the GP-B flight mission will officially end.Tony Lyons, NASA's program manager for GP-B from Marshall SpaceFlight Center in Huntsville, AL, has been here at Stanford all week,participating with our team in these final days of the mission.GP-B MISSION NEWS—THE FINAL DAYSThis past week marks a major transition point in the GP-B program.Exactly 16.4 months after a picture-perfect launch on 20 April 2004,we have come to the end of the GP-B flight mission. Literally any daynow, the supply of superfluid helium—the coolant that hasmaintained the cryogenic temperatures necessary for our SQUID gyroreadouts to function—will be exhausted.Right now, there is an ever-thinning layer of superfluid liquid heliumlining the dewar, and there will come a point where the last of thisliquid helium has “sweated” out through the porous plug, leaving onlyhelium gas inside the dewar. (See our Mission News story of 29 July2005 for a description of the porous plughttp://einstein.stanford.edu/highlights/hlindex.html.) As this finalhelium gas begins to pas through the porous plug, the pressure in thedewar will begin to drop, causing the temperature in the Probe to rise.We don't know whether this transition will happen gradually orsuddenly (as was the case with NASA's COBE spacecraft), but we willknow that we have reached this point when our telemetry data shows arise in the temperature sensors on the bracket that houses the SQUIDgyro readout controllers and the temperature sensors on the telescopedetector packages. When the temperature in the Probe reaches about 7kelvin, we will begin to lose superconductivity in the niobium gyrorotor coatings, and the SQUID gyro readouts will gradually cease todetect the London moments in the gyros. At this point, it be no longerbe possible to determine the spin axis orientation of the sciencegyroscopes, although the Gyro Suspension Systems (GSS) willcontinue to indicate the position of each gyro rotor within its housingto great precision. Also, with no helium propellant left, the spacecraftwill no longer be able to maintain a drag-free orbit.The 650 gallons of helium that filled the dewar at launch has lastedexactly the length of time planned. We collected 50 weeks of sciencedata-just two weeks short of the year's worth of data we had hoped tocollect. Furthermore, we lost less than 1% of the data collected totelemetry and/or spacecraft hardware problems. We have nowcompleted all of the vital planned calibration tests that needed to bedone with the gyro readouts still functioning, and we are using thesefinal days of helium to perform additional tests that will be valuable tothe data analysis now in process. In short, GP-B has been a remarkablysuccessful mission.There is an air of triumph here at Stanford, but there is also a note ofsadness. Having become a very tight-knit team, we are saddened thattoday is the last day of work on GP-B for a number of our colleagues,especially those from Lockheed Martin Corporation. Thus, thisafternoon, we all gathered in the conference room where we've beenholding daily status meetings since before launch, and we spent a fewminutes raising toasts to our successful mission and to the futureendeavors of our colleagues who will be moving on to new jobs nextweek.Regarding the GP-B spacecraft, the experiment, and the remainingmembers of the team, following is a brief overview of what's in store.Next Tuesday, when we return from the Labor Day holiday, GP-B willbe a much quieter place. Our science team is already well into the long,painstaking process of analyzing the data. A small spacecraft514 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission