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

Gyro Suspension System (GSS): All 4 gyros digitally suspendedDewar Temperature: Not AvailableGlobal Positioning System (GPS) lock: Greater than 99.8%Attitude & Translation Control (ATC): 137.8 marcs rms (as of 9/15)Y-axis error: 176.4 marcs rms (as of 9/15)Command & Data Handling (CDH): B-side (backup) computer incontrolMulti-bit errors (MBE): 0Single-bit errors (SBE): Not AvailableGyro #1 Drag-free Status: Backup Drag-free mode (OFF during somecalibration maneuvers)As of Mission Day 514, the Gravity Probe B vehicle and payload are ingood health, with all subsystems performing nominally. We still havehelium remaining in the dewar, and the spacecraft is currently flyingdrag-free around Gyro #1.This coming weekend, if we still have helium remaining, we will visitsimilar virtual star locations, or possibly real stars, within a 4-degreeradius of IM Pegasi, each time visiting a neighboring location forseveral hours and then returning to IM Pegasi. And, as long as there isstill helium, we will continue similar tests into next week…andbeyond.GP-B MISSION NEWS—THE HELIUM JUST KEEPS ONFLOWING…Like Energizer Bunny® that “keeps on going,” the helium in the GP-Bdewar just keeps on flowing. Our dewar specialists assure us that weare, indeed, well into the final days—or possibly hours—of helium.But when will the helium actually run out? That's anybody's guess (wehave a pool going amongst the remaining team members here atStanford).Estimating the amount of liquid helium remaining in a 650 gallonThermos bottle, orbiting 400 miles above the Earth, requires complexcalculations, and there are many variables at play. Also, there is aconsiderable margin of error in these calculations. Our GP-B dewarspecialists, who modeled the dewar's life cycle and have been trackingits performance, have purposely remained conservative in theirlongevity estimates of the helium, so as to ensure that there would beenough helium to perform the most important calibration tests.Following the initial tests, we have been working our way through aprioritized list of further tests that will be beneficial in analyzing thescience data. As long as the helium continues to flow, we will continueworking our way through this list. The consensus here is that thehelium will very likely run out by the beginning of next week. But, if itlasts longer, we will simply continue performing these tests.The helium in the dewar has now surpassed its estimated lifetime bymore than two weeks. We have thus continued working our waythrough a prioritized list of calibration tests. Most of these tests haveinvolved slewing the telescope (and spacecraft) to “visit” stars (bothreal and virtual) in the neighborhood around the guide star, IM Pegasi.In each case, we visit a neighboring location for a period of time andthen lock back onto IM Pegasi for a number of hours. The purpose ofthese excursions to neighboring stars is to precisely calibrate thetorques imparted onto the science gyroscopes by purposelymisaligning the telescope axis (and spacecraft roll axis) by varyingamounts from the direction of the gyros' spin axes.Last Friday, we visited a virtual star (place where no stars visible to thetelescope exist) located 0.1 degrees west of our guide star, IM Pegasi, inthe direction of neighboring star HR Pegasi. We remained fixed onthat virtual star location for 48 hours and then returned to IM Pegasi,where we remained for 24 hours.While locked onto IM Pegasi, we switched drag-free control from gyro#1 to gyro #3 in order to perform some calibration tests on gyro #1.During this calibration test, the pressure in the dewar dropped, andthe spacecraft experienced thruster instability in the z-axis direction(along the spacecraft/telescope roll axis). This behavior was similar toinstabilities that we had previously seen in June 2004, during theInitialization and Orbit Checkout (IOC) phase of the mission. Theroot cause is under investigation, but there is some speculation thatthis may be the result of a micrometeorite striking the spacecraft. Torectify this situation, we switched the operating mode of our microthrusters to “open loop”—that is, turning off the thruster pressuresensors and relying instead on pre-defined thruster calibration curvesto control thruster flow. We will leave the micro thrusters in open loopmode until the helium is depleted.When the helium actually does run out, we will post a notice on ourWeb site and send out a message to the subscribers of our GP-BUpdate email list. NASA will also issue a news release, and we will thenpost the content of that release on our Web site and send it to ouremail subscribers. Stay tuned….16 SEPTEMBER 2005—GRAVITY PROBE B MISSIONUPDATEMission Elapsed Time: 514 days (73 weeks/ 16.85 months)—IOC Phase: 129 days (4.2 months)—Science Phase: 352 days (11.6 months)—Final Calibration Phase: 33 daysCurrent Orbit #: 7,586 as of 5:30PM PSTSpacecraft General Health: GoodRoll Rate: Normal at 0.4898 rpm (2.04 minutes per revolution)516 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission