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

emained in the digital suspension science mode. A preliminaryassessment by the science team indicated minimal impact to anyexperiment accuracy. An investigation is under way to determine thecause of the drag-free gyro #3 transition to analog mode and safemodeactivation. The operations and engineering teams did an excellent jobworking through the night to safe the vehicle and quickly develop arecovery plan.By Friday afternoon the spacecraft was back in science collecting modewith gyro #1 performing the drag-free operation. On Saturday theoperations team successfully transitioned Gyro #3 from analogsuspension to the digital suspension science mode. All other spacecraftsystems are in excellent health. The spacecraft is rolling at a rate of0.77419 rpm (one revolution every 77.5 seconds). The Attitude andTranslation Control (ATC) system is maintaining a drag-free orbitaround gyro #1, and it is properly tracking the guide star, IM Pegasi.The dewar temperature is nominal (1.82 Kelvin), and the flow ofhelium, venting from the dewar through the micro thrusters hasremained within expected limits.spacecraft was back in science mode, with gyro #1 performing thedrag-free operation. The effect of this event on the science experimentwas insignificant. The most likely cause of this event was a data spikein the Gyro Suspension System (GSS) position sensor for gyro #3.Gyro #3 was returned to digital suspension as part of the recoveryplan, and it has remained digitally suspended, with no furtherproblems, since that time. However, we are continuing to monitor itsperformance and investigate the root cause. Thus, for the foreseeablefuture, the spacecraft will continue to fly drag-free around gyro #1.During the week the team performed a heat pulse test on thespacecraft dewar to obtain further information to help predict lifetime.Early indications are that the helium in the dewar will last another 10months, which should be sufficient to satisfy the missionrequirements.1 OCTOBER 2004—GRAVITY PROBE B MISSIONUPDATE: Day 164The spacecraft is in good health, flying drag-free around gyro #1. Allfour gyros are digitally suspended, and their SQUID readouts arecollecting relativity data during the portion of each orbit when thescience telescope is locked onto the guide star, IM Pegasi. Thespacecraft’s roll rate remains at 0.7742 rpm (one revolution every 77.5seconds). The dewar temperature is nominal (1.82 Kelvin), and theflow of helium, venting from the dewar through the micro thrusters iswithin expected limits.At this time of year in North America, at around 8:30 PM PacificDaylight Time, the constellation Pegasus, in which the guide star IMPegasi resides, is clearly visible above the Eastern horizon. With amagnitude ranging from 5.6 – 5.85, IM Pegasi is too dim to see withthe naked eye, but in a good viewing location, it should be visible withbinoculars.In last week’s update, we reported that at 8:30 PM local time onThursday, 23 September 2004, gyro #3—which was then serving as thedrag-free gyro—suddenly transitioned to analog backup suspensionmode, automatically triggering a safemode that stopped the missiontimeline. The GP-B operations and engineering teams quicklydeveloped a recovery plan, and by the following afternoon, theAlso last week, we reported that the team had performed a heat pulsetest on the spacecraft dewar in order to determine the mass of liquidhelium remaining inside, which determines the remaining lifetime ofthe mission. The heat pulse test works in the following way: Theamount of heat that it takes to warm an object by a specified amountdepends on the type of material, its temperature, and its mass. Thus, ifthe “specific heat” (the amount of heat needed to warm a kilogram ofmaterial by one degree kelvin) is known, it is a simple matter tomeasure the mass by applying a known amount of heat (usually withan electric heater) and measuring the resultant temperature rise. In thecase of the GP-B dewar, the situation is simplified by the fact that heatdistributes itself virtually instantaneously throughout superfluidhelium. The amount of heat used in the test must be large enough tocause a measurable temperature change (approximately 10millikelvin) but not so large as to appreciably shorten the missionlifetime. The heat pulse test yielded a remaining superfluid heliummass of 216 kg, which translates into a remaining mission lifetime of9.9 months. This means that the science (data collection) phase of themission will continue for a little over 8 more months, and then we willspend the final month re-calibrating the science instrument.Finally, this past Tuesday, 28 September 2004, we observed a slightincrease in the helium required by the micro thrusters to maintaindrag-free flight around gyro #1. To be conservative, we decided to turnoff drag-free mode and evaluate the situation. Over a period of 4-6hours, the oscillations that were causing the ATC to require excesshelium died out and have not returned. After some analysis anddiscussion, we have determined these oscillations were caused by asympathetic resonance between the ATC drag-free control efforts anda sloshing wave on the surface of the now reduced superfluid heliumin the dewar. This situation is somewhat analogous to placing avibrating tuning fork on the body of a guitar, which then causes guitarstrings tuned to harmonics of the same frequency to start vibrating.This is a transient situation, and the team has adjusted the ATC drag-Gravity Probe B — Post Flight Analysis • Final Report March 2007 487