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

neighbor star we chose has no proper name, only catalog numberssuch as HD 216635, and with an average V magnitude of 6.61 it issomewhat fainter than IM Pegasi (V magnitude 5.89). We kept thetelescope pointed at HD 216635 for about half an hour and thenslewed the telescope back to IM Pegasi. Over the course of last week,we repeated this procedure a total of five times. Then, this pastMonday, 22 August 2005, we slewed the telescope and spacecraft tothe star named Homam (aka Zeta Pegasi or HD 214923), which isabout seven degrees away from IM Pegasi and considerably brighter(average V magnitude 3.40). This time, we kept the telescope andspacecraft pointed at Homam for about 30 hours before returning toIM Pegasi.Ostriches” and Na’ir Sa’d al Bahaim, “The Bright Fortunate One of theTwo Beasts” were also in use among the Arabs. The Chinese, for someunknown reason, connected the star with thunder.Zeta Pegasi is computed to be about 210 light years away, with anactual luminosity about 145 times brighter than our Sun. On our Website this week, we have posted photos of HD 216635, IM Pegasi, andZeta Pegasi, along with sky charts produced by the Voyager III SkySimulator from Carina Software (http://www.carinasoft.com) thatshows the locations of these stars. The star photos, as well as much ofthe information about these stars came from the various Web pages ofthe Centre de Donnees astronomiques de Starsbourg—CDS(http://cdsweb.u-strasbg.fr/), including the Simbad astronomicaldatabase (http://simbad.u-strasbg.fr/sim-fid.pll) and the Aladininteractive sky atlas (http://aladin.u-strasbg.fr/aladin.gml).2 SEPTEMBER 2005—GRAVITY PROBE B MISSIONUPDATEThe reason we are performing these slewing tests is to measure andcalibrate the amount of torque (force) placed on the sciencegyroscopes by intentionally mis-aligning the telescope axis from thegyroscope spin axes by relatively large factors of first one, and thenseven degrees. At the beginning of the science phase of the mission, wealigned both the telescope and the spin axes of all four gyroscope withthe guide star, IM Pegasi to an accuracy level of a few arcseconds.During the science phase of the mission, we kept the telescope alignedwith the guide star to within 100-150 milliarcseconds. (an arcsecond is1/3600th of a degree, and a milliarcsecond is 1/1000th of anarcsecond). By design, the spin axes of the gyroscopes are aligned withthe axis of the telescope, which is also the roll axis of the spacecraft,and as long as these alignments are maintained, the non-relativistictorques on the gyros average out. However, during these finalcalibrations, we intentionally break these alignments, forcing thetelescope first one degree and then seven degrees away from the gyrospin axis alignments, and we can then determine the extent to whichthese misalignments place torques on the gyro spin axes. Clearly, giventhe nature of these final calibration tests, they could not be performedwhile we were still collecting science data, which is why they were leftto be performed at the very end of the experiment.Mission Elapsed Time: 500 days (71 weeks/ 16.4 months)—IOC Phase: 129 days (4.2 months)—Science Phase: 352 days (11.6 months)—Final Calibration Phase: 19 daysCurrent Orbit #: 7,376 as of 1:30PM PSTSpacecraft General Health: GoodRoll Rate: Normal at 0.7742 rpm (77.5 seconds per revolution)Gyro Suspension System (GSS): All 4 gyros digitally suspendedDewar Temperature: 1.82 kelvin, holding steadyGlobal Positioning System (GPS) lock: Greater than 98.5%Attitude & Translation Control (ATC): X-axis attitude error: 140.0marcs rmsY-axis error: 186.4 marcs rmsCommand & Data Handling (CDH): B-side (backup) computer incontrolMulti-bit errors (MBE): 0Single-bit errors (SBE): 8 (daily avg.)Gyro #1 Drag-free Status: Backup Drag-free mode (OFF during somecalibration maneuvers)On Mission Day 500, the Gravity Probe B vehicle and payload are ingood health and all subsystems are performing nominally.As of today, there is still an ever-thinning film of superfluid helium inthe dewar, and thus calibration tests, which began 19 days ago,continued throughout this past week. Over the course of last weekendand early this week, we thrice slewed the telescope (and spacecraft) to“visit” the star HD 216635, which is about one degree northwest of IMPegasi, and then back to IM Pegsi.The star, Homam or Zeta Pegasi, was not one of our initial candidatesto be used in the final calibrations, but it was selected by our telescopeteam because it emits light in the blue range of the spectrum, whereasIM Pegasi emits light in the red range of the spectrum. Homam is alsomuch brighter than IM Pegasi, as noted above, and thus this latestcalibration test was not only useful for examining torques on thegyros, but also for evaluating the telescope performance on a brighterstar with a different color of light.Burnham’s Celestial Handbook (a three-volume compendium ofastronomical information), gives the following information about thename Homam, by which Zeta Pegasi is also known:Homam, probably comes from the Arabic phrase Sa’d al Humam, the“Lucky Star of the Hero,” though Thomas Hyde derived it from AlHammam, which seems to mean “The Whispering One.” According toR. H. Allen, the names Sa’d al Na’amah, “The Lucky Star of theGravity Probe B — Post Flight Analysis • Final Report March 2007 513