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

18 JUNE 2004—MISSION UPDATE: DAY 59Just under two months into the mission, the spacecraft is in goodhealth, and all subsystems are performing well. The spacecraft’s orbitcontinues to be stable, meeting our requirements for transition intothe science phase of the mission. All four gyros remain digitallysuspended, and we are completing the planned series of calibrationtests at very low gyro spin rates. The science telescope remains lockedonto the guide star, IM Pegasi, and we are beginning the process ofdistributing and balancing the mass of the spacecraft at increased rollrates, as required for the science mission. For reasons discussed below,the Initialization and Orbit Checkout (IOC) phase of the mission hasbeen extended to 90 days.This past week, the team increased the roll rate of the spacecraft from0.1 rpm to 0.3 rpm, in preparation for “mass trim” and “bubble wrap.”These procedures are used to bring the entire spacecraft into balanceso that it rolls smoothly about its main axis, while continuing to focuson the guide star through the telescope. The mass trim operation issimilar to dynamically spin balancing a tire, using movable weights onthe spacecraft frame under computer control to adjust the spacecraft’scenter of mass. Bubble wrap is the process of uniformly distributingthe liquid helium around the dewar’s outer shell. We will accomplishthis by increasing the roll rate of the spacecraft in steps, from 0.3 rpmto 0.6 rpm.Also during this past week, the spacecraft/telescope re-visited guidestar neighbor HD 216635 (SAO 108242) as well as guide star neighborHR Peg (HR 8714), for further testing and brightness calibration.Last Saturday, the team was honored to have NASA AdministratorSean O’Keefe, Alaska Senator Ted Stevens, and some of their staffmembers visit the GP-B facilities here at Stanford University. The visitbegan with brief presentations by GP-B NASA Program Manager, RexGeveden from Marshall Space Flight Center, Principal InvestigatorFrancis Everitt, and Stanford Program Manager Gaylord Green. Next,the group viewed a display of spare flight gyroscopes, the flighttelescope, and other hardware and then toured the MissionOperations Center (MOC), talking with members of the GP-BOperations team who were on duty and observing them in action.While the visitors were watching, the MOC received data relayed fromthe spacecraft through the Tracking Data Relay Satellite System(TDRSS) to the ground tracking station at White Sands, New Mexico,as the guide star returned to view in the telescope during a validationcheck.Another important event this past week was using the results of priorgyro calibration tests to fine-tune the Gyro Suspension System (GSS)for each gyro. This significantly improved the suspension performanceof all the gyros, especially gyro #2. Parameters are now in place forspinning up the gyros to 5 Hz (300 rpm).We have received several email inquiries about how the spin rate ofthe gyroscopes is determined in orbit. It is determined using theSQUIDs. Even though the gyros are currently spinning very slowly,there is enough trapped magnetic flux on the gyro rotors for theGravity Probe B — Post Flight Analysis • Final Report March 2007 477