Hi,

Here is the Gravity Probe B Mission Update for September 25, 2004:

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GRAVITY PROBE B MISSION UPDATE FOR 25 SEPTEMBER 2004:
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GP-B had an eventful week. We began the 4th week of science with all four gyroscopes in the science mode. Then on Thursday we had an anomaly with gyro #3.

At 8:30 PM local time on Thursday, the drag-free gyro #3 transitioned to analog mode resulting in safemode that stopped the timeline. The GP-B vehicle and science gyros were safe. Gyro's #1, #2 and #4 remained in the digital suspension science mode. A preliminary assessment by the science team indicated minimal impact to any experiment accuracy. An investigation is under way to determine the cause of the drag-free gyro #3 transition to analog mode and safemode activation. The operations and engineering teams did an excellent job working through the night to safe the vehicle and quickly develop a recovery plan.

By Friday afternoon the spacecraft was back in science collecting mode with gyro #1 performing the drag free operation. On Saturday the operations team successfully transitioned Gyro #3 from analog suspension to the digital suspension science mode. All other spacecraft systems are in excellent health. The spacecraft is rolling at a rate of 0.775 rpm (one revolution every 77.5 seconds). The Attitude and Translation Control (ATC) system is maintaining a drag-free orbit around gyro #1, and it is properly tracking the guide star, IM Pegasi. The Dewar temperature is nominal (1.82 Kelvin), and the flow of helium, venting from the Dewar through the micro thrusters has remained within expected limits.

During the week the team performed a heat pulse test on the spacecraft Dewar to obtain further information to help predict lifetime. Early indications are that the helium in the Dewar will last another 10 months, which should be sufficient to satisfy the mission requirements.

Please Note: We will continue updating these highlights and sending out the GP-B email update on a weekly basis through the first few weeks of the Science Phase of the mission. Then, as mission operations become routine, we may reduce the frequency these updates to biweekly. However, from time to time, we intend to post special reports and special updates, as warranted by mission events.



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Tom Langenstein
tom@relgyro.stanford.edu
Fax: (650) 725-8312
Office: 5-4108
Cell: (650) 823-4906



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Regards,

LelandJ