Hi,

Here is the Gravity Probe B Mission Update for Oct. 22, 2004

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GRAVITY PROBE B MISSION UPDATE FOR 22 OCTOBER 2004:
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On Day #185, GP-B continues to perform well. We have switched back to flying the spacecraft drag-free around gyro #3, and all four gyros are digitally suspended and generating science data. The spacecraft's roll rate remains constant at 0.7742 rpm (77.5 seconds per revolution), and all subsystems are continuing to perform well. As we near two months of data collection, the flow of helium from the Dewar through the micro thrusters remains within expected limits, and the Dewar temperature is stable at 1.82 Kelvin. We have approximately seven months of data collection remaining, and the quality of the data received thus far continues to be excellent.

This past Tuesday, October 19th, gyro #1, which has been serving as the "drag-free" gyro for the past few weeks, transitioned into analog backup suspension mode. Having already worked through this same scenario with gyro #3 last month, we were able to smoothly switch drag free operations back to gyro #3 and restore digital suspension to gyro #1 within three hours of this event. Because gyro #3 has been performing well after we increased the signal to noise ratio in its Gyro Suspension System (GSS) position readout, we have made the same adjustment to gyro #1. We are now monitoring the performance of both gyros #3 and #1 to determine if the analog suspension transition will recur, or if this issue has been resolved..

The oscillations in the drag-free control force, which we have been reporting on in previous updates, have remained at an insignificant level over the past week. We are performing further analysis and adjustments on the drag-free suspension parameters to ensure the de-tuning of any harmonic coupling between the drag-free control system and helium sloshing in the Dewar. We are continuing to monitor the drag-free control system with regard to this issue.

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We have received inquiries about a recent letter to Nature by Ignazio Ciufolini and Erricos Pavlis claiming to have verified the Lense-Thirring frame-dragging effect of general relativity through laser ranging data observations of the LAGEOS I & II spacecraft. In their measurement, the frame-dragging effect needs to be separated by an extremely elaborate modeling process from Newtonian effects more than 10,000,000 times larger than the effect to be measured. The letter does not provide enough detail of the methods of verification and validation to allow a critical evaluation. We and other members of the relativity community look forward to a more complete account. If verified, their result will be of considerable interest.

The GP-B science instrument and spacecraft were specifically designed to create a pristine environment to perform direct measurements of both the frame-dragging and geodetic effects of general relativity with all Newtonian disturbances several orders of magnitude smaller than the effects to be measured. Theoretically, only one gyroscope is needed to make GP-B's measurements, but we use four gyroscopes to give highly accurate independent checks of the two effects. As a further validation, throughout the whole GP-B mission we conduct a continuing series of verification/calibration tests. In particular, during our final month-long instrument re-calibration following data collection, we will perform a series of tests in which certain classes of potential disturbances are deliberately increased in order to uncover any previously unknown effects. Tests of this kind, where possible disturbances are enhanced in order to calibrate and remove them, are a vital part of good experimental physics practice.

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NASA - Stanford - Lockheed Martin
Gravity Probe B Program
"Testing Einstein's Universe"
http://einstein.stanford.edu

Bob Kahn
Public Affairs Coordinator

Phone: 650-723-2540
Fax: 650-723-3494
Email: kahn@relgyro.stanford.edu
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Regards,

LelandJ