Hi,

Here is the latest update on the Gravity Probe B mission

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GRAVITY PROBE B MISSION UPDATE -- July 31, 2004
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Please Note: During the Initialization & Orbit Checkout (IOC) Phase
of the GP-B mission, we update our Web site and send out this email
update once a week (usually on Friday or Saturday) to keep you
apprised of our progress. From time to time, we may send out extra
updates, as warranted by mission events.

Gravity Probe B successfully accomplished a major milestone this week
as the final two gyroscopes were spun up to high spin speed. All
four gyros are now spinning at high spin speed. The spacecraft is in
excellent health, and all subsystems are continuing to perform well.
With this major accomplishment behind us the Program is entering the
home stretch of the Initialization and Orbit Checkout (IOC) phase of
the mission.

This week's high speed spin-up operations were performed on gyro #3
(Tuesday) and gyro #1 (Friday). There were no anomalies during the
spin up. The SQUID readout FFT tracked the spin up in real-time. As
expected, the other gyros spun down approximately 15% with each high
speed spin-up operation due to pressurization of the Probe caused by
gas leakage during the spin-up. Final spin speeds of the gyros are
now: gyro #1 (80.0 Hz - 4800 rpm), gyro #2 (62.3 Hz - 3738 rpm), gyro
#3 (82.7 Hz - 4962 rpm) and gyro #4 (65.5 Hz - 3930 rpm). At these
spin speeds, GP-B's relativity measurement is expected to be
significantly better than specification. Therefore, no further gyro
high speed spin-up operations are planned.

Fine-tuning of the Attitude and Translation Control system (ATC) is
nearly complete, and the ATC is performing well. This past week, we
continued fine-tuning the drag-free software used by the ATC to
optimize its performance at the current spacecraft roll rate of 0.52
rpm. Tests from parameter changes we made to the ATC system indicate
that we have reduced the time it takes to re-lock onto the guide star
from as much as 15 minutes to less than a minute. We have also
increased the number of reference stars seen by the star tracker from
3 to 8.

Next week we will be performing a few final operations prior to
beginning taking science data. The most significant operation left
to be performed is the spin axis alignment of each gyro. This is
performed using the Gyro Suspension System in a special oscillation
mode to slowly align each gyro spin axis to be in nearly perfect
alignment with the guide star. Having the gyros aligned at the
beginning of the mission makes any precession caused by general
relativity simpler to measure. We are nearly complete aligning the
spin axis of gyros #2 and #4 and plan to align #3 and #1 next week.
Another operation for next week is to perform a final bake-out
operation to remove any residual helium molecules from around the
gyros. This operation is performed by heating up the instrument area
slightly (to approximately 6 degrees Kelvin) , which will cause
residual helium to migrate to a collector device in the Probe. And
finally, the final operation prior to science is to command the
spacecraft to begin drag-free operations.

The spacecraft is being controlled from the Gravity Probe B Mission
Operations Center, located here at Stanford University. The
Stanford-NASA/MSFC-Lockheed Martin operations team is continuing to
perform superbly.

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Regards,

LelandJ