Hi,
Here is the 7/23/2004 update on the Gravity Probe B mission:
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GRAVITY PROBE B MISSION UPDATE -- July 23, 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 Thursday or Friday) to keep you
apprised of our progress. From time to time, we may send out extra
updates, as warranted by mission events.
One day #94 of the mission, Gravity Probe B is poised to enter the
home stretch of the Initialization and Orbit Checkout (IOC) phase of
the mission. The spacecraft is in excellent health, and all
subsystems are continuing to perform well. All four gyros are
digitally suspended, with gyros #2 and #4 spinning at science mission
speed-greater than 80Hz (4,800 rpm). Gyros #1 and #3 are spinning at
less than 1.5 Hz (90 rpm) and are ready for full-speed spin-up next
week. Fine-tuning of the Attitude and Translation Control system
(ATC) is still in progress, and the ATC is performing well. The
spacecraft's roll rate is 0.52 rpm, and the science telescope is
locked onto the guide star, IM Pegasi.
Last Friday, the full-speed spin-up of gyro#2 went smoothly, with a
final spin rate of 87 Hz (5,220 rpm). Helium gas leakage from the
spin-up of gyro #2 caused gyro #4 to slow down from 105.8 Hz (6,348
rpm) to 91 Hz (5,460 rpm). We had hoped that gyro #2 would achieve a
spin rate above 100 Hz (6,000 rpm), with less leakage effect on gyro
#4. Thus, rather than spinning up gyros #1 and #3 as originally
planned, we spent the past week doing analysis and running tests-both
on the spacecraft and here at Stanford-in order to ensure that the
upcoming spin-up of gyros #1 and #3 will result in higher speeds,
with less leakage effect on the remaining gyros.
The spin rate of the gyros during the Science Phase of the mission
affects the signal-to-noise ratio in the SQUID readouts of the
experimental data. The noise level is quite small, but constant. The
higher the gyro spin rate, the larger the London moment (magnetic
field created by a spinning superconductor), and thus, the greater
the signal-to-noise ratio. In ground testing prior to launch, we
determined that a spin rate of 80 Hz (4,800 rpm) or greater for each
gyro would provide a good signal-to-noise ratio for the science
mission. However, the threshold of 80 Hz (4,800) rpm is not a hard
and fast limit, so if the final spin rate of one or more gyros falls
slightly below this value, this will not appreciably compromise the
science data.
One way to potentially increase the spin-up rate of the remaining two
gyros, while reducing the amount of helium gas leakage during spin-up
is to use the Gyro Suspension System to position the gyro rotors
closer to the spin-up channel in the gyro housing. Tests and analysis
performed this past week indicate that we can move the rotors of
gyros #1 and #3 up to 30% closer to the spin-up channels than gyros
#2 and #4, and still have a safe margin of clearance from the
suspension electrodes and the gyro housings. We have also determined
that opening a second exhaust valve during spin-up may help to reduce
the pressure in the probe caused by helium leakage, thereby reducing
the spin-down effects on the remaining gyros. Both of these changes
will be implemented in the spin-up of gyros #1 and #3 next week.
Also, this past week, we continued fine-tuning the drag-free software
used by the Attitude and Translation Control system (ATC) to optimize
its performance at the current and final 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 2 minutes.
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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NASA - Stanford - Lockheed Martin
Gravity Probe B Program
"Testing Einstein's Universe"
http://einstein.stanford.eduBob Kahn
Public Affairs Coordinator
Phone: 650-723-2540
Fax: 650-723-3494
Email: kahn@relgyro.stanford.edu
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Regards,
LelandJ
