Hi,

Here is the most recent update I received today about the Gravity Probe B mission:

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GRAVITY PROBE B MISSION UPDATE -- June 18, 2004
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Just under two months into the mission, the spacecraft is in good
health, and all subsystems are performing well. The spacecraft's
orbit, which will remain in full sunlight through August,
continues
to be stable, meeting our requirements for transition into the
science phase of the mission. All four gyros remain digitally
suspended, and we are completing the planned series of
calibration
tests at very low gyro spin rates. The science telescope remains
locked onto the guide star, IM Pegasi, and we are beginning the
process of distributing and balancing the mass of the
spacecraft at
increased roll rates, as required for the science mission. For
reasons discussed below, the Initialization and Orbit Checkout
(IOC)
phase of the mission has been extended to 90 days.

Last Saturday, the team was honored to have NASA Administrator
Sean
O'Keefe, Alaska Senator Ted Stevens, and some of their staff
members
visit the GP-B facilities here at Stanford University. The visit
began with brief presentations by GP-B NASA Program Manager, Rex
Geveden from Marshall Space Flight Center, Principal Investigator
Francis Everitt, and Stanford Program Manager Gaylord Green.
Next,
the group viewed a display of spare flight gyroscopes, the flight
telescope, and other hardware and then toured the Mission
Operations
Center (MOC), talking with members of the GP-B Operations team
who
were on duty and observing them in action. While the visitors
were
watching, the MOC received data relayed from the 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 validation check.

This past week, the team increased the roll rate of the
spacecraft
from 0.1 rpm to 0.3 rpm, in preparation for "mass trim" and
"bubble
wrap." These procedures are used to bring the entire spacecraft
into
balance so that it rolls smoothly about its main axis, while
continuing to focus on the guide star through the telescope.
The mass
trim operation is similar to dynamically spin balancing a tire,
using
movable weights on the spacecraft frame under computer control to
adjust the spacecraft's center of mass. Bubble wrap is the
process of
uniformly distributing the liquid helium around the Dewar's outer
shell. We will accomplish this by increasing the roll rate of the
spacecraft in steps, from 0.3 rpm to 0.6 rpm.

Also during this past week, the spacecraft/telescope re-visited
guide
star neighbor HD 216635 (SAO 108242) as well as guide star
neighbor
HR Peg (HR 8714), for further testing and brightness calibration.

Another important event this past week was using the results of
prior
gyro calibration tests to fine-tune the Gyro Suspension System
(GSS)
for each gyro. This significantly improved the suspension
performance
of all the gyros, especially gyro #2. Parameters are now in
place for
spinning up the gyros to 5 Hz (300 rpm).

We have received several email inquiries about how the spin
rate of
the gyroscopes is determined in orbit. It is determined using the
SQUIDs. Even though the gyros are currently spinning very slowly,
there is enough trapped magnetic flux on the gyro rotors for the
SQUIDs to detect their rotation speed to an accuracy of 10
millionths
of a Hertz. Because the measurement is made between the gyro
rotors
and their housings, and because the gyro housings are connected
to
the spacecraft itself, the roll rate of the spacecraft was also
measured using the gyros. In fact, two of the gyros spin in the
same
direction as the spacecraft roll, and two spin against the roll.
Thus, as the roll rate of the spacecraft was increased, two of
the
gyros appeared to spin faster, and two appeared to slow down in
accordance with the increased roll rate.

As chronicled in previous weekly updates, two difficulties--now
both
overcome--have made the task of locking onto the guide star take
longer than anticipated, and as a result, the IOC phase of the
mission has been extended from 60 to 90 days. First, the
side-facing
star trackers on the spacecraft required an extended search
period to
properly identify the known field of stars. Feedback from these
star
trackers is used to orient the spacecraft in the vicinity of the
guide star, similar to using a spotting telescope to position a
high-powered telescope on a particular part of the sky. The
second is
from malfunction of two of the spacecraft's 16 ultra-sensitive
micro
thrusters. Redundancy built into the system enables the
spacecraft to
fly without the two problematic thrusters, but to optimize
performance with 14 instead of 16 thrusters, it was necessary to
revise the control software. This software change has now been
implemented, and after a Flight Readiness Review on June 25,
2004, it
will be uploaded to the spacecraft. At a later stage, we will
explore
partial re-activation of the two problematic thrusters.

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