Hi,
Here is the Gravity Probe B Mission update for September 3, 2004.
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GRAVITY PROBE B MISSION UPDATE -- September 3, 2004
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Please Note: We will continue sending these email updates, as well as
posting updated highlights to our Web site 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 may also send out
extra updates with special reports, or as warranted by mission events.
As of Day #136, GP-B has successfully completed its first full week
in the Science Phase of the mission, with gyros #1, #2, and #3 in
science mode. Gyro #4 is still undergoing alignment of its spin axis,
which we expect to be completed in about a week. For the past week,
the spacecraft has been in drag-free mode around gyro #3.
The spacecraft remains in excellent health, rolling at a rate of
0.7742 rpm, with all subsystems performing well. The telescope
continues properly tracking the guide star, IM Pegasi, during the
portion of each orbit when the guide star is visible. We are still
investigating a small force or bias along the roll axis of the
spacecraft, but this bias has no effect on science data collection.
Moreover, during the past two weeks, we have tuned the spacecraft's
Attitude and Translation Control (ATC) system to compensate for this
bias, with no excess expenditure of helium through the micro
thrusters.
Having just achieved the major milestone of transitioning into the
Science Phase of the mission, this is a good time to pause and look
back over the Initialization and Orbit Checkout (IOC) phase of the
mission. During this 9-week period, the GP-B mission has already
achieved a number of extraordinary accomplishments:
The orbit injection of the spacecraft was so close to perfect (within
6 meters of the target orbit plane) that none of the planned orbit
trim operations were necessary.
We've communicated with the spacecraft over 3,000 times during the
IOC phase, and the Mission Planning team has successfully transmitted
over 70,000 commands to the spacecraft without an error.
GP-B is the first satellite ever to achieve both 3-axis attitude
control (pitch, yaw, and roll), and 3-axis drag-free control.
Essentially, while orbiting the Earth, the whole spacecraft flies
around one of the science gyros.
The GP-B gyros, which are performing perfectly in orbit, will be
listed in the forthcoming edition of the Guinness Book of World
Records as being the roundest objects ever manufactured.
The spin-down rates of all four gyros are considerably better than
expected. GP-B's conservative requirement was a characteristic
spin-down period (time required to slow down to ~37% of its initial
speed) of 2,300 years. Recent measurements show that the actual
characteristic spin-down period of the GP-B gyros exceeds 10,000
years-well beyond the requirement.
Once tuned up, the spacecraft's Attitude and Translation Control
(ATC) system has been able to function at a spacecraft roll rate of
0.7742 rpm-more than twice the roll rate of 0.3 rpm initially
specified.
The magnetic field surrounding the gyros and SQUIDs (Super-conducting
QUantum Interference Device) has been reduced to 0.0000001 gauss,
less than one millionth of the Earth's magnetic field-the lowest ever
achieved in space.
The gyro readout measurements from the SQUID magnetometers have
unprecedented precision, detecting fields to 0.0000000000001 gauss,
less than one trillionth of the strength of Earth's magnetic field.
The science telescope on board the spacecraft is tracking the guide
star, IM Pegasi (HR 8703), to superb accuracy, and it is also
collecting long-term brightness data on that star.
A number of people have asked the following two-part question about
GP-B: "Given that our gyros are spinning about half as fast as we
originally anticipated, and that the IOC phase took about twice as
long as originally anticipated, how will these two situations affect
the success of the GP-B experiment?"
Regarding the gyros, several of the accomplishments above-especially
the extremely low SQUID noise and higher than planned spacecraft roll
rate-have effectively reduced the error factor in the GP-B science
experiment, thereby partially compensating for the reduced spin rates
of the gyros.
Regarding the extended length of the IOC phase, the GP-B mission is
unique because it is truly a physics experiment in space. As such,
there are trade-offs that can be made. The primary trade-off we had
to wrestle with was: More optimization/calibration of the instrument
prior to entering science, with a shorter data collection period; or,
less optimization/calibration, with a longer data collection period.
We concluded that the best overall accuracy would be achieved by
ensuring that the science instrument was optimally calibrated from
the start, even if this meant collecting data for a shorter period
than we had hoped. And, in fact, we now have a considerably better
understanding of the instrument than originally anticipated at this
stage of the mission.
The original ideal duration of the experiment was 13 months of
relativity data gathering, but this is not essential, especially in
view of the work we have now done on the optimization/calibration
phase. After two months of science data collection, we can make a
very good measurement of the geodetic effect and a significant
measurement of the frame-dragging effect. The data improves as the
3/2 power of the time (i.e. double the time, and the result will
improve by a factor of ~3). In the near future, we will make another
measurement of the residual helium in the Dewar, which will provide
an accurate determination of its cryogenic lifetime. This, in
combination with the observed instrument performance, will indicate
the final expected accuracy of the experiment.
The GP-B program will not release the scientific results obtained
during the mission until after the science phase has concluded. It
is critically important to thoroughly analyze the data to ensure its
accuracy and integrity prior to releasing the results. After more
than 40 years of development, we have learned the value of
thoroughness and patience.
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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
