Hi,
Here is the Gravity Probe B Mission Update for September 10, 2004:
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GRAVITY PROBE B MISSION UPDATE -- September 10, 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.
Just over 20 weeks in orbit, GP-B remains in the Science Phase of the
mission. Gyros #1, #2, and #3 are in science mode, generating
relativity data. Alignment of the spin axis of gyro #4 with the guide
star, IM Pegasi, is continuing and will be completed by early next
week.
The spacecraft remains in excellent health, rolling at a rate of
0.7742 rpm (one revolution every 77.5 seconds). The Attitude and
Translation Control (ATC) system is maintaining a drag-free orbit
around gyro #3, and it is properly tracking the guide star, IM
Pegasi, during the portion of each orbit when the guide star is
visible. The Dewar temperature is nominal, and the flow of helium,
venting from the Dewar through the micro thrusters to maintain the
drag-free orbit has remained within expected limits. All other
spacecraft subsystems are continuing to perform well.
On Tuesday, September 7, a double-bit error occurred in a
non-critical memory location of the spacecraft's main (A-side)
computer due to a proton hit. This type of error is likely to occur
from time to time during the mission. The necessary correction was
prepared and uploaded to the proper memory location of the computer.
This incident had no effect on science data collection or spacecraft
subsystems.
Our statement in last week's update that the GP-B program will not be
releasing any scientific data or results until after the completion
of the science and instrument re-calibration phases of the mission
prompted several inquiries that essentially boil down to two
questions: 1) What steps have we taken to ensure the integrity of the
raw data and the accuracy of the results? And, 2) Why is it necessary
to wait so long before releasing any results?
GP-B's principal investigator, Professor Francis Everitt, has been
quoted on several occasions as saying: " I don't care whether
Einstein was right or wrong; what I want is the experimental truth."
This is our guiding philosophy here at GP-B. The results of the GP-B
experiment will be invaluable to science whether or not they agree
with Einstein's predictions. Also, given that this experiment is
unlikely to ever be repeated, everyone on the GP-B team feels a great
sense of personal responsibility to be as careful and thorough as
possible in collecting the data and cross checking our instruments
and procedures for accuracy.
More specifically, in answer to the first question above, many
safeguards are built into the GP-B experiment to ensure the integrity
of the raw data, the correctness of the data analysis, and the
accuracy of the results. These safeguards include:
GP-B is a joint effort between NASA, Stanford, and Lockheed Martin.
People from NASA and Lockheed Martin work on site here in the GP-B
facilities at Stanford, and NASA managers review all GP-B mission
activities.
GP-B has an external Science Advisory Committee, comprised of
renowned physicists and space scientists. That committee has overseen
the preparations for data collection, and it will review all of the
data and analysis before the results are published.
GP-B also has an independent data analysis team that will be
analyzing the data in parallel with the GP-B science team to
corroborate our results. Any differences in the results of these two
teams will be reconciled before the results are announced.
The incoming raw data is fed directly into a database and is only
available on a "read-only" basis to the scientists who will be
analyzing it.
Multiple cross checks are performed to identify any potential
systematic experimental errors. For example, the results of all four
gyros are compared and correlated for consistency.
During the GP-B experiment, the spin axis deflections of our
gyroscopes is measured in reference to our guide star, IM Pegasi.
However, in order to determine the actual relativistic drift rate of
the gyros, we must relate the proper motion of IM Pegasi to quasars
(the most distant objects in the universe). The Harvard-Smithsonian
Center for Astrophysics (CfA) has mapped the proper motion of IM
Pegasi to an unprecedented level of precision. By design, the CfA
will not release this proper motion data to the GP-B science team
until after the Science Phase of the mission and post-science
instrument calibrations have been completed, and the data analysis
has begun.
Regarding the second question of why it is necessary to wait so long
before releasing any results, one reason is now apparent: the guide
star proper motion safeguard requires that all the GP-B data be
collected and the analysis begun before the CfA will release the
proper motion data on IM Pegasi. A related technical reason is that
while we should have a very good measure of gyro drift after 2-3
months of data collection, it is essential that this data be cross
checked and calibrated to ensure its accuracy. We use the annual
aberration of our guide star signal (the difference between the
actual position of IM Pegasi and its apparent position due to the
Earth's rotational motion around the Sun) to calibrate the
telescope/gryo readout scale factor. Collecting this calibration data
over a substantial portion of a year will ensure the greatest
accuracy.
In summary, we are using utmost care and rigor in our experimental
methods and analysis, and only after the analysis has been completed
and thoroughly checked will we announce and publish the results. At
that time, all of the GP-B data will become available to the public
through the National Space Sciences Data Center (NSSDC), located at
NASA's Goddard Space Flight Center in Greenbelt, MD.
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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
