Hi,
Here is a correction to the previous Gravity Probe B Mission Update.
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All,
It appears that my brain took a proton hit when I was writing last week's update. Following is a corrected version of the last three paragraphs.
Bob Kahn
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CORRECTIONS TO September 10, 2004 GRAVITY PROBE B MISSION UPDATE
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During the GP-B experiment, the spin axis deflections of the gyroscopes are measured relative to the guide star, IM Pegasi. But, in order to determine the actual relativistic drift rate of the gyros, we must also measure the proper motion of IM Pegasi relative to quasars (the most distant objects in the universe). The Harvard-Smithsonian Center for Astrophysics (CfA) is measuring the proper motion of IM Pegasi with respect to quasars to an unprecedented level of precision. However, by design, the CfA will not release this proper motion data to the GP-B science team until after the scientific data collection, post-science instrument calibrations, and the data analysis have all been completed. In other words, the relativistic drift rates of the gyros may only be determined once the GP-B gyro drift results and CfA proper motion results are combined.
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 analysis be completed before we can receive and combine the CfA proper motion data and release/publish the final result. 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 revolution around the Sun) as one means of calibrating the telescope/gryo readout scale factor. Collecting this aberration data over a substantial portion of a year will ensure the greatest accuracy of this calibration.
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
