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Gravity Probe B

Testing Einstein's Universe



Note: We've received several inquiries about a news story on GP-B in the current issue of Nature (Vol. 444, 21-28 December 2006, pp. 978-979). You'll find our response in this month's GP-B Mission News below.

During the 50-week science phase of the GP-B mission and the 7-week instrument calibration phase, which lasted from August 2004-September 2005, we collected over a terabyte of experimental data. Analysis of this data has been steadily progressing through a 3-phase plan, each subsequent phase building on those preceding it.

In Phase I, which lasted from the end of September 2005 through February 2006, the analysis focused on a short-term, day-by-day or even orbit-by-orbit, examination of the data. The overall goals of this phase were to optimize the data analysis routines, calibrate out instrumentation effects, and produce initial "gyro spin axis orientation of the day" estimates for each gyro individually. At this stage, the focus was on individual gyro performance; there was no attempt to combine or compare the results of all four gyros, nor was there even an attempt to estimate the gyro drift rates.

Phase II, which lasted from March-August, 2006, focused on understanding and compensating for certain long-term systematic effects in the data that spanned weeks or months. During this phase, the team was accurately able to model the time-varying polhode paths of the four gyros, as reported in last month's GP-B Mission News story, yielding increased precision for gyro precession rates over short intervals. This modeling of the gyro polhode behavior plus the development of a geometric interpretation of the data has enabled the team to make significant improvements in the precision of the analysis. Phase II concluded with the 15th meeting of our GP-B Science Advisory Committee (SAC) here at Stanford on 8-9 September 2006. During this important meeting, our data analysis team presented a complete progress report to the SAC.

As we come to the end of 2006, we are well into Phase III, in which the data from all four gyros is being integrated over the entire experiment. Now that the gyro polhode behavior is well understood, we are able to focus on identifying and addressing some subtle sources of noise and interference that are buried in the data, along with the relativity signals. During this final analysis phase, we are continuing to pursue both geometric and algebraic interpretations of the data, which is enabling us to make further improvements in the accuracy of the results.

The Phase III results will be relative to the position of our guide star, IM Pegasi, which changed continually throughout the experiment. This has been measured on our behalf by the Harvard-Smithsonian Center for Astrophysics (CfA). Thus, the final step in the analysis will be to combine our gyro spin axis orientation results with data mapping the proper motion of IM Pegasi relative to the unchanging position of a distant quasar.

At the conclusion of phase III, playing the role of our own harshest critic, our science team will perform a careful and thorough final review of the analysis and results, checking and cross-checking each aspect to ensure the soundness of our procedures and the validity of our outcomes. We will then turn the analysis and results over to the SAC, which has been closely monitoring our experimental methods, data analysis procedures, and progress for the past eight years, to obtain its independent review. Moreover, we will seek independent reviews from a number of international experts.

In addition to analyzing the data, members of our team are now in the process of preparing scientific and engineering papers for publication in 2007, including the reporting of the first results of this historic experiment at the American Physical Society (APS) Meeting in Jacksonville, FL on 14-17 April 2007. We have also begun discussions with NASA to plan a formal public announcement just prior to the APS meeting.



The GP-B space vehicle and payload continue to remain in good health. All active subsystems, including solar arrays/electrical power, Experiment Control Unit (ECU), flight computer, star trackers, magnetic sensing system (MSS) and magnetic torque rods, gyro suspension system (GSS), and telescope detectors, are performing nominally.

During the past few weeks, we completed final testing of the hibernation configuration and we have now “tucked in the spacecraft for a long winter's nap.” We are continuing to monitor the health status of the spacecraft on a weekly basis and archiving the status data, but we no longer performing any significant operations on the vehicle or its payload. Because it is now in hibernation, there is no reason to continue reporting detailed status information.

The United States Air Force Academy (USAFA) is planning to conduct training for staff and cadets on the operation of the spacecraft during the month of January. Members of our GP-B Mission Operations Team will continue to support the USAFA to help them get their Mission Operations Center up and running.

As we conclude a very active and productive 2006, all of us here at GP-B would like to express our sincere appreciation to everyone who has been following this program--both on our Web site and via our email status updates. We look forward to sharing our experimental results with you in 2007. Until then, we wish you a joyous holiday season and a very happy new year!



A recent story about GP-B in Nature

The December 21-28 2006 issue of Nature (v. 444, p. 978-979) contains a short news article stating that Nature has learned that “two unanticipated effects are clouding the [GP-B] team's frame-dragging results” and also that “results were expected by last summer but the announcement never came.”

The two issues referred to in Nature have been regularly reported to NASA and our GP-B Science Advisory Committee (SAC) and publicly via these status updates. They are: 1) The effect of polhode motion of the gyros on readout calibration (see the polhode story in last month's update) and 2) misalignment torques observed and calibrated during the post-science instrument calibration phase in August-September 2005 (see the four weekly updates of September 2005).

In August 2005, a three-phase data analysis plan was devised in order to properly handle these and other issues. As first reported in May 2006, our intent--reached in agreement with NASA--has been to make the first science announcement in April 2007. This is still our plan.



Our next regularly scheduled update will be towards the end of January. Of course, we will send out a timely update if noteworthy events occur here at GP-B in the meantime.


For a two-page, up-to-date overview of GP-B in Adobe Acrobat PDF format, click here to view/download "Gravity Probe B in a Nutshell." In addition, you'll now find our 6-page NASA/GP-B Fact Sheet (PDF format) listed as the last navigation link under "What is GP-B" in the upper left corner of this Web page. You can also click here to download a copy.


On Thursday evening, May 18, 2006, GP-B Principal Investigator, Francis Everitt, gave a 90-minute free public lecture entitled: “Testing Einstein in Space: The Gravity Probe B Mission.” The lecture was sponsored by the Stanford Continuing Studies program, as part its Brainstorms: New Frontiers in Science & Engineering lecture series.

Click here to view an MPEG4 streaming video of Professor Everitt's May 18th lecture.

Click here to view/download a PDF file containing Professor Everitt's slide presentation from this lecture.

Both audio only and video versions of this lecture are also available on the Stanford on iTUNES U Web site. This Web page automatically launches the Apple iTunes program on both Macintosh and Windows computers, with a special Stanford on iTunes U "music store," containing free downloads of Stanford lectures, performances, and events. Francis Everitt's "Testing Einstein in Space" lecture is located in the Faculty Lectures section. People with audio-only iPods can download the version under the Audio tab; people with 5th generation (video) iPodfs can download the version under the Video tab.

Photos, Drawings, and Video: The composite photo of the GP-B spacecraft orbiting above the Earth, the photos of our new Mission Operations Center, and Francis Everitt's lecture were created/taken by GP-B Public Affairs Coordinator, Bob Kahn. The group photo of the team from the U.S. Air Force Academy was taken by former GP-B Program Manager, Gaylord Green. All other photos and graphics, both on this page and in the polhode story, are part of the GP-B Image Archive here at Stanford. The MPEG-4 video of Francis Everitt's lecture was created by Stanford Video. Click on the thumbnails of any photo or graphic to view these images at full size.


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