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Gravity Probe B

Testing Einstein's Universe



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 has been 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. The results of the Phase II analysis enabled the team to improve the accuracy of the analysis, yielding increased precision for gyro precession rates over short intervals. Phase II culminated 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.

We are now proceeding with Phase III, the final phase-of the data analysis, which will last until January-February, 2007. Whereas in Phases I and II the focus was on individual gyro performance, during Phase III, the data from all four gyros is being integrated over the entire experiment. The results of this phase will be both individual and correlated changes in gyro spin axis orientation covering the entire 50-week experimental period for all four gyros.

These results will be relative to the position of our guide star, IM Pegasi, which changed continually throughout the experiment. Thus, the final step in the analysis, currently scheduled to occur early in the spring of 2007, 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. The proper motion of IM Pegasi has been mapped with unprecedented precision using a technique called Very Long Baseline Interferometry (VLBI) by Irwin Shapiro and his team at the Harvard-Smithsonian Center for Astrophysics (CfA), in collaboration with Norbert Bartel at York University in Toronto and French astronomer Jean-Francois Lestrade.

At the end of Phase III, playing the role of our own harshest critic, our science team will then 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 late 2006-2007. We have also begun discussions with NASA to plan a formal public announcement of the results of this unprecedented test of General Relativity. We expect to make this announcement of the results in April 2007.


Item Current Status
Mission Elapsed Time 903 days (129.0 weeks/29.6 months)
IOC Phase
129 days (4.2 months)
Science Phase
352 days (11.6 months)
Final Calibration Phase
43 days (1.3 months)
Extended Science Phase
4 days
Post Mission Phase
375 days (53.6 weeks/12.3 months)
Current Orbit # 13,320 as of 12:00 PM PDT
Spacecraft General Health Good
Roll Rate 0.15 rpm (6.67 minutes per revolution)
Gyro Suspension System (GSS) Gyro #1 in analog backup suspension mode; gyros #2, #3, and #4 digitally suspended
Gyro Spin Rates N/A; gyro rotors "tumbling" slowly rather than spinning
Dewar Inside Temperature ~251 kelvin (-26C) and falling slowly
Dewar Outer Shell Temperature ~229 kelvin (-48C) and falling
Global Positioning System (GPS) lock Nominal
Attitude Control System (ATC)

Nominal for post-mission operation
Pointing Error (XY/Pitch-Yaw): 2.0 degrees rms
Roll Phase (Z Axis) Error: 5.8 degrees rms

Telescope Readout (TRE) Pointing performance too low to lock onto guide star
Command & Data Handling (CDH) B-side (backup) computer in control
Multi-bit errors (MBE):
4 in CCCA Backup computer
9 in GSS computers
0 in SRE computer (turned off)


On Mission Day 903, both the GP-B space vehicle and payload 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. The spacecraft is now ready to be placed in a hibernation state.

During September, our small mission operations team finished upgrading various on-board systems and software that will enable the spacecraft to be placed in a hibernation state. These preparations included reducing the roll rate of the spacecraft from 0.04 rpm to 0.01 rpm and optimizing the Attitude and Translation Control system (ATC) to be consistent with this roll rate. Note that in comparison with the science mission roll rate of 0.7742 rpm (77.5 seconds per revolution) the spacecraft is now barely rolling at all (100 minutes per revolution).

However, the most important hibernation change, described in several previous GP-B updates, is the reconfiguration of the spacecraft's communications equipment to prevent it from automatically turning itself on and transmitting data in response to a safe mode condition or on-board computer reboot. During the science phase of the mission, just about any anomalous behavior on board the spacecraft would immediately trigger the spacecraft's communication system to begin streaming data so that our mission operations team could detect and troubleshoot anomalies as quickly as possible. Now that the mission has ended and we are only monitoring the spacecraft's status once a week, it is important that the spacecraft not be sending out signals that no one is monitoring and that could interfere with the operation of other spacecraft. Thus, we have now set up the spacecraft to transmit only when commanded to do so from the ground, using a robust communications channel.

All of the hibernation modifications to spacecraft systems and software have now been implemented and tested, and we plan to place the spacecraft in hibernation mode beginning next week.

Arrangements for the United States Air Force Academy (USAFA) to use the spacecraft part time--shared with our use here at Stanford--as a space operations training vehicle are now in the final stages. Members of the USAFA spacecraft operations will soon visit Stanford to take delivery of a ground station communications POD-a set of computer consoles-that they will take back to the academy in Colorado Springs to enable them to communicate with the spacecraft from there. It is anticipated that the USAFA will begin controlling the GP-B spacecraft sometime this month.


In 1916, shortly after completing his work on the general theory of relativity, Albert Einstein published a short book entitled, "The Special and General Theory of Relativity," to help the lay public understand his theory.

Now, some 90 years later, Penguin Books has re-published an English translation of this book in a paperback edition, including an introduction by physicist Roger Penrose, scientific commentary on the special and general theories of relativity by physicist Robert Geroch, and commentary on the cultural legacy of Einstein's theories by historian David Cassidy. For more information, see the listing for this book at The Penguin Group Web site or at or Barnes & Noble.



Our next regularly scheduled update will be at the beginning of October. Of course, we will post a timely update if there are any important changes in the spacecraft's status, or 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 GP-B data collection collage, 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. The cover photo of Einstein's book is courtesy of The Penguin Group. All other photos 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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