Skip navigation

Gravity Probe B

Testing Einstein's Universe

2007 Status Updates:



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 our November 2006 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.

Since the meeting with our Science Advisory Committee last September (SAC meeting #15), we have been proceeding through Phase III of the data analysis, in which the data from all four gyros is being integrated over the entire experiment. 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.

Now that the gyro polhode behavior is well understood, we have been able to shift our focus to identifying and addressing some subtle systematic sources of noise and interference that are buried in the data, along with the relativity signals. Identifying and removing as many of these subtle systematic effects as possible is critically important for reducing the margin of error in our final results—especially the frame-dragging result. While we have been making steady progress in these efforts, it has proven to be a slow and painstaking process, and it is now apparent that several more months of data analysis will be required to achieve the lowest possible margin of error.

At the SAC meeting #15 last September, committee members anticipated this situation and recommended that we ask NASA to create a contingency plan, and budget for an extension of the data analysis phase for several months past our scheduled results announcement at the American Physical Society (APS) meeting on 14-17 April 2007 in Jacksonville, FL. To this end, following a meeting with NASA in mid January, NASA has requested a proposal for extending the GP-B data analysis phase through December 2007, and this is in progress.

Consequently, we are now planning a two-phase announcement of the GP-B results. Our first announcement will be made at the April APS meeting, as planned for some time now. (For more information about our presentations at this meeting, see this month's GP-B Mission News story below.) In conjunction with this announcement, NASA is planning a press/media event at NASA Headquarters in Washington DC just prior to the APS meeting. The experimental results in this first announcement will have been presented to and vetted by our Science Advisory Committee during SAC meeting #16, which is scheduled for 23-24 March 2007. These will be preliminary results, representing the lowest margin of error obtainable by that date.

Concurrent with this preliminary results announcement in April, we will be releasing an initial science data set to the National Space Sciences Data Center (NSSDC) at Goddard Space Flight Center in Greenbelt, MD. The remainder of our science data, along with a complete archive of GP-B documents, images, video, and related program information will be released to the NSSDC by the end of May. All GP-B data and information archived at the NSSDC will be publicly available.

Following the APS meeting, our science team is planning to spend several more months removing further systematic sources of noise and interference, with the goal of reducing the margin of error in the result to the lowest possible level. These results will still be relative to the position of our guide star, IM Pegasi, which changed continually throughout the experiment. This proper motion of the guide star 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.

In late fall, 2007, 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 convene a final SAC meeting to obtain the committee's independent review of the final results. Moreover, we will seek independent reviews from a number of international experts.

We intend to announce the final experimental results of GP-B through a NASA press/media event towards the end of 2007. At that time it is also our intention to have submitted a number of papers on the GP-B results for publication in peer-reviewed scientific and technical journals.



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.

The spacecraft remains in a hibernation state. 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.

The United States Air Force Academy (USAFA) has begun conducting training for staff and cadets on the operation of the spacecraft. Members of our GP-B Mission Operations Team are continuing to support the USAFA to help them get their Mission Operations Center up and running.


The GP-B Results Announcement & Presentations at the APS Meeting in April

aps logoGP-B will have a strong presence at the American Physical Society (APS) meeting in Jacksonville, Florida, on 14-17 April 2007. During this meeting, we will emphasize three main themes:

  • Successful completion of most challenging space-based experiment in NASA's history
  • First scientific results from this historic mission
  • Public release of Level2 science data (via NSSDC)

Four members of the GP-B team have been invited to speak at the APS meeting, beginning on Saturday morning, April 14th, with GP-B Principal Investigator, Francis Everitt, giving the plenary conference talk, entitled First Results from Gravity Probe B.

In addition, on Saturday afternoon, two papers related to GP-B will be delivered in Session C12: Experimental Tests of Gravity.

  • C12.00004: " Lessons Learned from Gravity Probe B for STEP, LISA and other experiments" by GP-B team members Paul Worden and Sasha Buchman
  • C12.00005: "Proper Motion of the GP-B Guide Star" by the Harvard-Smithsonian Center for Astrophysics Gp-B guide star tracking team: Irwin Shapiro, Daniel Lebach, Michael Ratner, Norbert Bartel, Ryan Ransom, Michael Bietenholz, Jerusha Lederman, and Jean-Francois Lestrade

On Sunday morning, April 15th, three members of the GP-B team have been invited to give special talks on three aspects of the GP-B program:

  • H7.00001: "The Gravity Probe B Science Instrument," by GP-B Co-Principal Investigator, John Turneaure
  • H7.00002: "The Development Challenges of Gravity Probe-B—an ongoing partnership between Physics and Engineering" by GP-B Co-Principal Investigator, Bradford Parkinson
  • H7.00003: "Gravity Probe B Data Analysis Challenges, Insights, and Results" by GP-B Co-Investigator and Chief Scientist, George (Mac) Keiser

Finally, on Sunday afternoon, April 15th, a large part of the GP-B team and associated scientists and engineers will present 22 poster sessions on a host of scientific and technology topics, as listed below.

Session L1: Poster Session II L1.00011: GRAVITATION

  • L1.00012: "Radio Imaging of the Gravity Probe B Guide Star IM Pegasi" by Michael Bietenholz, Ryan Ransom, Norbert Bartel, Daniel Lebach, Michael Ratner, Irwin Shapiro, Jean-Francois Lestrade
  • L1.00013: "The 'Core' of the Quasar 3C454.3 as the Extragalactic Reference for the Proper Motion of the Gravity Probe B Guide Star" by Norbert Bartel, Ryan Ransom, Michael Bietenholz, Jerusha Lederman, Daniel Lebach, Michael Ratner, Irwin Shapiro, Leonid Petrov
  • L1.00014: "Performance of the Gravity Probe B Inertial Reference Telescope" by Suwen Wang, John Goebel, John Lipa John Turneaure
  • L1.00015: "Gravity Probe B Timing System and Roll Phase Determination" by Jie Li , Jeffery Kolodziejczak
  • L1.00016: "The Gravity Probe B SQUID Readout Detector" by Barry Muhlfelder, Bruce Clarke, Gregory Gutt, James Lockhart, Ming Luo
  • L1.00017: "SQUID Control, Temperature Regulation, and Signal Processing Electronics for Gravity Probe B" by James Lockhart, Barry Muhlfelder, Jie Li, Bruce Clarke, Terry McGinnis, Peter Boretsky, Gregory Gutt
  • L1.00018: "Gravity Probe B Science Instrument Assembly (SIA)" by Saps Buchman, Barry Muhlfelder, John Turneaure
  • L1.00019: "Polhode Motion of the Gravity Probe-B Gyroscopes" by Michael Dolphin, Alex Silbergleit, Michael Salomon, Paul Worden, Daniel DeBra
  • L1.00020: "Evidence for Patch Effect Forces on the Gravity Probe B Gyroscopes" by Dale Gill, Saps Buchman
  • L1.00021: "Gravity Probe B Orbit Determination" by Paul Shestople , Huntington Small
  • L1.00022: "Simulator Technology of the Gravity Probe-B Mission" by David Hipkins , Robert Brumley , Yoshimi Ohshima , Thomas Holmes
  • L1.00023: "Achievement of the Magnetic Environment Requirements for Gravity Probe B" by John Mester, James Lockhart, Michael Taber
  • L1.00024: "The Gravity Probe B Gyroscopes" by Saps Buchman, Bruce Clarke, Mac Keiser, Dale Gill, Frane Marcelja, Robert Brumley
  • L1.00025: "Gravity Probe B Gyroscope Electrostatic Suspension System (GSS)" by William Bencze, David Hipkins, Tom Holmes, Saps Buchman, Robert Brumley
  • L1.00026: "The Gravity Probe B Relativity Mission (GP-B)" by C.W. Francis Everitt
  • L1.00027: "Gravity Probe B Experiment Error" by Barry Muhlfelder, G. Mac Keiser, John Turneaure
  • L1.00028: "Gravity Probe B Science Data Analysis: Filtering Strategy" by Michael Heifetz, Thomas Holmes, David Hipkins, Alex Silbergleit, Vladimir Solomonik
  • L1.00029: "Performance of the Gravity Probe B Cryogenic Sub-System" by Michael Taber, David Murray
  • L1.00030: "The Gravity Probe B Drag-free and Attitude Control System" by Michael Adams, Daniel DeBra
  • L1.00031: "Features of the Gravity Probe B Space Vehicle" by William Reeve, Gaylord Green
  • L1.00032: "Classical Torques on Gravity Probe B Gyroscopes" by Alex Silbergleit, G. Mac Keiser, Yoshimi Ohshima
  • L1.00033: "Trapped Flux Mapping for the Gravity Probe B Gyroscopes" by Michael Salomon, John Conklin, Michael Dolphin, G. Mac Keiser, Alex Silbergleit, Paul Worden



Our next regularly scheduled update will be towards the end of March, following SAC meeting #16. 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 image portraying the GP-B experimental measurements, 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 APS logo is courtesy of the American Physical Society. All other photos and graphics on this page 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.


Previous Update
Index of Updates