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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 - Septermber 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.

We are continuing to progress through Phase II of the data analysis process, which began at the beginning of March and is scheduled to run through mid-August 2006. During Phase II, our focus is on understanding and compensating for certain long-term systematic effects in the data that span weeks or months. The primary products of this phase will be monthly spin axis orientation estimates for each gyro, as well as refined daily spin axis orientation estimates. In this phase, the focus remains on individual, rather than correlated gyro performance.

In May, our telescope team completed a careful analysis of data collected from the science telescope over the course of the mission. We now have a thorough understanding of the telescope system performance. Consequently, some subtle systematic errors introduced into the science data by the telescope are now being addressed in the data analysis process. Likewise, we are studying the performance of the SQUID gyro readout system, the gyro rotor dynamics, and the gyro suspension system.

During June, the team made significant progress modelling the polhode motion of the gyroscopes. This polhode motion—a natural, periodic exchange of rotational energy among the inertial axes of a spinning body—does not affect the ability of the gyroscopes to measure relativistic precessions, but does introduce some subtle systematic errors that need to be removed to obtain the most accurate measurements. Using SQUID measurements of the trapped magnetic flux on the rotor, a very precise measurement of the polhode period history was identified. This information, together with the history of the spin speed of the gyroscope has allowed the team to build accurate physical models of the polhode motion and how it has evolved for each gyroscope over the mission. These models will form the base from which the effects of this class of systematic errors can be largely eliminated.

In Phase III, which is scheduled to run from late August 2006 through December 2006, data from all four gyros will be 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 in January 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.

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 our GP-B Science Advisory Committee (SAC), that has been closely monitoring our experimental methods, data analysis procedures, and progress for 11 years, to obtain its independent review. In addition, we will seek independent reviews from a number of international experts.

Throughout phases II and III, members of our team will be preparing scientific and engineering papers for publication in late 2006-2007. In addition, we have already 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 808 days (115.4 weeks/26.5 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
280 days (40.0 weeks/9.2 months)
Current Orbit # 11,922 as of 4:00 PM PDT
Spacecraft General Health Good
Roll Rate 0.04 rpm (25 minutes per revolution)
Gyro Suspension System (GSS) All four gyros digitally suspended
Gyro Spin Rates ~0.52 rpm (nominal spacecraft roll rate before roll down)
Dewar Inside Temperature ~257.0 kelvin (and rising ~0.13 kelvin/day)
Dewar Outer Shell Temperature ~264.0 kelvin (average)
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): 1 in CCCA Backup computer; 4 in GSS computers; 0 in SRE computer (turned off)


On Mission Day 808, 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. Preparations for placing the spacecraft in a hibernation state are still in progress.

The temperature inside the Dewar has now warmed to ~257.0 kelvin, and now that the spacecraft is past its full-sun season, the rate of temperature rise has decreased to ~0.13 kelvin per day. Likewise, the temperature of the dewar's outer shell has decreased from an average temperature of ~282 kelvin (9 degrees centigrade) last month to ~264 kelvin (-9 degrees centigrade) this month. The dewar's inner and outer temperatures are now very close to being in thermal equilibrium.

A total of five multi-bit computer memory errors (MBEs) occurred during the month of June: one in the CCCA (main) computer and four in the gyro suspension (GSS) computers. These memory locations have been patched via commands sent from our Mission Operations Center (MOC). The SQUID readout (SRE) computers are now turned off.

A few other systems on-board the GP-B spacecraft have also been turned off and, we are still completing the process of placing the spacecraft in a very low-maintenance hibernation state, described in previous updates. Because our main focus is the data analysis, the hibernation work is proceeding slowly, and we are continuing to communicate with the spacecraft once each week to ensure that it is remaining in “good health.”

Once hibernation is fully achieved, the spacecraft can remain in this state indefinitely. Should funding become available for one or more post-mission experiments, we can re-activate any on-board systems required. Ultimately, if it is determined that there are no further uses for the spacecraft, we will simply stop communicating with it.

As reported in last month's update, a professor and small team of cadets from the Space Systems Research Center at the U.S. Air Force Academy (USAFA) came to Stanford for two weeks to learn about the GP-B spacecraft and evaluate the feasibility of the USAFA using it on a part-time basis for research and training purposes. The USAFA team completed their exploratory work here about two weeks ago, and they reported their findings to members of the school's administration upon returning to Colorado. At present, the USAFA continues to express interest in pursuing this part-time use of the GP-B spacecraft, but a final decision has not yet been reached.


In a special ceremony held on 21 June 2006 at the Marshall Space Flight Center in Huntsville, AL, NASA recognized 245 employees and contractors who made important contributions to the nation's space program. NASA Associate Administrator (and former GP-B Program Manager) Rex Geveden, from NASA Headquarters in Washington, and Marshall Center Director David King made the presentations to civil servants and contractors. The awards were presented to individuals and groups for their achievements.

Among the Marshall Center employees recognized was our current NASA GP-B Program Manager, Tony Lyons, who was awarded one of six NASA Outstanding Leadership Medals. The certificate accompanying this medal states: “…for sustained contributions and effectiveness in advancing the Agency's quality results, and building the organization's capacity for future performance while exemplifying the NASA values in the everyday work environment.”

All of us on the GP-B team here at Stanford congratulate Tony on receiving this well-deserved award.



Our next regularly scheduled update will be at the beginning of August. 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.

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 and the composite photo of the GP-B spacecraft orbiting above the Earth, as well as 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 photo of Tony Lyons receiving a leadership award is courtesty of NASA. All other photos and graphics are from 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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