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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 currently progressing 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.

During the month of 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.

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 778 days (111.1 weeks/25.5 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
250 days (35.7 weeks/8.2 months)
Current Orbit # 11,464 as of 3:00 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 Temperature ~249.0 kelvin (and rising ~0.23 kelvin/day)
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; 2 in GSS computers; 0 in SRE computer


On Mission Day 778, both the GP-B space vehicle and payload continue to be 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 will be completed in about two weeks.

The spacecraft is currently in the middle of its 5th full-sun "season." During this 15-20 day period, the plane of the spacecraft's orbit is orthogonal to the sun, and the sun shines broadside on the spacecraft throughout each orbit around the Earth. Thus, as we noted in last month's update, this is a good time to view the spacecraft if it passes overhead in your neighborhood. The best viewing times are the dawn and twilight hours.

The temperature inside the Dewar has now warmed to ~249.0 kelvin, and its rate of temperature rise has increased slightly to ~0.23 kelvin per day. Because the spacecraft has been in full-sun for over a week now, the temperature of the dewar's outer shell has warmed to an average temperature of ~282.4 kelvin (9.2 degrees centigrade). As the spacecraft moves out of its full-sun season next week, it will be eclipsed from sunlight for part of each orbit, causing the dewar's outer shell to cool somewhat and continue approaching thermal equilibrium with the rising inner temperature.

Three multi-bit computer memory errors (MBEs) occurred once again during the month of May: one in the CCCA (main) computer and one in each gyro suspension computer (GSS1 and GSS2). These memory locations have been patched via commands sent from our Mission Operations Center (MOC).

The GP-B spacecraft has performed exceptionally well since its launch on April 20, 2004, with no major failures to date. However, the extent of its continued post-mission use is still being determined. Our Stanford Mission Operations Center (MOC) is still fully functional, and at least through the data analysis period, we plan to activate the spacecraft's communication system once a week to monitor its status. Regarding longer term, post-mission use of the spacecraft, three options are currently under consideration:

  1. Spacecraft Hibernation. We are now in the final stages of preparing the GP-B spacecraft to enter a very low-maintenance hibernation state, described in last month's update. As required by NASA, we are in the process of re-configuring the spacecraft's communication system to safeguard it from automatically turning itself on and polluting the already crowded space communications channels with unexpected and un-monitored signals following an on-board computer reboot. Thus, in the hibernation state, it will only be possible to communicate with the spacecraft by explicitly sending commands from the ground to power on its communications system. Ground station communications with the spacecraft have now been discontinued, so all further communications will be via the NASA Tracking & Data Relay Satellite System (TDRSS).

The spacecraft can remain in this hibernation 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.

  1. Stanford Planet-Finding Proposal. Stanford Research Physics Professor, John Lipa, one of the GP-B Co-Investigators, has submitted a proposal for part-time use of the GP-B spacecraft to identify planets orbiting stars outside our local solar system. If this proposal is funded, various systems on-board the spacecraft will be re-activated for collecting the necessary data, and a small mission operations team will monitor and control the spacecraft from the Stanford MOC.
  2. Air Force Academy Space Operations Training. The Space Systems Research Center at the U.S. Air Force Academy (USAFA) in Colorado Springs, CO, offers a satellite engineering program called FalconSat in which Air Force cadets design, build, and learn to operate small satellites. This past March, cadets in the program packed the flight operations control room at the privately-owned Space Exploration Technologies Corporation (SpaceX), to watch the long-awaited launch of their FalconSat-2 satellite, the second in a series of satellites from the FalconSat program. Unfortunately, shortly after liftoff, there was a problem during the second stage burn, and FalconSat-2 was destroyed.

Can GP-B offer the USAFA cadets an alternative satellite to operate? Former GP-B Program Manager, Gaylord Green, thinks so. While GP-B is orders of magnitude larger and more complex than FalconSat-2, it has proven to be very robust and reliable on-orbit, and it is ready and available for alternative uses. To determine the feasibility of using GP-B on a part-time basis for research and training purposes, a professor and several cadets from the USAFA are spending three weeks here at Stanford, learning about the GP-B spacecraft and evaluating the feasibility of the USAFA using it. If the USAFA decides to proceed, a communications module will be sent to Colorado Springs, enabling cadets to communicate with the spacecraft in realtime from the academy. However, we will also retain the ability to jointly communicate and control the spacecraft from our Stanford MOC. Thus, if Professor Lipa's proposal for extra-solar system planet identification is funded, that research—and possibly other research programs—can proceed in conjunction with the USAFA program. A decision from the USAFA on their use of the GP-B spacecraft is expected at the end of this month.


Testing Einstein in Space—A Public Lecture by GP-B PI Francis Everitt

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 & Technology lecture series. The 500-seat Hewlett Teaching Center in the Science & Engineering Quad here on the Stanford campus, where Professor Everitt delivered his lecture, was filled beyond its capacity, with some people sitting in the aisles.

Prior to the evening lecture, Stanford's Dean of Research, Arthur Bienenstock, hosted a reception in honor of Professor Everitt and the Gravity Probe B program. Among the dignitaries present at the reception and lecture were Rex Geveden, Associate NASA Administrator, responsible for all technical operations at NASA Headquarters and formerly a GP-B Program Manager, and Tony Lyons, current NASA GP-B Program Manager at the Marshall Space Flight Center in Huntsville, AL.

Towards the end of the reception, Dean Bienenstock praised the GP-B program, and its leader, Francis Everitt, noting that “…[GP-B] is unique in many ways: It's the single longest-running project in Stanford's history [and at NASA], one of the first (if not THE first formal interdisciplinary project at Stanford); it has generated more PhDs than any other single project in the University's history. It is also one of the most scientifically complex projects ever undertaken at the University, with some remarkable spin off inventions that are benefiting a wide range of other disciplines today.” Dean Bienenstock then went on to acknowledge NASA's support and funding and the vision of GP-B founders Leonard Schiff, Bill Fairbank, and Bob Cannon. Finally, Dean Bienenstock singled out Francis Everitt for his devotion, single mindedness of purpose, and staunch leadership: “No where in our whole lives have we ever seen another Francis! He is truly special in so many ways.”

At the lecture, Dean Bienenstock introduced Professor Everitt, with remarks similar to those he made at the reception. Professor Everitt then talked for about an hour, telling the complex story of GP-B by weaving together seven interfolded themes:

  1. Testing Einstein
  2. The invention of many new technologies
  3. Collaboration between university departments
  4. Highly successful student involvement in a long-running space program
  5. A remarkable range of spin-offs, some of which made possible other NASA missions, including IRAS, COBE, WMAP, and the Spitzer telescope
  6. Collaboration between NASA, academia, and industry
  7. The challenge of managing a flight program with a very highly integrated payload and spacecraft

The Walls Came Tumbling Down

Stanford's master plan for the campus calls for a new 8.2 acre science and engineering quad, dubbed SEQ2, to be built around the spot where the two-story blockhouse building that has housed GP-B for the past 11 years resides. The overall project calls for the construction of an environment and energy building, a new School of Engineering center, a replacement for the Ginzton Applied Physics building, and a bioengineering/chemical engineering building. The new quad is expected to cost between $375 million and $420 million. The phased construction process is tentatively scheduled to begin in July 2006 with groundbreaking on the environment and energy building, and officials hope to finish the final building by 2014. Once completed, SEQ 2 will give many of the university's science and engineering efforts state-of-the-art new facilities in a desirable campus location that more cohesively links the western side of campus with the Main Quad and beyond.

As a result of this new construction, our GP-B team spent the last two weeks in May moving out of our GP-B building, into offices in three nearby buildings, where we will all reside through the end of the GP-B program. Our Mission Operations Center has been relocated to a new building, where it is now up and running. Also, two computer networks and four major computer installations had to be moved, in addition to the staff and our 40+year archive of documents, photos, drawings, technical papers, and GP-B memorabilia.

Given that our science team is right in the middle of Phase II of the data analysis process, orchestrating such a move was quite a challenge. But, our move coordinator and systems administrator rose to this challenge and somehow managed to accomplish this move with almost no downtime—and for this, they deserve the highest praise.

On Thursday last week, the area surrounding our old GP-B home was fenced off, and this past Monday, the bulldozer arrived—a mechanical Tyrannosaurus Rex, right out of Jurassic Park, with Jaws of steel. At 8:30 AM, the bulldozer began chomping, and by the end of the day, the physical home of GP-B—the place where we dreamed, designed, debugged, theorized, analyzed, met, talked, ate, slept, monitored and controlled our spacecraft—had been reduced to a pile of rubble. Needless to say, it was a melancholy day for our GP-B team members. However, we are now up and running in our new quarters, and for the most part, life here at GP-B has returned to normal.


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

Photos, Drawings, and Video: The GP-B data collection collage, the composite photo of the GP-B spacecraft orbiting above the Earth, and the composite "Seasons of GP-B" diagram, 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 photos of the GP-B building demolition were taken by Margot Tuckner. 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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