NASA Gravity Probe B Mission Update 7 July 2006

GP-B DATA ANALYSIS & RESULTS ANNOUNCEMENT STATUS

Note: A more complete status overview of the GP-B data analysis process & results announcement is posted at the top of our GP-B Home page: http://einstein.stanford.edu. Following is a brief summary of our activities and accomplishments during the past month.

As of July 7, 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 late 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 precession 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 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.

GP-B SPACECRAFT & PAYLOAD STATUS AT A GLANCE

• 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: Normal at 0.04 rpm (25 minutes per revolution)
• Gyro Suspension System (GSS): All four gyros digitally suspended
• Gyro Spin Rates: ~0.52 rpm (spinning at nominal roll rate prior to spacecraft roll down)
• Dewar Temperature: ~257.5 K and rising ~0.13 K/day; Outer shell temperature: ~264 K.
• Global Positioning System (GPS) lock: Nominal
• Attitude Control System: Magnetic Sensing System (MSS) control
• Pointing Error: (XY/Pitch-Yaw Axes) 2.0 degrees RMS;
• Roll Phase (Z Axis) Error: 5.8 degrees RMS
• Telescope Readout: 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 computers (turned off)

MISSION DIRECTOR’S SUMMARY

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.

GP-B MISSION NEWS–NASA GP-B PROGRAM MANAGER, TONY LYONS, AWARDED OUTSTANDING LEADERSHIP MEDAL

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: “Šfor 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.

NEXT SCHEDULED GP-B UPDATE IN EARLY AUGUST, 2006

Our next regularly scheduled update will be at the beginning of August. Of course, we will send out 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.

PREVIOUS GP-B UPDATES

If you wish to read any of our previous updates, our GP-B Web site includes a chronological archive of all the updates/highlights (with photos and drawings) that we have posted over the past 8 years: http://einstein.stanford.edu/highlights/hlindexmain.html

OTHER LINKS THAT MAY INTEREST YOU

Our GP-B Web site, http://einstein.stanford.edu contains lots of information about the Gravity Probe B experiment, general relativity, and the amazing technologies that were developed to carry out this experiment.

Visual tour of the GP-B spacecraft and payload from our GP-B Web site: http://einstein.stanford.edu/content/vehicle_tour/index.html

PDF file containing a 1/20 scale, paper model of the GP-B spacecraft that you can download print out, and assemble: http://einstein.stanford.edu/content/paper_model.

NASA’s Marshall Space Flight Center also has a series of Web pages devoted to GP-B: http://www.gravityprobeb.com

The Harvard-Smithsonian Center for Astrophysics (Cambridge) and York University (Toronto), with contributions from the Observatoire de Paris, have been studying the motions of the guide star, IM Pegasi for over a decade. To find out more, visit: http://www.yorku.ca/bartel/guidestar/

In addition, you’ll find information in the Guide Star FAQ on our Web site: http://einstein.stanford.edu/content/faqs/faqs.html#guidestar and on pages 18-20 of the Gravity Probe B Launch Companion: http://einstein.stanford.edu/highlights/GP-B_Launch_Companion.pdf

The Einstein Exhibition at the Skirball Cultural Center in Los Angeles has closed.However, you can visit the American Museum of Natural History’s virtual Einstein exhibit on the Web at: http://www.skirball.org/exhibit/amnh_frame.html

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