Hi,

Here is this weeks update on the Garvity Probe B Mission:

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GRAVITY PROBE B MISSION UPDATE FOR 22 APRIL 2005
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GP-B STATUS AT A GLANCE
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Mission Elapsed Time: 367 days (52 weeks/12.00 months)
Science Data Collection: 238 days (34 weeks/7.80 months)
Current Orbit #: 5,419 as of 9:00PM PST
Spacecraft General Health: Good
Roll Rate: Normal at 0.7742 rpm (77.5 seconds per revolution)
Gyro Suspension System (GSS): All 4 gyros digitally suspended in science mode
Dewar Temperature: 1.82 kelvin, holding steady
Global Positioning System (GPS) lock: Greater than 88%
Attitude & Translation Control (ATC): X-axis attitude error: 163.0 marcs rms
Y-axis attitude error: 86.6 marcs rms
Command & Data Handling (CDH): B-side (backup) computer in control
Multi-bit errors (MBE): 0
Single-bit errors (SBE): 9 (daily avg.)
Telescope Readout (TRE): Nominal
SQUID Readouts (SRE): Nominal
Gyro #1 rotor potential: -4.3 mV
Gyro #2 rotor potential: -5.5 mV
Gyro #4 rotor potential: -5.7 mV
Gyro #3 Drag-free Status: Backup Drag-free mode (normal)

MISSION DIRECTOR'S SUMMARY
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As of Mission Day 367, the Gravity Probe B vehicle and payload are in good health. All four gyros are digitally suspended in science mode. The spacecraft is flying drag-free around Gyro #3.
Our mission operations team performed routine tasks over this past week related to the electrostatic discharge of Gyro #3 (the "drag-free" gryo) and minor Dewar pressure oscillations. After performing a number of tests on Gyro #3 early in the week, it was discharged on Thursday to -3.0 mV, using ultraviolet light, as explained in last week's update.

The Attitude and Translation Control (ATC) group reports that the time required to re-lock onto the guide star as the spacecraft emerges from behind the Earth each orbit continues to be excellent, including one guide-star capture (re-locking) this weekend that took only 20 seconds.

On Tuesday afternoon (19-Apr), GP-B Principal Investigator, Francis Everitt, delivered a Physics Colloquium to a packed auditorium at Stanford entitled: "The Gravity Probe B Flight Mission: A Stanford Physics-Engineering Partnership". In this lecture, Professor Everitt explained some of the finer points of GP-B's various technologies, emphasizing in each case the interdisciplinary collaborations required to produce these extraordinary technological innovations.


MISSION NEWS-GRAVITY PROBE B CELEBRATES LAUNCH ANNIVERSARY
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This past Wednesday, 20 April 2005, marks the one-year anniversary of the GP-B spacecraft in orbit. At 9:57:24 am PDT on 20 April 2004, a Boeing Delta II rocket carried the GP-B spacecraft, embodying over 40 years of dogged persistence in science and engineering, into a perfect orbit. That emotionally overwhelming day, culminating with the extraordinary live video of the spacecraft separating from the second stage booster meant, as GP-B Program Manager Gaylord Green put it, "that 10,000 things went right."
This past Wednesday, exactly one year later, the entire GP-B team gathered in the conference room where we hold our daily briefings for a launch anniversary celebration. We watched a 12-minute condensed video of the launch and reminisced for a few minutes about that historic event. Then, GP-B Principal Investigator, Francis Everitt, walked to the front of the room and made a poignant toast to the entire GP-B team.

Our first year in orbit has been quite an exciting ride. The 4-month Initialization and Orbit Checkout (IOC) period demonstrated the exceptional preparedness and dedication of our team, from dealing with computer reboots and thruster problems in orbit to spinning up the four gyros, which following the launch, was the second greatest milestone in the program.

On 28 August 2004, we began the 10+ month science phase of the mission, collecting data on the changing spin axis orientation of the four gyros that will ultimately confirm or disprove the geodetic and frame-dragging predictions that arise from Albert Einstein's general theory of relativity--the theory of gravity that he published in 1916, 11 years after publishing his special theory of relativity. It is fitting that the completion of the Gravity Probe B experiment, perhaps the most rigorous test to date of general relativity, will come to fruition this year--the 100th anniversary of Einstein's "miracle" year, in which he published four seminal papers, including the special theory of relativity and his paper on the production and transformation of light, for which he was awarded the Nobel Prize in 1921.

The GP-B spacecraft is now performing better than ever, but there have been some challenges during the science phase of the mission. For example, on January 17th, an X-class solar flare--the largest in thirteen years--caused the science telescope to be temporarily blinded by an associated high flux of energetic protons. When such an event occurs, the hum of pagers and a cacophony of cell phone ringers can be heard, alerting the team. In this case and other anomalous situations, including the automatic switchover from the A-side (main) flight computer to the B-side (backup) computer two months ago, our mission operations team has always been able to restore normal flight operations within a few hours, with minimal loss of science data. In fact, we have now collected nearly 8 months of science data, with a 99% success rate.

We are now in the process of measuring the amount of liquid helium remaining in the spacecraft's Dewar. Shortly before the helium runs out sometime late this summer, we plan to perform a series of critical instrument calibrations. The purpose of these calibrations is to place limits on potential sources of systematic errors. Most of these tests call for intentionally enhancing, in a controlled fashion, one or more possible error terms. By increasing a disturbance during the calibration phase, we are better able to place tighter limits on these errors.
So was Einstein correct? We don't know yet. It is critically important to ensure data accuracy and analysis integrity, and thus, we will not be releasing any scientific results until after the data analysis is completed next summer.

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UPDATED NASA/GP-B FACTSHEET
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A recently updated NASA Factsheet on the GP-B mission and experiment is now available on our Web site in Adobe Acrobat PDF format. You can download this 6-page fact sheet at: http://einstein.stanford.edu/content/fact_sheet/GPB_FactSheet-0405.pdf

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PREVIOUS GP-B UPDATES
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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

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OTHER LINKS THAT MAY INTEREST YOU
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* 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 )


* Photo, taken through a telescope by Swiss physics teacher and amateur astronomer Stefano Sposetti, of GP-B spacecraft in orbit, passing near IM Pegasi: http://aida.astronomie.info/sposetti.



* 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.


* Track the GP-B satellite on the Web using NASA's Java-based J-Pass satellite tracking application at: http://science.nasa.gov/realtime/JPass/ Also, you can track the GP-B satellite on Personal Digital Assistants (PDAs) using either the Palm OS or Pocket PC operating systems with software from Big Fat Tail Productions: http://www.bigfattail.com.


* The Einstein Exhibition at the Skirball Cultural Center in Los Angeles through May 2005: Information about the Einstein exhibition is available on the Skirball Center Web site: http://www.skirball.org/index.asp?s=exhibit&p=einstein.asp. If you can't make it to Los Angeles, you can visit the AMNH's virtual Einstein exhibit on the Web at: http://www.skirball.org/exhibit/amnh_frame.html.


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NASA - Stanford - Lockheed Martin
Gravity Probe B Program
"Testing Einstein's Universe"
http://einstein.stanford.edu

Bob Kahn
Public Affairs Coordinator

Phone: 650-723-2540
Fax: 650-723-3494
Email: kahn@relgyro.stanford.edu
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Regards,

LelandJ