Hi,

Here is the latest news from the folks of Gravity Probe B:

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GRAVITY PROBE B MISSION UPDATE FOR 11 OCTOBER 2005
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GP-B STATUS AT A GLANCE
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Mission Elapsed Time: 539 days (77 weeks/ 17.7 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: 11 days
Current Orbit #: 7,850 as of 2:00PM PST
Spacecraft General Health: Good
Roll Rate: Normal at 0.4898 rpm (2.04 minutes per revolution)
Gyro Suspension System (GSS): Gyros #1, #2, and #3 suspended in analog mode; Gyro #4 digitally suspended
Gyro Spin Rates: All gyros spinning at less than 2 Hz (<120 rpm)
Dewar Temperature: 57 K and rising
Global Positioning System (GPS) lock: Nominal
Attitude Control System: Nominal for post-mission operation
Pointing Error: (Pitch-Yaw) 0.35 degrees RMS; Roll Phase Error: 6.0 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): 0
Single-bit errors (SBE): 10 (daily average)

MISSION DIRECTOR'S SUMMARY
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On Mission Day 539, the Gravity Probe B vehicle and payload are in good health. All active subsystems, including solar arrays/electrical power, Experiment Control Unit (ECU), flight computer, star trackers and magnetic torquer rods, gyro suspension system (GSS), and telescope detectors--are performing nominally.

The Dewar is now empty of both liquid helium and helium gas, though small amounts of other residual gasses may still remain frozen inside. With all the helium propellant used up, the micro-thruster system is now inactive, and without thrusters, it is no longer possible to maintain drag-free flight. Furthermore, the SQUID Readout System (SRE), which can only function in a cryogenic environment below 7 kelvin, is now inactive.

Over the past 11 days, since the liquid helium was exhausted in the Dewar, the temperature of the Dewar and the probe within it has risen from 1.8 kelvin to 57 kelvin, and it is continuing to rise. As the temperature in the probe exceeded 7 kelvin, we lost superconductivity in the gyro rotors and SQUID readouts. Then, as the temperature increased further, the cryopump above the telescope in the probe began releasing the helium molecules that it had adsorbed, filling the probe with helium gas.

The release of gas into the probe had a braking effect on all four gyros, and their spin rates rapidly decreased from an average of 72 Hz (4,290 rpm) down to less than 2 Hz (120 rpm) over the period of a few days last week. In addition, as the probe warmed, thermal expansion (aka creaking) occurred, causing rapid position changes between the gyro rotors and their housings. In response to these rotor movements, the Gyro Suspension System (GSS) automatically switched all of the gyros into analog suspension mode, as a safety precaution. In analog suspension mode, the gyros are held more firmly, and with much less position control than with digital suspension, which is computer-controlled. As an analogy, analog suspension is like a basketball player guarding the ball securely in his palms, whereas digital suspension is like a basketball player holding the ball at the tips of his fingers to make a shot.

In order to maintain proper pressure inside the probe, we opened both the main exhaust valve at the top of the probe as well as the spin-up gas exhaust valves at the bottom of the probe, and these exhaust valves will remain open for the time being.

This past weekend, we re-suspended gyro #4 in digital suspension mode, but we have decided to leave the other three gyros in analog suspension until the temperature in the probe and Dewar has leveled off. Once thermal equilibrium has been re-established in the Dewar and probe, we will return the other three gyros to digital suspension.

In summary, we have placed the spacecraft in a safe configuration, and we are continuing to communicate with it regularly. In the near term, our plan is simply to monitor the Dewar and probe as they continue to warm up. For now, we are not planning any significant vehicle operations. Rather, we are focusing primarily on science data analysis and preparing our final report to NASA.


GP-B MISSION NEWS--END OF HELIUM PRESS RELEASES
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On Monday, 3 October 2005, NASA Headquarters put out a press release announcing that GP-B had run out of helium. You can read that press release on the NASA Web site at: http://www.nasa.gov/home/hqnews/2005/oct/HQ_05293_probe_b.html.
Later that day, Stanford University News Service put out the full news story from which the NASA release was excerpted. You can read the full Stanford News release on the main Stanford Web site at: http://www.stanford.edu/dept/news/pr/2005/pr-gpbempty-100505.html.

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NEXT GP-B UPDATE IN TWO WEEKS, THEN MONTHLY
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We will send out our next update will occur towards the end of October. Then, we will decrease the frequency of these updates to once a month.

Of course, we will send out special timely updates whenever warranted by important changes in the spacecraft's status or noteworthy events here at GP-B.

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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 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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ABOUT THE GPB-UPDATE EMAIL LIST
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To subscribe to this list, send an email message to "majordomo@lists.Stanford.edu" with the command "subscribe gpb-update" in the body of the message (not in the Subject line).

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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