STATUS UPDATE AS OF 11 OCTOBER 2005:

GRAVITY PROBE B MISSION STATUS AT A GLANCE

Item	Current Status
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 2:00 PM PST
Spacecraft General Health	Good
Roll Rate	Normal at 0.4898 rpm (122.5 seconds per revolution)
Gyro Suspension System (GSS)	Gyros #1, #2, & #3 suspended in analog mode; gyro #4 suspended digitally
Gyro Spin Rates	All gyros spinning at less than 2 Hz (< 120 rpm)
Dewar Temperature	57 kelvin (and rising)
Global Positioning System (GPS) lock	Nominal
Attitude Control System (ATC)	Nominal for post-mission operation Pointing Error (Pitch-Yaw): 0.35 degrees rms Roll Phase Error: 6.0 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): 0 Single-bit errors (SBE): 10 (daily average)

MISSION DIRECTOR'S SUMMARY

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 torque 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.

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

On Monday, 3 October 2005, NASA Headquarters put out a press release announcing that GP-B had run out of helium. Click here to read the NASA Press release .
Later that day, Stanford University News Service put out the full news story from which the NASA release was excerpted. Click here to read the full story from the Stanford News Service.

NEXT GP-B STATUS UPDATE & MISSION NEWS IN TWO WEEKS; THEN MONTHLY

We will post our next GP-B status update towards the end of October. Then, we will decrease the frequency of these updates to once a month.

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

UPDATED NASA/GP-B FACT SHEET AVAILABLE FOR DOWNLOADING

We recently updated our NASA Factsheet on the GP-B mission and experiment. You'll now find this 6-page document (Adobe Acrobat 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.

Drawings & Photos: The photo of a model of the GP-B spacecraft was taken by GP-B Public Affairs Coordinator, Bob Kahn. All other photos and drawings are from the GP-B photo & graphics archive here at Stanford. Click on the thumbnails to view these images at full size.

Gravity Probe B

Testing Einstein's Universe