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Gravity Probe B

Testing Einstein's Universe



Item Current Status
Mission Elapsed Time 500 days (71 weeks/16.4 months)
IOC Phase
129 days (4.2 months)
Science Phase
352 days (11.6 months)
Final Calibration Phase
19 days
Current Orbit # 7,376 as of 1:30 PM 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
Dewar Temperature 1.82 kelvin, holding steady
Global Positioning System (GPS) lock Greater than 98.5%
Attitude & Translation Control (ATC)

X-axis attitude error: 140.0 marcs rms
Y-axis attitude error: 186.4 marcs rms

Command & Data Handling (CDH) B-side (backup) computer in control
Multi-bit errors (MBE): 0
Single-bit errors (SBE): 8 (daily average)
Telescope Readout (TRE) Nominal
SQUID Readouts (SRE) Nominal
Gyro #1 rotor potential +1.6 mV
Gyro #2 rotor potential +0.9 mV
Gyro #3 rotor potential -0.3 mV
Gyro #4 rotor potential +1.8 mV
Gyro #1 Drag-free Status Backup Drag-free mode (OFF during some calibration tests)


On Mission Day 500, the Gravity Probe B vehicle and payload are in good health and all subsystems are performing nominally.

As of today, there is still an ever-thinning film of superfluid helium in the Dewar, and thus calibration tests, which began 19 days ago, continued throughout this past week. Over the course of last weekend and early this week, we thrice slewed the telescope (and spacecraft) to “visit” the star HD 216635, which is about one degree northwest of IM Pegasi, and then back to IM Pegsi.

On Wednesday, we visited the star HR Pegasi (HD 216672), which is about 0.4 degrees west of IM Pegsi. We had previously visited this star during the Initialization and Orbit Checkout (IOC) phase of the mission in June 2004, as a test to confirm that the star we were using as the guide star was, in fact, IM Pegasi. (You can read about our previous visit to HR Pegasi in our archived weekly highlights for 11 June 2004.

Thus far during these final calibration tests, our visits to HD 216635 and to Zeta Pegasi were predominately in a north-south plane, relative to IM Pegasi. Our visit to HR Pegasi this past Wednesday was our first calibration test with a star located in the east-west plane, with respect to IM Pegasi. Continuing this line of exploration, yesterday and today we completed several visits to locations less than one degree west, and less than one degree east of IM Pegasi. No stars visible to the on-board telescope exist in these recently visited locations, so we used data from our star trackers and our standard navigational gyroscopes, which are accurate to within 20 arcseconds, to determine the positions of these “virtual stars.” Since the purpose of these calibrations is to evaluate the effects of telescope mis-alignments on the science gyros, greater pointing accuracy is not required for these excursions.

We have postponed our previous plans to slow down the spacecraft's roll rate until after the helium in the Dewar is fully depleted. Over this weekend, we will continue making calibration tests similar to those we have been performing, and if the helium lasts into next week, we will perform other types of calibration tests. Once the helium is depleted, the GP-B flight mission will officially end.

Tony Lyons, NASA's program manager for GP-B from Marshall Space Flight Center in Huntsville, AL, has been here at Stanford all week, participating with our team in these final days of the mission.


This past week marks a major transition point in the GP-B program. Just over 16.4 months after a picture-perfect launch on 20 April 2004, we will come to the end of the GP-B flight mission. Literally any day now, the supply of superfluid helium--the coolant that has maintained the cryogenic temperatures necessary for our SQUID gyro readouts to function--will be exhausted.

Right now, there is an ever-thinning layer of superfluid liquid helium lining the Dewar, and there will come a point where the last of this liquid helium has “sweated” out through the porous plug, leaving only helium gas inside the Dewar. (See our Mission News story of 29 July 2005 for a description of the porous plug As this final helium gas begins to pas through the porous plug, the pressure in the Dewar will begin to drop, causing the temperature in the probe to rise. We don't know whether this transition will happen gradually or suddenly (as was the case with NASA's COBE spacecraft), but we will know that we have reached this point when our telemetry data shows a rise in the temperature sensors on the bracket that houses the SQUID gyro readout controllers and the temperature sensors on the telescope detector packages. When the temperature in the probe reaches about 7 kelvin, we will begin to lose superconductivity in the niobium gyro rotor coatings, and the SQUID gyro readouts will gradually cease to detect the London moments in the gyros. At this point, it be no longer be possible to determine the spin axis orientation of the science gyroscopes, although the Gyro Suspension Systems (GSS) will continue to indicate the position of each gyro rotor within its housing to great precision. Also, with no helium propellant left, the spacecraft will no longer be able to maintain a drag-free orbit.

The 650 gallons of helium that filled the Dewar at launch has lasted a few days more than the length of time planned. We collected 50 weeks of science data-just two weeks short of the year's worth of data we had hoped to collect. Furthermore, we lost less than 1% of the data collected to telemetry and/or spacecraft hardware problems. We have now completed all of the vital planned calibration tests that needed to be done with the gyro readouts still functioning, and we are using these final days of helium to perform additional tests that will be valuable to the data analysis now in process. In short, GP-B has been a remarkably successful mission.

There is an air of triumph here at Stanford, but there is also a note of sadness. Having become a very tight-knit team, we are saddened that today is the last day of work on GP-B for a number of our colleagues, especially those from Lockheed Martin Corporation. Thus, this afternoon, we all gathered in the conference room where we've been holding daily status meetings since before launch, and we spent a few minutes raising toasts to our successful mission and to the future endeavors of our colleagues who will be moving on to new jobs next week.

Regarding the GP-B spacecraft, the experiment, and the remaining members of the team, following is a brief overview of what's in store. Next Tuesday, when we return from the Labor Day holiday, GP-B will be a much quieter place. Our science team is already well into the long, painstaking process of analyzing the data. A small spacecraft operations crew, including several GP-B graduate students who are being cross-trained for these duties, will continue to monitor the spacecraft's system status, send commands to it when necessary, and regularly, though much less frequently, download various types of data via NASA telemetry satellites and ground stations.

We will use magnetic torquers--long electromagnets mounted on the spacecraft frame--to slow down the spacecraft's roll rate from 0.7742 rpm to 0.5 rpm or less and to control the spacecraft's orientation in orbit. For about two weeks, we will perform a number of tests on various electronic systems in the spacecraft. For example, we want to compare various performance characteristics of the GSS system after it has “warmed up” to its performance in a cryogenic superconductive state. Finally, in about a month, we will spin down all four science gyros to speeds slow enough that they will not be in danger of shattering, should they lose suspension and touch their housing walls. At this point, the spacecraft could be used by other scientists to make various non-relativitistic measurements. For example, it could be used for experiments in geodesy (measuring the shape of Earth's gravitational field) and aeronomy (measuring atmospheric density). We are in the process of exploring the interest in further use of the GP-B spacecraft for such experiments with a number of scientists.

One last note: All of us on the GP-B team are deeply saddened by the horrific disaster and unfathomable human tragedy that has been unfolding in Gulf Coast this past week. Our hearts and prayers go out to the countless victims.


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 layered composite photo of the GP-B spacecraft was created by GP-B Public Affairs Coordinator, Bob Kahn, using Adobe Photoshop and Adobe Illustrator. The photo of the Dewar was taken by Lockheed Martin photographer Russ Underwood. The photos of the porous plug and SQUID are from the GP-B Image Archive here at Stanford. The sky chart images, showing the guide star IM Pegasi and its neighboring stars, were generated by the Voyager III Sky Simulator from Carina Software. The star photos, as well as much of the information about these stars came from the various Web pages of the Centre de Donnees astronomiques de Starsbourg--CDS, including the Simbad astronomical database and the Aladin interactive sky atlas. Click on the thumbnails to view these images at full size.


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