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Gravity Probe B

Testing Einstein's Universe



Item Current Status
Mission Elapsed Time 493 days (70 weeks/16.2 months)
IOC Phase
129 days (4.2 months)
Science Phase
352 days (11.6 months)
Final Calibration Phase
12 days
Current Orbit # 7,272 as of 1:00 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 in science mode
Dewar Temperature 1.82 kelvin, holding steady
Global Positioning System (GPS) lock Greater than 98.0%
Attitude & Translation Control (ATC)

X-axis attitude error: 127.7 marcs rms
Y-axis attitude error: 181.5 marcs rms

Command & Data Handling (CDH) B-side (backup) computer in control
Multi-bit errors (MBE): 1 (in GSS#2 Computer on 8/26)
Single-bit errors (SBE): 10 (daily average)
Telescope Readout (TRE) Nominal
SQUID Readouts (SRE) Nominal
Gyro #1 rotor potential +1.2 mV
Gyro #2 rotor potential +0.1 mV
Gyro #3 rotor potential -2.2 mV
Gyro #4 rotor potential +1.4 mV
Gyro #1 Drag-free Status Backup Drag-free mode (OFF during calibration tests)


As of Mission Day 493, the Gravity Probe B vehicle and payload are in good health and all subsystems are performing nominally.

The Gravity Probe B mission has continued with the final calibration phase this week, which it began ten days ago. Last Friday, the space vehicle was pointed toward HD216635 (the one-degree star) for the third time in five days and then returned to IM Pegasi for the weekend. On Monday, we maneuvered the space vehicle towards Zeta Pegasi, which is our seven-degree star.

The calibrations are scheduled to be complete by next Wednesday (August 31). The projected helium depletion date remains Friday, September 2. With any remaining helium, further calibrations will be performed at a different spacecraft roll rate (0.5 rpm).

The calibrations are scheduled to be completed by 31 August. With any remaining helium, further calibrations will be performed at a different spacecraft roll rate.


As noted in the Mission Director's summary above, we are now in the final days of our instrument calibration testing. Last week, we slewed the on-board telescope (and the spacecraft) away from the guide star, IM Pegasi, to point at a neighboring star about one degree away. The neighbor star we chose has no proper name, only catalog numbers such as HD 216635, and with an average V magnitude of 6.61 it is somewhat fainter than IM Pegasi (V magnitude 5.89). We kept the telescope pointed at HD 216635 for about half an hour and then slewed the telescope back to IM Pegasi. Over the course of last week, we repeated this procedure a total of five times. Then, this past Monday, 22 August 2005, we slewed the telescope and spacecraft to the star named Homam (aka Zeta Pegasi or HD 214923), which is about seven degrees away from IM Pegasi and considerably brighter (average V magnitude 3.40). This time, we kept the telescope and spacecraft pointed at Homam for about 30 hours before returning to IM Pegasi.

The reason we are performing these slewing tests is to measure and calibrate the amount of torque (force) placed on the science gyroscopes by intentionally mis-aligning the telescope axis from the gyroscope spin axes by relatively large factors of first one, and then seven degrees. At the beginning of the science phase of the mission, we aligned both the telescope and the spin axes of all four gyroscope with the guide star, IM Pegasi to an accuracy level of a few arcseconds. During the science phase of the mission, we kept the telescope aligned with the guide star to within 100-150 milliarcseconds. (an arcsecond is 1/3600th of a degree, and a milliarcsecond is 1/1000th of an arcsecond). By design, the spin axes of the gyroscopes are aligned with the axis of the telescope, which is also the roll axis of the spacecraft, and as long as these alignments are maintained, the non-relativistic torques on the gyros average out. However, during these final calibrations, we intentionally break these alignments, forcing the telescope first one degree and then seven degrees away from the gyro spin axis alignments, and we can then determine the extent to which these misalignments place torques on the gyro spin axes. Clearly, given the nature of these final calibration tests, they could not be performed while we were still collecting science data, which is why they were left to be performed at the very end of the experiment.

The star, Homam or Zeta Pegasi, was not one of our initial candidates to be used in the final calibrations, but it was selected by our telescope team because it emits light in the blue range of the spectrum, whereas IM Pegasi emits light in the red range of the spectrum. Homam is also much brighter than IM Pegasi, as noted above, and thus this latest calibration test was not only useful for examining torques on the gyros, but also for evaluating the telescope performance on a brighter star with a different color of light.

Burnham's Celestial Handbook (a three-volume compendium of astronomical information), gives the following information about the name Homam, by which Zeta Pegasi is also known:

Homam, probably comes from the Arabic phrase Sa'd al Humam, the "Lucky Star of the Hero," though Thomas Hyde derived it from Al Hammam, which seems to mean "The Whispering One." According to R. H. Allen, the names Sa'd al Na'amah, "The Lucky Star of the Ostriches" and Na'ir Sa'd al Bahaim, "The Bright Fortunate One of the Two Beasts" were also in use among the Arabs. The Chinese, for some unknown reason, connected the star with thunder.

Zeta Pegasi is computed to be about 210 light years away, with an actual luminosity about 145 times brighter than our Sun.


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. Mr. Kahn also took the photo of the GP-B Mission Operations Center in action. 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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