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Gravity Probe B

Testing Einstein's Universe



As of Day #136, GP-B has successfully completed its first full week in the Science Phase of the mission, with gyros #1, #2, and #3 in science mode. Gyro #4 is still undergoing alignment of its spin axis, which we expect to be completed in about a week. For the past week, the spacecraft has been in drag-free mode around gyro #3.

The spacecraft remains in excellent health, rolling at a rate of 0.7742 rpm, with all subsystems performing well. The telescope continues properly tracking the guide star, IM Pegasi, during the portion of each orbit when the guide star is visible. We are still investigating a small force or bias along the roll axis of the spacecraft, but this bias has no effect on science data collection. Moreover, during the past two weeks, we have tuned the spacecraft’s Attitude and Translation Control (ATC) system to compensate for this bias, with no excess expenditure of helium through the micro thrusters.

Having just achieved the major milestone of transitioning into the Science Phase of the mission, this is a good time to pause and look back over the Initialization and Orbit Checkout (IOC) phase of the mission. During this 19-week period, the GP-B mission has already achieved a number of extraordinary accomplishments:

  • The orbit injection of the spacecraft was so close to perfect (within 6 meters of the target orbit plane) that none of the planned orbit trim operations were necessary.
  • We’ve communicated with the spacecraft over 3,000 times during the IOC phase, and the Mission Planning team has successfully transmitted over 70,000 commands to the spacecraft without an error.
  • GP-B is the first satellite ever to achieve both 3-axis attitude control (pitch, yaw, and roll), and 3-axis drag-free control. Essentially, while orbiting the Earth, the whole spacecraft flies around one of the science gyros.
  • The GP-B gyros, which are performing perfectly in orbit, will be listed in the forthcoming edition of the Guinness Book of World Records as being the roundest objects ever manufactured.
  • The spin-down rates of all four gyros are considerably better than expected. GP-B’s conservative requirement was a characteristic spin-down period (time required to slow down to ~37% of its initial speed) of 2,300 years. Recent measurements show that the actual characteristic spin-down period of the GP-B gyros exceeds 10,000 years—well beyond the requirement.
  • Once tuned up, the spacecraft’s Attitude and Translation Control (ATC) system has been able to function at a spacecraft roll rate of 0.7742 rpm—more than twice the roll rate of 0.3 rpm initially specified.
  • The magnetic field surrounding the gyros and SQUIDs (Super-conducting QUantum Interference Device) has been reduced to 0.0000001 gauss, less than one millionth of the Earth’s magnetic field—the lowest ever achieved in space.
  • The gyro readout measurements from the SQUID magnetometers have unprecedented precision, detecting fields to 0.0000000000001 gauss, less than one trillionth of the strength of Earth’s magnetic field.
  • The science telescope on board the spacecraft is tracking the guide star, IM Pegasi (HR 8703), to superb accuracy, and it is also collecting long-term brightness data on that star.

A number of people have asked the following two-part question about GP-B: “Given that our gyros are spinning about half as fast as we originally anticipated, and that the IOC phase took about twice as long as originally anticipated, how will these two situations affect the success of the GP-B experiment?”

Regarding the gyros, several of the accomplishments above—especially the extremely low SQUID noise and higher than planned spacecraft roll rate—have effectively reduced the error factor in the GP-B science experiment, thereby partially compensating for the reduced spin rates of the gyros.

Regarding the extended length of the IOC phase, the GP-B mission is unique because it is truly a physics experiment in space. As such, there are trade-offs that can be made. The primary trade-off we had to wrestle with was: More optimization/calibration of the instrument prior to entering science, with a shorter data collection period; or, less optimization/calibration, with a longer data collection period. We concluded that the best overall accuracy would be achieved by ensuring that the science instrument was optimally calibrated from the start, even if this meant collecting data for a shorter period than we had hoped. And, in fact, we now have a considerably better understanding of the instrument than originally anticipated at this stage of the mission.

The original ideal duration of the experiment was 13 months of relativity data gathering, but this is not essential, especially in view of the work we have now done on the optimization/calibration phase. After two months of science data collection, we can make a very good measurement of the geodetic effect and a significant measurement of the frame-dragging effect. The data improves as the 3/2 power of the time (i.e. double the time, and the result will improve by a factor of ~3). In the near future, we will make another measurement of the residual helium in the Dewar, which will provide an accurate determination of its cryogenic lifetime. This, in combination with the observed instrument performance, will indicate the final expected accuracy of the experiment.

The GP-B program will not release the scientific results obtained during the mission until after the science phase has concluded. It is critically important to thoroughly analyze the data to ensure its accuracy and integrity prior to releasing the results. After more than 40 years of development, we have learned the value of thoroughness and patience.


Drawing: The drawing/collage depicting the GP-B spacecraft in orbit was created by GP-B Web designer Kate Stephenson. Among other things, Kate also designed the paper model of the GP-B spacecraft that you can download from this Web site as a PDF file.

Please Note: We will continue updating these highlights and sending out the GP-B email update on a weekly basis through the first few weeks of the Science Phase of the mission. Then, as mission operations become routine, we may reduce the frequency these updates to biweekly. However, from time to time, we intend to post special reports and special updates, as warranted by mission events.


To the right, is a thumbnail of a photograph of the GP-B spacecraft in orbit, along with the guide star, IM Pegasi. (Click on the thumbnail to view the photo at full size.) The photo was taken and emailed to us by Stefano Sposetti, a Swiss physics teacher and amateur astronomer. Stefano used a 40cm newtonian telescope, with a CCD camera and 20mm wide field lens attached to make this photo. He then sent us the two versions shown — the normal (black sky) version on the right, and an inverse version in which the constellation, Pegasus, the guide star IM Pegasi, and the path of the GP-B spacecraft are highlighted for easy identification on the left.

We are grateful to Stefano for sending us this wonderful photo. You can view other astronomical photos that he has taken on his Web page: Following is Stefano's description of his photo:

In this picture one can see the quite dim GP-B satellite traveling from North (up) to South (down) direction. The bold line in the right part of the left image represents the satellite trail just before entering the earth shadow. (Every satellite becomes visible because it reflects the sunlight). The connecting lines show the constellation Pegasus. The small circle around the star is IM Pegasi, the guide star used by the spacecraft's telescope in his experiment! GP-B is a circumpolar satellite following a free fall trajectory about 640km above the earth surface. From my location the satellite passed that night at a maximum elevation of 87degrees, thus not exactly overhead. The brightness of the satellite was between 3mag and 4mag. The Moon, about in the last quarter phase, illuminated the sky and was a drawback for having a good signal/noise ratio of the satellite trace. I took this 60-seconds black and white CCD picture with a 20mm,f/2.8 lens on august 6 centered at 01:19:00 UT. North is up, East is left.

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