Skip navigation

Gravity Probe B

Testing Einstein's Universe



Six months into the mission, GP-B is performing remarkably well. The spacecraft is in fine health, flying drag-free around gyro #1. All four gyros remain digitally suspended, and they are all generating science data. The spacecraft is rolling at 0.7742 rpm (77.5 seconds per revolution), and all subsystems are performing well. The Dewar temperature remains nominal at 1.82 kelvin, and the flow of helium from the Dewar through the micro thrusters continues to be within expected limits. We are now in our seventh week of data collection, which is going very smoothly, and a recent test on the Dewar indicates that we have enough helium to continue collecting data for approximately eight more months. The quality of the data gathered thus far is excellent.

This past week has been a relatively quiet one for the GP-B spacecraft. The oscillations in the drag-free control force, which we have been reporting on for the past two weeks, have diminished to an insignificant level. Adjustments that we made to the drag-free suspension parameters to de-tune a harmonic coupling between the drag-free control system and helium sloshing in the Dewar, combined with reducing the gain in the drag-free controller to its initial level, seem to have mitigated this instability. To be certain, we are continuing to monitor the drag-free control system with regards to this issue.

Furthermore, gyro #3, which we have been monitoring over the past three weeks, since it unexpectedly transitioned from digital to analog back-up suspension, has remained digitally suspended, without further incident this past week. We suspect that the issue with gyro #3 is not related to the gyro rotor (sphere) itself, but rather with some noise spikes in its suspension system that have exceeded pre-set limits, automatically triggering a transition into analog back-up suspension mode. Increasing the signal-to-noise ratio of this gyro’s position readout seems to have mitigated this problem. However, we are continuing to monitor gyro #3, and we are still analyzing the root cause of this behavior.

The two issues mentioned above have prompted several inquiries about the effects of such events on the health of our data and the ultimate results of the experiment. The short answer is that neither of these events has had any significant effect on the data or the experimental results. The SQUID readouts for each gyro continue to provide spin axis orientation data in both digital and analog suspension modes, so no data was lost when gyro #3 transitioned to analog mode. Also, even in those few moments when the spacecraft is not in drag-free mode, we continue to collect data, but in this case, we must carefully evaluate any potential torques (forces) placed on the gyros to ensure that they did not alter the gyro’s spin axis alignment. For readers who are interested, we will provide a more detailed explanation of our data collection process in a future Weekly Highlights.

Drawings & photos: The diagrams of the GP-B experimental procedure and the cutaway drawing of the Dewar are from the GP-B Image Archives here at Stanford. Likewise, the photo of a complete gyro housing, including all of its wires and fiber optic cables and the photo of a SQUID (Super-conducting Quantum Interference Device) electronics box are also from the GP-B Image Archives. Click on the thumbnails to view these images at full size.

Please Note: We will continue updating these highlights and sending out the GP-B email update on a weekly basis—at least through the first few weeks of the Science Phase of the mission. As mission operations become more routine, we may reduce the frequency of 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.

More links on recent topics

Previous Highlight
Index of Highlights