WEEKLY HIGHLIGHTS FOR 13 AUGUST 2004:
GRAVITY PROBE B MISSION UPDATE
On day #115 of the mission, Gravity Probe B is within a week of completing Initialization and Orbit Checkout (IOC) and making the transition into the Science Phase of the mission. The spacecraft is in excellent health, and all subsystems are performing well. The spacecraft's roll rate is stable at 0.75 rpm, and final testing of drag-free orbital flight and fine-tuning of the Attitude and Translation Control system (ATC) is nearing completion. All four gyros are digitally suspended and spinning at science mission speeds, ranging from 61.8Hz (3,708 rpm) to 82.1 Hz (4,926 rpm). The spin axes of Gyros #1 and #3 have been aligned with the science telescope's sighting axis, which is locked onto the guide star, IM Pegasi, and these gyros have completed the transition to science data collection mode. Spin axis alignment is continuing on Gyros #2 and #4, which are expected to transition to science mode over the next few days.
- This past Monday, August 9, 2004, we successfully increased the spacecraft's roll rate from 0.52 rpm to 0.75 rpm. In last week's highlights, we noted that the decision to do this was made "after much analysis and discussion," triggering a number of email inquiries about the risks of increasing the roll rate. Three main issues influenced this decision: First, increasing the roll rate requires more fine-tuning of the ATC system, which would delay the transition into Science mode by a few more days. Second, because the star trackers used by the ATC are mounted on the sides of the spacecraft, an increase in roll rate means that the star trackers must perform their pattern-matching on a faster moving field of stars. Third, there was a concern that any effects of mass imbalances in the spacecraft might be exaggerated at a faster roll rate. In the end, we decided that the ~20% improvement in experimental accuracy outweighed these risks. All systems are performing correctly at the increased roll rate, and the only downside was a small delay in our transition to the Science phase of the mission.
- Also this past week, we have flown the spacecraft in both primary and back-up drag-free modes, using Gyro #1 as the proof mass (spacecraft's center of mass). Primary drag-free mode relies solely on the micro thrusters to create a drag-free orbit around the proof mass gyro, whereas in back-up drag-free mode, the ATC uses suspension force data from the Gyro Suspension System (GSS) of the proof mass gyro to "steer" the spacecraft so that residual GSS forces are nullified or canceled on this gyroscope. Either primary or back-up drag-free mode can be used in the Science Phase of the mission. However, the residual voltages applied by the GSS to suspend the drag-free gyro add a very small, but acceptable, amount of noise to the gyro signal in back-up drag-free mode, so the primary mode is preferable, all other things being equal.
- In fact, early in July, we had selected back-up drag-free mode as the nominal mode for the Science Phase of the mission because at that time, when the gyros were spinning very slowly, back-up mode was yielding better results than the primary mode. However, now that the gyros are spinning at full speed, primary drag-free mode is yielding the best results, as originally anticipated, so it will be the nominal mode for drag-free operation during the Science Phase of the mission. The reason for this performance difference is that while the GP-B gyro rotors (spheres) are the roundest objects ever manufactured, they are not perfectly spherical. The GSS is capable of controlling the position of the gyros to a level of one nanometer. However, some of the "bumps" or deviations on the rotor surfaces are as large as 20 nanometers. When the rotors are spinning slowly, these larger "bumps" are detected by the GSS, and they decrease its positioning accuracy. However, when the rotors are spinning fast, these deviations average out. Thus, at full speed, the average surface deviation on the gyro rotors is less than the one-nanometer precision of the GSS, so there is no degradation in positioning the gyro rotors.
- The final steps in the IOC, prior to beginning data collection, involve the transition of each gyro into science mode. There are three main steps in this transition. First, the spin-axis of each gyro is aligned with the science telescope/spacecraft roll axis. Then, ultraviolet light, beamed from lamps on the spacecraft frame through fiber optic cables into the gyro housings, are use to remove any residual static charge from the gyro rotor surfaces. Finally, the GSS voltage is reduced, in order to obtain the best signal on the SQUID readouts. Currently, Gyros #1 and #3 have completed this transition to science mode. Gyros #2 and #4 are finishing spin axis alignment, and they are expected to complete the transition to science mode over the next few days.
The spacecraft is being controlled from the Gravity Probe B Mission Operations Center, located here at Stanford University. The Stanford-NASA/MSFC-Lockheed Martin operations team is continuing to perform superbly.
Photos: The first three photos, taken from the GP-B Photo Archive here at Stanford University, show the Science Instrument Assembly (SIA) during its development, a GP-B gyroscope rotor (sphere) and housing on a red background, and a countour map of the surface of a GP-B gyro rotor, used to measure the roundness or sphericity of the rotor. In the last two photos, taken by GP-B Public Affairs Coordinator, Bob Kahn, team members in the GP-B Mission Operations Center here at Stanford are spinning up gyro #4 to full speed. Click on the thumbnails to view enlarged copies of these images.
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: http://aida.astronomie.info/sposetti. 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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