WEEKLY UPDATE FOR 23 SEPTEMBER 2005:
GRAVITY PROBE B MISSION STATUS AT A GLANCE
|Mission Elapsed Time||521 days (74 weeks/17.08 months)|
|129 days (4.2 months)|
|352 days (11.6 months)|
Final Calibration Phase
|Current Orbit #||7,688 as of 4:00 PM PST|
|Spacecraft General Health||Good|
|Roll Rate||Normal at 0.4898 rpm (122.5 seconds per revolution)|
|Gyro Suspension System (GSS)||All 4 gyros digitally suspended|
|Dewar Temperature||Not Available|
|Global Positioning System (GPS) lock||Greater than 99.4%|
|Attitude & Translation Control (ATC)||
X-axis attitude error: 128.0 marcs
|Command & Data Handling (CDH)|| B-side (backup) computer in control
Multi-bit errors (MBE): 1 9in GSS#3 computer on 9/23)
Single-bit errors (SBE): 7 (daily average)
|Telescope Readout (TRE)||Nominal|
|SQUID Readouts (SRE)||Nominal|
|Gyro #1 rotor potential||+4.5 mV (as of 9/22)|
|Gyro #2 rotor potential||+3.4 mV (as of 9/22)|
|Gyro #3 rotor potential||+1.9 mV (as of 9/22)|
|Gyro #4 rotor potential||+4.1 mV (as of 9/22)|
|Gyro #1 Drag-free Status||Backup Drag-free mode (OFF during some calibration tests)|
MISSION DIRECTOR'S SUMMARY
On Mission Day 521, the Gravity Probe B vehicle and payload remain in good health, with all subsystems performing nominally. We still have some helium in the Dewar, and the spacecraft is flying drag-free around Gyro #1.As of Mission Day 514, the Gravity Probe B vehicle and payload are in good health, with all subsystems performing nominally. We still have helium remaining in the Dewar, and the spacecraft is currently flying drag-free around Gyro #1.
The helium in the Dewar has now surpassed its estimated lifetime by three weeks, and thus, our Dewar team has been analyzing the calculations and underlying assumptions on which their helium lifetime predictions were originally made. (See the Mission News story below for more information about this.)
Meanwhile, we have continued working our way through our prioritized list of calibration tests. Last weekend, we completed the process of de-fluxing the SQUIDS--that is, removing electromagnetic flux from the SQUIDs by heating them up a few kelvins, and then allowing them to cool back down to their normal cryogenic operating temperature of 1.8 kelvin.
On Monday, we attempted to switch from backup to primary drag-free operation on gyro #1. For various reasons, including a mis-configuration of the ATC system prior to the mode switch, the attempt failed, and we reverted to backup drag-free mode on gyro #1.
On Tuesday, we visited a virtual star 0.1 degrees east of IM Pegasi, in the opposite direction from HR Pegasi. We remained locked in the position for 48 hours, while the telescope team performed some dark current calibration tests, and then we returned and re-locked the telescope onto IM Pegasi on Thursday. We then attempted again to switch from backup to primary drag-free mode on gyro #1, and again the attempt failed. We are currently investigating the possible causes of this failure, and we plan to try this switch again next week...assuming that we still have helium in the Dewar.
Our Dewar team made its initial helium lifetime predictions by making calculations based on the results of several heat pulse meter tests that were performed at various points throughout the mission and some assumptions derived from the scientific literature and experience of other spacecraft that used a helium-based cryogenic system.
The Dewar team's initial calculations suggested that the helium in the Dewar should have been depleted sometime around Labor Day (5 September 2005). Because the helium has now lasted three weeks longer than the initial predictions, the Dewar team has been re-examining their calculations and the underlying assumptions used to make them. The team has checked and re-checked their calculations, and it appears that no errors were made. This has led them to re-evaluate the assumptions underlying these calculations.
Inside the Dewar, there is some amount of superfluid liquid helium and some amount of helium vapor (gas). The initial longevity calculations were based on the assumption, as suggested by the literature and performance of other spacecraft, that the helium liquid and the helium gas inside the Dewar are in a state of thermal equilibrium. That is, they are both are maintaining the same temperature due to close thermal contact between the liquid and gas.
An alternative assumption is that there is poor thermal contact between the helium liquid and gas in the Dewar, due to the low thermal conductivity of the vapor. If this is the case, the temperatures of the liquid and gas are not necessarily in equilibrium over the time scale of the measurement. That is, over short time scales, a temperature increase in the liquid helium does not necessarily result in a corresponding temperature increase in the helium vapor bubble. (Equilibration does ultimately occur, but over a longer time scale.)
Taken to their extremes, these two assumptions result in two limiting cases for the helium depletion:
- Strong thermal contact between the two helium phases (the thin layer of liquid helium coating the Dewar wall and the bubble of helium vapor in the center of the Dewar are in thermal equilibrium over the short time frame of the measurement--the initial assumption)
- Weak thermal contact between the two helium phases (a change in temperature in the liquid phase has little effect on the temperature of the gas bubble over the time scale of the measurement; rather, it takes many hours for the helium vapor bubble to reach equilibrium with the liquid helium).
The Dewar team's initial predictions for the helium longevity were based on the first assumption, which yielded the 5 September helium depletion date. However, if the second assumption is used, the longevity of the helium increases by 5-6 weeks. In other words, the upper bound on the helium longevity, based on the second assumption, places the helium depletion date around mid October. It is most likely that the actual conditions inside the Dewar lie somewhere between these two bounding assumptions. And, judging from the Dewar's performance thus far, it appears that the thermal contact between the helium phases is weaker than was originally assumed.
As we have noted these past few weeks, as long as we still have helium in the Dewar, we will continue working our way through our prioritized list of calibration tests. When the helium actually does run out, we will post a notice on our Web site and send out a message to the subscribers of our GP-B Update email list. NASA will also issue a news release, and we will then post the content of that release on our Web site and send it to our email subscribers.
UPDATED NASA/GP-B FACT SHEET AVAILABLE FOR DOWNLOADING
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 orbiting Earth was created by GP-B Public Affairs Coordinator, Bob Kahn, using Adobe Photoshop and Adobe Illustrator. The photo of the Dewar at theend of the Mission News section was taken by Lockheed Martin photographer Russ Underwood. The photos of the micro thrusters, SQUID, and Dewar, and the cutaway drawings of the Dewar are from the GP-B Image Archive here at Stanford. Click on the thumbnails to view these images at full size.
MORE LINKS ON RECENT TOPICS
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- The GP-B Launch Companion in Adobe Acrobat PDF format. Please note: this file is 1.6 MB, so it may take awhile to download if you have a slow Internet connection.