Hi,
Here is the latest update of the Gravity Proble B Mission.
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GRAVITY PROBE B MISSION UPDATE FOR 1 OCTOBER 2004:
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On Day 164 of the mission, the spacecraft is in good health, flying drag-free around gyro #1. All four gyros are digitally suspended, and their SQUID readouts are collecting relativity data during the portion of each orbit when the science telescope is locked onto the guide star, IM Pegasi. The spacecraft's roll rate remains at 0.7742 rpm (one revolution every 77.5 seconds). The Dewar temperature is nominal (1.82 Kelvin), and the flow of helium, venting from the Dewar through the micro thrusters is within expected limits.
At this time of year in North America, at around 7:00 PM Pacific Daylight Time, the constellation Pegasus, in which the guide star IM Pegasi resides, is clearly visible rising above the Eastern horizon. With a magnitude ranging from 5.6 - 5.85, IM Pegasi is too dim to see with the naked eye, but in a good viewing location, it should be visible with binoculars.
In last week's update, we reported that at 8:30 PM local time on Thursday, 23 September 2004, gyro #3-which was then serving as the drag-free gyro-suddenly transitioned to analog backup suspension mode, automatically triggering a safemode that stopped the mission timeline. The GP-B operations and engineering teams quickly developed a recovery plan, and by the following afternoon, the spacecraft was back in science mode, with gyro #1 performing the drag free operation. The effect of this event on the science experiment was insignificant. The most likely cause of this event was a data spike in the Gyro Suspension System (GSS) position sensor for gyro #3. Gyro #3 was returned to digital suspension as part of the recovery plan, and it has remained digitally suspended, with no further problems, since that time. However, we are continuing to monitor its performance and investigate the root cause. Thus, for the foreseeable future, the spacecraft will continue to fly drag-free around gyro #1.
Also last week, we reported that the team had performed a heat pulse test on the spacecraft Dewar in order to determine the mass of liquid helium remaining inside, which determines the remaining lifetime of the mission. The heat pulse test works in the following way: The amount of heat that it takes to warm an object by a specified amount depends on the type of material, its temperature, and its mass. Thus, if the "specific heat" (the amount of heat needed to warm a kilogram of material by one degree kelvin) is known, it is a simple matter to measure the mass by applying a known amount of heat (usually with an electric heater) and measuring the resultant temperature rise. In the case of the GP-B Dewar, the situation is simplified by the fact that heat distributes itself virtually instantaneously throughout superfluid helium. The amount of heat used in the test must be large enough to cause a measurable temperature change (approximately 10 millikelvin) but not so large as to appreciably shorten the mission lifetime. The heat pulse test yielded a remaining superfluid helium mass of 216 kg, which translates into a remaining mission lifetime of 9.9 months. This means that the science (data collection) phase of the mission will continue for a little over 8 more months, and then we will spend the final month re-calibrating the science instrument.
Finally, this past Tuesday, 28 September 2004, we observed a slight increase in the helium required by the micro thrusters to maintain drag-free flight around gyro #1. To be conservative, we decided to turn off drag-free mode and evaluate the situation. Over a period of 4-6 hours, the oscillations that were causing the ATC to require excess helium died out and have not returned. After some analysis and discussion, we have determined these oscillations were caused by a sympathetic resonance between the ATC drag-free control efforts and a sloshing wave on the surface of the now reduced superfluid helium in the Dewar. This situation is somewhat analogous to placing a vibrating tuning fork on the body of a guitar, which then causes guitar strings tuned to harmonics of the same frequency to start vibrating. This is a transient situation, and the team has adjusted the ATC drag-free suspension parameters to de-tune this harmonic coupling. We have since returned the spacecraft to drag-free operation around gyro #1, and it is performing nominally.
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.
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NASA - Stanford - Lockheed Martin
Gravity Probe B Program
"Testing Einstein's Universe"
http://einstein.stanford.eduBob Kahn
Public Affairs Coordinator
Phone: 650-723-2540
Fax: 650-723-3494
Email: kahn@relgyro.stanford.edu
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Regards,
LelandJ
