Hi,
Here is the Nov 12 Gravity Probe B mission update.
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GRAVITY PROBE B MISSION UPDATE FOR 12 NOVEMBER 2004
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In its 30th week in orbit, the GP-B spacecraft is in good health and is performing well. We have completed 2.5 months of data collection, and the quality of the data continues to be excellent. The spacecraft is flying drag-free around gyro #3, while maintaining a constant roll rate of 0.7742 rpm (77.5 seconds per revolution). As we enter the second portion of the year when the spacecraft remains in full sunlight for approximately a month, the shell of the Dewar is warming up slightly, and thus rate of helium boiling off from the Dewar has increased slightly, resulting in a uniform increase of helium flow through the micro thrusters, in order to maintain the appropriate pressure inside the Dewar to keep its temperature stabilized at 1.82 kelvin.
On 7 November 2004, the National Oceanic & Atmospheric Administration's (NOAA) Space Environment Center (SEC) issued the following advisory bulletin:
A large and moderately complex sunspot group, NOAA Region 696, was responsible for a series of eruptions on the surface of the Sun known as solar flares, including a significant eruption on 7 November 2004 at 9:06 a.m. MST. A radiation storm, currently at the moderate (S2) level, began soon after the eruption was observed on the Sun. Solar flare activity over the past few days has resulted in a series of Earth-directed coronal mass ejections (CMEs) that have impacted Earth's geomagnetic field with severe (G4) geomagnetic storming. Today's large eruption on the Sun may produce further periods of strong geomagnetic storming on 9-10 November.
Solar radiation storms, which result in increased proton bombardment of the Earth, coupled with the ensuing geomagnetic storm, has been responsible for an extraordinary display of the aurora borealis (northern lights) all over the Northern Hemisphere this past week. The aurora is typically only visible in places like Alaska and Canada, which are located at far northern latitudes. But this past week, people have reported seeing the aurora as far south as Alabama. In fact, NASA's Photo of the Week for this past week
http://antwrp.gsfc.nasa.gov:80/apod/astropix.html is an extraordinary photo of the aurora borealis, taken by amateur astronomers in Warrensburg, Missouri on 7 November 2004.
In this week's highlights on our GP-B Web site, we have included two plots from NOAA/SEC, showing a side-by-side comparison of normal solar activity from one month ago versus the activity during the geomagnetic storm this past Wednesday and Thursday. The difference is dramatic. Note especially the large increase in highest-level (green) proton flux on November 10th. The increase in these particles rarely rises above the 0.1 (first) level on this plot.
One of the effects of geomagnetic storming is a significant increase in high-energy proton bombardment in any part of the Earth facing the Sun, and especially in the region known as the South Atlantic Anomaly, which normally experiences heightened levels of solar particle bombardment . Thus, it was not surprising that at 4:58 PM PST on 10 November 2004, as the GP-B spacecraft was entering the South Atlantic Anomaly region, a proton hit a critical memory location in the SQUID Readout Electronics (SRE). This event triggered a chain reaction of safemodes that, among other things, halted the GP-B mission time line, caused the SRE computer to reboot, and transitioned all four gyros to analog backup suspension mode.
Over the past few weeks, the SRE electronics had sustained two proton hits to non-critical memory locations, and we were already in the process of planning the necessary steps to manually reboot it, but as it happens, nature apparently took care of this issue for us. Fortunately, the effects of this event were limited to the SRE electronics, and the spacecraft's main (A-side) computer was not affected. Our team worked through the night this past Wednesday and most of Thursday, sending commands to the spacecraft to reset affected systems and restore all four gyros to digital suspension modes. As of this afternoon, the spacecraft has returned to normal operation.
The effect of this event on the experimental data is not yet known. A few data points from this period may have to be omitted, but such an omission will have no significant effect on the overall experimental results.
For more information and photos of recent aurora activities, see the NASA feature story at
http://www.nasa.gov/vision/universe/solarsystem/aurora1110.html. For more information on solar activity and geomagnetic storms, see the NOAA/SEC Web site:
http://www.sec.noaa.gov.
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NOTES & CORRECTIONS ON LAST WEEK'S TELESCOPE STORY
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Last week's update on the subject of the GP-B telescope prompted some of the scientists/engineers who worked on the telescope to email some elaborations and corrections. First, the prototype telescope shown in the photos in last week's highlights, plus the actual flight telescope now in orbit, were fabricated from the fused quartz, not fused silica. Our first prototype telescope was fabricated from fused silica. Fused quartz and fused silica have the same chemical properties, but slightly different physical properties. Specifically, a fused quartz boule is made from hand-selected pieces of natural quartz that are flame-fused together. By contrast, a fused silica boule is from by the vapor phase hydrolysis of a silicon halide. The fused quartz was purchased from the German company, Heraeus. It was graded "Herasil 1 Top," the highest grade obtainable in the size boule required for the telescope. The highest grade of fused quartz, called Homosil, is also made by Heraeus, but it is only available in small boules. Thus, Homosil was used to fabricate the GP-B gyro rotors (spheres).
The second correction in last week's update is that the reticle plate covering the Image Divider Assembly (IDA) functions as an aperture stop, not a field stop.
By the way, I really do appreciate all of the comments suggestions, and corrections I receive by email each week, after sending out these updates. I always strive for clarity and accuracy in my writing, but this experiment is full of nuances, which I sometimes miss.
Please Note: We will continue sending out this 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
