Hi,

Here is the Gravity Probe B mission update for May 27, 2005

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GRAVITY PROBE B MISSION UPDATE FOR 27 MAY 2005
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GP-B STATUS AT A GLANCE
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Mission Elapsed Time: 402 days (57 weeks/13.18 months)
Science Data Collection: 273 days (39 weeks/8.95 months)
Current Orbit #: 5,933 as of 5:00PM PST
Spacecraft General Health: Good
Roll Rate: Normal at 0.7742 rpm (77.5 seconds per revolution)
Gyro Suspension System (GSS): All 4 gyros digitally suspended in science mode
Dewar Temperature: 1.82 kelvin, holding steady
Global Positioning System (GPS) lock: Greater than 98.0%
Attitude & Translation Control (ATC): X-axis attitude error: 222.4 marcs rms
Y-axis attitude error: 216.0 marcs rms
Command & Data Handling (CDH): B-side (backup) computer in control
Multi-bit errors (MBE): 0
Single-bit errors (SBE): 15 (daily avg.)
Telescope Readout (TRE): Nominal
SQUID Readouts (SRE): Nominal
Gyro #1 rotor potential: -0.9 mV (As of 5/26)
Gyro #2 rotor potential: -1.7 mV (As of 5/26)
Gyro #4 rotor potential: -1.4 mV (As of 5/26)
Gyro #3 Drag-free Status: Backup Drag-free mode (normal)

MISSION DIRECTOR'S SUMMARY
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As of Mission Day 402, the Gravity Probe B vehicle and payload are in good health. All four gyros are digitally suspended in science mode. The spacecraft is flying drag-free around Gyro #3.

The spacecraft has entered its third solstice or full-sun period of the mission. During these periods, the plane of the spacecraft's orbit is perpendicular with respect to the sun's position for about two weeks. Thus, the sun shines broadside on the spacecraft throughout each orbit around the Earth. As in previous full-sun periods, some fine tuning of various spacecraft systems is required to counter the effects of temperature increases in the Dewar shell and the electronics boxes mounted in the bays of the spacecraft frame.

Preliminary SQUID calibration signal tests, described in last week's Mission Director's Summary, are continuing. These tests do not affect the science data collection, but they help us evaluate the performance of the four SQUIDS relative to each other.

Also, development of procedures for the final set of instrument calibration tests, which will occur in August, just before the liquid helium in the Dewar is exhausted, are continuing. Likewise, the plans for two full-day simulations, during which the Mission Operations Team will rehearse executing these procedures and work through any issues or problems, are in the process of being finalized.

MISSION NEWS--WHAT WILL ULTIMATELY HAPPEN TO THE GP-B SPACECRAFT?
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Over the past year, we've received a number of questions regarding the ultimate fate of the GP-B spacecraft after the mission ends this coming September. These questions include:

--Can the spacecraft be used to make any other measurements or perform any other kinds of experiments after the helium runs out?

--How long will the GP-B spacecraft continue to orbit the Earth? Can it be maneuvered after the helium runs out?

--Does NASA have any plans to retrieve the GP-B spacecraft and preserve it somewhere-like the Smithsonian Institution's Air and Space Museum in Washington DC? Can the Space Shuttle retrieve the GP-B spacecraft?

--Can I get one of the gyros?

Because these questions seem to be on many people's minds, I thought it would be appropriate to discuss this whole topic in this week's Mission News.

The relativity measurements that Gravity Probe B is currently making require that the science gyros and SQUID magnetometer pick-up loops be maintained in a cryogenic environment, with all other sources of noise and disturbance either eliminated or minimized to insignificant levels. This environment had to be maintained from before launch through the Initialization and Orbit Checkout (IOC), science (data collection), and final instrument calibration phases of the mission. In other words, there needed to be enough liquid helium in the Dewar to last at least 14 months and preferably 16-20 months. This is the reason that the spacecraft needed a 650-gallon Dewar, the size of a cement mixer.

Furthermore, in addition to maintaining the cryogenic environment inside the Dewar, the liquid helium also provides a constant source of propellant that enables the micro thrusters to both keep the spacecraft and telescope pointed at the guide star and also maintain the spacecraft in drag-free flight, using one of the science gyros (currently gyro #3), as the spacecraft's center of mass or "proof mass."

Recent measurements indicate that the liquid helium in the Dewar will be completely expended around the beginning of September. When this happens, the gyro rotors will continue to spin, but their niobium coatings will warm up and cease to be superconductive. Thus, the spinning gyro rotors will no longer generate magnetic moments around their axes of spin, and there will no longer be any signals for the SQUID magnetometers to readout. Moreover, there will no longer be any helium gas escaping from the Dewar to serve as propellant for the micro thrusters, so the spacecraft will cease to fly drag free in its orbit.

Without drag-free flight and superconductivity in the gyros and SQUIDs, it will no longer be possible to conduct further relativistic measurements based on measurements of the gyro's spin axes. However, the spacecraft's solar arrays will continue to provide power for many years, and the NiCAD batteries on-board are rated for a minimum of five years. While there would be no propellant for controlling the attitude of the spacecraft using the micro thrusters, the spacecraft has magnetic torquers, which can provide a small degree of attitude control. Furthermore, the Gyro Suspension System (GSS) electronics will continue to keep the gyros suspended, and if left untouched, the gyros would continue to spin for thousands of years. The GSS can also be used to measure torques or forces being exerted on the gyros.

Thus, the GP-B spacecraft could be used to provide other kinds of scientific data, such as measurements of variations in the Earth's shape. The decision as to whether or not this will ultimately be made by NASA, as a function of scientific interest, available funds, and agency priorities. There is no way that the Space Shuttle, when it begins flying again this summer, could retrieve the GP-B spacecraft. All shuttle flights a launched from Cape Kennedy in Florida, out over the Atlantic, into equatorial orbits, about 200 miles above the Earth. By contrast, GP-B is in a polar orbit, perpendicular to shuttle orbits, and it is orbiting 400 miles above the Earth. Thus, shuttle retrieval is not a possibility.

As an historical note, the original Space Shuttle program was scheduled to launch shuttles in both equatorial orbits (from Cape Kennedy) and in Polar Orbits (from Vandenberg Air Force Base in California). Originally, GP-B was slated to be launched into orbit from the cargo bay of a shuttle in polar orbit. However, after the Challenger accident inn 1986, NASA did away with all plans for West Coast, polar orbiting shuttle launches. Thus, GP-B had to be re-designed as a satellite that could be launched from atop a rocket, rather than being deployed from the cargo bay of a Space Shuttle. Furthermore, even if GP-B had been deployed from a shuttle, it would have to have been boosted up to its 400 mile orbit, so the shuttle could not have retrieved it anyway.

If NASA chooses not to fund any further research uses of the GP-B spacecraft, then one of the final actions of the GP-B Mission Operations Team will be to spin down the gyros and then shut down all spacecraft systems, using a special double-safe shutoff procedure that ensures the spacecraft's computers and telemetry system will never spontaneously re-start themselves (which once happened with another satellite).

Once this procedure has been run on the GP-B spacecraft, it will basically become another piece of orbiting "space junk." The spacecraft will continue to orbit the Earth for about 70-80 years. Eventually, its orbit will decay, and it will re-enter the Earth's atmosphere and burn up. Most likely, the whole spacecraft will burn up, but if anything were to survive re-entry, it would undoubtedly be the quartz block, housing the four gyros. Thus, although very unlikely, it is conceivable that sometime in our grandchildren's generation, a fiery piece of space junk containing four of the roundest objects ever made may come crashing down in a field somewhere, and someone could bring one or more of the GP-B flight gyros to the Smithsonian Institution to keep for posterity.

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CORRECTION TO LAST WEEK'S UPDATE
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In last week's update, I inadvertently described the photovoltaic (PV) cells in the GP-B solar arrays as SUPERconductors. This is incorrect; the PV cells are SEMIconductors. The niobium coated rotors of the GP-B science gyroscopes are superconductors at temperatures approaching absolute zero.

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PREVIOUS GP-B UPDATES
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If you wish to read any of our previous updates, our GP-B Web site includes a chronological archive of all the updates/highlights (with photos and drawings) that we have posted over the past 8 years: http://einstein.stanford.edu/highlights/hlindexmain.html

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OTHER LINKS THAT MAY INTEREST YOU
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* Our GP-B Web site, http://einstein.stanford.edu contains lots of information about the Gravity Probe B experiment, general relativity, and the amazing technologies that were developed to carry out this experiment.


* Visual tour of the GP-B spacecraft and payload from our GP-B Web site: http://einstein.stanford.edu/content/vehicle_tour/index.html


* PDF file containing a 1/20 scale, paper model of the GP-B spacecraft that you can download print out, and assemble: http://einstein.stanford.edu/content/paper_model.


* NASA's Marshall Space Flight Center also has a series of Web pages devoted to GP-B: (http://www.gravityprobeb.com)


* Photo, taken through a telescope by Swiss physics teacher and amateur astronomer Stefano Sposetti, of GP-B spacecraft in orbit, passing near IM Pegasi: http://aida.astronomie.info/sposetti.



* The Harvard-Smithsonian Center for Astrophysics (Cambridge) and York University (Toronto), with contributions from the Observatoire de Paris, have been studying the motions of the guide star, IM Pegasi for over a decade. To find out more, visit: http://www.yorku.ca/bartel/guidestar/


* In addition, you'll find information in the Guide Star FAQ on our Web site: http://einstein.stanford.edu/content/faqs/faqs.html#guidestar and on pages 18-20 of the Gravity Probe B Launch Companion: http://einstein.stanford.edu/highlights/GP-B_Launch_Companion.pdf.


* Track the GP-B satellite on the Web using NASA's Java-based J-Pass satellite tracking application at: http://science.nasa.gov/realtime/JPass/ Also, you can track the GP-B satellite on Personal Digital Assistants (PDAs) using either the Palm OS or Pocket PC operating systems with software from Big Fat Tail Productions: http://www.bigfattail.com.


The Einstein Exhibition at the Skirball Cultural Center in Los Angeles through May 2005: Information about the Einstein exhibition is available on the Skirball Center Web site: http://www.skirball.org/index.asp?s=exhibit&p=einstein.asp. If you can't make it to Los Angeles, you can visit the AMNH's virtual Einstein exhibit on the Web at: http://www.skirball.org/exhibit/amnh_frame.html.


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NASA - Stanford - Lockheed Martin
Gravity Probe B Program
"Testing Einstein's Universe"
http://einstein.stanford.edu

Bob Kahn
Public Affairs Coordinator

Phone: 650-723-2540
Fax: 650-723-3494
Email: kahn@relgyro.stanford.edu
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Regards,

LelandJ