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Gravity Probe B

Testing Einstein's Universe



Item Current Status
Mission Elapsed Time 423 days (60 weeks/13.87 months)
Science Data Collection 294 days (42 weeks/9.64 months)
Current Orbit # 6,242 as of 4:30 PM 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 98.6%
Attitude & Translation Control (ATC)

X-axis attitude error: 274.6 marcs rms
Y-axis attitude error: 270.4 marcs rms

Command & Data Handling (CDH) B-side (backup) computer in control
Multi-bit errors (MBE): 0
Single-bit errors (SBE): 8 (daily average)
Telescope Readout (TRE) Nominal
SQUID Readouts (SRE) Nominal
Gyro #1 rotor potential +1.1 mV
Gyro #2 rotor potential +0.5 mV
Gyro #4 rotor potential +0.7 mV
Gyro #3 Drag-free Status Backup Drag-free mode (normal)


As of Mission Day 423, 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 GP-B spacecraft is beginning to move out its full-sun “season,” and it is once again being eclipsed from sunlight by the Earth for a portion of each orbit. Because of the geometry of the spacecraft's polar orbit around the Earth, and the Earth's orbit around the sun, the spacecraft experiences two “seasons” when the plane of the spacecraft's orbit is perpendicular with respect to the sun's position for about three weeks. Thus, the sun shines broadside on the spacecraft throughout each orbit around the Earth. During these full-sun periods, the Attitude Reference Platform (ARP) containing the star trackers and other equipment heats up, causing it to move slightly, and consequently we must adjust certain parameters to counteract this motion. Then, as the full-sun season comes to an end, we re-adjust these parameters back to their normal settings.

A final heat pulse test was run on the Dewar this past Monday, 13 June 2005. The results of this test indicate that the liquid helium in the Dewar will run out towards the beginning of September. This result is consistent with previous heat pulse measurements, so this was the last heat pulse measurement we will perform.

Preliminary SQUID calibration signal tests described in previous Mission Director's Summaries 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.


Continuing a long-standing tradition, GP-B welcomes 16 students to our staff this summer. These students span a wide range of academic levels, as shown in the following table:

Stanford Doctoral Students 3
Stanford Masters Students 4
Stanford Undergrads 2
Undergrads from Other Institutions 4
Local High School Students 3

The other academic institutions from which this summer's students come include Princeton University, Washington University, the University of Montana, and San Jose State University. For some of these students, this summer is their first experience working with the GP-B team; others are returning for their second, third, and even fourth year.

The three Stanford doctoral students are all on our staff year round, and each of them is working on a research project, as well as working with our Mission Operations Team. Each of the other students is supervised by one of our staff members and is assigned one or more projects to work on during the summer in accordance with their grade level, knowledge, and experience. This summer's projects include: Developing technology for mission data presentations, writing a report on our SQUID technology, assisting with data analysis, working on orbital data and orbit calculations, modeling the polhode effects on our science gyros, post-experiment calibration planning, and preparing sections of our final NASA report.

Every Wednesday afternoon during the summer, we hold a Student Presentation session. During these two-hour sessions, each of our Summer Students gives a short presentation on what they have accomplished over the past week, what they are currently working on, and what their work will lead to. Many members of our staff attend these presentations and give feedback to the students. These sessions can be enlightening both to the students and to the staff.

Along the same lines, this month two students who have been working part time on our staff during this past school year graduated from Stanford--one with a Bachelor's Degree in Mechanical Engineering, and the other with a Masters Degree Aeronautics and Astronautics. Both students will now continue as full-time members of our GP-B staff through the end of the mission.

Perhaps the most important and invaluable spin-off of GP-B over the years has been trained minds. The program has produced 79 doctoral dissertations in seven departments at Stanford University and 13 at other universities, 25 Master's degrees, and 5 Engineer's degrees. No less important, GP-B has provided rare and exciting frontier research opportunities for about 350 undergraduates from a dozen departments and 40 high school students over the years. Of these Stanford undergraduates, eight went on to become graduate students working on the program. Also, GP-B has hosted 45 students in Stanford's Rotation Program, where mostly first year students get an in-depth one-quarter exposure to research being conducted in various departments and laboratories here on campus.

GP-B alumni include the first American woman astronaut in space, the CEO of a major aerospace company, professors at Harvard, Princeton, Stanford and elsewhere, a recent Nobel Laureate in Physics, and many other scientists and engineers in disciplines ranging from theoretical and experimental physics to agriculture (e.g. GPS control of tractors to plow fields automatically without a farmer on board.)


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: Tthe diagrams of the GP-B experiment and the Season of GP-B were created by GP-B Public Affairs Coordinator, Bob Kahn using Adobe Photoshop and Adobe Illustrator. The cross sectional drawing of the Dewar is from the GP-B Image Archive here at Stanford. Click on the thumbnails to view these images at full size.


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