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Gravity Probe B

Testing Einstein's Universe



Item Current Status
Mission Elapsed Time 479 days (68 weeks/1.31. years)
IOC Phase
129 days (4.2 months)
Science Phase
352 days (11.6 months)
Final Calibration Phase
5 days
Current Orbit # 7,169 as of 1:00 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 98.0%
Attitude & Translation Control (ATC)

X-axis attitude error: 166.9 marcs rms
Y-axis attitude error: 254.5 marcs rms

Command & Data Handling (CDH) B-side (backup) computer in control
Multi-bit errors (MBE): 1 (in GSS#1 Computer on 8/17)
Single-bit errors (SBE): 10 (daily average)
Telescope Readout (TRE) Nominal
SQUID Readouts (SRE) Nominal
Gyro #1 rotor potential +0.2 mV
Gyro #2 rotor potential -0.6 mV
Gyro #3 rotor potential -1.8 mV
Gyro #4 rotor potential +1.4 mV
Gyro #1 Drag-free Status Backup Drag-free mode (normal)


As of Mission Day 486, the Gravity Probe B vehicle and payload are in good health and all subsystems are performing nominally.

On Monday, 15 August 2005, the Gravity Probe B mission concluded the science phase of the mission and transitioned to the final calibration phase of the mission. In total, the mission collected science data for 352 days (11.6 months) during its 7,000+ orbits around the Earth with an extremely high data capture rate for that time (99.0%).

The final calibration phase officially began at 6:26am (PST) on Monday when the drag free gyro (Gyro #1) was transitioned to “drag free off.” After setting the gyro preloads to the IOC levels of 10V (Initialization & Orbit Checkout Phase), we maneuvered the space vehicle to point at HD216235, a star one degree away from the guide star. On Tuesday, we returned to the guide star and resumed drag-free operation. On Wednesday and Friday, we are repeating these maneuvers to the neighboring star (HD216235). The aim of these calibrations is to place tight constraints on potential systematic errors.

The calibrations are scheduled to be completed by 31 August. With any remaining helium, further calibrations will be performed at a different spacecraft roll rate.


Two notable events related to GP-B occurred this past week. First, this past Tuesday and Wednesday, we were honored with a visit from Dr. Nancy Roman, one of the nation's leading scientists in the space program.

Roman received her PhD in astronomy from the University of Chicago in 1949. She began her career doing astronomy research in stellar distances and motions at the University of Chicago's Yerkes Observatory and also teaching graduate courses there. In 1959, Roman joined NASA, and from 1960-1979, she served as Chief of the Astronomy and Relativity Programs in the NASA Office of Space Science. In her position at NASA, Roman was very influential in creating astronomical research satellites such as the Cosmic Background Explorer (COBE) and the Hubble Space Telescope (HST). The photo to the right shows Dr. Roman with a model of the Orbiting Solar Observatory, one of the many NASA spacecraft launched under her guidance and leadership.

She also oversaw the development of a number of ground and space research programs, including Gravity Probe B. In fact, Roman helped organize a NASA-sponsored two-day seminar, held at Stanford in July 1961, in which over 30 distinguished physicists, engineers, and aerospace experts from all over the U.S. gathered to discuss the possibilities of testing Einstein's theories of relativity in space. Some of the ideas discussed at that seminar--most notably the concept of a drag-free satellite--are currently being used in the GP-B mission. After retiring from NASA in 1979, Dr. Roman continued working as a contractor at the Goddard Space Flight Center. Throughout her career, Dr. Roman has been a spokesperson and advocate of women in the sciences.

This past week was Dr. Roman's first trip to Stanford since 1979, but she has been following the progress of Gravity Probe B through our weekly updates. During her visit, she sat in on several mission status briefings, she met with scientists and engineers on the GP-B team, she attended a student presentation session and had lunch with the students and staff, and she toured the GP-B development labs and Mission Operations Center. The photos to the right were taken during Dr. Roman's visit to GP-B at Stanford this past week. Click here to read a NASA biography about Dr. Nancy Roman.

Also in the news this past week, NASA Administrator Michael Griffin named Rex Geveden as the agency's associate administrator. In this capacity, Geveden has oversight for all the agency's technical missions' areas and field center operations. He will be responsible for programmatic integration between NASA's mission directorates and field centers. In November 2004, Geveden moved from NASA's Marshall Space Flight Center (MSFC) in Huntsville, AL to NASA Headquarters in Washington DC to become NASA Chief Engineer. He has been serving as acting associate administrator since June 2005.

Prior to moving to NASA Headquarters, Geveden held various leadership positions at MSFC. He served as deputy director of the Marshall Center from July 2003-November 2004. Prior to that, he was deputy director of the MSFC Science Directorate, leading research and development projects in space science, materials science, biotechnology, earth science and space optics. Geveden was project manager for several successful efforts, including the Optical Transient Detector and Lightning Imaging Sensor satellites, which produced data for the world's first global map of lightning.

However, we here at GP-B have a special connection with Rex Geveden. From 1995-2003, Geveden was the NASA MSFC Program Manager overseeing GP-B's final development and testing, and readying the spacecraft for launch. Geveden has long been a staunch supporter of GP-B, and we wish him every success in his new position at NASA.

Click here to read a NASA press release about Rex Geveden's new appointment.


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 the Earth was created by GP-B Public Affairs Coordinator, Bob Kahn using Adobe Photoshop and Adobe Illustrator. Mr. Kahn also took the photo of the GP-B Mission Operations Center in action and the photos of Dr. Nancy Roman visiting GP-B at Stanford. The sky chart images, showing the guide star IM Pegasi and its neighboring stars, were generated by the Voyager III Sky Simulator from Carina Software.The photos of Dr. Nancy Roman with the model of the Orbiting Solar Observatory and Rex Geveden with a model of the GP-B spacecraft are courtesy of NASA. Click on the thumbnails to view these images at full size.


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