Skip navigation

Gravity Probe B

Testing Einstein's Universe



Item Current Status
Mission Elapsed Time 325 days (46 weeks/10.75 months)
Science Data Collection 196 days (28 weeks/6.50 months)
Current Orbit # 4,767 as of 2:00 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 1.83 kelvin, decreasing slowly
Global Positioning System (GPS) lock Greater than 98.4%
Attitude & Translation Control (ATC)

X-axis attitude error: 351.8 marcs rms as of 3/10/05
Y-axis attitude error: 355.4 marcs rms as of 3/10/05

Command & Data Handling (CDH) Now using B-side (backup) computer & guidance systems
Multi-bit errors (MBE): 0
Single-bit errors (SBE): 7 (daily average)
Telescope Readout (TRE) Nominal
SQUID Readouts (SRE) Nominal
Gyro #1 rotor potential -0.7 mV as of 3/10/05
Gyro #2 rotor potential +8.3 m as of 3/10/05V
Gyro #4 rotor potential -6.0 mV as of 3/10/05
Gyro #3 Drag-free Status Backup Drag-free mode (normal)


On mission day #325, the GP-B spacecraft is in good health, but it is now running on its B-side, or back-up computer and guidance systems, including the main on-board computer, control gyros, attitude control electronics, and thruster coils.

The switch-over to the backup systems occurred automatically at 7:17 am PST last Friday morning, 4 March 2005, due to two or more multi-bit errors (MBE's) in the flight computer, occurring within a 0.2 second interval. The MBEs were caused by radiation hits in the south magnetic pole. This event triggered pre-programmed safemodes, which in turn resulted in the automatic switch-over. We have provided a more complete description of this anomalous event and the ensuing recovery efforts in the Mission News Section below.

The possibility of a B-side switch during the science phase of the mission was anticipated, and the team had recently rehearsed the procedures for dealing with such an event. As a result, much of the recovery process--which otherwise might have taken two weeks or longer--was accomplished last weekend. This event has caused us to lose about a week’s worth of science data. Our data collection rate has been running above 99% up to this point in the science phase of the mission, and the loss of a week’s data will only reduce that level by about 2%, which is still well within the mission requirement of 90%. Assuming that this event did not place any non-relativistic torques (forces) on the gyros, the loss of this small amount of data will not have any significant effect on the outcome of the experiment. Analysis to ensure that this is the case is underway.

This week, the team is still in the process of fine-tuning the backup control systems in order to provide the same level of response and quality of science data that we had been obtaining from the main (A-side) systems prior to this event. These fine-tuning efforts are proceeding well, and we have already resumed science data collection.


As a reminder, just under two weeks after GP-B’s spectacular launch on 20 April 2004, the spacecraft was hit by radiation while passing over the Earth’s south magnetic pole. This radiation caused data errors in the spacecraft’s primary (A-side) computer, which exceeded its capacity for self-correction. This event triggered a number of safemodes, and by design, the spacecraft automatically switched over to the backup (B-side) computer and put the planned timeline of events on hold.

The automatic switch over from primary to backup computer worked flawlessly. Since this event occurred in the very early stages of initialization and checkout of all the spacecraft systems and instrumentation, the mission operations team, over the course of a week, sent a series of commands to the spacecraft that re-booted the primary computer, restored its data parameters, and then switched control of the spacecraft back to the main computer.

After this early safemode event occurred, the team recognized that, although unlikely, such an event could happen during the science phase of the mission, interrupting the collection of science data. To prepare for such a possibility, the team created a set of recovery procedures, designed to minimize data loss should such an event recur. A few weeks ago, the GP-B mission operations team conducted a complete rehearsal of these recovery procedures—not unlike an emergency preparedness drill—to ensure that the team was ready to handle the recurrence of a so-called “B-side switch.”

As it turns out, the timing for this rehearsal couldn’t have been better. At 7:17 AM PST last Friday, 4 March 2005, automated safemodes were activated, causing an autonomous switch to B-side hardware and to "gyro hold" for attitude control. This event was declared a "major" anomaly. The cause of the safemode activation was a memory checkout detection of at least two multi-bit errors (MBE's) in the flight computer occurring within a 0.2 second interval.

The safemode responses proceeded as designed, and the vehicle remained safe during the autonomous switch to the B-side control and guidance hardware. Not all of the systems on board the spacecraft switched to the B-side. For example, the payload systems--Gyro Suspension System (GSS), SQUID Readouts (SRE), Experimental Control Unit (ECU), Telescope Readout (TRE), and Dewar--remained on the primary or A-side. The science gyro data indicated that the vehicle remained pointed to the guide star within 500 arcseconds, with a coning of 50 arcseconds. This is consistent with the “gyro hold” state, which minimizes the impact to science data collection.

Being fully rehearsed, the team followed the recovery procedures to re-configure the spacecraft for collecting science data using the B-side computer and control systems and to return as quickly as possible to collecting science data. In order to obtain additional NASA/TDRSS relay satellite communications passes that would assist in the recovery, a spacecraft emergency was declared with NASA’s Goddard Space Flight Center (GSFC) last Friday.

After the anomaly occurred, as anticipated, the vehicle roll rate decreased from the nominal 0.7742 rpm. As part of the recovery procedure the roll-down was stopped at 0.67 rpm and was subsequently commanded to roll back up. This roll-up took about 6 hours and returned to the nominal 0.7742 rpm at 11:00 PM PST last Friday. By Saturday morning (less than 27 hours after the B-side switch), we returned to drag-free operation. Much of this work involved transferring files and software piece-by-piece from the A-side to the B-side computers—much like the process of migrating all your files and programs when upgrading your home computer.

During the recovery sequence, when we rebooted the SRE to resynchronize the spacecraft’s clocks, one of the command blocks for restarting drag-free operations was inadvertently omitted. This omission was discovered during a subsequent routine system checkout, and drag-free operation was once again restored on Tuesday. Also, the SRE reboot sequence momentarily changed the gyro suspension system (GSS) to analog mode, and it corrected the existing MBE's in the SRE. The B-side GPS receiver was started for the first time on-orbit and acquired the GPS constellation within 30 minutes.

On Sunday morning, we adjusted various parameters to improve A-side star tracker performance. We also adjusted the control gyros (navigational gyros used to maintain the spacecraft’s pointing direction when the telescope is not locked onto the guide star), and by early Sunday evening, we successfully locked onto the guide star. Then, on the next telemetry pass, the telescope achieved a clean guide star acquisition! The attitude control team is continuing to make adjustments to improve the pointing accuracy of the spacecraft under B-side control.

The Stanford/Lockheed Martin/NASA Gravity Probe B team responded to this event in a highly professional manner. The team continuously demonstrated extraordinary dedication and expertise, and they remained focused on achieving flawless execution. Thanks to a robust design and an outstanding operations team, Gravity Probe B is again collecting valuable science data to test Einstein's theory.


If you're going to be in Los Angeles anytime before 30 May 2005, and if you’re interested in Einstein’s life and work, the Einstein Exhibition at the Skirball Cultural Center (just north of the Getty Museum on Interstate 405) is the most comprehensive presentation ever mounted on the life and theories of Albert Einstein (1879-1955). It explores his legacy not only as a scientific genius who re-configured our concepts of space and time, but also as a complex man engaged in the social and political issues of his era. It examines the phenomenon of his fame and his enduring status as a global icon whose likeness has become virtually synonymous with genius.

In this exhibit, you can examine Einstein's report card, inspect his FBI file, and enjoy his family photographs, love letters, and diary entries. Exhibition highlights include scientific manuscripts and original correspondence—including original handwritten pages from the 1912 manuscripts of the special theory of relativity and his 1939 letter to President Roosevelt about nuclear power—and a wealth of other documents from the Albert Einstein Archives at the Hebrew University of Jerusalem.

In addition to these displays of Einstein memorabilia, the exhibit also features a number of interactive components that help provide an understanding of Einstein's revolutionary theories. Furthermore, several “explainers,” identified by their red aprons, are on hand to discuss various aspects of the exhibit and to explain and demonstrate difficult concepts, such as time dilation and warped spacetime. At the end of the exhibit, you’ll find one of GP-B’s gyro rotors on display.

The Einstein exhibition was jointly organized by the American Museum of Natural History (AMNH), the Hebrew University of Jerusalem, and the Skirball Cultural Center. It was designed by the AMNH under the supervision of Dr. Michael Shara, curator of the exhibit and chairman of the museum’s Astrophysics Department. It opened in November 2002 at the AMNH in New York and then traveled to Chicago and Boston, spending about 8 months in each location. It will remain at its final U.S. stop at the Skirball Center in Los Angeles through 29 May 2005, after which time it will move permanently to the Hebrew University in Jerusalem.

Information about the Einstein exhibition is available on the Skirball Center Web site. If you can’t make it to Los Angeles, you can visit the AMNH’s virtual Einstein exhibit on the Web.

Drawings and photos: The diagram of the GP-B experiment created by GP-B Public Affairs Coordinator, Bob Kahn. The photos of the GP-B daily all-hands meeting, the Anomaly Room, and the Mission Operations Center were also taken by Bob Kahn. The photos from the Einstein Exhibit are courtesy of the Skirball Cultural Center. Click on the thumbnails to view these images at full size.


Previous Highlight
Index of Highlights