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Gravity Probe B

Testing Einstein's Universe



Item Current Status
Mission Elapsed Time 651 days (93.0 weeks/21.3 months)
IOC Phase
129 days (4.2 months)
Science Phase
352 days (11.6 months)
Final Calibration Phase
43 days (1.3 months)
Extended Science Phase
4 days
Post Mission Phase
123 days (17.6 weeks/4.0 months)
Current Orbit # 9,605 as of 5:30 PM PST
Spacecraft General Health Good
Roll Rate Normal at 0.4898 rpm (122.5 seconds per revolution)
Gyro Suspension System (GSS) Gyro #1 digitally suspended; Gyros #2, #3, & #4 in analog suspension
Gyro Spin Rates ~0.5 rpm (same as spacecraft roll rate)
Dewar Temperature ~195 kelvin (and rising ~0.6 kelvin/day)
Global Positioning System (GPS) lock Nominal
Attitude Control System (ATC)

Nominal for post-mission operation
Pointing Error (XY/Pitch-Yaw): 0.39 degrees rms
Roll Phase (Z Axis) Error: 7.6 degrees rms

Telescope Readout (TRE) Pointing performance too low to lock onto guide star
Command & Data Handling (CDH) B-side (backup) computer in control
Multi-bit errors (MBE): 1 in CCCA Backup computer; 3 in GSS computer
Single-bit errors (SBE): Data Not Available


On Mission Day 651, the Gravity Probe B vehicle and payload continue to be in good health. All active subsystems, including solar arrays/electrical power, Experiment Control Unit (ECU), flight computer, star trackers and magnetic torque rods, gyro suspension system (GSS), and telescope detectors, are performing nominally. We continue to communicate with the spacecraft regularly, though less frequently, monitoring the Dewar and probe as they continue to warm up, and collecting status data from various instruments on-board.

The temperature inside the Dewar has now warmed to ~195 kelvin, and its rate of temperature rise has slowed to ~0.6 kelvin per day. The temperature inside the Dewar will eventually reach thermal equilibrium with the outside temperature of ~0 centigrade (~273 kelvin), but this will occur very gradually, over a long period of time.

As a result of the CCCA backup computer re-boot on December 21, 2005, the attitude of the spacecraft shifted approximately 90 degrees, so that instead of pointing in the direction of its orbit, the spacecraft is now pointed broadside or perpendicular to its orbit plane (orbit normal orientation). Because we are no longer tracking the Guide Star, IM Pegasi, there is no need to maneuver the spacecraft back to its Guide Star orientation. Rather, we have been stabilizing the spacecraft in its orbit normal orientation, and we are preparing to reduce its roll rate to 0.04 rpm (25 minutes/revolution) in order to collect sample planet eclipse data from the two star trackers on-board. (See today's Mission News story below for more information about the planet eclipse data and other sample data we will be collecting over the next few weeks.)

Because the spacecraft has been in orbit normal orientation for the past month, its two antennae have been oriented less favorably for communication with the NASA TDRS (Tracking Data Relay Satellite) system and with the NASA ground network tracking stations. Even though the spacecraft's antennae are omni-directional, their optimal transmission/reception path is contained within cone-shaped areas to the front and rear of spacecraft, and their gain is somewhat diminished in the orbit normal orientation. Over the 3-day Martin Luther King holiday weekend of 14-16 January, this diminished antenna communications link, coupled with reduced weekend/holiday monitoring from our Mission Operations center (MOC), triggered a safemode that automatically re-boots the on-board CCCA Backup computer if it does not receive any commands from our MOC within a 36-hour period. We have since recovered from this re-boot.

Another unsurprising consequence of the spacecraft's recent position changes, as well as thermal changes in the quartz block where the gyros are housed, is that the Gyro Suspension System (GSS) automatically transitioned gyros #2, #3, and #4 from digital (highest control) to analog (safe and secure) suspension after the computer re-boot two weeks ago. The ultra-sensitive GSS interprets spikes in rotor position due to thermal stresses during warm-up as excessive gyro motion, and it automatically transitions the suspension mode from digital to analog to ensure the safety of the rotors. In due course, we will return these three gyros to digital suspension.

All of these recent spacecraft behaviors--spotty communication, computer re-boots, gyro safekeeping transitions--are the expected results of the spacecraft operating outside the limits of its controlled experimental environment, along with reduced communications and monitoring from our MOC. As we've stated many times in recent status updates, our main focus now is analyzing the science data we have collected. However, we will continue to perform minimal maintenance on the spacecraft, so that it is ready and available for other post-mission experiments, as described in the Mission News story below.


Our GP-B team has now completed all planned, as well as some extended post-mission analyses on our spacecraft and its component systems. We are in the process of stabilizing the spacecraft in its current orbit normal orientation, and the spacecraft is now ready and available for use by other scientists to perform various types of experiments. The GP-B spacecraft is a state-of-the-art orbiting laboratory, and it has performed extraordinarily well throughout the GP-B mission and beyond. While it no longer maintains the cryogenic environment necessary for testing the geodetic and frame-dragging effects of general relativity, the on-board star trackers, magnetometers, and science gyros are still functioning perfectly, and they can be used individually or in combination for a number of other types of experiments.

Thus, we are actively seeking scientific partners around the world who would be interested in using this space borne laboratory to perform additional post-mission experiments. To this end, for the next few weeks we will be collecting sample data to demonstrate and validate the spacecraft's post-mission experimental capabilities in the following five areas.

  1. Occlusion of Stars by Planets. Within the next few days, we will begin the process of slowing down the spacecraft's roll rate to 0.04 rpm (25 minutes per revolution). At this very slow roll rate, the on-board star trackers can be switched into a more sensitive tracking mode, in which the light from stars can be integrated over much longer time periods. By observing brightness variations of various star systems, the star trackers can detect the presence of orbiting planets in those systems.
  2. Measuring Residual Drag on the Spacecraft. The four GP-B science gyroscopes can be used as 3-axis accelerometers that are capable of measuring solar pressure and upper atmospheric drag on the spacecraft to an accuracy of 5 x 10-12 g. 
  3. GPS Satellite Accuracy Measurements. Because the orbit plane of our GP-B spacecraft is very well established, we can use the four science gyroscopes as 3-axis accelerometers to determine the spacecraft's precise inertial position without GPS reckoning. We can then compare this internally-calculated spacecraft position information with corresponding position information generated by various GPS satellites to determine their level of accuracy.
  4. Subtle Aurora Borealis Effects. Using a combination of the on-board magnetometers, the proton monitor, and the four science gyros as accelerometers, we can investigate what happens when the upper atmosphere heats up as a result of bombardment by charged particles. We can also measure the buffeting effects of the upper atmosphere as the spacecraft passes through a region containing charged particles.
  5. Latitude Axis Gravity Gradient. As a complement to the GRACE mission that measured gravity gradients along a longitudinal axis between a pair of orbiting spacecraft, the GP-B science gyros can be used as 3-axis accelerometers to measure latitude-axis (cross-orbit) gravity gradients.

As noted earlier, for the next few weeks, we will be collecting sample data to illustrate the feasibility and limits of performing all of the above experiments with the GP-B spacecraft. Because the spacecraft is already in orbit and functioning, funding requirements for such “experiments of opportunity” will be minimal. The cost for using the spacecraft to perform any or all of these experiments amounts to the cost of a 4-5 person mission operations staff over the period of the experiments.

In summary, the GP-B spacecraft has performed exceedingly well to date. It has experienced no serious failures, and all the systems required for performing post-mission experiments are operational and ready for use. In a few weeks, after we have finished collecting the sample data, if there is no interest or funding for performing any of these experiments, we will place the spacecraft in a safe, hibernation configuration and reduce our maintenance monitoring of its health to once a week.


Our next regularly scheduled update will be at the end of February. Of course, we will send out a timely update if there are any important changes in the spacecraft's status, or if noteworthy events occur here at GP-B in the meantime.


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.

Photos & Drawings: The "GP-B Acronym Sky" graphic, the Orbit Normal composit graphic, and the Data Analysis collage were created by GP-B Public Affairs Coordinator, Bob Kahn. The photo of the Dewar, NASA TDRS and Ground Communications stations, and the photos of a gyro housing assembly and the Gyro Suspension System (GSS) electronics are from the GP-B Image Archive here at Stanford. Click on the thumbnails to view these images at full size.


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