Skip navigation

Gravity Probe B

Testing Einstein's Universe



Item Current Status
Mission Elapsed Time 395 days (56 weeks/12.95 months)
Science Data Collection 266 days (38 weeks/8.72 months)
Current Orbit # 5,830 as of 5:00 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 98.3%
Attitude & Translation Control (ATC)

X-axis attitude error: 186.6 marcs rms
Y-axis attitude error: 214.7 marcs rms

Command & Data Handling (CDH) B-side (backup) computer in control
Multi-bit errors (MBE): 1 (Gyro Suspension System on 5/20/05)
Single-bit errors (SBE): 8 (daily average)
Telescope Readout (TRE) Nominal
SQUID Readouts (SRE) Nominal
Gyro #1 rotor potential -1.7 mV
Gyro #2 rotor potential -2.6 mV
Gyro #4 rotor potential -2.1 mV
Gyro #3 Drag-free Status Backup Drag-free mode (normal)


As of Mission Day 395, 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.

At the most recent weekly review of all spacecraft subsystems, one of our GP-B mission directors remarked that this past week has been one of the quietest and smoothest since launch. Our telescope pointing and guide star capture times continue to be excellent. To minimize noise in the SQUID Readout Electronics (SRE), the Experiment Control Unit (ECU) is now only being powered on for a few hours each week in order to obtain a set of readouts, such as the Dewar temperature, that are provided by the ECU. 

We have also begun some preliminary SQUID calibration signal tests. These tests involve turning on calibration signals on one pair of SQUIDs for one day, while turning these signals off on the other pair of SQUIDS. The next day, we reverse the procedure, turning off the calibration signals on the first pair of SQUIDs and vice versa. At the end of the test, we will compare the results. These tests do not in any way affect the science data collection, but they help us evaluate the performance of the four SQUIDS relative to each other.

Preparations for the end of the GP-B mission are now in full swing. Various chapters of our final mission report, which we will deliver to NASA late this summer, are currently being drafted. 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 now being created. The mission operations team will practice using these procedures during two upcoming calibration phase simulations. Because these final instrument calibration tests place controlled torques (forces) on the gyros, they cannot be performed until after the data collection has been completed.


If you've seen photos or drawings of the GP-B spacecraft with its solar arrays (panels) fully deployed, you know that it has a rather unique shape compared with other satellites. This distinctive shape is partly due to the 650 gallon Dewar that comprises most of its body. But, it is the 3.5 x 1.3 meter (11.5 x 4.25 foot) solar arrays, extending out at 90 degree angles from each other and canted (tilted) at seemingly odd angles, along with its rotation along its main axis, that cause the GP-B spacecraft to resemble a flying windmill (although the canting of the solar arrays was specifically designed to preclude the spacecraft from windmilling, due to interaction with the solar stream).

The Electrical Power System (EPS) is the heart of the GP-B spacecraft, providing power to maintain all of its systems and payload operations, as well as enabling sustained communication and tracking of the spacecraft throughout its mission. The solar arrays are the sole source of power for the EPS, converting light from the Sun into electricity through photoelectric and photovoltaic processes. The electricity is then stored in re-chargeable Nickel-Cadmium (NiCAD) batteries and delivered in a controlled flow to all of the spacecraft's systems by a Standard Power Regulation Unit (SPRU).

The solar arrays must be able to deliver sufficient power to keep the EPS running continuously as the spacecraft orbits the Earth, while the Earth revolves around the Sun. Most satellites are designed to face the sun year around, or they have moveable solar panels that can be re-oriented to face the sun, thus maintaining positive power throughout each orbit. This is not the case with GP-B. Due to the nature of the GP-B science mission, GP-B always points at the guide star, IM Pegasi; it never points directly at the Sun, because sunlight overwhelms the telescope's ultra-sensitive photon detectors. Furthermore, the GP-B spacecraft itself must be as free as possible from any motion disturbances that could affect the gyros and SQUID readouts. Thus, once opened, the solar arrays on the GP-B spacecraft had to be locked into fixed positions. For these reasons, GP-B's solar arrays are unique, both in their design and in their positioning on the spacecraft.

GP-B is equipped with four fixed, double-sided solar array panels, canted at such angles as to always be able to receive some solar power regardless of the season. Additionally, these double-sided panels were designed to minimize the thermal effects of the spacecraft entering and exiting eclipses, when the Earth blocks the Sun's light for up to 36 minutes per orbit. Each GP-B array contains 9,552 individual single-junction Gallium Arsenide (GaAs) solar cells that convert sunlight into electrical power with 18.5% efficiency, the standard for this type of eclipsing satellite. As sunlight is absorbed by the solar cells, its photons, or bundles of energy, are transferred to the electrons of the superconductive photovoltaic (PV) cells. The electrons in the PV cells become excited enough to escape their atoms and begin flowing, producing the current that powers the GP-B electronics. 

Because the GP-B spacecraft always points at the guide star, rather than at the Sun, the angle between the spacecraft's main axis (also the telescope axis) and the Sun--called the gamma angle--varies throughout the year. Thus, the solar arrays do not receive consistent direct sunlight, and this is the reason for the unusual angles at which the solar arrays are canted.

Engineers at Lockheed Martin, who designed and built the GP-B solar arrays, had to ensure that the arrays would produce sufficient electricity in the worst-case gamma angle of approximately 22 degrees, which occurred once during the mission, on 12 September 2004. The Lockheed Martin engineers also had to design and mount the solar arrays in such a way that there would be sufficient power during the maximum 36-minute eclipse times, which occur when the gamma angle is at its maximum and minimum values (158 degrees and 22 degrees, respectively). At these angles, GP-B's orbit plane directly faces the sun--in other words, the Sun appears directly below the guide star, instead of off to one side or the other. This alignment only happened twice during the actual mission--on 12 September 2004 and 20 March 2005--and the greatest power usage and deepest depth of discharge (DOD) by the batteries occurred at these times.


Taking these worst-case alignments into account, the Lockheed Martin designers arranged the orientations of the solar panels on the spacecraft to optimize the amount of incident sunlight. Looking down on the spacecraft from the top of the telescope (that is, looking down the spacecraft's z-axis), the arrays are arranged 90 degrees apart, forming an “X” pattern. Panels 1 and 3 are canted 90 degrees into the Z-plane of the spacecraft, so that you can only see their edges looking down on the spacecraft. Panel 2 is canted 25 degrees forward in the Z-plane, towards the top of the spacecraft, and Panel 4 is canted in the opposite direction, 25 degrees backwards in the Z-plane, towards the bottom of the spacecraft. Because of this asymmetry, the spacecraft is free to rotate about the Z-axis of the telescope with minimal perturbations due to the dragging or torquing of the solar panels.

GP-B maintains a Low Earth Orbit (LEO) at an altitude of only 640 km (400 miles) about the Earth's poles. At this altitude, it has an orbital period of 97.5 minutes, eclipsing behind the Earth 14-16 times a day--almost 5,000 times a year. While in these eclipses, GP-B relies on its Super NiCAD batteries to provide power for all of the spacecraft's subsystems for the duration of each eclipse, lasting up to about 36 minutes during the peak eclipse seasons. This constant cycle of charging and discharging puts a large amount of strain on the batteries. Because of this high discharge and recharge rate, NiCAD batteries, which have been the standard in space since the 1960s, were chosen due to their extensive space heritage and robustness in high eclipse cycle missions. GP-B's batteries have an expected lifetime of five years, and data analysis of battery performance throughout the mission has been consistent with this prediction.

Currently, the spacecraft is just about to enter its third 3-week solstice “season” of the mission. During this period, the spacecraft's orbit plane is perpendicular to the Sun's position, and there are no solar eclipse periods. In other words, the spacecraft remains in full sunlight throughout each orbit. During these solstice periods, the solar arrays provide the maximum amount of power to the EPS.

The solar arrays and batteries are capable of providing almost three times more power than the entire spacecraft requires, demonstrating the robustness of the system. The EPS has performed flawlessly since launch and has exceeded all expectations--not only by supporting all mission operations, but also doing so with more than ample margin.

Note: In 1921, Einstein was awarded the Nobel Prize for his 1905 paper explaining the photoelectric effect. This year, 2005, is the 100th anniversary of Einstein's “Miracle Year”--the year in which he not only wrote his photoelectric paper, but also the special theory of relativity, and two other seminal papers


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.



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 & Photos: The schematic diagram of the GP-B experiment was created by GP-B Public Affairs Coordinator, Bob Kahn. The photos of the ECU electronics box, the SRE box, the spacecraft solar array installation at Vandenberg Air Force Base prior to launch, and the gamma chart are from the GP-B Photo & Graphics Archive here at Stanford. The photos of the solar arrays under development at Lockheed Martin are courtesy of Lockheed Martin Corporation. The birdseye view photo of the solar arrays in place on the spacecraft, prior to launch, was taken by Lockheed Martin photographer Russ Underwood. The drawing of the GP-B seasons and the composite photos showing the GP-B spacecraft in orbit above the Earth were created with Adobe Photoshop by GP-B Public Affairs Coordinator, Bob Kahn. Finally, 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