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Gravity Probe B

Testing Einstein's Universe



Item Current Status
Mission Elapsed Time 304 days (43 weeks/10.0 months)
Science Data Collection 175 days (25 weeks/5.75 months)
Current Orbit # 4,486 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 1.82 kelvin, holding steady
Global Positioning System (GPS) lock Greater than 98.6%
Attitude & Translation Control (ATC)

X-axis attitude error: 173.2 marcs rms
Y-axis attitude error: 264.0 marcs rms

Command & Data Handling (CDH) Multi-bit errors (MBE): 0
Single-bit errors (SBE): 4,704 (daily average)
Telescope Readout (TRE) Nominal
SQUID Readouts (SRE) Nominal
Gyro #1 rotor potential -2.7 mV
Gyro #2 rotor potential +6.1 mV
Gyro #4 rotor potential -7.8 mV
Gyro #3 Drag-free Status Backup Drag-free mode (normal)


As of Mission Day 304, the Gravity Probe B vehicle and payload are in good health, with all systems functioning nominally and more than five months of science data collected. All four gyros are digitally suspended in science mode and the spacecraft is flying drag-free around Gyro #3.

This past week was again a relatively quiet one for GP-B. Solar flare activity is low, and solar radiation levels are normal. Guide star capture times (time required to re-lock the telescope onto the guide star as the spacecraft emerges from behind the Earth each orbit) are averaging approximately one minute, which is excellent. The brightness table was updated for two of the stars that are monitored by the spacecraft’s star trackers, and this significantly improved the star tracker’s performance. Brightness tables for other stars in the star tracker catalog will be updated next week.


During this time of year for our orbit, the front end of the spacecraft is pointed towards the sun, warming the Forward Equipment Enclosure (FEE) slightly more than our models had predicted. Thus, for the past few weeks, the flow rate of the helium escaping from the Dewar has been slightly higher than anticipated, which may slightly reduce the length of the mission. When the Dewar has only 3-5 weeks of helium remaining, we will conclude the science phase and begin our instrument re-calibration, and this requires at least 3 weeks.

To determine how much helium is currently left in the Dewar, the GP-B Dewar team is in the process of performing another heat pulse meter operation. The heat pulse test works in the following way: The amount of heat that it takes to warm an object by a specified amount depends on the type of material, its temperature, and its mass. Thus, if the "specific heat" (the amount of heat needed to warm a kilogram of material by one degree kelvin) is known, it is a simple matter to measure the mass by applying a known amount of heat (usually with an electric heater) and measuring the resultant temperature rise. In the case of the GP-B Dewar, the situation is simplified by the fact that heat distributes itself virtually instantaneously throughout superfluid helium. The amount of heat used in the test must be large enough to cause a measurable temperature change (approximately 10 millikelvin) but not so large as to appreciably shorten the mission lifetime.

The Dewar team expects to complete their analysis of the heat pulse data next week, and we will report the results in next week’s update. Our current expectation is that the science phase of the mission will conclude towards the end of June, and the results of the heat pulse meter operation will help us determine whether or not the current mission time line is correct.



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 photo: GP-B Public Affiars Coordinator, Bob Kahn, created the diagrams of the GP-B experiment and the Seasons of GP-B. The photo of the Dewar was taken by Russ Underwood at Lockheed Martin Corporation. The photos from the Einstein Exhibit are courtesy of the Skirball Cultural Center. Click on the thumbnails to view these images at full size.


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