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Gravity Probe B

Testing Einstein's Universe



On the eve of the 2004 Thanksgiving holiday weekend—mission week #32--the GP-B spacecraft is in good health, with all subsystems performing well. We have now been collecting data for three months. Data collection is proceeding smoothly, and the quality of the data is excellent. The spacecraft continues to fly drag-free around gyro #3, maintaining a constant roll rate of 0.7742 rpm (77.5 seconds per revolution.) The temperature inside the Dewar remains steady at just under 1.82 kelvin.

Last week’s update prompted questions about seasonal temperature variations in the GP-B spacecraft and the effects of these variations on the Dewar and the spacecraft’s various subsystems. Seasons on the Earth are due to the 23-degree tilt in the Earth’s axis relative to the orbital plane in which we circle the Sun. This tilt results in varying intensity and duration of sunlight at different latitudes during the four seasons of the year. For example, sunlight shines more directly, and for more hours (longer days) on locations in the Northern Hemisphere in the summer months, versus the winter months, when the days are shorter and the sun shines more obliquely on the Northern Hemisphere. This situation is reversed in the Southern Hemisphere.

The GP-B spacecraft experiences four “seasons” as well, but they are not directly due to the tilt in Earth’s axis. Rather, the spacecraft alternates between hot and cold seasons through the course of a year due to the changing relationship of the spacecraft’s orbital plane with respect to the Sun, as the Earth and spacecraft revolve around the Sun. The changing geometry between the Sun and the spacecraft’s orbital plane affects the amount of sunlight shining on the front, side, or back of the spacecraft. Because the spacecraft is in a polar orbit, with the plane of its orbit continually aligned with the guide star, there are two 3-week periods when the Sun shines broadside on the spacecraft throughout all of its daily orbits. These are the spacecraft's "hot" seasons.

The first of these “hot” seasons occurred shortly after launch, between the end of May and the middle of June. The second “hot” season began last week and will last through the middle of December. Likewise, another 3-week “hot” season will begin in towards the end of May, next year. At all other times of the year, the spacecraft is in its “cold seasons,” moving behind the Earth—out of sunlight and into the cold void of space—for a part of each orbit. The time when the spacecraft moves behind the Earth is called the orbital eclipse period, and the maximum eclipse period occurs in mid-September and again in mid-March. At these times, the spacecraft is out of sunlight for approximately half of each orbit. The two diagrams to the right, one depicting Earth’s seasons and the other depicting the spacecraft’s seasons, will help you visualize the geometric relationships between the Sun, Earth, and spacecraft. (Note: The Seasons of GP-B diagram is drawn 90 degrees out of phase from the Earth's Seasons diagram. Thus, in this diagram, the Earth's axis is not shown tilted, because you are looking in the direction of the tilt.)

One effect of the spacecraft’s “hot” seasons is the warming of the Dewar shell, which we have been reporting in the past two weekly updates. In order to maintain the cryogenic temperature and pressure conditions inside the Dewar requisite for superconductivity in the gyro rotors and SQUID readouts, the flow of helium boiling out of the Dewar increases as the Dewar shell warms up. In turn, the Attitude and Translation Control system (ATC) null-dumps (uniformly vents) this excess helium through the micro thruster system in order to maintain the spacecraft’s proper attitude and pointing direction.

Another effect of the “hot” season is a temperature rise in the Forward Equipment Enclosure (FEE) at the front end of the spacecraft. Among other things, the FEE includes the telescope and SQUID readout (SRE) control electronics. In the spacecraft’s “cold” seasons, the heat generated by these electronics boards is radiated to the outer wall of the FEE and then into space when the spacecraft cools off during the eclipsed portion of each orbit. However, in the “hot” seasons, when the sun is shining broadside on the spacecraft throughout each orbit, there is no eclipse period to cool off the FEE, so cooling of this equipment must be accomplished by increasing the set points of heaters in the FEE, which creates a temperature differential for radiating heat away from these important circuits. For the past two weeks, as the spacecraft has been entering a “hot” season, the team has been monitoring and adjusting these temperature controls in order to maintain the proper temperature in the FEE. As orbital eclipsing of the spacecraft returns towards the middle of December, the team will re-adjust these thermal controls for the oncoming “cold” season.

One nice side effect of the “hot” seasons is that the sunlight, shining broadside on the spacecraft, makes it more prominent to see from various locations on Earth in clear skies. The spacecraft is best viewed at times when it passes nearly overhead to your location, in the pre-dawn or evening twilight sky. A typical GP-B pass lasts about 15 minutes from horizon to horizon. From a viewing location that is devoid of city lights, you can see the GP-B spacecraft with your naked eye; in locations with more ambient light, binoculars may be helpful. See the links in our GP-B FAQ to view satellite tracking Web pages for computers and PDAs (Palm Pilot, Blackberry, and so on).


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 diagram of the GP-B experiment is from the GP-B Image Archives here at Stanford. The seasonal diagrams were created by GP-B Public Affairs Coordinator, 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.

Please Note: Until further notice, we intend to continue posting these GP-B highlights on a weekly basis. Also, from time to time, we may post special reports and special updates, as warranted by mission events.

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