Bulletin of the American Physical Society
2007 APS April Meeting
Volume 52, Number 3
Saturday–Tuesday, April 14–17, 2007; Jacksonville, Florida
Session H7: Gravity Probe B |
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Sponsoring Units: GGR Chair: Clifford Will, Washington University at St. Louis Room: Hyatt Regency Jacksonville Riverfront Grand 8 |
Sunday, April 15, 2007 8:30AM - 9:06AM |
H7.00001: The Gravity Probe B Science Instrument. Invited Speaker: The Gravity Probe B (GP-B) experiment employs a unique state-of-the-art science instrument to measure the geodetic and frame-dragging precessions predicted by Einstein's theory of general relativity for gyroscopes orbiting a massive spinning body, in this case the Earth. The GP-B instrument comprises four electrostatically suspended gyroscopes, each of which is independently subject to both the geodetic and frame-dragging precessions, and a telescope that tracks the guide star, IM Pegasi. Each gyroscope is read out with a dc SQUID system utilizing the London magnetic moment of the spinning gyroscope. The two axes of the telescope are read out with an image divider assembly, solid-state photo detectors and JFET preamplifiers. The telescope and gyroscopes are mechanically and thermally linked by a fused quartz block, which forms the metrology bench for the experiment. The instrument is located in a probe/helium dewar system, which provides a low-temperature environment of about 2.7 K for the instrument, as well as the ultra-low magnetic field, the ultrahigh magnetic shielding of the on-orbit ambient magnetic field, and the ultrahigh vacuum environments. The instrument was designed to allow a measurement of the geodetic and frame-dragging precessions to an accuracy of better than 0.5 mill-arc second/year for one year of science data collection. The instrument also provides the signals needed for drag-free and attitude control of the space vehicle. This presentation will include a description of the instrument and its principal on-orbit performance characteristics. Many persons at various institutions contributed to the development of the instrument. Numerous contributed presentations in a poster session will provide more detail. [Preview Abstract] |
Sunday, April 15, 2007 9:06AM - 9:42AM |
H7.00002: The Development Challenges of Gravity Probe-B -- an ongoing partnership between Physics and Engineering. Invited Speaker: Gravity Probe-B is very simple in concept: orbit four gyroscopes to measure the coordinate-frame deflections predicted by GR to milli-arc second accuracy. In reality it proved almost overwhelming. It took over 40 years to develop the Technology, and to engineer the complex payload which is intimately united with the spacecraft. This achievement was critically dependent on the partnership of Physics and Engineering at Stanford. Without this relationship, the mission would surely have failed. The late Professor Bill Fairbank described these challenges as the seven near-zeros of GP-B. The ``near zeros'' include magnetic field, atmospheric pressure, acceleration, and temperature. To be successful, the payload must meet all these requirements \textit{at the same time} for the duration of the 16 month experiment. This talk will describe some of these essential ``near zero'' technologies, and point out where both Physics and Engineering made vital contributions. Equally important, many of these developments are now being incorporated into other Fundamental Physics Experiments in Space. [Preview Abstract] |
Sunday, April 15, 2007 9:42AM - 10:18AM |
H7.00003: Gravity Probe B Data Analysis Challenges, Insights, and Results Invited Speaker: Telemetry data were collected from the Gravity Probe B satellite from its launch on April 20, 2004, through the depletion of the superfluid liquid helium on September 29, 2005. This interval included three distinct phases of the mission: the 123 day initialization phase, an eleven month science data collection phase, and a 45 day calibration phase. During the science data collection phase, the satellite rolled about the direction to the guide star, IM Pegasi, at a 77.5 second period. Data from the SQUID readout systems at the satellite roll rate were used to determine the orientation of each of the four gyroscope spin axes relative to the quartz block metrology reference frame. When the guide star, IM Pegasi, was not occulted by the earth, data from redundant readouts of the two axes of the cryogenic telescope were used to determine the orientation of the guide star relative to the quartz block. The SQUID and telescope data were combined to provide the orientation of the gyroscope spin axes relative to the apparent position of the guide star. The 5 arc sec optical aberration of the guide star due to the orbital velocity of the spacecraft was used to determine the gyroscope readout scale factor and to determine the orientation of the pickup loop relative to the measured roll phase of the satellite as provided by two star tracking telescopes. The eleven month data set was used to determine the rate of change of the gyroscope spin axes relative to the guide star. This talk will discuss the first results and sources of statistical and systematic error. [Preview Abstract] |
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