Fundamental Physics and Chemistry Space Science in the Twenty-First Century -- Imperatives for the Decades 1995 to 2015 (1988) / Chapter Skim
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6. Programs After 1995
6. Programs After 1995
Pages 38-54
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From page 38... ...
With major efforts at ~solating the antennas from ground noise and avoiding t~rne-varying gravity gradient effects, high sensitivity can be achieved down to 10 Hz or possibly somewhat below. For frequencies of 3 Hz and below, antennas In space seem essential to achieve sufficiently high sensitivity.
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From page 39... ...
For each type of binary the number of sources in our galaxy Is very large, so that even several years of observations would leave many sources contributing in each frequency resolution bin with similar signal strengths. Only a relatively small number of unusually nearby sources would give signals higher enough than the continuum level to be individually distinguishable.
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From page 40... ...
More exotic sources of Iow-frequency gravitational radiation such as phase transitions in the early universe also have been suggested. Despite our current limited knowledge concerning sources of low-frequency gravitational waves, it appears important scientifically to carry out a reconnaissance mission in space at an early date to search for gravitational waves over as broad a frequency range as possible in the region below 1 Hz.
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From page 41... ...
41 ,~ at" \ 106km ~ E ND SPACECRAfT /' / In ~~ 106km ~1 L ~ CENTRaL SPACECRaFT FIGURE 6.1 Laser heterodyne gravitational wave antenna. SOURCE: Courtesy of University of Colorado.
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From page 42... ...
Separate curves for the noise level are shown with the close white dwarf binaries (CWDBs) included or excluded, since the amplitude of their contribution is quite uncertain.
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From page 43... ...
FIGURE 6.2 Strain amplitude for gravitational wave pulses. SOURCE: Courtesy of University of Colorado.
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From page 44... ...
This approach would then be analogous to that being used in ground-based laser gravitational wave experiments, except for having a much lower frequency range. The main disadvantage is that any perturbations of the test masses on which the rn~rrors are mounted will cause proportionately larger fractional changes in the interferometer arm lengths than for the larger spacecraft separations.
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From page 45... ...
The technical challenge of the instrument Is to maintain dimensional stability, especially the angle between the optic axes, against the inevitable thermal drifts and changing gravity gradients over the integration time. Absolute stability is not required, nor possible.
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From page 46... ...
46 \\~? ~V: ~( ~~/: \ 1,,,, I I I ~ I 1 0.0 0.5 1.0 meters FIGURE 6.3a POINTS: Optical stellar interferometer.
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From page 48... ...
48 circular polar orbit at 250~km altitude, dual-frequency Doppler tracking with 5 x 10-~5 accuracy can determine the gravity field well enough that the Earth-satellite distance can be converted reliably to the desired Earth-Mercury distance. Phase modulation of the signals at frequencies of up to 50 MHz also is needed in order to measure the range to the satellite with 3-cm accuracy.
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From page 49... ...
The hydrogen maser flown on Gravity Probe A in 1976, which performed a gravitational red shift experiment in the field of the Earth to one part in 104, had a long-term stability of 2 x 10-~5 in an averaging period of about 3 h. The clocks have been unproved since then, exhibiting a stability better than ~ x 10-~5 in an averaging time of one day.
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From page 50... ...
High-Precision Prmciple of Equivalence Experiment The principle of equivalence experiment can be performed to a level of one part in 10~8 in a surround that minimizes the gravity gradient problem and produces a ~zero-g" environment to the order of 10-~2 g. One possible satellite in which to perform this experiment would be the refurbished Gravity Probe B (GPB)
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From page 51... ...
ENHANCED PROGRAM Easer Gravitational Wave Observatory After an initial reconnaissance mission to observe the general nature of astronomical sources of gravitational radiation, an observatory in space capable of much more detailed studies will probably be needed. Whether such a laser gravitational wave oh servatory should be planned during the period 1995 to 2015 clearly will depend on the results obtained from the reconnaissance mid sign.
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From page 53... ...
Such high sensitivity would greatly enhance the opportunities for detailed studies of gravitational wave pulse shapes, as well as for Recoveries of completely unexpected phenomena. Redight of GPB With improved superconducting quantum interference devices
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From page 54... ...
With such a system a refight of the gyroscope could make absolute measurements of several star positions in our galaxy at the two ends of the solar orbit to an accuracy of 10-4 see of arc. This would provide astrometry data on the distance and proper motion of these stars based on absolute gyroscope measurements rather than on comparison of several stars.
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