Pre-Phase A Report - Lisa - Nasa

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42 Chapter 2 Different Ways of Detecting Gravitational Waves 2.3 Pulsar timing Man-made gravitational wave detectors operate by detecting the effect of gravitational waves on the apparatus. It is also possible to detect gravitational waves by observing their effect on electromagnetic waves as they travel to us from astronomical objects. Such methods of detection are like “one-arm interferometers” – the second arm is not needed if there is another way to provide a reference clock stable enough to sense the changes in propagation time produced by gravitational waves. Pulsar timing makes use of the fact that the pulsar is a very steady clock. If we have a clock on the Earth that is as stable as the pulsar, then irregularities in the arrival times of pulses that are larger than expected from the two “clocks” can be attributed to external disturbances, and in particular possibly to gravitational waves. Since the physics near a pulsar is poorly known, it might be difficult to prove that observed irregularities are caused by gravitational waves. But where irregularities are absent, this provides an upper limit to the gravitational wave field. This is how such observations have been used so far. All pulsars slow down, and a few have shown systematic changes in the slowing down rate. Therefore, it is safer to use random irregularities in the pulsar rate as the detection criterion, rather than systematic changes. Such random irregularities set limits on random gravitational waves: the stochastic background. The arrival times of individual pulses from most pulsars can be very irregular. Pulsar periods are stable only when averaged over considerable times. The longer the averaging period, the smaller are the effects of this intrinsic irregularity. Therefore, pulsar timing is used to set limits on random gravitational waves whose period is of the same order as the total time the pulsar has been observed, from its discovery to the present epoch. Millisecond pulsars seem to be the most stable over these long periods, and a number of them are being used for these observations. The best limits come from the first discovered millisecond pulsar, PSR B1937+21. At a frequency of approximately 1/(10 yr) the pulsar sets an upper limit on the energy density of the gravitational wave background of ΩGW < 10−7 [71] (see Section 1.2.3). This is in an ultra-low frequency range that is 105 times lower than the LISA band and 10 10 times lower than the ground-based band. If one believes a theoretical prediction of the spectrum of a cosmic gravitational wave background, then one can extrapolate this limit to the other bands. But this may be naive, and it is probably wiser to regard observations in the higher-frequency bands as independent searches for a background. More-recently discovered millisecond pulsars are also being monitored and will soon allow these limits to be strengthened. If irregularities are seen in all of them at the same level, and if these are independent of the radio frequency used for the observations, then that will be strong evidence that gravitational waves are indeed responsible. These observations have the potential of being extended to higher frequencies by directly cross-correlating the data of two pulsars. In this way one might detect a correlated component caused by gravitational waves passing the Earth at the moment of reception of the radio signals from the two pulsars. Higher frequencies are accessible because the higher intrinsic timing noise is reduced by the cross-correlation. Again, seeing the effect in many pairs of pulsars independently of the radio frequency would be strong evidence for gravitational waves. 3-3-1999 9:33 Corrected version 2.08
2.4 Spacecraft tracking 43 2.4 Spacecraft tracking <strong>Pre</strong>cise, multi-frequency transponding of microwave signals from interplanetary probes, such as the ULYSSES, GALILEO and CASSINI spacecraft, can set upper limits on low-frequency gravitational waves. These appear as irregularities in the time-ofcommunication residuals after the orbit of the spacecraft has been fitted. The irregularities have a particular signature. Searches for gravitational waves have produced only upper limits so far, but this is not surprising: their sensitivity is far short of predicted wave amplitudes. This technique is inexpensive and well worth pursuing, but will be limited for the forseeable future by some combination of measurement noise, the stability of the frequency standards, and the uncorrected parts of the fluctuations in propagation delays due to the interplanetary plasma and the Earth’s atmosphere. Consequently, it is unlikely that this method will realise an rms strain sensitivity much better than 10 −17 , which is six orders of magnitude worse than that of a space-based interferometer. 2.5 Space interferometer The LISA measurement concept is, in essence, a space-borne implementation of a Michelson interferometer for the purpose of measuring the fluctuations in the distance between widely separated mirrors. There is, however, a fundamental distinction between LISA and the ground-based interferometers: LISA will search for the distinctively low-frequency (milli-hertz) gravitational waves (Chapter 1) which will probably never be detectable by any terrestrial detectors — existing or planned — because of unshieldable gravitational disturbances. These disturbances are due to the motion of bulk matter in the Earth and the atmospere which will pull gravitationally on the interferometer mirrors, producing undesirable phase shifts. Since gravity can not be shielded, and there does not seem to be a feasible way of independently measuring the gravitational effects of seismicity, these effects impose a strict lower limit on the gravitational wave frequencies observable on Earth. With its wide separation from Earth, LISA is completely free from these terrestrial disturbances. 2.6 Early concepts for a laser interferometer in space The earliest concept for a laser gravitational wave detector in space appears to have been a Shuttle-launched monolithic Gravity Wave Interferometer (GWI). R. Weiss was informed in 1974 about NASA studies of producing such a device with up to 1 km arm lengths by using an orbiting machine to extrude aluminium beams. A NASA publication in March 1978 [72] described an interferometer with a total launch mass of 16.4 t, which included four 1000 kg test masses at the ends of a cross-shaped device (see Figure 2.3). The GWI’s sensitivity was calculated as δl/l =10−21 in the frequency range from 10−1 to 102 Hz. The total cost of the project was estimated at that time to be $ 49.5 M. The idea of a monolithic space gravitational wave interferometer presented to Weiss started discussions in 1974 with P.L. Bender, R.W.P. Drever and J.E. Faller of a much larger space interferometer. The approach considered was to send laser beams between Corrected version 2.08 3-3-1999 9:33
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LISA Laser Interferometer Space Ant
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LISA Laser Interferometer Space Ant
Page 5 and 6: Foreword The first mission concept
Page 7 and 8: The result of the Team-X study was
Page 9 and 10: Contents Foreword iii Executive Sum
Page 11 and 12: Contents ix 4.2.3 Acceleration nois
Page 13 and 14: Contents xi 9 Science and Mission O
Page 15 and 16: Executive Summary 1 Executive Summa
Page 17 and 18: Executive Summary 3 Complementarity
Page 19 and 20: Mission Summary 5 LISA Mission Summ
Page 21 and 22: Chapter 1 Scientific Objectives By
Page 23 and 24: 1.1 Theory of gravitational radiati
Page 33 and 34: 1.2 Low-frequency sources of gravit
Page 51 and 52: Chapter 2 Different Ways of Detecti
Page 53 and 54: 2.2 Ground-based detectors 39 2.2 G
Page 55: 2.2 Ground-based detectors 41 Count
Page 59 and 60: 2.7 Heliocentric versus geocentric
Page 61 and 62: 2.8 The LISA concept 47 telescopes
Page 63 and 64: 2.8 The LISA concept 49 Figure 2.6
Page 65 and 66: 2.8 The LISA concept 51 seem rather
Page 67 and 68: Chapter 3 Experiment Description 3.
Page 69 and 70: 3.1 The interferometer 55 Fiber to
Page 71 and 72: 3.1 The interferometer 57 3.1.4 Sys
Page 73 and 74: 3.1 The interferometer 59 Figure 3.
Page 75 and 76: 3.1 The interferometer 61 3.1.6 Las
Page 77 and 78: 3.1 The interferometer 63 signal sh
Page 79 and 80: 3.1 The interferometer 65 for a sin
Page 81 and 82: 3.2 The inertial sensor 67 design (
Page 83 and 84: 3.2 The inertial sensor 69 and the
Page 85 and 86: 3.2 The inertial sensor 71 Figure 3
Page 87 and 88: 3.2 The inertial sensor 73 sufficie
Page 89 and 90: 3.3 Drag-free/attitude control syst
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Page 93 and 94: Chapter 4 Measurement Sensitivity 4
Page 95 and 96: 4.1 Sensitivity 81 averaged transfe
Page 97 and 98: 4.2 Noises and error sources 83 The
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4.2 Noises and error sources 93 ele
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4.2 Noises and error sources 95 is
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4.2 Noises and error sources 97 and
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4.2 Noises and error sources 99 Eq.
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4.3 Signal extraction 101 here that
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4.3 Signal extraction 103 phase loc
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4.4 Data analysis 105 • LISA obse
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4.4 Data analysis 107 for ground-ba
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4.4 Data analysis 109 that are defi
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4.4 Data analysis 111 with the phas
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4.4 Data analysis 113 density Sn(f)
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4.4 Data analysis 115 box (in stera
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4.4 Data analysis 117 LISA’s dist
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Chapter 5 Payload Design 5.1 Payloa
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5.2 Payload structural components 1
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5.2 Payload structural components 1
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5.4 Mass estimates 125 5.4 Mass est
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5.7 Thermal analysis 127 the Y-shap
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5.8 Telescope assembly 129 surface
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5.9 Payload processor and data inte
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5.9 Payload processor and data inte
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Chapter 6 Mission Analysis 6.1 Orbi
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6.3 Injection into final orbits 137
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6.4 Orbit configuration stability 1
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6.5 Orbit determination and trackin
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Chapter 7 Spacecraft Design 7.1 Sys
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7.1 System configuration 145 2220 m
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7.1 System configuration 147 7.1.4
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7.2 Spacecraft subsystem design 149
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7.3 Micronewton ion thrusters 151 b
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7.3 Micronewton ion thrusters 153 F
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7.3 Micronewton ion thrusters 155 (
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7.3 Micronewton ion thrusters 157 F
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7.4 Mass and power budgets 159 The
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Chapter 8 Technology Demonstration
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8.1 ELITE - European LISA Technolog
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8.3 ELITE Technologies 165 8.2.2 Co
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8.4 ELITE Satellite design 167 inco
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Chapter 9 Science and Mission Opera
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9.2 Mission operations 171 9.2 Miss
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9.3 Operating modes 173 Power savin
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Chapter 10 International Collaborat
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10.3 Schedule 177 The ESA Project M
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References [1] C.W. Misner, K.S. Th
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References 181 [35] C. Cutler, T.A.
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References 183 [77] J.E. Faller and
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References 185 [104] E. Grun, H.A.
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List of Acronyms - and other abbrev
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List of Acronyms 189 LHC Large Hadr
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Rear cover figure : This Report has
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