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5 Expected Research Prior to 1995 SHUTTLE TEST AND FLIGHT OF GRAVITY PROBE B (GPB) Gravity Probe B (GPB) is a gyro test of genera] relativity made with four cryogenic gyroscopes whose spin axes are compared with the position of a star to 10-3 see of arc in a zero-g satellite for a period of at least one year (see Figure 5.1a). For the first time the experiment tests the dragging of inertia] frames (magnetic gravity) in general relativity due to the rotating Earth and the geodetic precession due to the motion of the gyros around the Earth (see Figure 5.1b). It is planned that the experimental package will be tested on a Shuttle flight in 1989, followed by a free flyer zero-g experiment to perform the actual relativity experiment in 1991 or 1992. The Shuttle test is an important step in the GPB program. It will test the completely integrated package consisting of the instrument, dewar, and electronics. The Shuttle test will provide some information on gyro drift at reduced g but will not be able to establish the low drift rates of the gyros required to carry out the actual experiment. 30
31 LEAD BAG MU -MET.A L SH IELD QUARTZ_~ RI t1CK PODS~ 4 GYROSCOPESJ SPI N UP H ELI UM r T AN K ~ <~k,4/'' ~DEWAR WPROBt / KW I N DOWS ( 3) \ ,,,/~ - ~ ,~W ',,,;t >~~~~/~ ., ~ / r ~>~ 1 ~ ~ f':~ Ji~ 7~li~ Ji;>,/' .'' 1 '~ ~/ / /~ DRAG FREE J / LTELESCOPE PROOF MASS LSUPERFLUID HELIUM TAN K l NORMAL LIQUID HELI UM TAN K PROBE NECK WPROBE TUBE VALVE FIC:URE 5.1a Gra~rity Probe B (GPB) experiment module. SOURCE: Reprinted from D. Bardas et al., aHardware Development for Gravity Probe B,~ in Procec~nge of SPIE, thc Intcrn~ond Socict~for Optied Enginecrin,, volume 619, page 33, 1986. SHUTT[E F[IGHT OF ~ CRYOGENIC PRINCIPLE OF EQUIVALENCE EXPElilMENT . The principle of equivalence requires that the ratio of the ~ner- tial to gravitational ma" of all bodies be the same. An exper~ment done in the mnd-1960s has establ~hed that this ratio is the same for gold and alum~num to one part in 10~. New experunents to set better lim~ts are in progre" at Joint Institute for Laboratory Astrophysics (~LA) and at Stanford University. The Stanford experunent is being designed to be placed in space. A superconducting equivalence experunent has been designed with the potential of testing the equivalence of gravitational and inertial mass to a projected sensitivity of one part in 10~5 on a Shuttle flight and one part in 10~8 on a zero-g free flyer. At
32 me= 6.6 sec/yr (GEODETIC) RIGEL 'at / ''_ ~ ~ · ~ /~\ AB-.042 sec/yr (MOTIONAL) it ~ ~~: ~ ~ FIGURE 5.1b Gyro experiment orbit. Relativistic effects as seen in gy- roscope with spin vector oriented as shown and lying parallel to the line of sight to Rigel. SOURCE: Reprinted from W. Fairbank, Near Zero: New Fro not of Physics, W. H. Freeman, New York, 1987. t this level the experunent tests with an improvement of 3 and 6 orders of magnitucle, respectively, the foundation on which the geo- metr~zation of space-time in Einstein's general theory of relativity is based. A violation at any level would pose a problem for general relativity. A new long-range force coupled to baryon number or further peculiarities in the weak interactions are posited sources of such a violation. Conceptually, the experiment ~ similar to Galileo's purported experiment of dropping different weights from the Leaning Tower of Pisa, except that instead of falling a few tens of meters, the objects fall all the way around the Earth. Two concentric test masses, a solid rod and a hollow cylinder, are constrained by mag- netic bearings so that they are free to move only along a common axis (see Figure 5.2~. As they orbit the Earth with fixed orien- tation in inertial space, they are subjected simultaneously to the centrifugal acceleration of the orbital motion and the gravitational attraction of the Earth. If the ratios of gravitational to inertial
33 SE NSE COI LS ~ INNER MASSE OUTER MASS SUPPORT STRUCTURES/ 1t / ~ ~~ /: ALIGNMENTS SCREW ,~\ ,SHIELD _~SQUID ~ I ~ -A ~ ~ ~~ I D ~ I Ah/ it' '/ / ~RlJPERCONDUCTING BEARINGS LEVELING CAM FIGURE 5.2 Equivalence principle accelerometer; cryogenic Eolvos experi- ment. SOURCE: Reprinted from C.W.F. Everitt and Paul W. Warden, Jr., A Prcli~unarf,' Study of a Cryogenic Equivalenec Pnncipic E::pcnmcnt on Shells, page 20, 1985. mass of the test bodies are different, there vnI] be a periodic differ- ential acceleration between them, which will show up as a relative displacement at orbital period along their axm. On the Shuttle the experiment is limited to one part in 10~5 by the changing gravitational gradients due to motion of mass on the Shuttle. On a specially designed drag-free satellite the potential accuracy of the experiment is one part in 10~. One feature of the experunent is that the perturbing ejects due to gravitational field gradients can be reduced to negligible levels by making the center of mass of the two test masses coincident. This ~ done by sensing the differential displacement signals at twice the orbital frequency and using the signature in a null servo system. MICROWAVE RANGING TO THE MARS OBSERVER SPACECRAFT The planned orbit for the Mars Observer spacecraft is sun- synchronous and nearly circular, with degree inclination and 361-km altitude. The telecommunications system anti permit
34 X-band Doppler measurements, but no ranging signals or dual- frequency capability is planned. Thus, unless at least ranging signals are added, no information useful for gravitational physics can be obtained. With dual-frequency capability and ranging signals added, the Mars Observer mission could improve on present knowledge of so- lar system dynamics. The first requirement would be to determine the gravity field for Mars accurately so that the Earth-spacecraft distance could be converted to the Earth-Mars center-of-mass sep- aration. Despite the low altitude, and the corresponding high- degree and high-order gravity field solution required, valuable results could be obtained if the range and Doppler measurements were sufficiently accurate. There also may be an opportunity for accurate lower degree and lower order gravity field solutions and thus improved Earth-Mars distance determinations when the spacecraft orbital altitude is raised at the end of the planetary observation period. This would be possible only if the operation of the spacecraft altitude control system and tracking system could be continued. New Earth-Mars distance measurements starting in 1991 with the arrival of the Observer spacecraft at Mars could be combined with the Viking Lander tracking data during the period 1976 to 1982 to make the duration of the observations 3 times longer. This would result in better knowledge of the orbits of Mars and Earth, as well as improved masses and densities for a number of the asteroids that significantly perturb the motion of Mars. For testing gravitational theory, the accuracy of the precession of perihelion for Mars and the limit on the possible rate of change of the gravitational constant would be improved significantly. The greatly improved gravity field for Mars also would be of high value for planetary studies. However, in view of the present lack of plans for ranging to the Mars Observer spacecraft, we cannot expect information of the kind d~cumed above to be obtained during the next decade. The task group does not know when another comparable opportunity may occur. X-1lAY TIMING EXPERIMENT AND A MEDIUM-AREA FAST X-RAY DETECTOR ON THE S1IUTTLE X-ray astronomy is closely linked to gravitational physics be- cause of the fact that most bright x-ray sources are identified either
35 with compact objects such as neutron stars, black holes, and active galactic nuclei or tenth large clusters of galaxies in which missing mass influences the spatial extent and temperature of the diffuse x-ray emitting gas. X-ray emission in compact objects is produced deep in the potential wells near or on the surfaces of the neutron stars and near the Sch~varzschild radius in the black holes. Effects such as gravitational radiation, the stability of orbits near the Schwarzechild radius, gravitational red shifting, and gravitational tensing become important to understanding how x-ray emission Is produced, and the x-ray observations can be utilized to test basic physical principles. The x-ray sources available for such purposes are very bright. Advanced exper~rnents can be undertaken with this abundant flux by building instruments with appropriate performance character- istics. X-ray sources have other desirable properties; for example, the spherical neutron stars should be gravitational lenses of high optical quality, and their symmetry simplifies tensing calculations. Black holes are thought to be even more perfectly symmetrical than neutron stars, with the metric near such objects being well- specified when given only the mass, angular momentum, and net charge. Accreting material probes that metric. In general, tim- ing and spectral measurements are expected to provide the best probes, provided that there is sufficient sensitivity to observe at the dynarn~cal time scales of the sourcesoften milliseconds or shorter. The use of secreting neutron stars to study gravitational radi- ation instabilities appears to be a field particularly ripe for experi- mental development. Several theoretical lines of argument lead to the expectation that neutron stars secreting from binary compan- ions should be able to spin up to angular frequencies in which they are subject to relativistic instabilities, with the angular momen- tum supplied from the accreting disk continually radiated away in gravitational waves. Such objects would be pulsars in both gravi- tational waves and x-rays. Continuous-wave signals in both types of radiation would be precisely phase-Iocked with one another. Detection of the x-ray signal would permit a gravity wave antenna to be constructed so as to be optimally tuned for direct detection of the gravity wave flux. It has become apparent that many extremely fast processes occur in compact x-ray sources, but progress in understanding has been limited by data quality. Larger x-ray detector collecting areas
36 and the capability to deal with very high data rates are needed in order to make timing observations of sufficient detail to advance our understanding of the physics of compact, highly relativistic objects. A desirable program to unplement before 1995 would include the Shuttle flight of a proportional counter array of a few square meters of collecting area with thin windows to allow detection of x-rays from 0.25 keV to 50 keV. The data bit rate may be as large as 10 Mbps. Later, on the Space Station, a possible exper- iment would entail the construction of a large (100 m2 effective collecting area) x-ray detector array devoted to fast-timing and tune-resolved broadband spectral studies. The Shuttle instrument development can be used to engineer a proportional counter mod- ule that could be replicated inexpensively for the large array on the Space Station. A Shuttle instrument for timing observations of the brightest compact x-ray sources offers the exciting possibility of discovering new phenomena in Secreting binary systems containing a neutron star. For example, there are strong reasons to believe that in such a system a neutron star with a weak magnetic field can be spun up by accretion to periods so short that the star is subject to general relativistic instabilities. The nonax~symmetry resulting from this instability can produce coherent gravitational waves and modulation of the x-ray flux generated by accretion. If even one instance of this phenomenon is found, it will have profound conse- quences for the gravity wave detection effort. A detection in x-rays wavelengths would allow a coincident, phase-sensitive search with ground-based gravity wave detectors now under development. A Shuttle flight of a medium-area instrument would also pro- vide a much improved capability to study rn~second x-ray burst activity from the inner part of the accretion Ask around massive objects such as black holes. These bursts provide a probe of the metric very near the innermost stable orbit of a black hole. SPACECRAFT OBSERVATIONS OF [ONG-PERIOD GRAVITATIONAL WAVES The gravitational wave spectrum at periods from a few hours to several minutes may be explored by observing the motions of interplanetary spacecraft. The technique Is most sensitive to grav- itational wave bursts, with periods shorter than the propagation
37 time between the Earth and the spacecraft. The gravitational wave burst is seen in the Doppler data as a dual pube with a time difference determined by when the gravitational wave hits the Earth and the spacecraft. The technique is not limited to bursts and could be used in a search for periodic sources. With several spacecraft operating simultaneously, it is possible to carry out co- incident searches as well as to observe a stochastic background of gravitational radiation. Searches for long-period gravitational waves will be made by the Galileo and Ulysses missions. The launch dates for these m~s- sions are now in doubt, but simultaneous observations will still be carried out when possible. The root-mean-square strain sensitiv- ity In these searches is anticipated to be 3 x 1o-~5, with the noise budget determined almost equally by uncertainties in the electron- ics, the transmission by the troposphere, and the fluctuations in the interplanetary plasma. In order to measure the fluctuations in the column density of the interplanetary plasma, dual-frequency transmission at both S- and X-band from the spacecraft to the Earth is incorporated in both Galileo and Ulysses. Galileo has the option of S- or X-band transmission from the Earth to the space- craft, while Ulysses can use only S-band. Similar experiments have been proposed for the Mars Observer mission and the Comet Rendezvous Asteroid Flyby (CRAP) using X-band transmission. The sensitivity of this type of search can be improved with modest effort on several fronts. The Deep Space Net has a stated goal of unproving frequency standards, transponders, and trans- mission technology to make use of frequency stabilities of 10-~7 in periods of thousands of seconds. Should this be implemented, the other noise sources will dorn~nate. The noise from phase fluctua- tions due to the interplanetary plasma could be further reduced by simultaneous two-band transmission in both uplinks and down- links. It is estimated that dual-frequency S- and X-band in both links could reduce the plasma noise by at least a factor of 10 in the antisolar direction. Larger improvements would be made if K-band is used. The tropospheric contribution to the phase noise could be reduced substantially by measurement of the column density of water vapor along the receiving antenna beam. Given the very few opportunities to carry out interplanetary missions in the U.S. space program, it seems prudent to make as many of these improvements as possible and to take every opportunity to continue the search for long-period waves by this technique. The incremental costs on a mission to carry out this research appear small.
