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6 Programs After 1995 BASELINE PROGRAM LAGOS: Baser Gravitational Wave Obeenatione m Space Important opportunities for valuable new types of scientific observations in space exist in the area of gravitational wave astron- omy. GrouncI-based programs for observing gravitational radiation are being pursued actively in a number of countries, as described in Chapter 3. Given proper support, the prospects appear good for detecting signals that give information about several kinds of sources within the next 5 or 6 years. However, the frequency range that can be covered relatively easily in ground-based observations is only from roughly 100 Hz to 10 kHz. With major efforts at ~so- lating 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. The types of low-frequency gravitational wave signals that are likely to be observable fall into several classes. The only one that is certain to be observed is radiation from various kinds of binary star systems. Sources of this kind include normal main sequence 38
39 binary stars, contact binaries, cataclysmic variables, neutron star binaries, and close white dwarf binaries. The radiation level ex- pected for the last type is not well known because of uncertainties in the initial distribution of masses and separations. 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. Thus the observations would be limited by confusion, since types of antennas suggested so far have rather poor angular resolution. A very important class of signals that could be present, but about which the theoretical predictions are highly uncertain, is pulses of gravitational radiation resulting from the formation or collisions of very massive (102 to 105 MC)) or supermassive (>105M<3) black holes. If stars with masses greater than a few hundred Ma form at any time, they are expected to evolve quite rapidly and may well collapse to form black holes. The efficiency of gravita- tional racliation emission during collapse for some levels of initial rotation has been estimated to be roughly 0.1 percent. However, whether stars with such high masses ever form is a major question. Although a Population II] of massive stars is often suggested to have formed early In the history of the galaxies and to have pro- duced the initial heavy element abundance, there is no evidence for current formation of stars more massive than perhaps 150A,, even in regions of rapid star formation. Whether conditions were favorable for the formation of much more massive stars in early galactic times is not known. Another possible scenario for the formation of very massive or supermassive black holes involves pregalactic density inhomo- geneities. The probable scale for such inhomogeneities is roughly 105 to 106 M<3. However, it is not clear whether objects in this mass range that initially were very dilute would fragment, form normal stars, or collapse. Lignite on the density of black holes with masses of 102 to 107M<' in galaxies currently are about 1 percent of the closure density. The absence of many such objects in the cores of normal galaxies is not an argument against their existence, since the time for them to spiral into the center due to viscous drag is very long.
40 The other frequently mentioned possible source for the emis- sion of strong gravitational wave pulses with frequencies below 1 Hz is collisions between very massive or supermassive black holes. If two such objects approach each other with substantial angular momentum, calculations indicate that the efficiency for gravita- tional wave emission is likely to be fairly high. Whether such collisions are likely to occur even as frequently as a few tunes per galaxy lifetime ~ not clear. However, the chances for such collisions might be substantial if rotating supermassive objects ex- isted in pregalactic or early galactic times. Then bifurcation could occur because of the bar instability, followed by collapse of each mass, and a spiraling into a finli collision. If as much as 10-4 of the mass of the galaxies was put into very massive or super- massive black hole binaries with relatively close separations, the chance of detecting them by their gravitational radiation seems good. Such evidence Knight include fairly strong periodic signals with a drift toward higher frequencies as the black hole binaries spiral in toward each other. As ~ emphasized above, the probability of being able to oW serve pulses of gravitational radiation due to the formation or in- teraction of very massive or supermassive black holes ~ not known at present. If observable, the shapes of such pulses might well be able to give new information about gravitational interactions at distances of the order of the Schwarzachild radius, where general relativity has nearer been tested before. The absence of such pulses would make it seem unlikely that large numbers of high-mass black holes played a substantial role in the pregalactic or early galactic history of the universe. More exotic sources of Iow-frequency grav- itational 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. The basic approach for laser gravitational wave observations in space is the measurement of changes in the distances between three separate spacecraft with extremely high accuracy. For the baseline case that has been studied over the last few years, the three spacecraft are placed in similar one-year-period nearly circular orbits around the Sun with separations of about 106 km. The central spacecraft is in the plane of the ecliptic and has very
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. small eccentricity. The other two "remote" spacecraft have almost equal eccentricities of e = 0.003 and inclinations of ~ e, but different longitudes for their nodes and perihelia. By choosing the differences in node and perihelion correctly, the two remote spacecraft always wall be very close to 90 degrees apart, as seen from the central spacecraft. For these orbits, the distances from the central spacecraft to the two remote spacecraft can be kept equal to about one part in 103. With this geometry, a beam-splitter and two end murrors can be mounted in free-floating test masses inside the three space- craft to form a Michelson interferometer. As shown in Figure 6.1, a continuous wave (cw) laser beam is sent to the beam-splitter in the central spacecraft, and then the two resulting beams are transmitted through a pair of 5~cm-diameter telescopes to the two remote spacecraft. There the beams are received by sunilar telescopes and sent to the end rn~rrors. The return beams are generated by similar lasers in the two remote spacecraft, which are phase-Iocked to the received beam. At the central spacecraft,
42 the phases of the beats between the return beams and the central laser are measured as a function of time. The antenna ~ sensitive to gravitational waves, since one arm of the interferometer will increase in length and the other will decrease for favorable directions of propagation and polarization of the waves. For 100 mW of transmitted laser power, the expected shot noise limit for the detectable gravitational wave amplitude ~ 10-2°/~. It appears likely that this sensitivity can be achieved for gravitational wave frequencies ranging from 0.1 Hz to about 10-4 Hz. For a 104-s integration time, the gravitational wave sensitivity would be 10-22. Useful but lower sensitivity would be achievable over the rest of the frequency range from 3 Hz to 10-6 Hz. Thus, a 106-km LAGOS antenna would have extremely high sensitivity for detecting and observing astronomical sources of gravitational waves with periods of roughly 0.3 s to 10 days. The sensitivity of the antenna for short pubes of gravitational radiation is shown in Figure 6.2, along with the expected noise level due to random variations in the power from binaries in our galaxy. 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. Also shown are the signal strengths expected for short gravitational wave pulses if very massive or supermassive stars collapse to form black holes at red shifts of z = 2.5 or z = 30. It is assumed that the efficiency is 0.1 percent, the Hubble constant is 55 km/s/Mpc, and the expansion parameter q(0) = 1/2. The interferometer sensitivity will be degraded from the curve shown by a factor In(T/r), where T is the time between pulses near that frequency, and ~ us the pulse length. If frequent pulses are generated at z = 30, it is clear from the figure that they would be observable for initial masses greater than about 104 A(,, provided that the efficiency is 0.1 percent and the noise from CWDBs is somewhat lower than estimated. For z = 2.5, the initial masses would have to be greater than about 3 x 104 MA Laser stability requirements for LAGOS are not as severe as might be expected at first. Although each arm of the ~nterferome- ter changes its length at a rate of up to about 20 cm/s due to the solar attraction and planetary perturbations, there are no solar system sources of disturbing gravitational forces with periods in the range of 1 s to 10 days that would not be known accurately from their effects at longer periods. Thus, apparent changes in
43 15 -16 1 -17 -18 - ~n ' -19 o -20 -21 -22 _ -23 Pulse Nolse Limit Due to Galactic Blnarles \ '_~\ Close White Owart Blnarles excluded Massive Black Hole Formation E (efflclency) = 0.1%, qO (acceleration parameter) = 0.5 Z=2.5 7 \ Z=30 \ ~ ~10 Me / \ ~310 Mo \ ~ ~ W ~ it.... ~~ - \~. 1704M~ '~ 3 103Me \` Laser Interferometer Pulse Sensitivity <106km, 100 mW , 1 1 1 1 1 1 1 -6 -5 -4 -3 -2 -1 0 LOG FREQUENCY (Hz) FIGURE 6.2 Strain amplitude for gravitational wave pulses. SOURCE: Courtesy of University of Colorado. the length of one arm in the period range of interest can be used to correct for changes in the laser frequency. The difference in length of the two arms, after this correction, ~ much less sensitive to laser frequency variations. Preliminary studies indicate that a laser stability of 1 x 10-~3 from 1- to As period, and less stability below and above this range, would be sufficient to achieve the full gravitational wave sensitivity. It appears that the desired stability can be achieved with laser-diode-pumped Nd:YAG lasers, which are being investigated for other space applications requiring high efficiency, long lifetime, and high reliability.
44 The possibility of locating the LAGOS antenna in geosyn- chronous orbit instead of in solar orbit also should be considered. For 90 degree separations between the three spacecraft, the in- terferometer arm length is reduced to 60,000 km. The sensitivity of the antenna would be unproved at frequencies above 0.1 Hz and made worse at frequencies below 3 x 10-4 Hz because of the shorter arm length. However, changes in thermal gradients inside the spacecraft probably would generate large spurious signals at a number of harmonics of 1 cycle/day. The effect of orbit pertur- bations due to harmonics in the Earth's gravity field would have to be considered, and tune variations in the mass distribution of the Earth might cause appreciable perturbations. Whether there would be substantial savings in propulsion requirements by placing the antenna in geosynchronous orbit has yet to be investigated. A third possibility is to reduce the spacecraft separation to about 300 km, and to reflect the light from the remote spacecraft directly back to the central spacecraft instead of using phase-Iocked lasers. For this separation, many bounces of the light back and forth in each arm could be used with the Fabry-Perot type of inter- ferometer. 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 sep- arations. This is expected to be a problem mainly for frequencies below about 2 x 10-3 Hz. However, much unproved sensitivity can be achieved from higher frequencies. For 100 bounces in each arm, about 100 times higher sensitivity can be expected if the short node limut can be achieved. Such a multipass antenna might be desirable if it is decided to focus attention on frequencies above roughly 2 x 10-3 Hz. This antenna could be located either in solar orbit or in geosynchronous orbit. POINTS: Smut Astrometric Interferometer m Space The proposed instrument ~ designed to measure relative an- gular positions of stars to microarcsecond accuracy and would be able to detertn~ne the bending of starlight by the Sun to the second order in the gravitational field strength. The first-order deflection
45 is 1.7 arcsec at the solar limb and varies inversely with the dim tance from the limb. The second-order term is expected to be 11 microarcsec, falling inversely as the square of the distance. The astrometric precision would find application in many branches of astronomy, particularly in a search for planetary systems by mea- suring the motions of the star about the center of mass of the planetary system. The instrument concept is a pair of Michelson interferometers of 2-m baseline employing 25-cm-diameter telescopes (see Figure 6.3a). The two interferometers are mounted so their optic axes are approximately 90 degrees apart on the sky (see Figure 6.3b?. The nominal 90 degree separation angle is chosen to max~nize the number of stars that can be referenced to a given star in establishing an astrometric grid on the sky. The relative position measurement ~ carried out by executing small changes in the angle between the optic axes. The instrument should be capable of cleterm~ning the relative position of a tenth magnitude star to an accuracy of 5 microarcsec in an integration time of 10 An. 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. The consistent solution of multiply measured angular differences in the astrometric grid can be used to correct for long-term drifts in the instrument. An internal metrology system using frequency- stabil~zed lasers and fiducial surfaces would be used to determine the optical delay in the interferometers and the angle between the optic axes. In its present conceptual design the interferometer would take up about one-third of a Shuttle bay. Useful scientific information could be gained by observing from the Shuttle; however, the longer exposure and more stable pointing provided on a platform or free flyer could be used to advantage by this instrument. Mercury Relativity Satellite Major improvements in solar system tests of general relativity would be made possible by accurate tracking of a small relativity satellite in a nearly circular orbit around Mercury. One of the primary benefits would be an improvement by 2 to 3 orders of
46 \\~? ~V: ~( ~~/: \ - 1,,,, I I I ~ I 1 0.0 0.5 1.0 meters FIGURE 6.3a POINTS: Optical stellar interferometer. SOURCE: Courtesy of Robert D. Reasenberg. magnitude in our knowledge of whether the gravitational interac- tion constant G is changing with time. Mach's principle suggests that local inertial properties can be influenced by distant matter in the universe. If so, it could be expected that the effective value of G would decrease as the universe expands. This would cause a quadratic variation with time in the angular position of each planet in its orbit.
47 GO A? FROM RIGHT TELESCOPE i, FROM LEFT TELESCOPE \ i) FIGURE 6.3b POINTS: Optical design utilizing double-plate beamsplitter. SOURCE: Courtesy of Robert D. Reasenberg. Tracking of a relativity satellite also would peanut improve- ments in tests of two other relativistic effects. One is the precession of perihelion for Mercury. This effect is an important test of the extent to which gravitational interaction energy ~ itself a source of curvature in space-time, as measured by the PPN parameter ,8. The other test would be an improved measurement of the rela- tivistic time delay for electromagnetic signals traveling along paths near the Sun. The extra time delay depends on the PPN parameter 7, which also determines the deflection of star light or radio waves by the Sun. In addition to these effects, the tracking data would peanut an independent measurement of the solar quadrupole mo- ment 32, which depends on the internal rotation rate of the Sun. And finally, the data would permit a very accurate determination of the gravity field of Mercury up to roughly degree and order 10. This is of great interest to planetary scientists. For relativity tests, it is necessary to know the spacecraft orbit well enough that the distance from the Earth to the center of mass of Mercury can be determined accurately. For a satellite in a
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. The resulting overall Earth-Mercury distance uncertainty would be approximately 10 cm. Tracking data are needed for about 3 periods per week of ~ h each over a minion lifetime of 2 to 8 years. The expected ma" requirement for a suitable relativity satellite using low-power X-band and K-band tracking is 50 kg. Recently, a new class of ballistic trajectories has been d~scov- ered that would permit sending substantial scientific payloads to Mercury. The trajectories involve swingbys of both Venus and Mercury in order to reduce the final approach velocity. A par- ticularly favorable launch opportunity exists in 1994. The long flight time involved would delay the arrival at Mercury until the year 1998. However, the payload would be large enough that it appears feasible to include a number of separate spacecraft, to be placed in separate orbits. These could include the following: a main spacecraft In a low circular orbit for planetary surface stud- ies; the small relativity satellite; a particles and fields satellite in an elliptical orbit; and possibly one or two small landers. The task group recommends the inclusion of a smai} relativity satellite on such a mission, or on any other mission that could put the satellite in a suitable orbit around Mercury. It also is possible to test relativity by using tracking signals from a lander on the surface of Mercury, provided that accurate tw~frequency tracking data and long active mission duration can be obtained. This would have the advantage of eliminating the need for determining the gravity field of Mercury. However, the eEects of systematic data loss because of the Today rotation period have not yet been investigated. Tradeoffs between the lander approach and a small relativity satellite should be considered if it is possible to include a lander on a Mercury mission. For a 2-year nominal mission lifetime with the small relativity satellite, the expected accuracy for testing the possible change in the gravitational constant is 1 x 1o-~3 per year. The accuracy for the precession of perihelion and for 32 are 1 x 10-4 and 3 x 10-8, respectively. For an 8-year extended mission lifetime, the corresponding accuracies would be 1 x 10-~4, 3 x 10-5, and 1
49 x 10-8 per year. The expected accuracy for 7 from the time delay would be 3 x 10-5, with only a weals dependence on the mission duration. The present uncertainties are approximately 1 x 10- per year for the rate of change of G. 5 x 10-3 for the precession rate, 4 x 10-8 for 32, and 2 x 10-3 for 7. However, the small quoted present uncertainty for 32 is from splittings of the roughly 5-m~n period normal modes of oscillation of the Sun, and an independent direct determination of ]2 is desirable. At the level of 1 x 1o-~4 per year for possible changes in G. the effect of the comparable rate of mass loss for the Sun by the solar wind would need to be considered. STARP ROBE: Second-Order Gravitational Red Shift Experiment The experunent proposed is to incorporate a hydrogen maser clock on the mission being planned to explore the plasma around the Sun to make the first measurement of the gravitational red shift to second order in the gravitational field strength (see Figure 6.4~. The experiment would provide a direct measure of the PEN parameter ,B without assumptions concerning values for the other PEN parameters and would have a weak dependence on a solar quadrupole moment. At its closest approach to the Sun the clock will be at 4 solar radii. At this position the first-order gravitational red shift is 5 x 10-7, while the second-order term is 3 x 10-~3. 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. A hydrogen maser appropriate to the my - ion has an estimated mass of 35 kg and would consume led than 30 W. The intrinsic stability of the clock ~ better than needed to perform a 10 percent measurement of the second-order red shift in the tune during closest approach to the Sun, about 10 hours. The requirements on spacecraft tracking are not severe; the range error at closest approach should be less than 100 m. However, the effects of the large spatial and temporal variations in the plasma around the Sun must be consiclered both in the ranging and on the measurements thernseIves. The angular position of the spacecraft
50 r OPTICAL Buff LES ~ / COINlIAMl~ATION BARRIER / Nail PRIMARY SHIELD me SECONDARY SHIELD / ~ - Rut :~ AN{ENNA . ~\/~ PLASMA AND ~ <fly r PARTICLE INSTRUMENTS :~>'J,s~,t, SPINNING PLATfORM ~ ~ ~ ~> ~ ~~ D " = \ <~G NETOMETER \ (BOOM ~ CANISTER) - X WAY TE LE SCO PE ELECTS NIC 5 BUS \ \\ VISUAL/1JV TELESCOPE FIGURE 6.4 STARPROBE: Hydrogen maser to the Sun. SOURCE: Cour- tesy of NASA. could be measured by very long baseline interferometry (V[Bl) using the tracking station antennae. A possible scheme for carrying out the measurement ~ to use the first-order Doppler cancellation technique employed in the Gravity Probe A experiment with the additional redundancy of a Plink system. The 4 links consist of signals originated on Earth and transponded back to the Earth by the spacecraft, and an inde- pendent pair originated by the spacecraft and transponded back to the spacecraft from the Earth. The individual carrier frequencies of the signab are chosen to allow a solution for both the first-order Doppler and the first-order plasma terms. Dual-frequency ranging at both K- and X-band will probably be required. Thermal control of the spacecraft is a key problem in the entire mission; the clock imposes no new special requirements. 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)
51 satellite after 1995. At present, it ~ planned to recover GPB, and it would require minunum refurbishment for the experunent. GPB is optimized so that the center of mass stays constant as the liquid helium boils away. It contains a complete guidance and "zeroing" system. An experiment performed aboard GPB to an accuracy of one part in 10~8 would be an unprovement of 106 over the best existing experiments. X-ray Large Array/Fast-tan;ng E~peranents and Correlation with Gravitational Radiation The program to develop a large array of proportional counters begun on the Shuttle before 1995 will come to fruition on the Space Station. The scientific importance of fast x-ray tinning experunents and their relation to gravitational physics has been discussed in Chapter 5. The fur-scale concept is described here. An array of 512 proportional counters, each 2000 cm2, for a total effective area of greater than 100 m2, would be attached to the Space Station (see Figure 6.5~. Mechanical collimators would provide a I-degree-square field of view. The x-ray large array could utilize the Space Station for assembly, operation, and maintenance and would need to handle a data base volume of up to 10~4 bits, and data rates of up to 108 bpe. The planned range of energy sensitivity is from 0.25 keV to 50 keV. ENHANCED PROGRAM Easer Gravitational Wave Observatory After an initial reconnaissance mission to observe the gen- eral 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. The task group wid discus here the type of observatory that should be considered if substantial evidence is found for pubed or periodic signab associated with black holes or for other types of sources of major scientific importance. The main experimental requirements for the observatory are likely to be in two areas. One is that there be multiple antennas for
52 o Z C' ~ 1- o .. r-~ 5~1~ in ~0 ~ @ // Z x I ha . :^ TO L. o v O ~ ~ _ ~ :^ ._ L. ~ X ·> 00= ._ ~ ._ O :^ O L. al in O = L AS 00 .,, · A= ~
53 determining the location of pubed sources, for polarization stud- ies, and for coincidence observations. Quite accurate information on the locations of periodic sources can be obtained with a single antenna by observing when the sources pa" through the sharp nulls in the antenna pattern. However, this cannot be done for pulses, unless they arrive frequently and from only a few sources. It generally will be necessary to have observations of the different amplitudes, shapes, and arrival times for signal pubes at different antennas in order to determine source location and polarizations. Whether or not there is any large-scale structure in the source locations is import ant to investigate. Coincidence measurements with multiple antennas also can substantially increase the sensitiv- ity and reliability of observations made of pulses of gravitational radiation. Three antennas would be sufficient to obtain a large amount of additional information. Separating them by a distance of many gravitational wavelengths is quite feasible if the antennas are placed in solar orbit. One possibility would be to use sepa- rations of about 108 km in order to achieve the highest angular resolution possible. The other foreseeable requirement would be to maximize the sensitivity in the frequency region that appears most important based on the observations from the reconnaissance mission. For example, if pulses are seen in the frequency region above 10-3 Hz but not at lower frequencies, then the use of roughly 300 km multipass antennas of the Fabry-Perot type probably would be desirable. Also, the achievement of additional power output and improved performance by efficient lasers with long lifetimes for space applications is highly likely by the year 2000. With 1 W of transrn~tted power, 100 passes per arm, and an integration time of 106 a, a sensitivity as high as 3 x 10-26 would be possible for periodic signals In the frequency range of roughly 10-2 Hz to 3 Hz. The corresponding pulse sensitivity would be 3 x lo-24 to 5 x 10-23 over the above frequency range. Such high sensitivity would greatly enhance the opportunities for detailed studies of gravita- tional wave pulse shapes, as well as for Recoveries of completely unexpected phenomena. Redight of GPB With improved superconducting quantum interference devices
s4 (SQUIDS) now available and a laser modulation technique devel- oped by Cabrera to modulate the superconducting pickup loop around the gyroscope, it appears possible to reduce the gyro read- out nome to a level of 10-4 see of arc per year. 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 astrom- etry data on the distance and proper motion of these stars based on absolute gyroscope measurements rather than on comparison of several stars. To make such measurements a 36~degree gyro readout could be developed to an accuracy of 10-4 see of arc. This would have unportant applications to other astronomical instru- ments In space. It is possible that the Gravity Probe B data will be suffi- ciently interesting to make a second flight, possibly in a nonpolar orbit, very worthwhile. Thus, the capability to bring back Gravity Probe B for refurbishment or to refill with helium in space seems desirable.
