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OCR for page 30
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
OCR for page 31
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
OCR for page 32
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
OCR for page 33
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
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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
OCR for page 35
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 sources—often 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
OCR for page 36
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
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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.
Representative terms from entire chapter:
gravitational waves
