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OCR for page 157
7
Programs for Study
and Development
In addition to the programs recommended in the preceding chapter
for approval and funding during the 1980's, the Astronomy Survey
Committee considered in detail many other proposed programs of
comparable scientific merit. Some of the most promising of these
differ from the programs already recommended primarily through
being at an earlier stage of technological development or through
falling at a later stage in a logically planned observational program.
However, planning and development are often time-consuming, es-
pecially for large projects. It is therefore important to begin, during
the coming decade, study and development of programs that appear
to have exceptional promise for the 1990's and beyond. The support
of such programs should include funding for design studies and,
where appropriate and timely, for the development and testing of
instrumentation. Projects and study areas recommended by this
Committee in this category include the following, in which the order
of listing carries no implication of priority:
A. Future x-ray observatories in space;
B. Instruments for the detection of gravitational waves from astro-
nomical objects;
C. Long-duration spaceflights of infrared telescopes cooled to cry-
ogenic temperatures;
D. A very large telescope in space for ultraviolet, optical, and near-
infrared observations;
157
OCR for page 158
158
ASTRONOMY AND ASTROPHYSICS FOR THE 1980's
E. A program of advanced spatial interferometry in the radio,
infrared, and optical spectral regions;
F. Advanced gamma-ray experiments; and
G. Astronomical observatories on the Moon.
The present chapter discusses these areas of study and development
and illustrates their role in addressing some of the long-range sci-
entific problems that may now be foreseen.
A. FUTURE X-RAY OBSERVATORIES IN SPACE
The Astronomy Survey Committee recommends the study and
development of space observatories beyond the Advanced X-Ray
Astrophysics Facility (AXAF) for pursuit of a number of impor-
tant future goals in x-ray astronomy. These investigations will
either demand observational capabilities exceeding those of AXAF
or will be more appropriate to observatories incorporating spe-
cialized, dedicated instrumentation.
The last decade has been a period of extraordinary advances in
x-ray astronomy, culminating in the impressive observations re-
turned by the Einstein Observatory. These studies will be accel-
erated and extended to much fainter objects by AXAF, recom-
mended by this Committee as the major new program of highest
priority in all of astronomy for the 1980's. However, a number
of important future investigations in x-ray astronomy will de-
mand greater sensitivity, spectral resolution, energy coverage, or
time resolution than even this powerful facility can provide.
Moreover, the rich variety of objects and problems uncovered by
the past decade of x-ray observations points to the need for fu-
ture studies that are too specialized and diverse to be accom-
plished by a single observatory. Such studies should include at
least the following scientific goals:
1. An all-sky survey at x-ray wavelengths comparable in scope
with the Palomar Sky Survey, to facilitate the identification and
study of objects in various wavelength regions and to provide the
data base for detailed studies of the statistics, distribution, and
evolution of x-ray sources.
2. Comprehensive measurements of variability, over a wide
range of time scales, of the x-ray emissions from faint Galactic
and extragalactic sources. This work will be of crucial impor-
tance for a detailed understanding of virtually every kind of
OCR for page 159
Program for Study and Development
159
compact x-ray source, ranging from nearby flare stars to distant
quasars.
3. Low- to moderate-resolution spectroscopy of faint objects, to
obtain such data as x-ray red shifts of very distant clusters of
galaxies and plasma diagnostics for large numbers of x-ray
sources.
4. Detailed study of the large-scale x-ray structure of the Uni-
verse through measurements of diffuse features as well as study
of structure on smaller scales through observations of clusters of
galaxies.
5. High-resolution spectroscopy and polarimetry of faint ob-
jects to determine elemental and ionic abundances, temperature
distributions, morphologies, and dynamical behaviors. The tar-
gets of such studies will include stellar coronas, supernova rem-
nants, active galactic nuclei, and galaxy clusters.
6. Comprehensive investigation of x-ray sources in the energy
range above 10 keV, which will be inaccessible to AXAF. Pre-
vious balloon and satellite studies have demonstrated the impor-
tance of high-energy x-ray measurements in the analysis of the
physical conditions in x-ray stars and active galactic nuclei and in
the evaluation of the contributions of discrete and diffuse sources
to the high-energy x-ray background.
7. Detection, location, and detailed study of transient x-ray
sources, including sources of hard x rays associated with gamma-
ray bursts and supernova explosions in external galaxies.
8. Measurements of the spectra and variability of soft x-ray and
extreme-ultraviolet (EUV) sources, including stellar coronas, single
hot white dwarfs, and accreting white dwarfs in binary systems
(cataclysmic variables), together with detailed mapping of soft
x-ray and EUV emission from the interstellar medium.
The first four of these scientific goals (an all-sky survey,
measurement of variability, low- to moderate-resolution spectros-
copy, and study of low-surface-brightness features, all directed at
faint sources) require collecting areas much larger than that of
AXAF but angular resolutions sufficient only to avoid source con-
fusion and to permit unambiguous identification of optical and
radio counterparts. One may therefore pursue these goals through
deployment of a moderate-resolution array of reflectors of large
area, at a cost modest by comparison with that of a monolithic-
mirror telescope. Individual modules of such arrays can be de-
signed, tested, and deployed as part of the Spacelab program.
OCR for page 160
160
ASTRONOMY AND ASTROPHYSICS FOR THE 1980's
The Committee encourages the continued development of large-
area modular arrays of reflectors through Spacelab flights and the
eventual assembly of a large-area facility for long-duration obser-
vations, perhaps upon a space platform.
High-resolution x-ray spectrometry and polarimetry of faint
objects will require specialized instruments with very large effec-
tive areas. Preliminary design studies suggest that both kinds of
measurements can be pursued in the same long-duration mis-
sion, and the Committee urges continued development of the
concepts for such a mission.
The study of objects that are most luminous at x-ray energies
above 10 keV was begun through balloon and HEAo-1 observa-
tions, but a greatly expanded and more systematic study of such
sources will be needed during the 1990's. Instruments providing
imaging spectrophotometry up to 100-keV photon energies, with
concurrent precise location and high-resolution x-ray spectros-
copy of such sources, are now becoming technologically feasible;
these and other instruments should be studied for possible in-
corporation into such a mission, which may prove to be an at-
tractive candidate for location on a space platform.
A comprehensive study of the brighter members of the var-
ious classes of transient and variable sources will be initiated by
the X-Ray Timing Explorer (XTE) satellite. It is already evident,
however, that a long-term observatory with enhanced capabili-
ties will be needed during the 1990's to extend the observation
of variability to a substantial fraction of the objects accessible to
AXAF.
Finally, the Committee believes that planning should begin for
soft x-ray and EW observations beyond those anticipated from the
EUV Explorer. Such observations, with improved sensitivity, an-
zular resolution, and spectral resolution, will be needed for ana
v . ,
~ . - ~ . ~ - ~ . 1 1 · . 1 ~ ~ 1~ _ 1 1 _ ~ ~ _
lytical studies of the objects detected by EUVE, as weft as to
complement the optical and near-ultraviolet observations made by
Space Telescope and the far-ultraviolet observations to be made
by the far-ultraviolet spectrograph in space recommended as a
new program in the present report. Coordinated observations of
a wide variety of nearby stars at x-ray, EUV, UV, and optical
wavelengths will furnish decisive tests of stellar-atmosphere
models and theories of coronal activity. Measurement of the
structure and physical conditions in the hot component of the
interstellar medium by means of soft x-ray and EUV surveys and
OCR for page 161
Program for Study and Development
161
spectrometry with high angular resolution will be an essential part
of future studies of star formation and chemical evolution in the
Galaxy.
B. INSTRUMENTS FOR THE DETECTION OF GRAVITATIONAL
WAVES
The Astronomy Survey Committee recommends the study and
development of instruments for the detection of gravitational
waves from astronomical objects.
Detection and measurement of gravitational waves would be of
great importance to the achievement of a fundamental under-
standing of the gravitational field. Success would provide infor-
mation on important astronomical phenomena that is otherwise
unobtainable. This includes, in particular, rapid dynamical be-
havior in stars and other massive objects and advanced evolu-
tion of stellar systems. The great potential value of gravitational-
wave detection and the challenging technical problems it presents
justify current effort on the study and development of detectors
and on the theory of the generation of these waves. When this
development reaches a state that allows astronomical observa-
tions to become practical, instruments for such observations
should be constructed. These will probably include long-baseline
systems in space, which at present seem to promise the great-
est sensitivity. Meanwhile, advantage should be taken of oppor-
tunities for improvements in space-based efforts to detect gravi-
tational waves by upgrading the quality of equipment for tracking
the motions of solar-system space probes.
C. LONG-DURATION SPACEFLIGHTS OF INFRARED TELESCOPES
COOLED TO CRYOGENIC TEMPERATURES
The Astronomy Survey Committee recommends the study and
development of long-duration space observatories incorporating
infrared telescopes cooled to cryogenic temperatures. In particu-
lar, NASA should study the ways in which the Shuttle Infrared
Telescope Facility (SIRTF) may be most effectively incorporated into
such a long-duration observatory following a vigorous observa-
tional program carried out through Spacelab flights.
During the next decade, the most dramatic gains in astronom-
ical sensitivity will probably occur in the infrared region of the
OCR for page 162
162
ASTRONOMY AND ASTROPHYSICS FOR THE 1980's
spectrum as a result of observations carried out in space with
improved detectors and cryogenically cooled optical systems.
Programs currently under development, such as the Infrared As-
tronomy Satellite (IRAS) and SIRTF, will exploit these advantages
in part; by the end of the decade, infrared astronomy should ri-
val radio, optical, and x-ray astronomy in the depth and rich-
ness of its observable sky. It is therefore important to begin
planning now for the long-duration infrared observatories em-
ploying cryogenic optics that will be needed to exploit these op-
portunities during the 1990's and beyond.
As emphasized in Chapter 4, STRTF will be the cornerstone of
space infrared astronomy during the coming decade. This facil-
ity will be able to carry out a powerful observational program
even within the relatively brief, 7-day length of an early Shuttle
sortie mission. Frequent Shuttle flights of SIRTF, with appropriate
refurbishment between flights, thus constitute one of the high-
est priorities for U.S. astronomy during the 1980's.
However, the scientific return from SIRTF will almost certainly
provide an overwhelming case for long-duration spaceflights of
cryogenically cooled infrared telescopes. A possible way to achieve
this capability is to develop SIRTF itself as a facility for eventual
placement onto a space platform, to be refurbished at intervals of
6 to 12 months. Alternatively, development of a free-flying ob-
servatory for SIRTF may prove to be more effective. We urge NASA
to pursue the general development and design of detectors and
facilities for infrared astronomy with the aim of achieving long-
duration spaceflights of cryogenic infrared telescopes by 1990.
D. VERY LARGE TELESCOPE IN SPACE FOR ULTRAVIOLET,
OPTICAL, AND NEAR-INFRARED OBSERVATIONS
The Astronomy Survey Committee recommends the study and
development of the technology required to place a very large
telescope in space early in the next century.
The advances in ultraviolet, optical, and infrared astronomy
expected in the 1980's are extremely impressive and will cer-
tainly lead to great advances in our understanding of diverse
astrophysical phenomena. Yet, just as certainly, the instruments
of the 1980's will discover new- phenomena that will require new
and even more powerful facilities for their systematic investiga-
tion. By the turn of the century it may be possible to place in
orbit a very large telescope-perhaps 30 m in diameter with
OCR for page 163
Program for Study and Development
163
diffraction-limited performance from the far-infrared to the near-
ultraviolet regions.
The capabilities of such an instrument would be awesome by
today's standards. At visual wavelengths it would have a re-
solving power of about 4 rnilliarcseconds, a limiting magrutude for
medium-resolution spectroscopy of about 30, and a limiting
magnitude for imaging of about 3Wmore than a hundred times
fainter than the faintest object accessible to Space Telescope. A
telescope of such power could, for example, observe the planets
with resolutions ranging from 12 km at Jupiter to 90 km at
Neptune, permitting long-term studies of the atmospheric dy-
namics of the major planets; observe the star-forming regions in
the Orion nebula with a resolution of 10 astronomical units at
near-infrared wavelengths, permitting direct observation of the
process of star formation and detailed studies of preplanetary
nebulae; observe the main sequence in the Andromeda Galaxy
tour magnitudes Below tne oldest turn-off point and obtain
spectra one magnitude below this point; perform spectroscopy of
solar-type stars in nearby galaxies, permitting direct chemical-
abundance and abundance-history determinations for these gal-
axies; with suitable care and stability, obtain relative positions of
, . ~ ~ ~
~ - . . ~ ~ ~ . ~ ~ ~ .
stars and background quasars to a precision of about 100 mi-
croarcseconds-enough to provide accurate parallaxes out to a
distance of 1000 parsecs and to measure proper motions of stars
in globular clusters and in nearby spiral galaxies, permitting
studies of the structure and dynamics of these stellar systems;
obtain high-resolution ultraviolet spectra of the brightest stars in
galaxies of the Virgo cluster of galaxies, making possible de-
tailed studies of the intergalactic medium over long path lengths;
and observe the nearest quasars with a resolution of 10 parsecs
and the nearest active galaxies with a resolution of 0.2 parsec,
fine enough to reveal fundamentally new structural details.
These examples are only a few from a long list of important
problems that a very large space telescope could pursue. Such an
instrument could be the most powerful too! of astronomy at the
start of the next millenium. We therefore recommend that NASA
begin exploratory studies of this project and encourage the de-
velopment of the requisite technologies. Development of tech-
nology for the Large Deployable Reflector in space (recom-
mended in Chapter 6) should provide a strong impetus for the
further advances needed for this even larger, more powerful in-
strument.
OCR for page 164
164
ASTRONOMY AND ASTROPHYSICS FOR THE 1980's
E. PROGRAM OF ADVANCED SPATIAL INTERFEROMETRY IN THE
RADIO, INFRARED, AND OPTICAL SPECTRAL REGIONS
The Astronomy Survey Committee recommends the study and
development of advanced spatial interferometers for the radio,
infrared, and optical spectral regions.
Exploration of the structural details of astronomical objects from
planets to distant quasars requires ever higher angular resolu-
tion. Radio astronomers have pioneered in such studies with re-
fined interferometric techniques. The Very Large Array produces
radio images with resolutions of an arcsecond or better, and
continent-spanning very-long-baseline interferometry currently re-
solves detail down to a few milliarcseconds. Increases in base
lines for radio telescopes and extension of interferometric tech-
niques to infrared and optical wavelengths can achieve even
higher resolution.
Important next steps are the Very-Long-Baseline (VERB) Array, the
placement of a radio antenna in low-Earth orbit to complement
the ground-based VERB Array, and the development of a spatial
interferometer for the mid-infrared region, as presented in the
recommendations for new programs (Chapter 6~. These steps
would be followed by the placement of radio antennas in highly
elliptical orbits (increasing VERB} baselines by a factor of 10) and by
the construction of a ground-based infrared interferometric array
with baselines of a few kilometers. These instruments would
achieve angular resolutions of a few hundred microarcseconds in
the radio and a few milliarcseconds in the infrared region. In
both cases, detailed image reconstruction would be possible since
they would provide good coverage in the Fourier-transform plane,
a necessary condition for the accurate mapping of complex
sources.
Further improvements of infrared interferometric capabilities and
significant developments in optical interferometry will probably
require interferometers in space to escape the deleterious effects
of the Earth's atmosphere. A desirable goal is a program of space
optical and infrared interferometry leading, by the early part of
the next century, to an interferometer with baselines of a few
tens of kilometers and resolutions of 1 to 10 microarcseconds at
optical wavelengths. Development of interferometric techniques to
this extent will require the capability to place large structures in
space, significant strides in optical technology, and extensive ad-
vances in systems for space-vehicle control, communication, and
OCR for page 165
Program for Study and Development
165
information processing. With space-based infrared and optical
interferometers it would be possible to measure trigonometric
parallaxes of objects throughout the Galaxy, to detect Earth-like
planets orbiting nearby stars, and to resolve significant structure
in nearby quasars, to name only a few of the many important
projects that would be advanced with such powerful instru-
ments.
Advanced interferometry on the ground, and then in space, will
be among the most important and exciting areas of astronomy in
the coming decades. We therefore strongly recommend that NSF
and NASA begin preliminary planning, exploratory studies, and
technological development for such a program.
F. ADVANCED GAMMA-RAY EXPERIMENTS
The Astronomy Survey Committee recommends the study and
development of advanced gamma-ray experiments to follow the
program to be carried out by the Gamma Ray Observatory (GRO).
Subsequent to GRO, an advanced high-energy gamma-ray tele-
scope of very large area, high sensitivity, and high angular res-
olution will be needed for long-term observations of selected
sources and regions of special interest. This will be necessary to
achieve the statistical accuracy in the counting of gamma-ray
photons required to resolve spatial and spectral features of the
sources and to analyze their variations. The field of view of the
telescope need not be wide, and an appropriate goal for angu-
lar resolution is the order of 1 to 2 arcmin. A high-resolution
nuclear gamma-ray spectrometer should be included in the mis-
sion for the study of the gamma-ray lines from radioactivity in
supernova remnants, positron annihilation in the Galactic disk and
in extragalactic sources, nuclear excitations caused by cosmic rays
in dense matter, and nucleosynthesis in extragalactic superno-
vae; energy resolution sufficient to study line profiles will be
desirable. Development of such instruments, possibly for de-
ployment upon a space platform, should begin as soon as there
emerges a clear understanding of the observational requirements
from analysis of results from GRO.
G. ASTRONOMICAL OBSERVATORIES ON THE MOON
The Astronomy Survey Committee recommends that agencies of
the U.S. Government, working in concert with those of other
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ASTRONOMY AND ASTROPHYSICS FOR THE 1980's
nations, take steps to ensure the preservation of sites on the
Moon (particularly on the far side) for astronomical observa-
tions. In addition, the Committee urges NASA to set aside re-
sources for the establishment of lunar astronomical observatories
as an important corollary to the establishment of large-scale in-
dustrial or power-generation facilities in Earth orbit.
The Moon offers certain decisive advantages as a base for as-
tronomical observations. In particular, the far side of the Moon
provides protection from radio interference from sources on or
near the Earth and therefore has great potential value for radio
astronomy. Shielded at all times from earthlight, sites on the far
side of the Moon are also shielded from sunlight for substantial
parts of each month and thus offer advantages for optical and
infrared observations requiring the darkest possible sky. These
considerations become compelling if large military, industrial, or
power-generation facilities are constructed in Earth orbit, for the
electromagnetic pollution arising from the operation of such fa-
cilities may well make it difficult or impossible to observe faint
astronomical objects from the ground or from Earth orbit. The
utility of the Moon for astronomical observations must therefore
be protected as a unique resource for future generations on Earth.
The preservation of sites on the Moon for astronomical obser-
vations is clearly a task of international scope. However, the
Committee recommends that agencies of the U.S. Government
take the lead in such an effort and begin planning in the near
future for the establishment of lunar observatories early in the
next century. In addition, the U.S. Government should consider
carefully the potentially disruptive effects on Earth-based astron-
omy of the construction of large-scale military, industrial, or
power-generation facilities in Earth orbit.
Representative terms from entire chapter:
survey committee
