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OCR for page 19
Astronomy and Astrophysics in 1995
Expected Status
OVERVIEW
.
Astronomy Han advanced dramatically over the past two to
three decades, stimulated by the development of major ground-
based facilities and the introduction of space techniques. A promi-
nent feature of the astronomy program has been its reliance on
detailed, Tong-term planning, exemplified most recently by the As-
tronomy Survey Committee report, Astronomy and Astrophysics
for the 1980s. Looking forward to 1995, the task group antici-
pates progress in realizing the plans detailed in that report. Such
progress will involve the principal elements of the Shuttie-based
program, as well as HST, GRO, AXAF, and STRTF, which form
the first family of comprehensive observatories in space and have
been called "the Great Observatories." These instruments will
provide a powerful capability for detailed astrophysical studies at
optical, gamma-ray, x-ray, and infrared wavelengths. Thus, the
next decade will see space astronomy move strongly from an era
of exploration to a program of in-depth study.
The Great Observatories are the foundation of this program.
Their effective use will require the development of new state-of-
the-art instruments for these telescopes during their useful lifetime
and the replacement of the entire observatory when its useful life
19
OCR for page 20
20
ends, due either to out-of-date components or scientific obsoles-
cence. In the case of out-of-date components, it might be adequate
simply to replace the observatory with one of equal capabilities.
Replacement could also be seen as an opportunity for improve-
ments that would make the observatory more capable to investi-
gate the astrophysical questions of greatest interest at the time.
Scientific obsolescence can occur either when the unique capabili-
ties for which an observatory was built have been fully exploited
or when general improvement of other observational capabilities
diminishes the facility's productivity. The core program that the
task group envisages is one in which we would attempt to maintain
the four observatories as forefront research instruments. Different
considerations will apply to the different wavelength regions.
Along with this evolution of technical means and scientific
aims, the task group foresees two other trends a further shift in
emphasis from ground-based to space observations and a continued
strengthening of the international base of space astronomy, with
the realization of major European and Japanese missions, such as
the Roentgen Satellite (ROSAT), the Infrared Space Observatory
(ISO), and Japan's Explorer-cIass x-ray satellite (ASTRO-C).
In the event that the international base of space astronomy
continues to strengthen, as present indications in Europe and
Japan suggest, the task group also looks forward to further im-
provement in the cooperation and planning of the major programs
of this era on a worldwide basis. Such a development would be
fully in keeping with the international nature of astronomy.
During the coming decade, it will be vital to:
~ maintain adequate support for the ongoing, long-lived ob-
servatories;
~ provide, not only for major new initiatives, but also for
medium- and small-scale missions, for supporting analysis, and
for research and development; and
. introduce substantially less expensive ways of conducting
space activities so that new facilities such as those outlined in
Chapter 4 can become a reality.
RADIO ASTRONOMY
The results of extending observational capabilities into new
domains are well illustrated by examining the recent history of
OCR for page 21
21
radio astronomy. Naively, one might have expected that the rela-
tively long wavelengths characteristic of the radio spectrum would
have precluded the attainment of high angular resolution. This
turned out not to be the case, because of the exquisite control of
the time domain achieved by modern electronic techniques. Start-
ing in the late 1960s, very long baseline interferometry (V~Bl)
extended the effective size of observing apertures to worldwide
dimensions. Angular resolution of a milliarcsecond became com-
monplace, superior to the angular resolution achieved in any other
part of the electromagnetic spectrum, and a number of new discov-
eries were made. The relativistic jet phenomenon was clarified and
shown to have alignments over scale factors differing by more than
a million. Two puzzling phenomena velocities that apparently
exceed the speed of light, and interstellar masers still remain un-
explained; both could potentially influence our basic assumptions
about physical phenomena in the universe.
The first V[B! observations used simple two- and three-
element configurations, but it rapidly became evident that more
complete arrays were needed to give reliable pictures on the rnilli-
arcsecond scale. This led, eventually, to the establishment of the
V~BI array project of the National Radio Astronomy Observa-
tory (NRAO), which is expected to be in use by 1995. This gives
the angular resolution of an aperture nearly equal to the Earth's
diameter, provided a source is high in the northern sky. The prob-
lems at hand reading down toward the site of acceleration of the
relativistic jets, for example require still longer baselines, but
the Earth is too small. This led to the concept of space V[BI,
and it is expected that the launching of QUASAT in mid-1995
will represent an important early step in exploring this new ob-
servational realm. The orbiting antenna of QUASAT will have an
apogee of the order of 20,000 km, which will be further extended by
earthbound telescopes covering another 10,000 km baseline. The
wavelengths to be used are 1, 2, and 6 cm. Images will comprise up
to 2000 x 2000 picture elements (pixels), with pixel dimensions as
small as 90 ,uarcsec, and perhaps 40 parcsec if coordinated inter-
national programs are effected. This will probably provide a first
step toward understanding the problem of the "synchrotron-self-
Compton effect" that theoretically limits the brightness of sources
to 10~2K. It will lead to greater understanding of superTuminal
OCR for page 22
22
motions in quasars and of the structure of interstellar and circum-
stelIar masers. When used in conjunction with the VLBA, the
system will have miTTijansky sensitivity, allowing the study of a
vast array of interesting objects.
INFRARED AND SUBMILI`IMETER ASTRONOMY
In 1983 the Infrared Astronomical Satellite (IRAS), the first
cryogenically cooled telescope to orbit in space, catalogued approx-
imately 300,000 new infrared sources. Among its major discoveries
were dust systems orbiting Vega and many other nearby stars; a
"cirrus" cloud component on a galactic scale extending well above
and below the galactic plane; two new zodiacal dust bands strad-
dling the ecliptic plane; hosts of infrared sources, which, because
of their very Tow luminosities, appear to be solar-type stars in their
earliest stages of formation; high infrared fluxes from interacting
galaxies; and large numbers of galaxies that are orders of magni-
tude brighter at infrared wavelengths than in the optical domain
and are rivaled in their energy production only by quasars.
There are galaxies that at first sight might appear much like
our own Milky Way-a spiral galaxy among billions of others. At
infrared wavelengths, however, these galaxies have exhibited an
enormous population of recently formed stars still shrouded by
clouds of the gas and dust from which they formed. Massive star
formation appears to have taken place there in a sudden outburst
of activity. We will need to understand the origins and underlying
mechanisms at work in these starburst galaxies.
{RAS discovered disks of finely structured material orbiting
about several nearby stars. These disks may provide clues to the
way in which planetary systems are formed around a star. SIRTF
will be able to study a number of these disks to determine the
spatial distribution and chemical composition of this circumsteliar
matter.
In order to understand the various physical processes that re-
sult in infrared emission under these diverse conditions, we need
considerably higher spatial resolution coupled with high sensitiv-
ity. We also need to obtain spectral information about the chemical
structure and physical conditions in many of these sources.
The task group expects that by 1995 a number of major space-
and ground-based infrared facilities will be available for these
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23
studies, as well as for future observations that we cannot yet
anticipate. These facilities are described briefly below:
~ Cosmic Background Explorer (COBE) will have searched
for deviations from the isotropic mucrowave background, examin-
ing the conditions when galaxies and stars first formed. It will also
search across the entire infrared domain, mapping the large-scale,
diQuse features of the galaxy.
The Space Infrared Telescope Facility (SIRTF) and In-
frared Space Observatory (ISO) will draw upon advanced detector
and detector-array technology to provide imaging at sensitivities
orders of magnitude higher than TRAS, together with greatly en-
hanced spectroscopic capability.
~ A near-infrared imaging and spectrometry experiment will
extend the Hubble Space Telescope's (HST's) advantages of in-
creased angular resolution and reduced background from space
into the short-wavelength infrared region of ~ to 3 ~m. At the
3-,um wavelength there is a natural window that opens on the uni-
verse. In that region of the spectrum both the optically scattered
zodiacal light and the infrared emission from zodiacal dust are
particularly low, and our search deep into the universe will extend
out to exceptionally great distances.
~ A new generation of large, ground-based, optimized in-
frared telescopes, such as the loom Keck telescope, will provide
large collecting area for spectroscopic observations and high angu-
lar resolution (~0.2 arcsec) imaging with high sensitivity to point
sources.
A number of 1~ to l~m-cIass subm~limeter telescopes
will be in operation at high mountain sites, providing limited
access to the submillimeter regime through selected atmospheric
windows. Observations in the far-infrared and submillimeter spec-
trum carried out from aircraft and balloons will continue to fill in
the spectral coverage at lower angular resolution and sensitivity.
. High-resolution spectroscopic studies will be possible
throughout the subrn~llimeter range with the 8-m-clam Far-Infrared
Space Telescope (FIRST).
The major shortcomings in infrared observational capabilities
in the mid-1990s will be the limited angular resolution from space
observatories other than HST, the lack of a major submillimeter
space observatory, and the absence of milliarcsecond interferomet-
ric capability.
OCR for page 24
24
ULTRAVIOLET AND OPTICAL WAVELENGTHS
By 1995 one or two ground-based telescopes of the 15-m class
are expected to have been in operation for several years. In ad-
dition, a few 8-m-cIass ground-based telescopes should have been
developed by the United States and Japan, and located at good
observing sites in the northern and southern hemispheres. Despite
major efforts to improve astronomical seeing, angular resolution
from the ground will be limited to about 1 arcsec, with occasional
glimpses of 0.3- to 0.5-arcsec image quality. The Hubble Space
Telescope (MST) was planned to overcome these limitations by
providing better than 0.1-arcsec resolution in the ultraviolet and
optical wavelength regions.
By 1995 the HST will have been in operation for nearly a
decade. If funded adequately, it will by then be equipped with
second-generation instruments approaching detector quantum ef-
ficiencies of 80 percent over the spectral range 1200 ~ to 2 ,um.
By 1995 the Extreme Ultraviolet Explorer (EUVE) and ROSAT
will have surveyed the sky between 80 and 900 ~ and the Far Ul-
traviolet Spectroscopic Explorer (FUSE/LYMAN) telescopes will
perform detailed studies of objects between 900 and 1200 A.
By the mid-199Os important astrophysical problems to be
addressed with ground-based and space facilities will be the deter-
mination of:
An accurate cosmological distance scale.
Evolutionary and chemical states of normal and active
galaxies to modest red shifts.
. Dynamics of disk and bulge regions in nearby galaxies and
the distribution of associated dark matter.
.
Physical and chemical properties of the interstellar and
intergalactic media including deuterium in the local neighborhood.
. Physical and chemical states of degenerate stars and hot
plasmas in stellar chromospheres and coronas.
Activity and mass loss in every kind of star and stellar
.
system.
.
Initial search for protoplanetary disks and planets.
These studies will be aided by:
. Deep surveys with detection capabilities of point sources
to large red shifts in small angular areas.
OCR for page 25
25
. Follow-up to sources discovered in new surveys (COBE,
EUVE, and ROSAT).
Although point sources will be detectable to very large red
shift limits with the HST, studies of galaxies will be limited by the
rapid dimming of surface brightness for these highly red-shifted
objects. Thus, for studies of galaxies, the HST will not be capable
of exploiting the ultraviolet/optical region over substantial cosmic
Took-back times. Moreover, by 1995 there will be many funda-
mental astrophysical problems that will be limited by the angular
resolution capabilities even of the HST.
Undoubtedly the next step will require higher angular resolu-
tion combined with large collecting areas so that our studies can
answer the many questions that will have arisen about the spatial
structure of the objects that HST will likely discover and examine.
X-RAY ASTRONOMY
X-ray astronomy will achieve a substantial increase in obser-
vational capability with the anticipated launch of the Advanced
X-ray Astrophysics Facility (AXAF) in about 1994. AXAF will be
the first major observatory for x-ray astronomy to be operated on
a Tong-term basis. It will permit detailed, high-angular-resolution
(about 0.5 arcsec) studies at x-ray energies from about 0.1 to 10
keV. It will observe the broad range of astronomical objects shown
by the Einstein Observatory to be x-ray emitters a range that
runs the gamut from coronas around nearby coo! stars to distant
quasars. AXAF will not only permit detection of distant Quasi-
Steller Objects (QSOs) and galaxy clusters but also will enab]
moderate-resolution spectral analyses of those sources. These ob-
servations will begin to address problems such as the formation
and evolution of galaxies and galaxy clusters.
In studies of brighter objects, AXAF will allow high-resolution
x-ray spectra to be obtained for the first time on objects as diverse
as supernova remnants, accreting neutron stars, black holes, and
normal stars. Once again, the exploratory spectroscopic studies
carried out with the Einstein Observatory herald the rich poten-
tial for discovery and detailed astrophysical measurements that
AXAF will conduct. In detailed spectroscopy of bright halos of
galaxies such as M87, AXAF would determine the composition,
temperature, and density profiles of the halo; that would greatly
OCR for page 26
26
constrain the origin and mass profile of the galaxy halo. It would
then be possible to determine whether the halo and its apparently
large, dark mass component was supplied by accretion from the
surrounding cluster or was intrinsic to the galaxy. Similarly, stud-
ies of dark matter in other nearby galaxies and clusters of galaxies
will be initiated by AXAF. More-sensitive instruments, however,
will be required to extend these studies to more distant objects
of cosmological interest. The overall sensitivity of AXAF is well
matched to that of the major optical (HST), infrared (SIRTF),
and radio (VLA and V~BA) observatory capabilities anticipated
for the early l990s. AXAF will draw heavily on observations made
with these facilities as well as on the moderate or Explorer-cIass
missions In x-ray astronomy launched in the period from 1987 to
1994. These include the following:
. ROSAT (1987~: A West German imaging soft x-ray (~2
keV) telescope, and a British XUV telescope make up this mission.
ROSAT will carry out the first all-sky survey in these wavebands
at high sensitivity, as well as a program of pointed observations in
which U.S. observers will be directly involved. This mission should
observe 100,000 x-ray sources.
· X-Ray Explorers (1987 to 1992~: These are planned for
the study of relatively bright compact x-ray sources. In 1987,
Japan will launch a large-area (about 0.5 m2), nonimaging x-ray
detector for broadband spectrophotometry and the study of rapid
variability in x-ray sources. It will probe the physics of accreting
neutron stars, black holes, and white dwarfs in our Galaxy and
L ~ml T T ~
~ ~ ~ _,_,,%) ~
oeyona. l ne u A. Array l lmlng Explorer (XTE), to be launched in
1992, will extend the high time-resolution studies of such compact
galactic x-ray sources and permit similar studies to still higher
photon energies. The internal structure of neutron stars will be
probed with sustained observations of x-ray pulsars and bursters,
and physical conditior~s in accretion disks will be explored.
. Space Shuttle Experiments (1988 to 1990~: These will be
carried out primarily on the OSS-2 mission, to develop new con-
cepts tor large-area x-ray telescopes with greatly increased sensi-
tivity. These short-duration flights with limited capability will be
vital for the development of future x-ray missions.
· XMM (1995~: This is a first-generation, high-throughput
x-ray telescope now under study by the European Space Agency
(ESA) as a "cornerstone" mission. It will have a large collecting
, . ~
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27
area (about 1 m2) for imaging and spectroscopy of soft x-ray
sources at moderate spatial and spectral resolution. XMM is thus
strongly complementary to AXAF and will also serve as an ideal
springboard for the planning of x-ray facilities of the future (see
Chapter 4~.
The powerful capabilities of AXAF and the wealth of fun-
damental problems it can address suggest that this facility will
advance research for a long time to come. When the need arises
for major refurbishment or when the technology of producing sig-
nificantly larger diameter and still higher-resolution x-ray mirrors
has sufficiently advanced, the entire AXAF facility could be up-
graded or replaced.
GAMMA-RAY ASTRONOMY
Small Astronomy Satellite-2 (SAS-2) and COS-B, following
earlier pioneering measurements, have provided maps of the high-
energy gamma-ray emission from the galactic plane and an inter-
pretation of the origin and propagation of the cosmic rays. In
addition, they have made observations of pulsars as well as other
galactic objects, measured the basic properties of extragalactic dif-
fuse gamma radiation, and discovered gamma-ray emission from
a quasar. Active galactic nuclei, the galactic center region, and
various sources, including transients, have been observed in the
low-energy gamma-ray region by telescopes on the High Energy
Astronomical Observatory-3 (HEAO-3), the Solar Maximum Mis-
sion (SMM), and other instruments. Reported observations of
gamma rays at energies above 10~i eV using ground-based tech-
niques bear on the physics of pulsars and active galactic nuclei.
Thus, over the entire spectral range from 105 to above 10~4 eV,
there are now interesting, known sources of gamma radiation.
The next major advance in gamma-ray astronomy will come
with the launch of the Gamma Ray Observatory (GRO), scheduled
for 1988. Covering the energy range from about 105 to 3 x 10~°
eV, it will have a sensitivity at least an order of magnitude greater
than previous experiments over this whole energy range and a
viewing program designed to scan the entire sky. Currently, the
lifetime of the GRO will be from 2 to 4 years; the lifetime would
be extended if refueling capabilities are provided. With GRO,
gamma-ray astronomers will obtain the first comprehensive full-
sky survey in moderate detail. In particular, GRO should further
OCR for page 28
28
open the Tow-energy gamma-ray astronomy window by recording
nuclear emission lines and investigating the properties of gamma-
ray bursts in more detail. At the same time, it will extend studies
in high-energy gamma-ray astronomy to a far greater depth.
Results from GRO will undoubtedly help to focus require-
ments and objectives for future experiments, but the general thrust
of garnrna-ray astrophysics in the post-GRO period is already rea-
sonably clear. The Astronomy Survey Committee in its report
Astronomy and Astrophysics for the 1980s already anticipated
the need for at least two gamma-ray instruments beyond those
on GRO. An advanced high-energy gamma-ray telescope of fiery
large area, high sensitivity, and high angular resolution will be
needed for Tong-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 re-
solve spatial and spectral features of sources and to analyze their
variations. The field of view of the telescope need not be wide, and
an appropriate goal for angular resolution is of the order of 1 to 2
arcmin. A high-resolution nuclear gamma-ray spectrometer would
also be needed for the study of the gamma-ray lines from radioac-
tivity in supernova remnants, positron annihilation in the galactic
disk and in extragalactic sources, nuclear excitations caused by
cosmic rays in dense matter, and nucleosyntheses in extragalactic
supernovae. The instruments for this era are discussed in Chapter
4. It is envisioned that these instruments would be well into their
development stage by 1995 and ready for a flight opportunity soon
thereafter.
. . ~
COSMIC-RAY ASTROPHYSICS
Cosmic-ray particles provide us with the only direct sam-
pling of matter of extra-solar-system origin. The great range of
particle energy, covering about 15 decades, and the variety of
species, encompassing the nuclei of all elements, as well as elec-
trons, positrons, and antiprotons, require a number of different
observational approaches. These approaches must be pursued on
the ground, above the atmosphere, and in deep space.
Most of the instruments flown previously on spacecraft are
of relatively small size and are therefore restricted to studies at
low energies (~1 GeV), where solar modulation strongly affects
OCR for page 29
29
the particles' energy spectra. Several of these spacecraft are ex-
pected to remain active into the 1990s. These investigations are
providing: (1) definitive measurements with very high precision
of the elemental composition of galactic cosmic rays (up to iron)
at Tow energies; (2) exploratory data on the isotopic abundances;
(3) direct sampling of particles in interplanetary space that are
accelerated in solar flares or in planetary magnetospheres; and
(4) perhaps, if our current interpretation is correct, a sampling of
local interstellar neutral gas through the observation of "anoma-
lous" cosmic rays (i.e., helium, nitrogen, oxygen, and neon, and
possibly other elements with high ionization potential that may
be accelerated at the boundary of the heliosphere).
The continuation of these studies through the 1990s will be
essential in order to understand the spatial and time variations
in the cosmic-ray flux through changing levels of solar activity.
Instruments of modestly increased size will be used during this
period in order to enhance the accuracy of the observations and to
explore the solar system outside the ecliptic plane. Most important
will be studies of the nucleosynthetic origin of the Tow-energy
cosmic rays through a more precise determination of their isotopic
abundances.
A continuing series of observations of galactic cosmic rays at
higher energies is being conducted on balloons and on the Space
Shuttle. These provide important exploratory data, but definitive
measurements have to await the availability of spacecraft capa-
ble of carrying very large payloads for extended periods of time
(years). Objectives of such measurements are to determine: the
isotopic abundances at high energies; the composition of the rare
ultraheavy nuclei; the energy spectra of nuclei and electrons over
a wide range; and the abundances of positrons and antiprotons.
The Astronomy Survey Committee had recommended carrying
out a series of such observations (A stronomy and Astrophysics for
the t980s), but only a few investigations are being implemented.
Thus, by 1995 we will probably still be short of decisive data that
could specify the character of particle acceleration in the galaxy, or
that could define the nucleosynthesis processes leading to cosmic-
ray matter and governing evolution of the galaxy. We will probably
also lack definitive data that could further our understanding of
the structure and properties of the interstellar medium, and that
could address some fundamental cosmological questions such as
matter/antimatter symmetry in the present universe.
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30
The investigations listed in the following subsections are under
way at present or are planned by 1995.
Interplanetary Spacecraft
Particle detectors on Pioneer 10 and 11 and Voyager 1 and
2 will continue to explore the solar system at ever-increasing dis-
tances from the Sun. Space scientists will use instruments on In-
terplanetary Monitoring Platform-8 (IMP-8) and perhaps on the
Wind spacecraft of the International Solar-Terrestrial Program
(ISTP/Wind) as references closer to Earth. The Ulysses Mission
will provide in situ measurements outside the ecliptic plane for the
first time. An important time period for cosmic-ray observations
at low energies will be the next minimum in solar activity, around
1988-1989.
Cosmic-Ray Composition Explorer
The exposure of a sensitive particle detector onboard an
Explorer-cIass mission outside the Earth's magnetosphere has been
highly endorsed by the scientific community, and should be im-
plemented before 1995. In this instrument a large-area solid-state
detector telescope, combined with trajectory measuring devices,
will obtain an order-of-magnitude improvement in sensitivity and
resolution over previous instrumentation. We thus expect qualita-
tively new insight into the elemental and isotopic composition of a
variety of particle populations at low energies (~1 GeV): galactic
cosmic rays, the "anomalous" component, and nuclei accelerated
in solar flares. The first two are thought to be rather contempora-
neous samples of the interstellar medium, while the third is a more
ancient sample that has been stored in the Sun for almost 5 billion
years. The comparison between the compositions of these particle
populations will therefore either reveal characteristics of the chem-
ical evolution of the interstellar matter for instance, a continuing
metal enrichment or indicate if the solar system composition is
atypical, perhaps owing to the admixture of fresh supernova ejecta
during its formation.
Space Shuttle and Space Station
A very large detector to measure elemental abundances and
energy spectra of cosmic-ray nuclei at very high energies has been
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31
developed for sortie flights on the Space Shuttle. A first flight
was performed in 1985, and several reflights are expected that will
yield data on the cosmic-ray composition up to energies around
10~3 to 10~4 eV. These measurements will constrain models of
galactic particle acceleration by determining, for instance, the en-
ergy dependence of the relative abundances of primary cosmic rays
(e.g., the iron/carbon ratio). They will also provide decisive infor-
mation on the interaction of particles and interstellar matter and
fields by measuring the relative abundances of spalIation-produced
particles (e.g., light nuclei such as lithium, beryllium, and boron)
over an extended energy range. A Tong- duration flight (approxi-
mately 1 year) of this same instrument could cover another decade
in energy, close to the "knee" of the cosmic-ray spectrum around
10~5 eV. This may be accomplished onboard a Space Station or
Space Platform in its initial operational configuration. This ex-
periment would reach into an energy regime where the cosmic-ray
composition is entirely unexplored at present.
Instrument Development
A cosmic-ray facility centered around a large superconducting
magnetic spectrometer in space has been proposed by the scien-
tific community and is currently under study as a high-priority
new initiative. The spectrometer will be the common, central
component for a succession of specific investigations, directed,
for instance, toward measurements of protons and electrons and
their antiparticles, antiprotons, and positrons. Other investiga-
tions might include searches for heavy antimatter, and studies of
isotopic composition at high energies. During the coming years,
detector development for such a facility is expected to be in ac-
tive progress. The first elements of the facility are to be deployed
during the initial stage of the Space Station in the early 1990s.
GRAVITATIONAL PHYSICS
Gravity is one of the fundamental forces of nature. Its ac-
tions in the neighborhood of relatively small masses like the Sun
and Earth are well approximated by Newton's laws. These laws,
however, have been superseded by Einstein's general relativistic
description of gravity. General relativity predicts small deviations
from Newtonian behavior in weak gravitational fields. Some of
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32
these deviations are being measured accurately in the solar system.
General relativity also predicts both new classes of phenomena in
strong gravitational fields, and the generation of gravitational
waves by accelerating massive objects such as compact stars in
close binary orbits (e.g., the millisecond pulsar) and supernova
explosive events in which enormous masses are hurled into space
. . . · ~
in gigantic exp oslons.+
One test of the first of these two classes of effects will be carried
out in NASA's gravity probe B (GPB). This experiment in earth
orbit will involve a rapidly rotating, superconducting, nioblum-
coated sphere whose pole direction will be affected a minuscule
amount a precession of 0.04 arcsec/yr by the rotation of the
Earth. The detection of this change should provide an important
test for the magnetic-like effects of mass predicted by general
relativity.
The already successful solar system tests of general relativ-
~ty could be extended to second (post-Newtonian) order in the
solar potential. Instruments to perform such tests could be de-
veloped by the mid-199Os. NASA has also considered a mission
(STARPROBE) to place a precision clock in a near-Sun orbit to
measure the second-order gravitational red shift. In addition, a
proposal exists to build a small astrometric optical interferome-
ter (POINTS) to measure the deflection of starlight by the Sun.
Each of these experiments also has many other scientific objec-
tives. Should either experiment be conducted and fait to confirm
the predictions of general relativity, we would be forced to re-
think our understanding of a fundamental part of physics and its
implications for astrophysics.
Tests for the existence of at least a restricted frequency range
of gravitational waves will be provided by spacecraft equipped with
dual-frequency transponders in distant trajectories throughout the
solar system. Passage of a gravitational wave between these space-
craft and Earth should provide a signature in the arrival time of
transponder response uniquely identifying the passage of such a
wave. Current tests are orders of magnitude too insensitive to
* A separate task group on fundamental physics and chemistry has
addressed the topic of gravitational physics in the context of this study. Its
findings are published in the volume Space Science in the Twenty-Fir~t Century:
Fundamental Physics and Chemistry. However, this subject is of great interest
to astronomers and astrophysicists and is therefore discussed briefly here.
OCR for page 33
33
detect such waves, but future observations of this kind may be suf-
ficiently improved to provide a useful complement of ground-based
instruments developed to detect gravitational waves of far higher
frequencies. To date, the only indication that we have for the
existence of gravitational waves comes from the secular decrease
in the orbital period of the binary radio pulsar.
NASA OPERATIONAL STATUS
The post-1995 program for astrophysics is strongly dependent
on the state of NASA technologies. The task group anticipates
routine Shuttle operation and the ability to transport large masses
to optimum scientific orbits.
With regard to the state of astrophysics projects, the task
group assumes the following:
.
HST will be operational with at least one, and probably
two, instrument upgrades completed.
.
GRO will be nearing the end of its operating lifetime with
about 50 percent of the scientific instrumentation operating.
. AXAF will be in orbit and nearing the time for its first
· ~
servicing.
.
SIRTF will be operating with the first cryogen servicing
planned for the near term.
LDR will be undergoing intensive design study.
. Among the moderate-scale projects, FUSE, QUASAT, and
HTM will also be operating.
INTERNATIONAL PROGRAMS
There are a number of programs originating outside the United
States in Europe, Japan, and the Soviet Union. These should be
listed here as well. Some have already been approved; others are
planned or only under discussion. Further, we must understand
that some of the missions both within and outside the United
States will be launched before 1995, while some might be delayed.
Others might be logical continuations of missions started before
1995. A clear-cut demarcation therefore cannot be drawn at 1995.
A further complication is the difference between international Eu-
ropean missions as distinct from autonomous national European
missions. Lumping all these varied efforts together, we then have
the following array of instruments:
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. Radio: There is a Japanese version of QUASAT (space
VLBI) under study by a working group. The plan is to launch a
5- to 10~m diameter antenna at 20 GHz into a 500- to 40,000-km
orbit around the year 1995. There also is an approved Soviet plan,
RADIOASTRON, to launch several radio telescopes into orbit for
VERB! purposes, possibly coordinated with QUASAT.
. Infrared and Submillimeter: ESA is constructing an In-
frared Space Observatory (ISO) to be launched around 1992. It
will have the capability for producing maps and obtaining low-
resolution (R = 1000) spectra over a wavelength range of 2 to 200
,um. There will also be polarization sensing capabilities aboard. A
further effort is the Far Infrared Space Telescope (FIRST), a het-
erodyne spectroscopy mission planned for the mid-199Os. It will
have high-precision capability for determining chemical composi-
tion and velocity structure within coo! clouds, at submillimeter
wavelengths.
· Optical Astronomy: The ESA astrometric mission HIP-
PARCOS will improve optical astrometric accuracy by an order
of magnitude and survey about 100,000 stars. It will provide
parallaxes for a variety of astrophysical studies including a re-
fined color-luminosity relationship and an improved distance scale
based on direct measurements of a few Cepheid and RR Lyrae
variables. HIPPARCOS will also provide proper motions with an
uncertainty of about 1.5 milliarcsec/yr. These will find use in
the study of the dynamics of the Hyades and in the determina-
tion of the birthplaces of young stars. In Japan, a survey-type
ultraviolet telescope, UVSAT, is on the menu of the series of
small-to-moderate satellite missions. The hope is that UVSAT
will complement HST with a sub-Lyman-alpha, moderate spatial-
and spectral-resolution capability. Currently, it is intended for
launch in 1995; the exact schedule will depend on that of Japan's
next generation of ground-based optical telescopes.
. High-Energy Astrophysics: Japan's Explorer-ciass x-ray
satellite, ASTRO-C, is being prepared for launch in 1987 in a col-
laborative venture with the United Kingdom. Whenever possible,
Japanese satellites will carry a small gamma-ray burst monitor
as well, as a matter of policy. The gamma burst monitor for
ASTRO-C will be provided by Los Alamos National Laboratory.
ASTRO-C will be nonimaging with an area of 0.5 m2. ASTRO-C
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may be followed by ASTRO-D. The plan is to carry an imaging de-
vice with an area in excess of 500 cm2 and with moderate angular
resolution (about 1 arcmin).
The West German ROSAT mission of 1987 will be followed
by a hard x-ray mission (SAX) being prepared in Italy and by a
gamma-ray imaging facility, SIGMA, being prepared in France for
flight on a Soviet mission.
In addition, ESA is planning a high-throughput x-ray spec-
trometer, XMM, within their future space science plan (Horizon
2000 Program).
Representative terms from entire chapter:
cosmic rays
