Space Science in the Twenty-First Century

Imperatives for the Decades 1995 to 2015

Overview

5

Astronomy and Astrophysics

BACKGROUND

The universe we perceive today appears much more complex in its design and more mysterious in its ways than anyone could have predicted in generations past. Early in this century, the stars were thought to form an unchanging cosmic tapestry, remote and inaccessible. But within our lifetime modern technology and physical theory have let us glimpse a far grander cosmological scheme. Our Milky Way galaxy is one of a myriad of island universes, flying apart after an initial "big bang" that not only determined the structure of the universe, but seems to have determined the physical laws that govern the behavior of elementary particles. Today, astronomers address questions that would have been framed in earlier times only by philosophers.

A major contribution to our expanded world view has come from new generations of astronomical instruments. Larger optical telescopes collect more photons, and electronic detectors and advanced spectrographs yield more detailed information about the physical state of matter throughout the visible universe than their predecessors. Observations using wavelengths outside the optical window began with radio astronomy and its revelation of entirely new aspects of the universe. A dramatic improvement came with the advent of space vehicles that, by carrying detectors above the atmosphere, allowed observations throughout the range of the electromagnetic spectrum—from x ray to infrared—that had previously been blocked by the Earth's atmosphere. High-energy astronomy using x-ray and gamma-ray observations has revealed violent phenomena invisible at other wavelengths. Ultraviolet astronomy allows stellar astronomers to study that region of the spectrum in which the spectral lines of key elements occur, while the development of infrared astronomy reveals yet another aspect of the universe: the interiors of the dark dust clouds where stars and planets are born.

The present rapid expansion in astronomy is not a transient phenomenon. There is no evidence that we are approaching a state of complete scientific knowledge; in contrast, a new age of space astronomy is just beginning—the era of "great observatories" in space. The period from 1995 to 2015 will be a pivotal one, since the time scale for planning observatories of the future is a decade or more. In this study, therefore, the steering group first examines the scientific considerations that drive the program. It then sets out the expected state of space astronomy 10 years hence and projects the classes of instruments that will be necessary during the succeeding 20 years. The program is bold but realistic.

Three basic principles guided the steering group's planning: (1) astronomy requires access to the entire electromagnetic spectrum, an access available only through space techniques; (2) the ability to obtain higher angular resolution will result in powerful new insights into stars, planets, and galactic nuclei; (3) telescopes with greater collecting area, higher resolution, and more efficient spectrographs will be needed in every wavelength band to observe the farthest and faintest objects. These basic thrusts provide the framework and the focus of the proposed program.

MAJOR SCIENTIFIC QUESTIONS

The astronomy and astrophysics program is designed to answer a set of fundamental questions that deal with three general topics:

1. The early universe, including the large-scale structure of the universe, dark matter, and the formation of galaxies.

2. Strong force regimes: physics of gravitational collapse and attendant active processes.

3. The formation of stars, planetary systems, and the origin of life.

Interest in these questions has persisted over the last two decades, and they remain as valid guides for the next several decades.

The questions related to the events of the early universe—starting with the basic puzzle, "How did the universe begin?"—appear to have a surprising connection with current theories of the fundamental forces between elementary particles. Various versions of current "Grand Unified Theories" of the fundamental physical forces lead to evolutionary models of the universe that require the average particle energy to be 10¹⁵ GeV (an equivalent temperature of 10²⁸ K) only 10^-35 s after the creation event (accelerators of the sort that physicists now use would need to be one light year long to produce particles of that energy). Before that instant of time, theories assert that the universe was composed of rapidly expanding matter in a primeval state; the tiny volume of the universe suddenly cooled down and was thus transformed into a hot gas in a manner somewhat analogous to supercooled water expanding when it freezes. After a brief period of extremely rapid inflation, the rate of expansion then settled down to the level that is deduced from present observations.

Such theories, resulting in a cosmology dubbed "inflationary" due to the initial period of rapid expansion, require a geometrically flat universe in which expansion slows down forever, growing to larger and larger dimensions but never halting. The amount of matter in such a universe must be about 100 times that which is deduced from all the visibly luminous material in all the stars of all the galaxies, and about 10 times the amount of "ordinary" matter, comprised of the familiar protons, neutrons, and electrons, believed to be present, although largely invisible. A number of independent arguments support the idea that a large fraction of the matter in the universe is "dark matter," whose nature is still a subject of speculation. Some "ordinary" dark matter could be very faint stars (brown dwarfs). But the additional dark matter required by inflationary theories cannot be ordinary matter. Rather, it must exist in some exotic form, such as massive neutrinos, or conjectured particles such as axions, photinos, or gravitinos. None of these particles has yet been observed. At an even more speculative level, the dark matter could be in the form of massive black holes or cosmic strings-infinitesimally thin (about 10^-30 cm) and enormously massive (about 10²² g/cm)stretching across the entire universe. Produced abundantly in the "big bang," cosmic strings would not be directly visible, but might be detectable by their large gravitational lensing effects.

In the past year, an even more exotic possibility has arisen with the "superstring" theory of matter. Highly conjectural, this theory has the attraction of being specific, since it leads to the concept of an 11-dimensional space-time with well-specified internal symmetry. The most straightforward argument leads to two symmetry groups, one of which gives rise to the universe of particles with which we are familiar. The other symmetry group would generate a completely different set of particles that we can detect only through their gravitational interaction. Thus, our universe might be coexistent with a second "shadow universe" of particles that interact through forces we can experience only through their gravitational effects. The methods of astronomy alone can measure these.

These explanations are far from established, yet the underlying fact is that the "dark matter" in the universe is present and remains to be understood. The subject of cosmology has always had close ties to theories of fundamental physics, and this continues to be the case. Our knowledge of the universe on a cosmic scale is still limited, and the methods of space astronomy, using the instruments proposed here, will bring vital new knowledge and understanding.

The second major topic, the behavior of matter under extreme astrophysical conditions, also emphasizes the important relationship between astronomy and modern physics. We know that white dwarf stars and neutron stars exist, and that supernovae mark the end of the life of a star. The physical processes associated with these phenomena are far from understood, but are of the most fundamental interest. Neutron stars, for example, are in a sense the largest nuclei of all, and their behavior is determined by the forces that act when matter is as dense as that in the atomic nucleus. Such behavior is far from simple. The complex phenomena associated with pulsars—which are rotating neutron stars—demonstrate this.

The explosion of a supernova is also ill-understood, and is equally important. The heavy elements that make life possible are generated in these explosions, but theory is only beginning to show how the explosion occurs and proceeds. Another intriguing question is the physical state of the stellar remnant of a supernova—a black hole or a neutron star. There are x-ray sources in binary star systems in which an unseen companion body is so massive that theory implies it might well be a black hole. Verification, it appears, will come only through space astronomy.

On a still larger scale, the powerful energy machines in quasars and active galactic nuclei seem to require a black hole of a million to a billion solar masses at the core. The program outlined for the period from 1995 to 2015 will probe ever closer to the black hole (or other large concentration of mass) at the heart of these mighty engines. There the principles of physics will be tested to their limits, for strong gravitational fields such as those near black holes are the least understood and tested of the fundamental force fields. In the study of black holes, Einstein's theory of general relativity receives its most severe test.

The third major topic has a number of aspects that have developed only within the past few years. The Infrared Astronomical Explorer Satellite (IRAS) has sent back a treasure of surprising information relative to the formation of planets and stars; interferometric astronomy has been used to detect planets by the wobble they induce in their companion star. Finally, it appears that even planets as small as Earth might be detected by large telescopes and imaging interferometers. These can then study the characteristics of the planetary atmospheres. If life—particularly earth-like life—is present, we may find evidence for it in the molecular constituents of those planetary atmospheres.

THE EVOLUTION OF SPACE ASTRONOMY

The early years of the space age brought great surprises. Some of the developments came from ground-based discoveries that at first seemed to be unrelated to the subjects of space astronomy; later developments have shown a multitude of surprising crosslinks. The picture now evolving of the unity of modern astrophysics explains the steering group's emphasis on the need to have simultaneous access to all regions of the electromagnetic spectrum.

Consider, for example, the events that followed the discoveries of the early 1960s. On the ground, radio and optical astronomers, working together, discovered quasars. The precise nature of quasars is still not understood, but they are the most powerful celestial engines that have been found in the universe, capable of radiating the power of thousands of Milky Ways from a volume that is only a trillionth that of an ordinary galaxy. It turned out that they are powerful emitters of x rays and gamma rays as well. The physics of their excitation may be closely related to the observed x-ray behavior of active binary stars, and it is clear that progress will come from a union of high-energy space astronomy with radio and optical observations.

The first evidence of cosmic x rays came in 1962. It was a complete surprise, unanticipated by any theories. As observations proceeded, it became clear that many stellar x-ray sources are in systems of binary stars, with the x rays being generated by matter from one member falling onto its companion star. The companion is generally a highly compressed star, sometimes a white dwarf or a neutron star-perhaps, in some cases, a black hole. These identifications were made possible when x-ray, optical, and radio astronomers joined forces.

Other surprises marked the early years of the space age. The discovery of the microwave background showed that the "big bang" concept of cosmology was fundamentally correct, and led to the construction of the Cosmic Background Explorer (COBE) mission, an Explorer-class satellite that will probe fundamental aspects of the relict radiation from the early universe. Pulsars were discovered by radio astronomy, and early rocket observations showed that the pulsars were emitting x-ray pulses as well. One of the earliest and most surprising observations made at gamma-ray wavelengths was that of gamma-ray bursts, detected by instruments aboard the Vela satellites, a series of satellites launched to monitor the nuclear test ban treaty of 1963.

As the era of telescopes in space began, the surge of discovery continued. The first x-ray astronomy satellite, Uhuru, generated a comprehensive catalog of x-ray stars, galaxies, and clusters of galaxies, and strong evidence was found for a black hole in the constellation Cygnus. In 1973, the Copernicus mission offered the opportunity for ultraviolet spectroscopy of galactic sources and the interstellar medium. Hot interstellar gas (about 500,000K) traced a lacy web along colliding fronts of expanding cosmic gas bubbles, the debris of supernova explosions. The first direct evidence for interstellar heavy hydrogen (deuterium) was also obtained in these pioneering observations. Very-long-baseline interferometry, using radio waves, revealed fine structure in the tight nuclei of quasars. Motions there were measured that appeared to be faster than the speed of light. The law of physics that prohibits this behavior for real motion is presumed valid, so the current belief is that this phenomenon is an optical illusion caused by bulk relativistic motions generated by the core of quasars. Furthermore, interferometry methods developed for radio astronomy appear to be directly applicable to high-resolution optical studies. The ambitious plans for infrared and optical interferometry during the period covered by this study will draw directly on this experience.

With the launch of COS-B in 1975, gamma-ray astronomy came into its own. Only 4 of the 26 high-energy gamma-ray sources discovered have been identified with known quasars and pulsars. The nature of the remaining sources, forming a catalog of UGOs (unidentified gamma-ray objects), is bating.

Discoveries in all wavelength bands revealed the need for various space telescopes. The first of these, a powerful x-ray telescope, was orbited aboard the Einstein Observatory (HERO-2) in 1978. At the limits of the universe, x-ray quasars were found to shine so powerfully that they were detected more readily than their optical counterparts. Close by, even the faint dwarf stars of the Milky Way were detectable x-ray sources, sometimes flaring to thousands of times the brightness of the largest solar flares. Early-type giant stars were found to be such prolific x-ray sources that often their entire surface seemed to be excited as though by a giant flare.

Early in the 1980s, the Infrared Astronomical Satellite (IRAS) opened a new wavelength band to investigation when it discovered a quarter of a million new infrared objects. As it focused on the young star Vega, 27 light years from Earth, IRAS detected what appears to be a protoplanetary system extending out to about 15 billion miles from the star. Thus opened a new era in planetary astronomy.

The IRAS infrared telescope was the latest space-age telescope to be launched. It represents, in a sense, the transition to a new era in space astronomy—the age of the great observatories. We now realize that we need long-lived telescopes in space at all wavelengths. In projecting the status of space astronomy in 1995, the start of the two-decade period addressed by this study, the general outlines of the program are clear.

The Hubble Space Telescope (HST), the first major observatory, is now awaiting launch. It will bring two major advances to astronomy by freeing a telescope from the limitations imposed by Earth's atmosphere. First, the telescope can observe far into the ultraviolet part of the spectrum, where many of the most important elements emit their fundamental spectral lines. Second, it will be free of the atmospheric blurring effect called "poor seeing," and will capture the finer details of celestial objects. At the same time, it will detect much fainter, more distant stars and galaxies, because the sharp images it can produce will stand out with greater contrast against diffuse sources of light in the night sky.

Gamma-ray astronomy exploits the highest energy range of the electromagnetic spectrum, a difficult band to study since the sources yield so few photons on Earth. The Gamma Ray Observatory (GRO) will cover the spectral band from about 1 to 1000 MeV, and represents perhaps the ultimate capable with present-generation instrumentation.

The Hubble Space Telescope and the Gamma Ray Observatory are due to be launched as soon as the consequences of the Challenger accident are resolved. The evolution of space astronomy in the years that immediately follow these launches has been set out in the NRC report Astronomy and Astrophysics for the 1980s, in which the Astronomy Survey Committee (ASC) formulated a program for the next decade. The steering group found in that report a reliable road map for the next 10 years of space astronomy. With the exception of the Large Deployable Reflector (LDR), which is one of the steering group's major recommendations for the period from 1995 to 2015, all the components cited by the ASC should be well advanced by 1995.

As the ASC suggests, the next major step in x-ray observatories will be the Advanced X-ray Astrophysics Facility (AXAF). The AXAF aperture will measure 1.2 m in diameter, twice that of the Einstein Observatory, and will contain a nest of seven reflectors. It will have 10 times the angular resolution, 50 times the sensitivity, twice the spectral range, and 1000 times the energy resolution of the Einstein Observatory. The kind of work conducted earlier by the Einstein Observatory will be greatly accelerated with AXAF.

The development of infrared astronomy may soon rival radio, optical, and x-ray astronomy. The Space Infrared Telescope Facility (SIRTF), which will be 1000 times as sensitive as IRAS, will join the family of great observatories when it has been placed in orbit. It will bring millions of infrared sources within observing range.

The design technology for each of the great observatories to follow the Hubble Space Telescope is well in hand. This entire constellation of spacecraft can be in place before the mid-1990x. The ability to conduct coordinated observations at various wavelengths will be one of the great benefits of flying these instruments simultaneously.

The Astronomy Survey Committee recognized that there is an equally important, far less expensive, component of the space astronomy program that must not be neglected. This "exploratory" program consists of smaller, ad hoc projects that prepare the way for major thrusts of the future. Explorer-class missions exemplify this kind of project, and several of these will become operational during the coming decade. The X-Ray Transient Explorer (XTE) will allow in-depth investigation of the bursts, pulse, and other transient phenomena characteristic of active x-ray sources. The Far Ultraviolet Explorer (LYMAN) will allow observation of atomic and molecular spectra over a wide range of energies that are beyond the wavelength limit of the Hubble Space Telescope. The orbiting very-long-baseline interferometry radio telescope (QUASAT) will permit the expansion of interferometry to baselines larger than Earth, and will obtain resolution of quasars and other active objects that approach 1 arcsec. In the spirit of the Explorer program, other interesting exploratory projects will surely arise over the coming decade, and NASA should stand ready to exploit these opportunities.

RECOMMENDED PROGRAM: POST-1995

The advance of space technology, lifting power, and space assembly capability offers great promise for a number of new ventures in space astronomy. In the first section of this chapter, it was shown that these can be classified, broadly, into two categories: instruments that will give the kind of breakthrough in angular resolution that will allow the study of fundamental phenomena, and telescopes of great collecting area and spectroscopic capability that will carry on the tradition exemplified by the large telescopes of Earth such as the 100-inch Mt. Wilson telescope and the 200-inch "glass giant" of Palomar.

The program for astronomy and astrophysics can be classified more explicitly as follows:

1. Imaging Interferometry

(a) Large Space Telescope Array

(b) Long Baseline Optical Space Interferometer

(c) An array of. very-long-baseline interferometry (VLBI) stations in space

2. Large-Area and High-Throughput Telescopes

(a) A large deployable reflector (LDR) for submillimeter studies

(b) An 8- to 16-m optical apace telescope

(c) Large-area telescopes for the energy range 20 keV to 2 MeV

(d) A large Compton telescope for spectroscopy, 0.1 to 10 MeV

(e) Large gamma-ray telescopes for energies above 2 Me V

3. AstroMag, a massive cosmic-ray analyzer in space

Some of these projects, which are explored more fully below, are logical successors to those now under way. On the other hand, some are novel and will require new research programs. Realization of the recommendations for interferometry, for example, will require a variety of preparatory technological studies during the coming decade to establish the background for major missions in this field.

Imaging Interferometer (Optical and Infrared)

A two-step plan for interferometric projects can be foreseen. The earliest mission would probably be a large array of telescopes. A reasonable projection would be for an array of several telescopes mounted on a structure 100 m or so in diameter. This structure could be tetrahedral, supporting nine 1.5-m telescopes, three along each leg of the base with a signal-processing cabin at the fourth vertex. This array could map a field of about 0.5 arcmin with a resolution of 0.5 milliarcsec at a wavelength of 5000 angstroms. It would have a sensitivity higher than that of the Hubble Space Telescope and would provide images 100 times sharper in angular detail.

The next step, to a long-baseline space interferometer, is more challenging and demands far more engineering study. It would consist of telescopes whose baselines would range from tens of meters to many kilometers. If station-keeping technology and the art of metrology can advance, these instruments might be independently orbiting, perhaps at the stable Lagrangian point. They might also be constructed on the Moon.

The VLBI radio array in space would be a logical extension of the QUASAT project. The project could be an evolutionary one, with standard radio telescopes placed in successively higher orbits. It might well be a cooperative international project.

The Large Deployable Reflector (LDR)

One of the highest priority missions for the United States astronomical community is the Large Deployable Reflector (LDR) being designed for work in the far-infrared and submillimeter regions. The desired aperture is in the 20- to 30-m range, and the instrument would need to be assembled by astronauts at a space station. It will provide high angular resolution (1 to 2 arcsec at 100 m and a diffraction limit of 0.3 to 0.6 arcsec at 30 m. Improving our present capabilities nearly 1000 times, the LDR would join the suite of great observatories. It would be a natural sequel to the SIRTF mission.

An 8- to 16-m Telescope for Ultraviolet, Optical, and Infrared Wavelengths

About the same time as we deploy the LDR, we will require a filled-aperture telescope of 8- to 16-m diameter, with ambient cooling to 100K for. maximum infrared performance. Such an instrument will monitor the range of wavelengths from 912 angstroms to 30 m and will follow on 10 to 20 years of study with the HST and ground-based 8- to 10-m telescopes. It will complement the space interferometer and provide images 6 times sharper than the HST. It will also be far more sensitive than HST because of its large aperture and small image size (10^-2 arcsec for a 15-m diameter telescope in visible light). In addition, it would surpass the HST in the study of distant quasars and the evolution of galaxies, including the formation of binary systems and planets. Finally, direct imaging of sources with a filled aperture will offer substantial advantages over images reconstructed with model-dependent techniques from interferometric data.

X-Ray Instruments of Large Area and Throughput

The Advanced X-Ray Astronomy Facility (AXAF) will elicit new scientific questions we cannot yet foresee. Nevertheless, we will probably need an instrument that can perform high time-resolution studies and high-resolution spectroscopy and make observations at higher x-ray energies. This might take the form of a Very High Throughput Facility (VHTF) that could perform the spectroscopy, or an x-ray timing facility that could look for rapidly varying events correlated with the results from gravity wave detectors. A High Energy Imaging Facility (HXIF) might allow the first in-depth exploration of the hard x-ray/soft gamma-ray region of the spectrum, roughly from 20 keV to 2 MeV. This instrument would address fundamental questions about anisotropy in the early universe. It would also give us insights into compact objects, stellar collapse, and star formation. At present, there is a gap in the electromagnetic spectrum in the 20-keV to 2-MeV range that must be filled. An instrument observing in this region must have a large area, since incoming photons are few.

Gamma-Ray Telescopes

We will have gained several years of experience with the Gamma-Ray Observatory (GRO) by 1995, and the results will surely influence plans for the gamma-ray observatories of the future. The gamma-ray domain is of vital interest for two reasons. First, the nuclear lines characteristic of supernovae and other high-energy phenomena appear there. Second, the character of very high energy gamma rays is quite unexpected. An Advanced Compton Telescope or other spectroscopic device can provide the capability to carry out gamma-ray spectroscopy effectively. Above 50 MeV, where basic pair production processes do not generate a limiting background, we should be able to realize an angular resolution approaching 1 arcmin.

Cosmic-Ray Research

Programs in particle astrophysics will explore new regions of the spectrum at greatly improved levels of sensitivity and resolution. Many of the current cosmic-ray problems should be accessible to ASTROMAG, a superconducting magnet spectrometer with capabilities comparable to those used by laboratory physicists at the large accelerators. The facility should be an early project in the era 1995 to 2015.

CROSS-LINKS WITH OTHER DISCIPLINES

The Sun is our closest star, and it provides, along with the solar plasma, a basic reference point for many astrophysical problems. Since the Sun is a star, solar astronomy and stellar studies are closely linked. The interferometric instruments projected for the period from 1995 to 2015 are likely to open a new era in stellar studies, as spots, flares, and other phenomena begin to be studied directly on other stars. The Einstein Observatory has already demonstrated that studies of stellar coronas can be carried out by x-ray instruments.

Studies of other planetary systems and, possibly, of life outside the solar system will be a goal of the various interferometric systems and for the 16-m optical telescope. The instruments, if properly designed, should be capable of detecting planets—even small planets like Earth—in orbit about nearby stars. They might be able to study the atmospheric constitution of these planets as well. If abundant atmospheric oxygen or other evidence suggestive of biological processes is found, there is potential for a new link both with planetary science and the life sciences.

CONCLUSIONS

The recommendations of the Astronomy Survey Committee form a valid basis for assessing the expected status of astronomy and astrophysics in 1995. The major science goals for astronomy can be formulated with some reliability. A period of great scientific productivity during the time from 1995 to 2015 can be expected. The guiding principle is to assure access to the entire electromagnetic spectrum, to obtain high (milliarcsec to arcsec) angular resolution from radio to ultraviolet wavelengths, and to build telescopes with large collecting areas and spectrographs of high throughput. At the same time, a vigorous exploration program, carried out by Explorer-class satellites, promises to supply a sound basis for studies in the even more distant future.