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214 C. General Recommendations The Panel urges that maintaining and upgrading the large investment in the National Astronomy Centers be given a high general priority in the 1980's. It recommends, in particular, strengthening the computing facilities and receivers of the Very Large Array, operated by NRAO, and urges that serious consideration be given to a 100-ft ex- tension of the large Arecibo telescope, operated by NAIC. III. DESCRIPTION OF RECOMMENDED PROJECTS AND FACILITIES A. Very-Long-Baseline (VLB) Array Description of the Array. The United States, because of the long north-south and east-west extent of its terri- tory, is better suited than any other nation for a VLB Array. The extremities of the proposed Array are in New England, Alaska, and Hawaii, the maximum spacing being 7500 km, or slightly larger than the radius of the Earth. Twenty-five equal antenna spacings are needed for uniform sampling, but in practice about twice as many are required to give adequate coverage at low declinations. Thus 10 antennas representing 45 antenna pairs or baselines are needed. The Array will give high-contrast images over a large part of the sky visible from the northern hemi- sphere. Although not coherently phased owing to atmo- spheric and other environmental fluctuations, closure around the 36 independent triangles yields 80 percent of the phase information, and it is known from recent experi- ments that very accurate maps can be made. Only a dedi- cated Array will provide the flexibility required for observing transient phenomena and the frequency agility needed for observations in many wavebands. Cost. The Array will cost about $35 million (1980) if constructed with all new equipment, but $3 million to $5 million might be saved by using existing antennas, equip- ment, and sites. No new technology is involved. All components are based on existing designs; prototypes generally exist and are in use. The cost of the system has been calculated in considerable detail and includes site development, antennas, instrumentation, computers, and software. Scientific Justification. The most fundamental problem in extragalactic astronomy is to understand the structure of the energy source in quasars and galactic nuclei and

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215 the nature of the physical processes responsible for the enormous energy released by these objects. Observations with the VLB Array are of critical importance since the Array's angular resolution is vastly superior to that of any other astronomical instrument. In a 1979 personal communication furnished to the panel, Sir Martin Ryle, the Astronomer Royal, has written, n If we are to have any hope of solving the problem of the origin and transmis- sion mechanism of the huge energies involved in the main components of radio galaxies, then it will be by comparing the most compact nuclear structure with the more extended features. . . ." Crude maps, with only a few picture elements, often show that the nuclear radio structure is aligned with features that extend over several megaparsecs, the most famous example being NGC 6251. Thus the directivity is established by the central object and is maintained for periods of 108 years and longer. The VLB Array will give high-contrast maps that should greatly improve our knowl- edge of the connection between the inner and outer components. Central radio components are almost always asymmetric and in a few cases show superluminal transverse motions, properties that can be explained in terms of radiation from a symmetric relativistic beam. m e VLB Array will give an improvement of an order of magnitude in the con- trast with which these phenomena can be studied, and for the first time it will be possible to make frequent repeated observations and follow the complex changes occurring in the brightness distribution. In addition, polarization observations will enable us to map the mag- netic field and its changes on scales of a few parsecs. More than half of the extragalactic objects detected in high-frequency radio surveys are self-absorbed, com- pact, and variable, and ordinary spiral and elliptical galaxies frequently contain such sources in their nuclei. The VLB Array will follow the evolution of such objects in detail. m is is not possible in other wavelength bands or with conventional radio aperture synthesis arrays. Both radio and optical observations show evidence of dramatic cosmic evolution. The probability of a galaxy or quasar being an extended radio source increases rapidly with distance, but compact sources appear to be distributed more uniformly. The relation between the compact and extended radio sources and the reason for their apparently different spatial evolution is not under-

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216 stood. Complementary observations made with the VLB Array and the Very Large Array (VLA) Will have a profound impact on our understanding of galactic nuclei and quasars and how they evolve. One of the great scientific attractions of the VLB Array is the possibility of using it for direct trigono- metric distance measurements on Galactic and even extra- galactic scales. Clusters of tens to hundreds of small H2O maser sources have been observed whose relative velocities are apparently random. m e distance to such a cluster can be determined by the classical method of statistical parallax--that is, by comparing the disper- sion in radial velocity with the dispersion in proper motion. Very recently, the distances to the H2O masers in the Orion nebula at 500 parsecs and in the giant H II region W51 at 7000 parsecs have been measured with VLBI to an accuracy of 20 percent. The dedicated VLB Array is essential if this technique is to be applied to a wide class of distant and faint maser clusters, because sensi- tive aperture synthesis maps are required. Within a decade proper motion studies with a VLB Array may permit direct measurements of distances over the entire Galaxy, to an accuracy of 10 percent. Extension to extragalactic H2O masers is possible to comparable accuracy. Gravitational bending of light, one of the main tests of General Relativity, can be measured most accurately by VLBI. The characteristic resolution of the VLB Array for Galactic sources is of the order of an astronomical unit (AU). The intense emission from astronomical masers has great potential as a probe of temperature, density, mag- netic fields, and dynamics on scales as small as 1 AU. Since masers are found in interstellar regions of star formation and in the envelopes of evolved stars, their study provides unique information on the birth and death of stars. A variety of stellar objects including novae, supernovae, binary stars, flare stars, and x-ray sources emit transient radio emission from regions of interplane- tary dimensions. Peculiar objects such as SS433 and the continuum source at the Galactic center have been observed by VLBI and found to have many similarities to the radio sources in active galactic nuclei and quasars. There are also applications of VLBI in geophysics, since it provides very accurate astrometric and geodetic measurements. Among the more important applications are the measure- ments of time, polar motion, and continental drift The technique of VLBI has reached maturity with the development of procedures that enable us to make true .

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217 aperture synthesis maps with an instrument of global dimensions. m e resolution of the VLB Array will exceed that of the VLA by two and a half orders of magnitude and that of the Space Telescope by two orders of magnitude. The time is ripe for a modern instrument that will exploit the potential of VLBI by providing high-contrast, high- resolution maps of a wide variety of astronomical objects The VLB Array will constitute one of man's great innova- tions in terms of scientific technology. B. 10-Meter Submillimeter-Wave Telescope . Submillimeter astronomy is a rapidly developing field of research that will have a major impact on astronomy in the 1980's. The opportunity,exists to observe a substan- tial portion of the submillimeter spectrum by combining recently developed high-altitude sites such as Mauna Kea with techniques for making large, ultraprecise antennas and very sensitive receivers. A submillimeter telescope of about 10-m aperture at a high site would provide the first opportunity for detailed studies of this rich region of the electromagnetic spectrum and would give angular resolutions of as small as 8.5 arcsec--a tremendous step forward in the study of interstellar molecules and con- tinuum sources. Within the submillimeter band lie three major areas of interest: first, the studies of spectral lines of mole- cules (mainly rotational lines) and of atoms (fine struc- ture lines); second, studies of continuum emission due to dust; third, the continuum radiation with a supposedly thermal spectrum of the 3 K cosmic background. Much work has been done at radio wavelengths on the rotational lines of interstellar molecules. However, the fundamental transitions of many molecules, particularly hydrides, lie in the submillimeter band and have not yet been observed. Also, atomic transitions such as the recently detected line of neutral carbon (C I) lie in this range, and these will provide new probes into the nature of the interstellar medium, its density in par- ticular. Metal abundances might be traced throughout the Galaxy, and across external galaxies, by means of the lines of simple diatomic hydrides. The role of carbon in interstellar chemistry can be traced by comparing the relative abundances of C I and CO. The ability to study high transitions of abundant molecules such as CO will be of great value. These

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218 lines, together with the atomic carbon lines, provide a cooling mechanism for interstellar clouds, thereby deter- mining the tendency toward collapse and fragmentation into stars. Further, the presence in the spectrum of many lines of the same molecule permits a much greater understanding of radiation transfer within molecular clouds, and tests theories of cloud structure and dynamics. The high resolution available will promote studies of molecules and atoms not only with the Galaxy and its neighbors but also within more distant galaxies. With a resolution of 8.5 arcsec, submillimeter observations will approach 21-cm maps from the VLA and optical photographs in the spatial detail that can be observed. Good sub- millimeter resolution will also be of great value in permitting observations of molecular envelopes around stars with mass loss. In observations of continuum radiation from quasars and bright radio sources there is a serious gap in the submillimeter band that must be filled if we are to understand the relationships among, for example, the sources of radio emission in quasars, BL Lac objects, and the sources of optical nonthermal radiation. Such studies, plus observations from dust clouds throughout the Galaxy, would be greatly facilitated by a 10-m tele- scope. An equally important source of continuum emission is the cosmic background radiation. Here the telescope would be capable of measurements above the peak of the 3 K blackbody spectrum and would be especially useful for measurements of small-scale anisotropy, such as might arise from primordial density fluctuations in the early Universe at a red shift of about 1000. The current technological status of telescopes and receivers in the submillimeter band is well matched to the opportunities offered by the best high-altitude sites. For a precipitable water-vapor content of 1.5 mm or less the atmosphere is reasonably transparent down to 300 Am, except at certain water lines. As shown by the Westphal survey, such conditions are more often found at Mauna Kea than at any other well-developed U.S. observatory. R. B. Leighton has demonstrated that a 10-m telescope can be constructed that is diffraction limited at 300 ~m. Such a telescope is already operational at the Owens Valley Radio Observatory at 1200 m. For several years continuum detectors have been capable of near-background-limited performance in the submilli-

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219 meter band. Now heterodyne receivers, suitable for observing spectral lines, have demonstrated the fea- sibility of submillimeter operations. The ground-state fine-structure transition of C I was recently detected at 610 am using an InSb heterodyne detector with a noise temperature of only 350 K. Schottky diode and new super- conducting mixer detectors are also under development and are expected to perform well. In summary, new developments within the United States have led to world leadership in telescope and receiver technology at very high frequencies. A 10-m-class sub- millimeter telescope at a good high-altitude site such as Mauna Kea offers no serious technological obstacles and would be an astronomical facility of great power with many exciting applications. C. Space VLBI Why Go To Space? The size of a VLBI Array can be extended far beyond the confines of the Earth. VLBI observations of quasars, galactic nuclei, x-ray stars, active binary systems, and interstellar masers show that there are unresolved structures for interferometer base- lines approaching an Earth diameter. Time variations of radio sources suggest that many of the structures are either so compact that baselines far longer than the Earth's diameter are needed to study them properly or that the structures are changing at highly superluminal velocities. Each new advance in angular resolution probes more closely to the seat of the action. The space VLBI system is the next step in the advance- ment of VLBI studies, leading to a significant improve- ment in angular resolution over ground-based systems. A single station in near-Earth orbit, together with the stations of the ground-based VLBI network, form the inter- ferometric system. This system has the multiple advan- tages of mapping very quickly, with higher angular resolu- tion, with a large dynamic range, and at the same time will yield essentially complete coverage of the southern sky. Conceptual and engineering studies are now under way that could lead to an orbiting VLBI facility in the late 1980's. The concepts were examined in depth in 1979 by the Space Science Board Workshop on Space Radio Astron- omy and by the National Aeronautics and Space Administra- tion's (NASA) Study Group on Advanced Programs, which met at Woods Hole to consider new long-range scientific ini-

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220 tiatives. Both groups concluded that there existed a wide variety of challenging scientific problems for near- Earth VLBI and that technical specifications for such a facility could now be met. Concept of the Orbiting VLBI Station. m e proposed system would consist of a 25-m deployable radio telescope, with receivers at standard VLBI bands. It could be used in conjunction with all major ground-based radio tele- scopes in the world, but the dedicated VLBI Array would be the principal ground-based component. m e data system would be of the Mark III type, with variable bandwidth in 2-MHz increments up to 56 MHz and a wideband 112-MHz option for problems requiring the highest sensitivity. Data transmission would probably be via the standard Tracking and Data Relay Satellite System (TDRSS), with the data stream being recorded on the ground for reduc- tion at the VLBI data-processing center of the ground- based array. With the ground-based VLBI network of 10 antennas, the orbiting station forms a VLBI system that samples the Fourier-transform plane with baselines up to a full Earth diameter. Because of the motion of the orbiting antenna relative to the ground-based elements, the sampling is much denser than that achieved by the ground array alone. By the use of closure phase methods, effective beam pat- terns should have a contrast ratio of over 100:1, allowing the study of complex sources having a large range of intensity (dynamic range). The addition of an orbiting station gives new capabili- ties to the ground-based array. ~~ ~ ~ · ~ · · . ~ . . · . __~ No longer is high-quality mapping limited to high northern declinations, since the synthesized beam is nearly homogeneous over most of the sky. Most of the sky is accessible to the southernmost ground station in Hawaii, and by adding southern hemi- sphere NASA stations, combined with international arrange- ments, the detailed VLBI study of radio sources in the rich southern sky would be possible for the first time. The Galactic center, the Magellanic Clouds, the x-ray source Circinus X-1, and the bright radio galaxy Cen- taurus A are examples of the important southern sources that become accessible to VLBI study. Secondly, the increased baseline enhances the angular resolution in maps of northern hemisphere sources by approximately a factor of 3, leading to an order-of-magnitude increase in the information content (picture elements) of the result- ing maps. Thirdly, the dense coverage of the Fourier-

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221 transform plane should yield enhanced contrast ratios. Fourthly, the orbiting station passes rapidly through the Fourier-transform plane in each orbital passage, and the multiple baselines from space to the ground array give the capability to make two-dimensional maps of rapidly time-varying phenomena. The system performance can be estimated from present practice. If a 25-m orbiting telescope is used in con- junction with a single 25-m ground-based telescope, the rms fringe detectability is 3 may (assuming 50-see inte- gration, 112-MHz bandwidth, and 20 K geometric-mean system noise). This assures a wide range of potential objects for study. Relatively simple sources can be studied when the source flux is approximately 16 times the minimum fringe visibility, and the source counts of Kellermann and Pauliny-Toth predict approximately 60,000 sources at that level. For detailed high-quality mapping, a source flux 50 times the minimum fringe detectability is needed, and about 10,000 sources will meet that requirement. one can expect that about half of these sources will be accessible to VLBI studies. Scientific Applications of Space VLBI. The general classes of VLBI studies have already been considered in the discussion of the ground-based Array; here we briefly summarize the relevant conclusions. The study of quasar phenomena, critically dependent on the construction of densely sampled maps, can be advanced greatly when a factor of 3 in angular resolution is gained. Two well-known examples can be cited: the super- luminal source 3C345 and the well-known galaxy NGC 1275 (3C84). The best maps of 3C345 show that the superluminal components are strung out like pearls on a string, and it must be determined whether the individual knots are moving out of a superluminal velocity or a wave of excitation is successively exciting new concentrations. If the best VLBI maps are degraded by only a factor of 3, this crucial detail is completely lost--the map then shows only an elongated, cigarlike distribution. Similarly, a degrada- tion by a factor of 3 destroys the detail in the maps of 3C84, which has a complex, time-varying structure. The concentrations are distributed in two dimensions, but a loss of a factor of 3 in resolution degrades the map to that of a slightly irregular elliptical blob. Radio galaxies share many of the properties of quasars. They show most dramatically the organization of radio jets in hierarchies of scale, with a remarkable degree of

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222 co-alignment. There is great current interest in this phenomenon, which is probably linked closely to the nature of the central energy source. The organization is hier- archical, and each new step presses closer to the level at which relativistic plasma acceleration occurs. Time-varying Galactic sources are a further class of phenomena about which we have more questions than answers and that the capability of the orbiting station can address. The x-ray sources Sco X-1, Cyg X-1, and Cyg X-3 and mass-transfer binaries such as Algol and RS CVn stars exhibit radio flares that vary in intensity within a few hours. The satellite moves rapidly, forming a good two- dimensional image in half an hour or less. For rapid variables like Cyg X-3 and Algol, which can exceed a Jansky in flux (Cyg X-3 is sometimes one of the brightest sources in the radio sky), the time development of these explosive phenomena would be studied on a scale of a few astronomical units by the VLBI system under discussion. The intention would be to relate the radio output to the dynamics of the mass transfer process. The sensitivity of the system depends on the source complexity, but radio star outbursts with a peak flux of about 50 mJy would certainly be accessible to study. The study of interstellar masers profits enormously from the enhanced capability of the space VLBI system. The masers are Galactic objects, and with the space VLBI system, the entire southern Milky Way, plus the Magellanic Clouds, is brought into view. Galactic astronomers know all too well the hazards of studying galactic objects without reference to the phenomena of the southern sky The general considerations already discussed under the heading of the VLB Array therefore apply with special . emphasis in this case. In summary, we expect that the scientific results of the proposed system will lead to fundamental advances in many areas. Furthermore, the work will give convincing justification for the more ambitious elliptic-orbit mis- sions of the 1990's. These missions will be both complex and expensive and demand strong scientific justifications. 1. State of Technological Readiness The principal components of the space VLBI station are the antenna, the receiver, the control and guidance system, the telemetry system, the recording system, and

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223 the data-reduction system. All subsystems are essentially at state-of-the-art readiness. Deployable mesh antennas 10 ft in diameter have been used in space, and a 25-m deployable mesh paraboloid with good 1-cm performance is well within the limits of present practice. The pointing requirement would be 10 arcmin, which is also a reasonable specification. Telemetry and data recording would rely on the TDRSS now under development by NASA. The Mark III VLBI data system is now in use, and a straightforward extension of the system will accommodate space VLBI requirements. 2. Elliptic Orbit Studies The freeing of VLBI from near-Earth orbits leads to mani- fold possibilities, in which virtually every point already discussed is further strengthened. The project is com- plex and expensive, with no engineering definition yet, but the plans should start in this decade. The precise antenna size, number of spacecraft, orbit choice, fre- quency range, and associated ground-based system are all uncertain. The lead times are so long that serious studies should be undertaken soon; NASA centers and universities and industrial laboratories should work together on the enterprise. One can also foresee that there may be natural opportunities for international cooperation. 3. Cost Estimates An orbiting VLBI station bearing a 25-m radio telescope can be carried on a dedicated special-purpose satellite or could be mounted on a large multipurpose platform such as the Space Science and Applications Platform now being studied by NASA. The cost estimates for the dedicated satellite can be based on the studies for various standard-module-spacecraft missions, with the antenna cost separate. The spacecraft, if it is of the standard modular type with TDRSS and pointing capability, would cost about $80 million, the deployable paraboloid would cost about $20 million, the electronic system should cost about $10 million, and an additional $10 million would be needed for ground-support and data-processing systems. Thus the total cost in 1980 dollars would be S120 million .

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224 D. 100-Meter Telescope Radio astronomy uses two fundamentally different types of telescopes: interferometers and filled-aperture antennas. Each instrument has limitations absent in the other, and the two are complementary. For this reason a large fully steerable filled-aperture telescope is a necessary tool to radio astronomy. An interferometer is needed for high angular resolution but suffers from relatively poor sensi- tivity to low-surface-brightness sources, and there is a certain size scale that it will "resolve out" and hence cannot see. A filled-aperture telescope, on the other hand, lacks high resolution but is good at mapping large areas and sources of low surface brightness. A filled- aperture instrument also has greater frequency agility, higher instantaneous bandwidth, the ability to handle rapidly fluctuating signals, lower instrumental polar- ization, and higher dynamic range. All previous studies of the needs of U.S. astronomy have recommended the construction of a large general- purpose radio telescope to work at wavelengths of roughly 1 cm and longer. In the judgment of the Panel on Radio Astronomy the arguments for such a facility remain very strong, and an instrument in the 100-m class is an impor- tant priority for the 1980's. In the Arecibo telescope the United States possesses the largest single-aperture radio telescope in the world, but one with severe limitations. Most of the sky (about 60 percent) is inaccessible to it because of its limited steerability--the southern Milky Way, the Galactic center and M31 cannot be observed--and, because of the short time it can track a source, it is of limited utility in VLBI studies. A steerable telescope in the 100-m class would overcome these limitations and would provide a decade of high frequency coverage not attainable at Arecibo. Receiver development has now progressed to the point where system noise is often dominated by Galactic back- ground at low frequencies and by the atmosphere at high frequencies. Improvements in the feed-antenna structure, which give rise to spillover and see-through contributions to the system noise, remain as the principal areas for improvements in signal-to-noise ratios. A new, fully steerable filled-aperture instrument would include such improvements. In addition, current techniques would take into account the ability to place many receivers at a focus of such an antenna, so that multibeam mapping experiments could be performed.

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225 The uses of such a telescope are many and are excit- ing. m ese include detecting and mapping nearby galaxies, mapping large cluster halos and cluster galaxies, and map- ping within our own Galaxy. A reduction in source confu- sion, because of the small beam available with a large filled aperture, would make such an instrument ideal for searching for small scale fluctuations in the 3 K back- ground as well as absorption of the background radiation by inverse Compton scattering in the intercluster medium. Molecular clouds too large to be mapped with a synthe- sis instrument would be ideal objects for a filled- aperture instrument. Frequency agility of the telescope would allow easy changeover for studying different lines. A particularly important experiment would be the search for highly red-shifted molecular features in absorption-- features comparable with the highly red-shifted neutral hydrogen line. Such molecular features would help to settle the ongoing argument as to whether the hydrogen clouds detected in absorption are intervening or intrinsic to the background source. Further, the finding of high- rea-sn'~t molecular Lines would help to fix the constancy of the electron-to-proton mass ratio in a manner analogous to the use of neutral hydrogen and optical absorption lines to study the constancy of other fundamental con- stants of nature. The small beam and high gain of a large filled aper- ture are both advantageous for emission-absorption obser- vations. Such experiments would be especially interest- ina for species with a partition function far from Boltzmannian. Determininq the partition functions of molecules would advance the science of interstellar ther- modynamics, i.e., the study of the effects of shock waves, ionization fronts, and other density and temperature inhomogeneities of the interstellar medium. The detailed study of recombination lines not only in our own Galaxy but in other galactic systems, and possibly in quasar envelopes where optical or heavy-element lines might appear, offers important research possibilities. Pulsars are an ideal field of study for a fully steer- , . able antenna. Both rapid time fluctuations as well as a long time base can be obtained with such an instrument. Pulsar scintillation observations would also be greatly advanced by such an instrument. A large filled-aperture telescope would also be a vital element in VLBI experiments, allowing such observations to go to much fainter limits.

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226 E. 10-pm Heterodyne Interferometer Heterodyne interferometry, developed by radio astronomers to increase the angular resolution of their telescopes, can be extended with lasers far into the infrared, and the first steps to do this have already been taken. Interferometric observations with the 24-inch auxiliary solar telescopes at Kitt Peak, using a CO2 laser as a local oscillator, have demonstrated the feasibility of a heterodyne interferometer working through the 10-m atmospheric window and have shown that there are a number of challenging applications for an instrument of this kind. m e work so far has been with a fixed baseline of 5.5 m; longer and variable baselines are needed to exploit the full power of the technique, and it is highly desir- able to employ somewhat larger telescopes to increase the sensitivity. The Panel on Radio Astronomy finds that such a dedicated 10-vm heterodyne interferometer can now be constructed at moderate cost, and the Panel believes that such an instrument would prove an innovative and produc- tive astronomical facility for the 1980's. The facility that the Panel recommends would consist of two mobile infrared telescopes, each about 60 inches in aperture. For observing, these are located at sta- tions of varying separation and orientation. Each tele- scope should be capable of blind pointing to high preci- sion, about 1 arcsec, and the optical path through the system should be constant to 1 Em or less. Desirable and feasible baselines range from a few meters to a few hundred meters, yielding angular resolutions from 0.5 X 10 3 to about 1 X 10-3 arcsec. Telescopes with these specifications have been well studied, and detailed designs already exist. The estimated cost for the pair of telescopes, the main expense of the interferometer, is $2 million. There are a number of important astronomical applica- tions for a dedicated infrared interferometer of this kind. In general, it should allow mapping of infrared fields, including regions obscured to visible light, with the kind of detail now familiar in the microwave region. It is possible to make measurements of stellar sizes and the infrared distribution over stellar disks, particu- larly of heavily obscured stars, and to study the circum- stellar distribution of dust and molecules--studies of great value in obtaining information on the activity of early or late stars or how and where planets form. Such examination of stars and stellar activity embedded in dust

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227 clouds may provide information on stars in stages of development not otherwise available to astronomy. The interferometer can also be used to search for an accre- tion disk around a black hole in the Galactic center--a possibility suggested by recent observations. It is finally a powerful astrometric instrument, which will allow the location of infrared sources and proper motions to be determined to high precision and will permit testing of the General meory of Relativity to a new order of accuracy by measuring the deflection of radiation from sources near the limb of the Sun. F. Steps toward a Submillimeter Telescope in Space The Panel on Radio Astronomy recognizes the enormous scientific potential of the submillimeter and far-infrared bands and believes that this potential can best be real- ized with a large (10- to 30-m-diameter) telescope in space. In order to enable such a facility to be launched in the 1990's, the Panel recommends that a broadly based program of technology development be started early in this decade. In particular, the Panel recommends that the development of submillimeter and far-infrared detec- tors and receivers be significantly increased during the 1980's and that a parallel effort in the design of antennas in the 10-m class for space applications, accurate to a few micrometers, be undertaken. m e Panel was impressed, in particular, by recent advances in high-frequency coherent detectors--semi- conducting and superconducting junctions used as hetero- dyne mixers--and strongly urges that increased research and development in this area be supported. The technology for manufacturing the large reflector of a space tele- scope, measuring and adjusting its shape, and deploying and pointing the telescope all must be developed and demonstrated soon in order to permit launch in the early 1990's. G. Solar Radio Astronomy Space radio astronomy would be best served in the 1980' by including radio spectrographs and polarimeters on appropriate spacecraft, such as the Star Probe and Solar Coronal Explorer. Such spectrographs and polarimeters should operate from decametric to kilometric wavelengths s

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228 and have direction-finding capability. The main objec- tive should be to observe the plasma physical processes in the solar corona and solar wind associated with par- ticle streams, collisionless shock waves, and shock- induced particle acceleration. Complementary plasma research, both experimental and theoretical, strongly guided by the radio observations, and direct measurements of plasma waves and electron distribution functions, are required to support the spacecraft radio observations. H. A Millimeter-Wave Telescope in the Southern Hemisphere The Panel on Radio Astronomy recommends the construction of a millimeter-wave telescope in the 5-10 m class to be located at high altitude in the southern hemisphere. An excellent site for this instrument is the Cerro Tololo Inter-American Observatory in Chile, a NSF-funded National Astronomy Center. This instrument will be used mainly for the study of galactic structure with interstellar molecular lines. The emphasis will be on the objects invisible from the northern hemisphere, such as the central region of the Milky Way and our nearest extra- galactic neighbors, the Magellanic Clouds. The cost of this facility is estimated to be $2 million. I. Upgrading National Facilities Recognizing that many of the most powerful and expensive instruments in U.S. radio astronomy are at the National Astronomy Centers, the Panel attaches a high priority to maintaining and upgrading this investment. It recommends especially that adequate funding be made available for maintaining and upgrading the VLA as the most powerful existing instrument for U.S. ground-based astronomy. An upgrading and enhancement of its computing facil- ities will be an ongoing need of the VLA. New experi- mental techniques and observing requirements will often call for computing capabilities beyond the present system which was in part defined in the early 1970's. Signifi- cant improvements in the system sensitivity are attainable by improved receiver design, especially at the higher fre- quencies. Masers or ferroelectric transistors offer such potential, with an expected improvement in system noise of a factor of 3. Longer-wavelength operations, for