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8 Recommendations In this chapter, the Space Science Board's Panel on Planetary Astronomy makes eight prime recommendations involving new equipment and better use of existing facilities. In addition to the specific recommendations, the Panel indi- cates areas requiring greater attention if ground-based studies are to be con- ducted most effectively. In making these recommendations the members of the Panel have en- deavored to project the needs of planetary astronomy over a ten-year period. Studies conducted by the Space Science Board * have indicated important gaps in our knowledge of the planets. Many of these can and should be par- tially filled by ground-based observations. A large amount of time is needed by qualified observers on moderate-size telescopes for optical measurements on the planets, satellites, and comets. As already mentioned (see Radar, Radio, and Optical Facilities, page 55), this need becomes acute at times of special events or at the most favorable opposi- tions and for those projects that require either superb seeing or large blocks of time. Such blocks of time are rarely, if ever, available on existing telescopes. For many planetary studies, a most important consideration is resolution. * Space Science Board, A Review of Space Research, NAS-NRC Pub. 1079 (Natl. Acad. of Sciences-Natl. Res. Council, Washington, D.C., 1962); Space Science Board, Space Research: Directions for the Future, NAS-NRC Pub. 1403 (Natl. Acad. of Sciences- Natl. Res. Council, Washington, D.C., 1966). 69
70 PLANETARY ASTRONOMY This problem has been discussed with reference to planetary surfaces (Chap- ter 3), but it applies equally to features in atmospheres and comets. New techniques are becoming available which promise to improve photographic resolution after exposures are made (see Chapter 6). The most important requirement, however, is to locate instruments at sites where the seeing is optimal. Recently, a site has been developed in northern Chile (Chapter 6) that is capable of providing resolution far superior to that at other observa- tories. It would be difficult to overemphasize the importance of such improved seeing for planetary studies. Special emphasis should also be laid on extensive and versatile spectroscopic facilities at a fixed (coude) focus. Many recent advances in planetary spectros- copy have been based on spectral resolution of approximately 100,000. Such performance can be obtained only with large, delicate equipment. Many of the requirements can be met by telescopes with apertures of ap- proximately 60 in. With the limited size of available gratings, a telescope of larger aperture does not provide more useful light but allows a fixed amount of light to be taken from a smaller area of the planet. Also, with a 60-in. aperture, the occasional periods of very steady seeing can probably be fully exploited. The study of highly variable phenomena in planetary atmospheres some- times requires continuous observation over many hours. It is therefore impor- tant that the longitude chosen for a planetary telescope of this aperture be such that it complements the longitudes of other planetary instruments. The length of time any given instrument can be used to observe a planet depends upon the planet's position in the sky and the latitude and elevation of the observatory. Low southern latitudes are particularly favorable for observing Mars at its closest approaches. At a high elevation there is less atmospheric absorption. These requirements could be met by a site in northern Chile. Because of its relationship to the space program, it is important that such a telescope be placed in operation as soon as possible, and certainly prior to the very close approach of Mars in August 1971. 1 We recommend that a 60-inch telescope with mirrors of superior optical quality and designed specifically for spectroscopy, interjerometry, photog- raphy, and photometry of the planets be erected as soon as possible at a dry site with exceptionally good seeing in the Southern Hemisphere. Chapters 2 and 3 have detailed the contributions that radar astronomy is making to planetary studies in testing gravitational theory and in determining orbital elements, planetary radii and rotations, and characteristics of planetary surfaces and atmospheres. Some of our most dramatic increases in knowledge in planetary astronomy have come from radar studies; and radar data have proved vital in the planning of space-probe missions. The further development
RECOMMENDATIONS 71 of radar planetary astronomy will depend upon the development of new facili- ties with expanded capabilities. 2 We recommend that maximum use be made of existing facilities capable of doing radar astronomical work. We also recommend the construction of a large filled-aperture radio-astronomical facility with maximum sky coverage and with capability for planetary radar astronomy. Although high-resolution work is becoming increasingly desirable, important infrared photometry and Fourier spectroscopy can be done at the coude focus of an instrument of lower resolution than that required in the visible wave- lengths. The recent development of inexpensive telescopes using all-metal mirrors has made significant contributions in the infrared region. A 120-in.- aperture telescope would collect larger amounts of radiation and would provide increased spectroscopic resolving power in the infrared windows and in the 1-mm regions of the spectrum. It would be essential, however, that image precision not be relaxed below 1 or 2 sec of arc in the visible because the signal-to-noise ratio of infrared detectors depends upon the detector size. The effectiveness of such a telescope will be highly dependent on the use of a site characterized by very low water-vapor content. 3 We recommend that a 120-inch, relatively inexpensive light collector with a coude focus for spectroscopic and photometric observations be con- structed at a site having exceptionally low water-vapor content. Studies made in the infrared reveal the great potential of this region for studies of planetary atmospheres (Chapter 4). If the full benefits of such studies are to be realized, it is necessary to be able to make observations from above much of the water vapor in the Earth's atmosphere. NASA has considered the instal- lation of a 36-in. infrared telescope on the Convair 990 jet airplane based at the Ames Research Center. Such an installation would provide planetary and stellar astronomers with a national facility capable of observations in the entire range from 1 to 1000 p above 99 percent of the water vapor that obscures much of this region from the ground. 4 We recommend that a 36-inch infrared telescope be installed in the Ames Convair 990 and that it be available to qualified users as a national facility. Radio astronomy has made important contributions to the study of planetary surfaces and atmospheres. Higher resolution must be achieved if we are to see marked improvement over observations already made, which tax existing capabilities. In addition to construction presently planned, a major facility is needed to provide resolution of a few seconds of arc in the region of the
72 PLANETARY ASTRONOMY spectrum from 3-cm to millimeter wavelengths. Such a national facility would contribute significantly to galactic, extragalactic, and planetary research. Greater collecting area than is now available is also necessary to aid the search for spectral lines in planetary atmospheres in the millimeter region of the spectrum. 5 We endorse the recommendation of the Whitford Committee * for con- struction of large radio-telescope arrays by the National Radio Astronomy Observatory and the California Institute of Technology. They will provide facilities for high-resolution planetary observations at centimeter and decimeter wavelengths. We also recommend that a high-resolution 3 cm-to-millimeter wavelength radio facility be constructed and made available for planetary studies; and that the feasibility of a radio facility able to provide large collect- ing area for low-resolution observations at millimeter wavelengths be studied. Recent work in the infrared spectroscopy of planetary atmospheres, discussed in Chapters 4 and 6, has emphasized the value of high-resolution Fourier spectroscopy. The technique makes possible new studies in planetary atmo- spheres: the determination of the rotational temperatures of the constituent gases and the detection of isotopes and trace elements. Fullest exploitation of this technique will be possible when observations can be made from airplanes above the tropopause. 6 We recommend the development of Fourier interferometers with reso- lutions of at least 10,000 for ground-based observations. One such instrument should be sufficiently rugged for later adaptation to high-altitude aircraft. Chapter 6 was concerned with some of the newer techniques being applied to planetary observations. Such technological improvements and new develop- ments often produce the most dramatic increases in our knowledge of the planetary system, yet it is virtually impossible to predict where such technologi- cal breakthroughs will occur. 7 We recommend that the importance of technological development be recognized by ensuring that funds are available to support it. Advances in our knowledge are particularly dependent upon the development of radiation detectors in the ultraviolet, visual, infrared, and millimeter regions of the spectrum; further development of image tubes; and devices for improving the quality of astronomical images. * Panel on Astronomical Facilities of the National Academy of Sciences Committee on Science and Public Policy, Ground-Based Astronomy: A Ten-Year Program, NAS-NRC Pub. 1234 (Natl. Acad. of Sciences-Natl. Res. Council, Washington, D.C., 1964), pp. 50 and following.
RECOMMENDATIONS 73 Cooperative observational programs are important in planetary astronomy. The variability of many planetary features must be studied by extensive photo- graphic patrol. Such a patrol can be expected to yield new understanding of the dynamics of the atmospheres of Jupiter, Mars, and Venus. It will also provide a necessary permanent and continuous record of planetary phenomena for timely comparison with more detailed or specialized technical studies. 8 We recommend the establishment of a worldwide photographic plane- tary patrol, appropriately distributed in longitude to ensure adequate coverage, for the period from January 1, 1969, to January 1, 1974. One or two new reflectors in the 24- to 30-inch class may have to be erected at appropriate sites to complete this patrol. In addition to the above recommendations, other areas are important in the development of planetary studies. Astrometric Observations of Comets and Asteroids The present, almost total, lack of astrometric observations of comets and asteroids made in the Southern Hemisphere limits study of comets whose orbits are only fractionally north of the ecliptic and restricts observations of asteroids that come to opposition during the short nights of northern summer. An astrographic telescope of moderate size (capable of reaching a limiting magnitude of at least 17.0) located in the Southern Hemisphere, and available for astrometric observations of comets and asteroids, would do much to remedy the imbalance produced by such selection effects. Observations of asteroids as faint as magnitude 17 are required on a regular basis to maintain the quality of orbits of known asteroids, to improve the orbits of recently discovered objects, and to extend orbital statistics to fainter asteroids. A larger number of competent astronomers could well enter this field. Observations of asteroids and comets contribute fundamental data that may be very important to our understanding of the evolution of the solar system and in the past have laid the foundations on which exciting discoveries were made. There is also a need for better measuring and data-handling equipment so that the more tedious aspects of reduction procedures can be automated as far as possible. The Recovery of Extraterrestrial Matter Meteorites are the only samples of extraterrestrial material that we can examine directly in the laboratory. This material comes to us without cost; it is necessary
74 PLANETARY ASTRONOMY only to identify and collect it. While considerable effort is already being ex- pended on this task, the great value and relative scarcity of the material calls for additional support and encouragement. In particular, existing photographic networks could be supplemented with microbarographs and radar installations to extend to daylight hours observa- tions of incoming objects. Existing radar facilities operated by the Air Force may be suitable for this work. A greater effort would be worthwhile for the recovery of truly unusual falls such as Revelstoke, where a powdery material was dispersed over a large area. This type of operation requires careful instruc- tion of the searchers to prepare them to identify the material and, occasionally, the use of sizable teams to cover an area thoroughly. With respect to this prob- lem of manpower, it may be useful to look into the possibility of mobilizing scouts, military personnel, or forest rangers to collect material from a special fall. Ultraviolet Observations of Planets and Comets The spectral region below 3000 A has only recently become the object of systematic investigation. At the time of this writing, there have been only five observations of the planets, and none of comets. Yet in many respects this spectral region is fully as important as the infrared for the potential informa- tion it contains about atmospheric characteristics. (See also Use of Airplanes, Balloons, Rockets, and Earth Satellites, page 50.) Resonance transitions of most atoms and molecules occur at these wave- lengths, with the result that very small quantities of a given substance can be detected; for some species such as helium and argon, it is the only region of the spectrum where we can hope to obtain direct evidence of their presence. In addition, because of attenuation in planetary atmospheres, one is generally probing only the upper strata, regions that are generally very difficult to explore at other wavelengths. Ultraviolet observations will also enable us to examine planetary coronas formed by escaping atomic constituents such as hydrogen and oxygen. Studies of these coronas with high spatial resolution will provide information on exospheric temperatures and thus the escape rate of atmospheric constituents. In the case of comets, we can observe molecules and atoms such as H2, N2, O2, N, O, and C with resonance transitions below 3000 A. We may be able to observe more complex moleculesâwhich are probably more closely related to the parent material of the nucleusâby means of their electronic transitions. We recommend that additional rocket flights be supported for planetary spectral studies at relatively high dispersion (~1 A) and that attention be given to the possibility of rocket firings at short notice to observe a bright comet. Furthermore, consideration should be given to the use of future
RECOMMENDATIONS 75 Orbiting Astronomical Observatories for ultraviolet observations of planets and comets. Laboratory Measurements There is need for laboratory intensity measurements of many of the gases thought to be constituents of planetary atmospheres. Equivalent widths or integrated intensities are needed, with much higher resolution than has been obtained, for band lines of CO2> H2O, CO, HD, CH4, and NH3 and for minor constituents that may be discovered in the future. Detailed analysis of the rota- tion-vibration spectrum of CH4 and NH3 is probably necessary because of complexities resulting from the dramatically different temperatures of outer planets and their satellites. An alternative to analyzing these spectra (at first in the atmospheric-window regions) is to measure the equivalent line-widths in the laboratory at the appropriate low temperatures. This is most desirable but very difficult because of the long path lengths required. For the outer planets, low-resolution spectra should probably be obtained in the laboratory with CH4 paths up to 10-km atmospheres. In many cases it would be desirable to simulate window-region spectra in the laboratory and measure them with a precision of from 0.001 to 0.002 cm-1, since the Mars and Venus spectra can be obtained with this precision. This is particularly desirable for CO2, for it is of interest to determine the "C/13C isotopic ratio. To obtain accurate abundances in the face of saturation produced by the strong absorptions, it will probably be necessary to determine curves of growth. Facilities, both long-path absorption cells and suitable spectrographs, are available at several places in the country to accomplish the necessary laboratory calibrations. This work involves only conventional grating spectroscopy, and available facilities are adequate to the task. Calibrations can alternatively be made using Fourier interferometers with their distinct quantitative advantages. A systematic, coordinated laboratory program should be supported to deter- mine the effects on spectral signatures (ultraviolet to microwave) of varying the physical and chemical parameters of likely planetary surface materials. In conjunction with this, programs of lunar and planetary direct sampling should be planned in such a way as to permit the calibration of Earth-based obser- vations. Personnel Requirements The following general recommendations arise from the statistical data and the discussion presented in Chapter 7. If the United States intends to carry on a substantial and effective planetary space program, and if the scientific com-
76 PLANETARY ASTRONOMY munity wishes to take full advantage of this unique opportunity to pursue studies in planetary science, then graduate schools where planetary studies are done, or those interested in developing programs in this field, should consider organizing interdisciplinary programs to provide broad training. Furthermore, emphasis and financial support should be given to postdoctoral training in the planetary sciences at centers at which planetary research is being actively pursued. The establishment of a national society for planetary sciences or of an affili- ate of an existing society would be highly desirable to serve as a forum for discussion and a cohesive force to facilitate recruitment of personnel, to assist in obtaining financial support or facilities for projects of unusual merit, and to encourage publication of results. SUMMARY OF RECOMMENDATIONS The Space Science Board Panel on Planetary Astronomy recommends ways in which ground-based planetary astronomy can advance our knowledge of the evolution of the solar system and contribute substantially to a significant plane- tary space program. These involve both instrumental advances and increases in personnel. On the instrumental side, several telescopes are recommended. One is a precision optical instrument to be placed in the Southern Hemisphere, and two are reflectors suitable for use in the infrared. For longer wavelengths, the Panel urges that existing facilities for planetary radar observations be used more extensively and supports previous proposals for radio-telescope arrays. It recommends the construction of a new large filled-aperture facility suitable for planetary radar astronomy and a high-resolution 3-cm-to-millimeter facility for high-resolution radio investigation. It urges that continuing study be made to determine the type of radio facility most appropriate for low-resolution measurements at millimeter wavelengths. The other recommendations include technological developmentsâparticu- larly Fourier interferometersâand an effective worldwide photographic plane- tary patrol. The Panel recommends greater effort in astrometry, recovery of extraterrestrial matter, and laboratory spectroscopy. It emphasizes the impor- tance of extraterrestrial observations. In the light of findings of a survey, the Panel urges that curricula or organi- zational boundaries of graduate schools be revised if the planetary sciences are to attract more and better-trained students. Also, it suggests that further emphasis be placed on postdoctoral training and that a planetary society or sub- group of an existing society be established. These last two recommendations would be of particular importance to those trained in other disciplines who wish to apply their talents to problems in planetary astronomy.
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