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Chapter 2 RECOMMENDATIONS 1. Goals of the Planetary Program The 1965 study identified three goals for the nation's planetary program. The planetary program should be designed to provide for progress in our understanding of: (1) The origin and evolution of the solar system (2) The origin and evolution of life (3) The dynamic processes that shape man's terrestrial environment We believe that these goals remain valid and that all three should be recognized in the development of the national program. In our view, the program of planetary exploration will contribute in major ways to the growth of science. Exploration of the planets is not an end in itself. In isolation, the discovery of new facts about the planets has little intrinsic interest. Scientific interest in the planets lies in the expectation that investigation of their atmospheres, surfaces, and interiors will contribute greatly not only to unraveling the complex his- tory of the solar system and problems of how life originated and developed, but also to an understanding of the Earth and of the processes which today take place in the at- mosphere, oceans, and deep interior. Our view is that no single goal, such as the determination of whether life exists in other parts of the solar Fystem, should be set for the planetary program. Rather, it should be emphasized that the scientific return from planetary exploration will flow into many areas of science and thereby strengthen them. Furthermore, there is every reason to expect that continued development of scientific understanding of the planets will lead to benefits for all mankind. For example, since the 1965 Woods Hole study, a series of investigations of the lower atmospheric dynamics of Mars has followed the preliminary determination by Mariner 4 of the character of Martian atmosphere. None of these studies is conclusive but nevertheless a remarkable relevance to terrestrial studies is revealed. Investigation of the Martian atmosphere has focused attention on a number of important aspects of the Earth's atmosphere that were not receiving the attention they deserved. On Mars, energy is transferred within the atmosphere through radiation. This mechanism is also important on Earth but to a great extent has been neglected. The interaction of the lower levels of the Martian atmosphere with the sur- face is of great importance in controlling the motions of the atmosphere. Greater under- standing of these interactions on Earth is being gained as a result. In the case of the atmosphere of Mars, there is thus a direct and demonstrable connection between study of that atmosphere and a better understanding of our own. From this better understanding, we can perhaps expect advances in the techniques of long-range weather forecasting and eventually the development of effective methods for weather modification. The signif- icance of the exploration of the Martian atmosphere to studies of the Earth is by no means unique: we can expect further advancement in the understanding of our own planet as a result of the study of the other planets. Because of the rich contribution that planetary exploration can make to a broad range of scientific subjects, we recommend that the planetary exploration program be presented not in terms of a single goal but rather in terms of the contribution that that exploration can make to a broad range of scientific disciplines. 2. Level of Support for Planetary Exploration The 1965 study recommended that an increasing fraction of the space program's re- sources be devoted to planetary exploration. The basis for that recommendation was the expected richness of the scientific return from such increased efforts. Since 1965, a -3-

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-4- number of groups external to NASA have made similar strong recommendations. In 1967, the President's Science Advisory Committee recommended that the exploration of planets and space astronomy should be primary objectives toward which the U. S. space program is oriented during the post-Apollo period. In the coming fiscal year, planetary exploration will receive at most 2 percent of the total space budget. We believe that this amount is totally inadequate to take ad- vantage of the opportunities available to us. The importance of planetary investigations is that these explorations provide information that cannot be secured by Earth-based studies. We cannot form an experimental planet with a light atmosphere composed largely of carbon dioxide to determine how its motions are influenced by radiation. We cannot easily reproduce the conditions that have existed on the surface of Mars over billions of years to determine whether life forms could develop. Instead, we can take the ex- isting planets and attempt to secure information most relevant to some of the great scientific problems of our time. The uniqueness of planetary studies makes any com- parison of the relevant benefit of expenditures devoted to these and Earth-based studies not only difficult but in many cases irrelevant. Study of the planets can provide under- standing about major questions that could never be achieved solely by Earth-based in- vestigations. Today we have developed the technology to place scientifically meaningful payloads near or on the planets and have developed the instrumentation with the long lifetimes required to carry out the complex planetary missions. We believe that we must take ad- vantage of these developments. While not competent to argue the complicated questions of national prestige, we certainly believe that we cannot abandon a broad area of space activities to our competitors. Therefore, we recommend that a substantially increased fraction of the total NASA budget be devoted to planetary exploration. 3. Priorities in Planetary Exploration At present, the planetary program is limited by resources rather than by technology, lack of competent scientists, or important ideas. The real fact of resource limitation makes it essential to examine the question of priorities with great care. In this ex- amination, we have been strongly guided by the concept that planetary exploration will strengthen broad areas of science, as has been emphasized above. (a) Redundancy In the past and in current planning, it has been customary to build considerable redundancy into a particular mission. This redundancy takes two forms. It is usual to plan for two spacecraft to accomplish a given mission. In addition, backup spacecraft for use on the ground are also manufactured. Several of the original reasons for such redundancy have vanished. Technological advances have made failures infrequent, whether at launch or during the mission. The many space endeavors have reduced the prestige of a single space shot and the public no longer requires success at every opportunity. Planetary exploration is no longer a primitive and risky art but rather a highly devel- oped technology. In view of these developments, and wi-th the great need to conserve resources, we recommend that duplicate missions for a particular opportunity be under- taken only when a clear gain in scientific information will result from such double launches. We do not find persuasive the argument that an additional launch costs only 20 or 30 percent more than the single launch. For the expensive planetary missions that have been proposed, this add-on is sufficiently large to support significant other missions. However, in view of the funding already committed to the 1969 Mariner Mars fly-by missions and the clear gain of covering different portions of the planet, we recommend that NASA proceed with this part of its program.

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-5- (b) Small Spacecraft for Planetary Missions Of central importance in the future of planetary exploration is the initiation of a series of relatively small and inexpensive spacecraft to orbit Venus and Mars, primar- ily, and to make exploratory missions to Jupiter (as currently planned in the Pioneer F and G missions) and perhaps to Mercury and to comets. Such a broad and well conceived series of opportunities for space experiments, with flights to the nearer planets at each opportunity, will make it possible to reach and maintain a high level of scientific interest and participation in the planetary program. This is particularly vital during these times of uncertainty concerning support of the larger missions. A continuing program with numerous opportunities for planetary research using relatively inexpensive planetary orbiters makes it possible to: (1) Involve a larger number of scientists, young researchers, and graduate students in planetary research (2) Plan and progress systematically to more ingenious and important exper- iments as more information is obtained (3) Respond quickly with a new payload in order to take advantage of new findings (4) Give group? of experimenters the opportunity to plan for the use of a total integrated payload (5) Conduct experiments that support or complement larger missions (U. S. and Soviet) or prove out a concept that may have high interest but also high risk (6) Carry out planetary and interplanetary particles-and-fields experiments on the same missions rather than instrumenting separate spacecraft for the two types of measurements; this will also reduce the load on the tracking facilities of the Deep Space Network The merits of small planetary orbiters have been discussed for several years and are recognized in particular in the Space Science Board's 1967 "Report of a Study on Explorations in Space with Sub-Voyager Systems." The success of small, spin-stabilized spacecraft has been well demonstrated in the IMP, Pioneer, and other programs. Pioneer A has now continued to operate in deep space for 2.5 years. The technical feasibility of placing such a spacecraft in orbit around an extraterrestrial body was proved when IMP-6 (Explorer 35) was placed in orbit around the Moon in July 1967. Experience from these programs can also be used to estimate costs with a high degree of reliability. There are both advantages and disadvantages to a spinning spacecraft as compared with a Mariner-class stabilized spacecraft, but recent technical advances in electron- ically phased antenna arrays, for example, can be used to simplify antenna designs for high gain communications with Earth. We recommend that NASA initiate now a program of Pioneer/IMP-class, spinning space- craft for orbiting Venus and Mars at each opportunity, and for exploratory missions to other targets. We endorse the proposed diversion of the existing Pioneer E spacecraft to orbital studies of Venus in 1970 and we recommend that NASA openly solicit scientif- ic experiments for this mission. We strongly support the Pioneer/IMP-class missions for investigation of the near-Jupiter environment in 1972 and 1973. The possibility of taking advantage of the 1973 Venus-Mercury opportunity with a Pioneer-class mission should be carefully examined. (c) Study of Mars The Space Science Board and its various panels have on frequent occasions emphasiz- ed the great importance of the investigation of Mars for the purpose of detecting possi- ble biological activity. The purely biological studies are of high risk but clearly the discovery of life on Mars would rank as one of the great events of this or any other century. Because of the great importance of such biological investigation we recommend the following for Mars:

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-6- Mariner orbiter mission in 1971 Mariner-type orbiter and lander mission based on a Titan-Centaur in 1973 The 1971 Mars orbiter is essential both for the significant information it will gain and for the support of subsequent landing missions. This orbiter will provide the first detailed maps of Mars and its sensors will be able to seek out persistent clouds as well as thermal and color anomalies indicative of water sources. This and other information will be of great value in selecting favorable sites for a lander, in the interpretation of data which a landed capsule will obtain, and in providing the first data on seasonal variations such as the intriguing wave of darkening. Therefore, the 1971 Mars orbiter should be accorded an especially high priority in the planetary program for it will make unique contributions to the search for extraterrestrial life. We believe that the program outlined above -- small planetary spacecraft and larger Mars missions -- is a minimal program. It is our view that such a program has greater priority both in terms of expected purely scientific returns and in long term benefits to society than other space ventures such as the qualifying of man for plan- etary voyages (see Section 7). (d) Other Programs Our emphasis on obtaining broad knowledge of the solar system leads us to give the next priority to a Mariner-class Venus-Mercury fly-by in 1973 or 1975. We note that the favorable position of the planets that permits this mission will not recur until the 1980's. Our incomplete knowledge of Venus coupled with the importance of understanding its atmosphere makes a multiple drop-sonde mission in 1975 a very high priority mission. Finally, we note the importance of a major lander for Mars, perhaps in 1975, which could take advantage of the opportunities opened up by the earlier investigations. 4. Payloads We have not attempted to structure a detailed program of experiments for each of the high priority missions discussed above. Instead we have examined in some detail possible sample payloads for the small planetary orbiters and for the Mars lander. In the case of the former we wish to demonstrate clearly that it is possible to carry out significant planetary observations with small spacecraft. In the case of the lander, this mission is of such importance that it is essential that detailed investigations of the instrumentation for that payload be undertaken at the earliest possible oppor- tunity. (a) Payloads for Small Planetary Orbiters of Venus and Mars The scientific payload for spinning spacecraft of the Pioneer/IMF class placed in orbit around Venus and Mars is on the order of 40 pounds. With such a payload there are a number of exciting possibilities for fundamental studies of the atmospheres and sur- faces of these planets as well as of the solar wind interaction regions and the inter- planetary medium. With an ongoing program that includes flights to both planets at every opportunity, it is possible to plan both integrated payloads that incorporate a number of related experimental packages of relatively small size and payloads consist- ing of a single, large instrument for imaging in, for example, the visual, infrared, radio, or active radar bands. There has been some tendency to think of the small, spinn- ing spacecraft as useful only for particles-and-fields experiments, but similar space- craft in Earth orbit have demonstrated visual imaging capabilities of high quality using the spin of the spacecraft itself to scan the field of view.

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-7- A high priority is placed on obtaining visual and infrared images of the Venus atmosphere near the tops of the clouds to help deduce dynamical and physical character- istics of this massive atmosphere. Both remote and in situ measurements of atmospheric constituents are possible for Venus and Mars, with local measurements being emphasized during the time when the orbit decays into the atmosphere. An early flight to Venus might be used to test the feasibility of obtaining radar images of the surface, using the S-band radio system, with an added instrument to sample wide-band characteristics of radio signals reflected from the surface in the bistat'ic mode. This could lead to the possibility of obtaining high resolution images of the surface of Venus using the total capacity of a spacecraft of the Pioneer/IMP class in a later flight. The bistatic radar echoes will also be of interest in establishing electrical and roughness properties of the regions on and near the surface. Recent results from the lunar-orbiting Explorer 35 suggest a variable thickness of the rubble layer (the regolith), thickest in the highlands and thinnest in the maria, and having anomalies of extreme thinness apparently correlating with infrared hot-spots. A high priority is also placed on obtaining synoptic particles-and-fields data in the solar wind interaction region around Mars. Recent results from Explorer 35 and data from the Mariner 5 fly-by of Venus lead us to believe that such a synoptic study of Mars will be of great interest and importance in establishing fundamental properties of planetary bodies and their environment in the solar wind. The radio links needed for communications have a high potential for scientific use, particularly if they incorporate range and range-rate capability and compatibility with the addition of a second frequency. Pressure and temperature profiles of the at- mospheres of Mars and Venus obtained by radio occultation in the Mariner 4 and 5 flights provided our first accurate measurements of these two markedly different samples of the types of atmospheres that can evolve from planetary bodies. In fact, occultation data in conjunction with radar studies from Earth, radio tracking of Mariner 5, and the results of the Soviet Venus 4 probe, have established that atmospheric par- ameters near the mean surface of Venus at low to medium latitudes and at all times of the day are quite different than was first thought on the basis of the direct Soviet measurements alone. Approximate pressure and temperature values are 100 atmospheres and 700° K, respectively, rather than the 20 atmospheres and 550° K values suggested by the Soviet experiments. Radio occultation measurements also provided initial profiles of the ionospheres of both planets. The peculiar, sharp, interface between the daytime ionosphere of Venus and the interplanetary medium, together with magnetic and plasma effects noted by Mariner 5, suggest that Venus and perhaps Mars represent a different class of planet, as compared with Earth, in terms of the ways a planetary body may interact with the solar wind. These differences could be of fundamental importance to eventual under- standing of the different manner in which the atmospheres of Venus and Earth have evolv- ed to have markedly different characteristics even though the two planets are very sim- ilar in size, mass, and distance from Sun. We assume that they probably formed from the same source of matter at about the same time. The radio measurements will take on added meaning with the small orbiters, as com- pared with previous fly-bys. The atmospheric and ionospheric measurements would become synoptic, covering large areas of the planets and extending over the lifetime of the spacecraft. Accurate tracking will be used to establish the mass and gravitational moments of the planets. Venus is of particular interest in this regard because of its strange dependence on Earth for locking its rotational period. Accurate tracking at two coherent radio frequencies will also provide data on the solar corona and interplan- etary medium, and will be used to improve planetary ephemerides. These same measurements, conducted when the planets are in regions around the opposite side of Sun, will yield information important to fundamental studies of general relativity.

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-8- Small Planetary Orbiters: Sample Payloads Venus 1970, 1972, 1973, 1975. The next opportunity to place a payload in the vicinity of Venus (1970) could only involve existing spacecraft and scientific instru- ments. Subsequent sample payloads are designed on the assumption that no large Venus- orbiting missions will be conducted during the period under consideration. Detailed payload choices will depend upon previous results and characteristics of the proposed Venus-Mercury fly-by (1973-75) and the Venus drop-sonde mission (1975). In each case it is assumed that the radio tracking and data acquisition system includes range as well as range-rate capability, and provision is made for incorporating a second radio frequency for dispersive measurements. Thus, based on the high precision radio cap- abilities, occultation measurements of the neutral and ionized portions of the atmos- phere will be conducted, as will experiments on celestial mechanics, relativity theory, the interplanetary medium, and the solar corona. Sample payloads are: 1972 (1) Spin-scan camera (ATS) and tape recorder (2) IR radiometer, spin scan (3) UV at 3600 A, spin scan (4) UV filter photometer (5) Modules tapped off S-band system for initial bistatic radar studies of the surface (uplink) (6) Particles-and-fields instruments, as many as can be incorporated, for study of the solar wind inter- action region. 1973-75 Depending upon previous results, the total spacecraft may be devoted to bistatic radar imaging of the surface, optical and infrared im- aging of the cloud tops, microwave emission (temperature) imaging, or to an integrated set of experiments to study the region of interaction between the upper atmosphere and the solar wind, includ- ing measurements of the interplanetary medium at 0.7 AU. In case there is no Mariner-class mission for multiple (three) atmospher- ic probes, it is important, and may be feasible, to design a small spacecraft for atmospheric entry. An orbiter may include a mass spectrometer to measure atmospheric constituents, particularly during orbit decay. Mars 1971, 1973, 1975. We assume the radio measurements described above for Venus. If the small planetary orbiter is flown at the same opportunity as a Mariner-class orb- iter, possible experiments for the solar wind interaction region, the upper atmosphere, and the interplanetary medium at 1.5 AU from the Sun include: (1) Magnetometer (2) Plasma probe (3) Trapped particle detector (4) Cosmic ray detector (5) Micrometeorite counter For the second and third missions, upgraded and new instruments will be considered. Should the small spacecraft be the only orbiter, its payload should be chosen from:

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-9- (1) Spin-scan camera (2) IR radiometer (3) UV photometer (4) Yellow-filter spin-scan camera (5) Bistatic radar for surface studies Particles-and-fields experiments are important, but probably of secondary priority in this case. (b) Payload for Direct Biological Investigation of Mars (Table 1) The central purpose of the lander mission is to determine the near-surface en- vironment of the planet and, if possible, whether biological activity is present. The environmental investigations are of direct importance to the life detection ex- periment but also provide information of interest in other disciplines. We regard the mass spectrometer experiments and the gas chromatograph of particular interest both because of the unique capabilities in this kind of instrumentation that exist in this country and the value of these experiments in interpreting biological observations. Instrumentation of Recommended Payload (1) Imaging. This is considered to be a life-seeking experiment on a gross scale. A resolution of a few centimeters at 10 m will suffice for a first lander, although higher resolution is desirable if it can be attained. A high resolu- tion view is needed of the sampling site, preferably while the sampler is being operated. (2) Sampler. The experiments which need a soil sample (Items 3, 4, 7, and 12) require a total sample weight not exceeding one gram. (3) Pyrolysis: Gas Chromatograph/Mass Spectrometer. This is a dual purpose instrument. The first use is for atmospheric analysis. A complete analysis of the atmosphere including trace constituents is needed, through a diurnal cycle if possible, to detect thermodynamic unstability (e.g., CH^ and Q£ present). The analysis of the atmosphere should include helium and the argon and neon isotopes since the relative proportions of these elements could yield information on processes that affected the primitive atmosphere and on the possibility of secondary outgassing, solar wind accretion and solar wind sweeping. The gas chromatograph of this instrument is not the same as that in Item 6 since a column suitable for the separation of organic compounds does not perform satisfactorily with the fixed gases. The second use of this instrument is for organic analysis. The information to be gained will indicate the presence of organic compounds, permit identification of the classes of compounds present and, in the case of compounds present in high relative amounts, will allow identification of individual compounds. Other valuable evidence suggestive of life is the detection of organic nitrogen compounds since these have not been found in meteorites in significant amounts. Both the atmospheric analysis and the organic analysis are considered to be life- seeking experiments. (4) Direct Biology. Experiments in this class involve the detection of growth, photosynthesis, or metabolism in the soil sample. Depending upon the number of controls, the number of different chambers, the number of media, etc., the detectors will range from simple, low-weight (1-lb) instruments to complex, higher-weight (10-lb) instruments. They require at least 3 days' operating time.

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TABLE 1 Suggested 1973 Mars Landed Payload, In Order of Priority Soil Sampler Pyrolysis (gas chromato- graph/mass spectrometer) Direct Biology 1^0 Detector Gas chromatograph Differential thermal analysis Penetrating subsurface H_0 probe Soil temperature Air temperature Air pressure Element analysis Neutron probe Air velocity Functional Est. Data Priority Range Weight Bits 1 1 gross scan upon 4 Ib 107 landing, 1 high resolution of sampler 1 Up to 1 g 2 Ib 102 1 10 to 140 amu 16 Ib 105 ) Dynamic range, 10 1 Growth/Metabolism 10 Ib* 103 1 Sensitivity, 10"5 mb 1 Ib 103 2 Atmosphere: H2, He, 4 Ib 2x103 CO, N2 , N-oxides, C02 HCN, 02, NH3, CH4, C2H2 2 10 mg at 0.01% HjO 1 Ib 2x103 2 - 3 Ib 103 3 +1°C 0 103 3 +1°C 0 103 3 4O.1 mb, range to 30 mb 1 Ib 103 3 X-ray fluorescence 20 Ib 104 X-ray diffraction 3 Permafrost(?) 3 Ib 103 4 Range up to 200 km hr -1 2 Ib 103 *A 1-lb life detection experiment may be feasible. -10-

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-11- (5) Water Detector. A knowledge of the amount of water present at the landing site is crucial since water is considered essential for biological processes. This instrument measures water vapor. One additional water detecting experiment (Item 8) will determine the presence of a permafrost layer while a second additional experiment (Item 7) will analyze for water in the soil sample. (6) Gas Chromatograph. This is a "gap-filling" instrument which analyzes the atmosphere for those gases that are difficult to analyze with the mass spectro- meter or which lie outside its mass range. These gases are H2, He, CO, and N2- Since the column would separate other possible atmospheric gases, the capability of the instrument can easily be extended to include the other gases listed in the table. (7) Differential Thermal Analysis (see Item 5). This instrument, which includes a water vapor sensor at the output of the DTA unit, will allow positive identification of water in the soil sample, including liquid water, ice, water of crystallization, and hydroxyl water. (8) Penetrating Subsurface Water Probe (see Item 5). This is a self- contained and self-powered probe which penetrates the soil and gives indication of a permafrost layer. Physically, it is part of the neutron probe (Item 13) (9) Soil Temperature. A knowledge of the temperature cycle of the soil at the landing site is useful in describing the environment. (10) (11) Air Temperature and Pressure. Measurement of air pressure and temperature during at least one diurnal cycle is desired to determine whether liquid water can exist at the landing site. (12) Element Analysis. This analysis will yield environmental data as well as information concerning planetary differentiation. (13) Neutron Probe (see Item 8). (14) Air Velocity. This measurement will help to determine whether aeolian erosion may be occurring at the landing site. It will also contribute to the descrip- tion of the biological environment. (c) Sterilization of Probes for Venus and Mars Direct impact probes are the most reliable and economical devices for explora- tion of the lower atmosphere of Venus. The development of this type of mission for the U. S. space program may have been inhibited in the past by the need to meet inter- national sterilization requirements. It now seems likely that these sterilization requirements can be satisfactorily fulfilled. Probes which detach from the main bus and which can impact in regions where surface temperature is uncertain can be heat- sterilized. Uniform heat sterilization of all components of the bus should not be necessary. For the probes, an equatorial impact point can be chosen where the surface tempera- ture will exceed 700°K. Clean construction, heat sterilization where possible, sur- face sterilization of all components, and the surface ablation during entry should then be sufficient to reduce the probability of contamination to acceptable limits. As information about Mars continues to become available a similar reassessment of sterilization requirements for this planet will also be desirable. We recommend that with regard to Mars and Venus, NASA continually reassess, in the light of current knowledge of the planets, its program, methods, and mathematical model for meeting the internationally agreed objectives for planetary quarantine.

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-12- 5. Ground-Based and Near-Earth Observations of the Planets In the years since the 1965 study, ground-based observations have produced a wealth of new information on the planets. We expect that the returns from investments in such studies will continue to be high. We urge that NASA pursue an active ground- based planetary program. (a) Radar Studies of the Planets Since 1961, considerable strides have been made in our knowledge of the nature of the terrestrial planets, much of which has come from ground-based observation. Earth-based radar observations of Mercury, Venus, and Mars have been especially fruit- ful. In this Section we review briefly the major contributions made using this tech- nique and outline extensions of the work that are possible with more sensitive radar systems. In view of the large number of remarkable discoveries already obtained by radar and its high potential for further advances, we advocate its support as part of the NASA planetary program. In particular we have in mind greater use of the Deep Space Network and improvement of its capability for radar observations, the upgrading of other radar facilities, and the construction of new ones. We believe that planetary exploration can best be carried out with a proper mix of space-probe and Earth-based observations. Those data of importance that can be obtained less expensively with, say, ground-based radar equipment ought to be so obtained. Such a cost-effective approach is especially important in a period of stringent budgetary limitations. Past Accomplishments. The study of planetary dynamics has benefitted enormously from the extreme precision -- several parts in 1(K -- of radar measurements. The value of the astronomical unit, in particular, has been refined by almost five orders of magni- tude. Perhaps most startling have been the discoveries of the spin-orbit resonances of Mercury and of Venus. Further, these results from Earth-based radar allow sensible inferences to be made about the existence of a fluid core in Venus. Radar refinements of the mass and radius of Mercury have established its density as equal to the Earth's within 1 to 2 percent, and provide important constraints on models of Mercury's internal composition. The accurate determination by radar of the radius of Venus, coupled with the Mariner 5 results, demonstrate that the Venus 4 probe did not transmit from the surface and that the surface temperature and pressure are closer to 700°K and 100 atmospheres than to 550°K and 20 atmospheres, the Soviet values. The absorption by the Venus atmosphere of high frequency radar waves observed from Earth-based measurements also favors strongly the former set of surface conditions. Clearly these radar results are of enormous importance to future investigations of the lower atmosphere and surface of Venus. Radar has also played an important part in studies of planetary topography, surface roughness, porosity, and composition. Notable discoveries include large altitude varia- tions (12 km) on the surface along 21°N latitude on Mars, and the near-circularity (variations < 2km) of the equatorial region of Venus. The only maps of the Venus sur- face, albeit at present with very poor resolution (100 to 200 km), have been produced using Earth-based radar and show a large number of features. It has been found that the surface of Venus backscatters radio waves about twice as efficiently as either Mercury, Mars, or the Moon, indicating a more compact surface or one made of a material with a higher dielectric constant. Surface slopes on the scale of the radar wavelength have been shown to be smaller on Venus, further accentuating the differences. Similar comparisons among the Moon, Mercury, and Mars are possible from the present measurements. Future Potential. The potential of radar for planetary exploration has not been fully exploited. New facilities, with improvements in sensitivity of nearly three orders of magnitude over existing radar systems, can be built for relatively modest capital

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-13- expenditures. With one such facility, detailed radar maps of Mercury, Venus, and Mars can be prepared, the Venus map having a resolution of 1 to 2 km which is comparable to that achievable in Earth-based optical photographs of the Moon. (A radar interferometer can be used to resolve the twofold ambiguity present in a map made with only a single radar receiver. The size of the second receiver is not critical.) Radar studies of the Galilean satellites of Jupiter will be possible, giving informa- tion on their rotation rates, surface properties, and densities. These measurements will also permit enormous improvements in the knowledge of the orbits of the satellites and of the orbit and gravitational field of Jupiter. Radar echoes from Jupiter may not in them- selves be detectable because of its very deep atmosphere. However, the upper regions of the Jovian atmosphere and magnetosphere can be studied by ground-based radar using signals reflected from the Galilean satellites near occultation with the planet. The largest satellite of Saturn, Titan, will also be accessible to radar study as will the two tiny moons of Mars. The inner planets themselves will be easily detectable at all points in their orbits, allowing important extensions of existing studies. For example, the Martian topography can be determined over the entire latitude and longitude region spanned by the subradar point. The important values of the fractional differences in the equatorial moments of inertia of Mercury and Venus -- vital to an understanding of their spin-orbit couplings and their interiors -- can be determined from radar measurements of the physical libration of these planets. Several asteriods and perhaps some comets can also be brought under surveillance by such a new radar system. In all cases, the knowledge gained from the radar investigations should aid significantly in arriving at the best choice of instru- ments for more detailed exploration with space probes. In view of the past accomplishments and future potential of the radar technique, we recommend strongly that NASA support radar astronomy as an integral part of its planetary exploration program. We further recommend that NASA fund the development and operation of a major new radar observatory required for the investigations discussed above. This observatory should provide an increase in sensitivity of about 1CP over existing systems, and such an increase seems readily obtainable for a capital expenditure of the order of $30 millions. (b) Planetary Observations from Near-Earth Orbit The NASA planetary program should take account of the Earth-orbital telescopes that are being planned for the mid-1970's. These all-reflecting telescopes having apertures of about 1 m, are being designed for diffraction-limited performance at X. = 5000 A and will therefore have a limiting resolution of 0.1 arc sec. High resolution imagery can be obtained particularly in the wavelengths from 2000 to 8000 5^ and long term observa- tions of planetary surface and atmospheric detail on the planets will be possible. For the mid-1970's it appears that fly-bys and orbiters are practical for only Venus, Mars, and Jupiter. In all these cases, a fly-by or orbiter is capable of at least 100 times more resolution than an Earth-orbital telescope, but over only a rather small frac- tion of the planet's surface at a given time. Therefore, the Earth-orbital telescopes should complement these detailed photographs by planet-wide photographs taken at appro- priate intervals. High resolution photography of planets from the Earth's surface at periods of exceptionally good seeing should also be vigorously pursued. The largest contribution of Earth-orbital telescopes to planetary imagery would appear to be imagery of the major planets. With the exception of Jupiter, fly-bys of these planets cannot be expected until at least 1980. Furthermore, the relatively long exposures required for these objects, particularly Uranus and Neptune, make it more dif- ficult to obtain ground-based photographs during intervals of good seeing. Many other solar system objects are photographically resolvable with a 40-inch dif- fraction-limited telescope. These include Pluto, about ten of the largest asteriods,

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-14- Titan, and the Galilean satellites of Jupiter. The latter have angular diameters of about one arc sec and should exhibit surface features on photographs having a resolution of 0.1 arc sec. Ultraviolet spectroscopy from Earth-orbiting telescopes should also be used to com- plement spectrographic data obtained by planetary missions. In the next few years, three Orbiting Astronomical Observatories (OAO's) are scheduled to carry a variety of instru- mentation for wide-band photometry and spectroscopy. Although primarily designed for stellar observations, each of these OAO's can give valuable supporting information on the planets. Attention should be directed toward the feasibility of specially modifying the Small Astronomical Satellite (SAS) for optical planetary observations. o High resolution (0.1 to 0.3 A) ultraviolet spectroscopy of the planets, utilizing the high spatial resolution of a diffraction-limited 1-m telescope would be extremely valuable. Instrumentation now being designed should allow such spectra to be obtained in the mid-70's. At present, there are no active plans to perform infrared spectroscopy of the planets from Earth orbit. However, NASA is equipping a Convair-990 jet aircraft with a 36-inch telescope, stabilized to about one arc sec, and to be flown at about 40,000 feet. There appears to be very little water vapor about this level with the result that the infrared spectrum is nearly completely transparent below 25 H and partially transparent above 25 H. This new facility, scheduled to become operational in 1970 or 1971, should be vigorously utilized for obtaining ir spectra of the planets at moderate spatial resolution (1 to 2 arc sec). In light of these considerations, we recommend that the NASA planetary program plan- ning be closely coordinated with the Earth-orbital telescopes being designed for the mid- 1970' s and with the infrared aircraft telescopes now under construction. (c) Ground-Based Optical Planetary Astronomy We commend NASA for its program of ground-based optical planetary astronomy, and for the construction of new telescopes being undertaken to further this program. The effectiveness of the program would be greatly increased by the addition of a Southern Hemisphere observatory: A number of planets will be in the southern sky three to ten years hence with the result that high resoltuion spectroscopy from northern observator- ies will be hampered by the large masses of air through which observations must be made. Moreover the highly successful technique of interferometric spectroscopy should be ex- tended to the fainter planets and comets. We recommend that this program continue to receive strong support and that its capabilities for planetary astronomical investiga- tions be increased by: (1) Construction of an intermediate-sized optical telescope in the Southern Hemisphere (2) Construction of an infrared telescope employing a very large collecting area and permitting interferometric measurements at a dry site (3) Development of new infrared devices including improved detectors and high resolution interferometers (d) Photographic Planetary Patrols We commend the implementation of a worldwide photographic planetary patrol, but we are concerned that the data obtained will not be sufficiently evaluated. Therefore, we recommend that steps be taken to facilitate the analysis by qualified investigators of these data for such things as presence and motions of clouds on Mars, Venus, Jupiter, and other planets, the seasonal wave of darkening on Mars, and unusual phenomena.

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-15- 6. Interaction of the Planetary Program with the Scientific Community At present, only a few groups in the country are directly involved with the scien- tific portion of the planetary program. There is, however, widespread interest; we believe that the program could be greatly strengthened by increased involvement. We have recommended certain steps -- institution of the small planetary orbiter program -- to help. Further initiatives will be valuable. (a) Principal Investigators or Teams The methods for selection of individual scientists to participate directly in past planetary missions have been criticized. This is due to ^e facto procedures which pre- cluded open announcement of flight opportunities or evaluation of proposals of experi- ments from all interested and qualified scientists, and led to the present situation in which only a very few individuals have access to planetary spacecraft as platforms for their experiments. An alternative method, the formation of scientific teams whose mem- bers have been chosen in several different ways, has had similar consequences, i.e., eliminating the participation of well-qualified and interested investigators. Panel's Views. We recognize that certain types of spacecraft experiments are sufficiently complex or costly so that no one individual scientist is able or interest- ed in utilizing the full capabilities of the experiment. Visual imaging experiments are an example of this class. By contrast, in situ measurements of the planetary en- vironment by ion spectrometer or magnetometer represent another class of experiments in which an individual scientist can and indeed is interested in both developing the in- strument and analyzing its results. We recommend that NASA openly solicit participation in all future planetary missions by announcing flight opportunities with adequate time for response from the scientific community. The assignment of experimental responsibility to a principal investigator or to a team of scientists cannot be made under a single policy that does not recognize the unique aspects that a given experiment may have from the point of view of different disciplines. Thus, at present, we recommend that responsibility be assigned to teams of investigators for imagery-type experiments and to principal investigators in all others. Because of the wide applicability of images to many disciplines, dissemination of the preliminary imaging information should be as widespread and prompt as possible. We recommend that these data be released immediately through the NASA Goddard Space Flight Center National Space Science Data Center. We recognize that the imaging team should have the exclusive opportunity and responsibility to carry out the complex pro- cedures, dependent on knowledge of the camera system, required to yield photometrically and geometrically correct scans. Similarly, for those experiments that demand complex treatment of the returned data before interpretations can be made, we recommend that the present well-tested principal investigator system, which is based on an exclusive-use period, be continued. (b) Summer Institute on Planetary Exploration It is essential, in planning missions and supporting specific experiments for the future, that the participation of the scientific community be broadened. Only a small number of scientists have had an opportunity to participate directly in the planetary exploration program and the experience they have gained is not generally familiar to the planetary science community. New ideas from young scientists must be included as must concepts from more established scientists not yet involved in the planning process- es of NASA. We recommend that NASA develop a summer institute program expressly designed to introduce interested scientists and engineers to the science, technology, and adminis- tration of the planetary program. Participants would include graduate students, junior and senior university faculty, NASA and industry scientists and engineers. The purpose of the institute would be to: (1) Summarize the present state of knowledge of the

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-16- planets and the kinds of scientific studies that can be made from either spacecraft or the Earth's surface; (2) Review the technology and operations of past, present, and future spacecraft and their support constraints; and (3) Discuss the mechanisms of NASA financial support of instrument development and missions studies. It is essential for a vigorous, viable program of planetary exploration that the future participation of individual scientists be encouraged. Such a summer institute will complement the existing post-doctoral research programs that NASA supports through the National Academy of Sciences - National Research Council and which should be main- tained. 7. Manned Planetary Exploration During the past few years, there have been numerous discussions of the possibility of manned planetary exploration. For example, in 1966, the NASA Office of Manned Space Flight prepared a detailed study for a manned round-trip mission to the immediate vicin- ity of Mars to collect information about the planet and, perhaps, to return a small sample of Martian surface material. It was suggested that such a mission could be flown as early as 1975. This study and other similar proposals supported the point of view that the next major space goal should be the placing of man on a planet, presumably Mars since the other planets appear to be even more inhospitable than the Moon. In order to maintain the option open of eventual manned exploration of the planets, several groups, including the Space Science and Technology Panels of President's Science Advisory Committee have recommended that NASA undertake a variety of programs. These include biomedical programs exposing man to space conditions for long periods (100 to 200 days) in Earth orbit to determine whether he is qualified to undertake planetary missions (these missions involve round trips of about 700 days). Such biomedical qual- ification requires the development of special vehicles since neither the present Manned Orbiting Laboratory nor Apollo could easily be adapted for long term missions; need- less to say these programs involve substantial funding. Panel's Views. We were unable to identify a need in planetary exploration, in the forseeable future, for the unique abilities of man. For example, in the proposal for a manned fly-by of Mars, man is not utilized in a unique role. In the face of a limited space budget, we favor reallocation to the unmanned exploration of the planets those resources directed to efforts preparatory to a manned planetary program. The rapid de- velopment of technology suggests that full automated systems of substantial complexity will be available for planetary exploration and that this technology should be capable of answering the major scientific questions that we can now pose about the planets. While at some time in the future it may be in the national interest to undertake manned missions to the planets, we do not believe man is essential for scientific plan- etary investigation at this stage. Therefore we recommend that those resources present- ly intended for support of manned planetary programs be reallocated to programs for in- strumented investigation of the planets. The scientific investigations recommended in the 1965 study and in this report apply directly to the proper planning of any eventual manned program, but they should be viewed in terms of their contribution to the major scientific goals of the NASA program rather than in support of manned exploration of the planets. 8. New Opportunities for United States-Soviet Cooperation in Planetary Exploration During the early years of the space program, President Kennedy sought cooperation in space with the Soviet Union. He believed that the development of areas of common interest could gradually be expanded as time went by, thus establishing a habit of cooperation between the two countries. Some of the early proposals for cooperation involved joint physical implementation in space ventures. For example, President Kennedy in a speech before the United Nations General Assembly in 1963 suggested that

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-17- the Soviet Union and the United States join in sending to the Moon representatives of a number of nations. The preliminary attempts at bilateral cooperation resulted in limited agreements for sharing meteorological data, examining possible joint efforts in obtaining magnetic data, and in using communication systems. Looking ahead to the near future, it is clear that there will be numerous opportun- ities for new and substantive collaboration with the Soviet Union in planetary explora- tion. Pressures placed on the programs of both nations by limited resources may add impetus to efforts for cooperation in space activities. Cooperation between them and eventually with other nations may take the form of planning for, rather than the actual joint physical implementation of, space experiments. Joint planning would permit the maximum use of the special talents of each of the countries involved, while at the same time providing prestige returns to each. Cooperative planning has the additional ad- vantage of not necessarily involving detailed hardware considerations; as a result, ques- tions of security, in the narrow military sense, are not as relevant as they would be in joint implementation of space flights. For example, the Soviets recently penetrated the atmosphere of Venus with a probe. Although this experiment was not entirely unexpected, U S. planning for the exploration of Venus had not taken into account possible Soviet results. The experiments aboard Mariner 5, which flew by Venus shortly after the entry of the Soviet probe, in part con- firmed by indirect radio techniques what the Soviets had obtained by in situ observation. Apparently the Soviet experiments were not carried through to completion and radar ob- servations carried out in the United States were needed fully to interpret the Soviet results. We do not know to what extent Soviet planning for their Venus probe recognized possible U. S. experiments, even though U. S. plans had been published several years earlier. Duplication of effort may have been valuable during the early stages of space explor- ation because of the high probability of failure. The great reliability of the present systems resulting from continued experimentation and advances in technology has elimin- ated the need for repetitive experiments. In the same sense, the rapid development of a broad area of space activities dilutes the prestige value to any nation of a particu- lar space success. There is no longer the great prestige advantage that accompanied a scientific discovery made in advance of competitors now that such discoveries have be- come more commonplace. This is so even with respect to the exploration of planets, in which considerations of celestial mechanics permit visits from Earth to a particular planet at intervals spaced as far apart as one or two years. Only if one nation were completely to dominate an area of exploration, such as planetary studies, would the present balance of space-related prestige be upset. The above considerations and the positive values to be derived from enlarging areas of contact suggest that in the future cooperation in planning for the exploration of the planets will be rewarding to both the Soviet Union and the United States. Planetary ex- ploration may be the earliest and most suitable candidate for such combined planning efforts. Journeys to the planets are expensive, they require long lead times because of limited opportunities for making the journey and great sophistication in instrumenta- tion if the instruments are to survive the lengthy voyage. Furthermore, planetary in- vestigations have no relevance to national security, nor has any nation as yet made a national goal of palnetary exploration. Implementation of such joint planning would, of course, require agreement between the two governments. Despite this need, it would appear best to proceed first through the mechanism of informal but coordinated contacts between American and Soviet scientists. We know little about how the Soviet space program is planned. Informal discussions with Soviet counterparts might provide a basis for such an information exchange. On the basis of such early discussions, it should be possible to draw up a specific proposal which then could be presented to the governments for agreement.

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-18- We recommend a coordinated effort involving representatives of NASA, the Department of State, and the National Academy of Sciences with the purpose of informally contacting knowledgeable Soviet scientists in regard to the possibility of joint planning of plan- etary exploration. Such early discussions should provide the basis for more formal in- tergovernmental agreements.