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Chapter 2 RECOMMENDATIONS SCIENTIFIC RETURN FROM VENUS MISSIONS In this report we examine the state of knowledge of the atmo- sphere of Venus, the important questions that can now be asked, and the relatively straightforward means by which these ques- tions can be answered. Noteworthy in this context is the extraordinary flow of ideas since the U.S. Mariner 5 and the Soviet Venera 4, 5, and 6 missions. Compared to the range of questions of first mag- nitude that remain to be answered the contribution from these space missions was small. Nevertheless, the few pieces of in- formation that they contributed made the investment of time and effort into the complex process of geophysical modeling worthwhile, and we now have stimulating quantitative theories about the atmosphere and surface of Venus. A strong scientific interest in Venus now prevails among geophysicists and astrophysicists. This interest can best be maintained by a series of missions that will permit broad par- ticipation by scientists in the U.S. planetary program. Another important reason to support the exploration of Venus is that we must have more information on that virtually unknown planet in order to obtain some of the data necessary to unravel the puzzle of the origin of the solar system--a question of major interest to laymen and scientists alike. Furthermore, a better understanding of the atmosphere of Venus will make important contributions to our understanding of generalized atmospheric problems and, hence, lead to added understanding of our own atmosphere. The problems of Hadley circulations and interactions between clouds and atmospheric dynamics are two cases in point. The long-range climatic ef- fects of pollutants on the transfer of solar and infrared ra- diation is another. We believe that the combination of scientific goals and the feasibility of contributing to these goals makes the ex- ploration of Venus one of the most important objectives for planetary exploration in the 1970's and 1980's. And we, 7

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8 therefore, recommend that exploration of Venus be prominent in the program of the National Aeronautics and Space Adminis- tration during the 19701s and 19801s. THE VALUE OF SMALL MISSIONS The idea of small, minimum-cost exploratory missions has immedi- ate appeal to large sectors of the scientific community. This is in part accounted for by a feeling of responsibility toward the taxpayer and society. It also results, however, from the real advantages that accrue from projects involving minimum funding and therefore minimum complexities of planning and or- ganization, personnel, and collaboration between organizations. Exploration 1968- The Space Science Board study,. PZanetar>y.. 19?5~ took place at a time when the Mariner program had estab- lished some of the fundamental physical parameters of the inner planets and most of the technology needed for future missions. As a result of this experience, the study chose, as the pre- ferred course for the early 19 IS to go in the direction of smaller, lower-cost missions. The first priority recommended by the study was lIaprogram of Pioneer/IMP-c.1ass [Planetary Explorer] spinning spacecraft for orbiting Venus and Mars at each opportunity, and for exploratory missions to other tar- gets.1i That entry probes were not included in this statement merely reflects. the prevailing climate of engineering thought at that time: entry probes did not come to mind as practical, inexpensive spacecraft. Since then, the success of the Venera probes and a number of engineering studies have greatly changed the situation; moreover, it has become clear that the entry probe is an ideal vehicle for attacking a number of prime scientific questions. There are many contributing reasons for this support for small missions: 1. If the cost is sufficiently low, a series of missions can be envisaged. This opens upm~ny opportunities, novel to planetary exploration. One is that relatively high-risk ex- periment.s can be undertaken if a high return of scientific information can be foreseen. Initiative is encouraged if the cost of failure is reduced. 2. A series of missions can be planned, Missions can build upon the observations of previous ones and upon the new

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9 theories and speculations to which they may give rise. Early observations can often be kept much simpler if there is an ex- pectation of more elaborate measurements to follow, based upon the preliminary results. It is, of course, necessary to de- velop organizational techniques to take advantage of this pos- sibility. It is barely possible to make some changes in in- strumentation from One VenuS opportunity to the next. It should, however, be relatively easy to ensure complete payload redesign, if necessary, every second opportunity (i.e., at approximately 3-year intervals). 3. A series of missions can stimulate the active parti- cipation and collaboration of many scientists and scientific disciplines. Moreover, with many opportunities, the value of the program to the educational process is enhanced because it becomes possible to involve less-senior experimenters--even to make use of graduate students under supervision. 4. A series of missions can, in times of fiscal strin- gency, be reduced in frequency without a complete cutoff of the program and the attendant loss of experimenters. Because of the nature of NASA funding, projects are not always funded as predicted, and it is therefore healthy to maintain the max- imum flexibility in this respect. 5. It is difficult to plan science far in advance. On the other hand, a very expensive mission is often inflexible. This can lead to a mismatch between the desire for unfettered and innovative thinking On the one hand and responsible tech- nical planning on the other. This gap closes as the cost of the mission decreases, although it cannot ever be expected to disappear. Scientists, engineers, and administrators each have to modify their attitudes when collaborating in a large project, but the Planetary Explorer concept should ensure that the difficulties are minimized. 6. The Planetary Explorer concept makes it possible to use probes, landers, and orbiters in combination, each sup- porting the other, in planetary exploration. There are ex- amples in this report in which orbiter science can prepare for better probe science and vice versa, and lander science can benefit from both. Thus, atmospheric-probe measurements can establish the wind-noise spectrum that might be encoun- tered by a landed seismometer--a parameter essential to de- fine sensitivity. When the above advantages of small missions are combined, the value of this concept becomes clear. We are not without previous experience of this kind of development. The

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10 exploration of the earth's atmosphere increased greatly in pacEc and became a different kind of program when the low-cost Aero- bee rocket replaced the V2 and Viking rockets in the early 1950's. Although the Aerobee has only arrow flight stability and a relatively small payload, and it was a long step back- wards in sophistication, it has been a workhorse for aeronomy and astronomy observations with which generations of experi- menters have been well satisfied. We may expect that the Planetary Explorer will playa comparable role in NASA's plane- tary program. The rest of this report summarizes the results of our ex- amination of the scientific questions posed by Venus, of the potential for rapid advance in our knowledge of the planet, and of the Planetary Explorer as the prime vehicle for this advance in the next 10 years. We peoommend the Planetary Explorer as the prime vehicle for the initial exploration of Venus by orbiters, atmospheric probes, and landers, because this inexpensive, Delta-launched spacecraft has the capability and versatility to obtain an- swers to most of our prime questions about Venus and to aid in defining the environment of Venus for possible bigger mis- sions in the future. OF MINIMUM COST ACHIEVEMENT As discussed below, the cost of a given set of hardware can depend greatly on the philosophy adopted, especially the amount of testing required. Most of the benefits discussed in the previous section are a direct consequence of small project size, which implies a proportionately low cost. Every effort should be made to keep testing and paperwork to the minimum required for a reasonable assurance of success. As the total cost of a mission diminishes, the ideas of what is reasonable can be relaxed. Recent practice in the planetary program has been to require extra flight instruments just for testing and to undertake environmental and life tests whose total cost may far exceed that of the instrument itself. In most rocket work, on the other hand, reliance is placed on the use of high-quality parts and careful design, with only brief tests in a simulated environment. The quality of the product is still high. For a given mission the philosophy chosen should lie somewhere between these extremes, but we

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11 suggest a considerable relaxation of the standards previously applied to the large planetary missions. Another area that can greatly affect costs is the choice of experiments. For example, complex mechanisms may need to be developed for some purposes~ but they tend to be expensive and unreliable. In general, such equipment should be avoided unless there is an overriding scientific requirement. We recommend that NASA be prepared to accept a somewhat higher risk for the Planetary Explorer than has been its prac- tice for previous planetary missions) if substantial over-all cost savings can be achieved. Because of our minimal experience in carrying out un- manned experiments on a remote planetary surface, and because of the high temperature and pressure of the surface of Venus, we recommend that experiments to be carried on early lander missions emphasize simplicity of operation and) where possible, avoid complex manipulation and processing of surface material. INSTRUMENT DEVELOPMENT Missions to Venus, particularly those entering the atmosphere, NASA will require a wide range of novel scientific equipment. has already shown its willingness to support the development of new instruments~ for example, those needed for cloud studies. To publicize this fact, we recommend that NASA announce the existence of a series of opportunities listing some of the more crucial needs. Such an announcement, coupled with a general de-. scription of the expected spacecraft and their capabilities~ should stimulate much of the necessary work. Current technology appears to be unable to provide a landed mission with a lifetime of more than a day or two, be- cause of the need to keep down the temperature of the instru- ments and batteries. Instruments such as seismometers can reap their greatest benefit only with a much longer period of operation. Therefore, we recommend that NASA encourage en- gineering research and development to provide~ if possible, lander systems and experiment components able to operate at temperatures up to 800 K.

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12 QUARANTINE REQUIREMENTS could A slight possibility exists that terrestrial organisms grow on airborne particles near to the cloud tops of Venus. The problem was discussed at the 1970 CaSPAR meeting, and some interest was expressed in investigations of airborne life. Life on Venus is no more than a remote contingency, but the possibility of contamination by terrestrial organisms must be considered. The saving feature of all Venus missions is that there is no longer any doubt that a temperature of about 700 K pre- vails over the entire surface of the planet. There is no pos- sibility that terrestrial organisms can grow at such tempera- tures, and we are therefore at worst concerned with a short period of transit through the cooler regions of the atmosphere. According to the CaSPAR agreements, the cumulative proba- bility up to 1988 of contaminating the planet must be less than 10-3. With 20 miss~gns, the probability per mission must then be less than 5 x 10 We are satisfied that this constraint . is readily met, even if the bus or orbiter should enter the atmosphere. These unshielded vehicles will mostly vaporize in the upper atmosphere, and at most a few charred members may fall rapidly through the temperate region of the cloud tops. For numerical estimates we may start with the figures given in the Planetary Explorer, Phase A Report (Goddard Space Flight Center, October .1969, Section 6 and Appendix C). The number of spores is taken as 104. The probability of release in the at- mosphere under the above circumstances is estimated to be less than 10-3; we regard the Goddard figure of 0.3 as far too high for atmospheric release, because it was based on a hard-surface of growth was given as 10-4, impact. The probability but this assumes the presence of a stable particle or droplet to grow on. However, droplets are subject to evaporation, while solid particles must be subject to rapid mixing to support them a- gainst fallout; they will therefore reach a hot region in a short time. We believe that the probability gf growth in the atmosphere should be amended to less than 10- for a total probability of contamination per impact of less than 10-5. We therefore see no reason why the bus or orbiter should not be permitted to impact the planet whenever a scientific benefit is to be gained thereby. Low-periapses orbiters should also be open to consideration. Surface-sterilized entry probes, hermetically sealed and with a fully sterilized heat shield, present a far lower probability of contamination than

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13 do the bus or orbi~er, and risk of contamination from them may be neglected. We therefore recommend that, with some precautions, space- craft be allowed to impact the planet when scientific benefit is to be gained thereby. CONTINUING PLANNING GROUP The fullest possible benefit of the Planetary Explorer con- cept can only be realized by an integrated series of missions, as discussed in Chapter 6. At any given time there will be a set of prime questions requiring answers; but some of them cannot be asked at present. Also, certain importantexperi- ments will require a further definition of the environment by a previous mission before they can operate to full efficiency. (For example, television is useless if there is dense fog at the surface.) The best available mechanism to assure that the scientific requirements are met is the planning group, which has already been used successfully for the Venus-Mercury Mari- ner and the Viking programs. But for a Planetary Explorer series it is.not enough to have such a group for each indi- vidual mission: at least some continuity is necessary over the series. In addition, the highly integrated nature of the experi- ments on a particular mission suggests that the traditional payload concept--a collection of separate experiments indi- vidually conceived--is not valid. Rather, each mission should be regarded as a single experiment with the individual experi- mental subsystems carefully chosen to complement one another and to maximize the scientific return within the mission con- Therefore, we reaommend that NASA set up and main- straints. tain a continuing planning group for the exploration of Venus which will advise on strategy for the mission series and on conceptual payloads for each mission. FUTURE MISSIONS The strength of the Planetary Explorer concept as discussed in this report is due in great part to quite recent improve-

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14 ments in launch capability~ communications~ and science instru- mentation. We have been able~ therefore~ to demonstrate how most of the important first-order questions about Venus can be answered by a series of Planetary Explorer missions. In a longer view, questions arise regarding the continuation of this strategy. As far as we wish to predict~ the study of planetary aer- onomy and particles and fields can be continued indefinitely within the Planetary Explorer concept. The two requirements for a full investigation of the first-order questions are (1) measurements within the upper atmosphere from entering vehi- cles and from low-periapsis orbiters and (2) measurements of particles and fields from orbit, preferably with two vehicles at the same time. Atmospheric studies made during the recommended Planetary Explorer missions will be extremely fruitful in defining the chemical and physical makeup of the atmosphere. The poten- tially great complexity of the cloud physics and of the cir- culation, however, leaves the possibility that these processes may remain poorly understood. Considering their great rele- vance to the determination of the high surface temperatures, it may become desirable, on the basis of the first missions, to have a more advanced effort to answer these questions. No guess as to the appropriateness of Planetary Explorer technol- ogy to advanced work is possible, although the recent rate at which capabilities have improved leaves ground for optimism. Most basic questions about the solid planet itself can be answered only by measurements made on the surface. It appears possible to make some important measurements of this kind with missions of the modest scale proposed. However, the lander mission suggested in this report could be as far as currently defined Planetary Explorer technology can go in this direction. If passive seismology proves viable, future visits to the sur- face ought to be made at three sites simultaneously. Analysis of surface materials can be definitive only if several sites are visited and if several fairly complicated experiments are carried out, involving, for example~ long lifetime, prepara- tion of samples, and vacuum conditions. As far as we wish to foresee, this sort of thing goes beyond the current Planetary Explorer concept in cost and complexity. We think that such a program will not be desirable until the 1980's~ and we sus- pect that improvements in launch vehicle and instrument tech- nology in the interim will greatly increase the viability of any more ambitious type of mission.

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15 INTERNATIONAL COLLABORATION International collaboration projects in geophy~ics--the Inter- national Geophysical Year, the Upper Mantle Project, and the International Year of the Quiet Sun--have contributed greatly to the development of knowledge of man's environment. Further, the International Geodynamics Project has recently been launched by the International Council of Scientific Unions to study the fundamental dynamical processes within the earth's interior which are responsible for crustal movements--e.g., earthquakes. Development of international collaboration in planetary explo- ration is a natural extension of these successful scientific policies in geophysics, in the development of which the United States has played a leading role. The proposed program of in- vestigation of Venus is scientifically the broadest yet proposed for a planetary investigation involving most of the subdisci- plines of geophysics. Thus it is most suitable for collabora- tion with scientists of other nations. We therefore recommend that NASA actively seek the col- laboration of other national space organizations in planning and carrying out these investigations. EARTH-BASED STUDIES Optical and radio (including radar) studies of Venus have been valuable in the past and are expected to remain so. We rec- ommend that NASA continue to support and develop earth-based studies, and we commend its past efforts in this respect. Im- provement of radio facilities can be expected to give better thermal maps, as well as radar maps of the surface comparable to earth-based optical photographs of the moon. Optical work at the cloud tops can give long-term information on inhomo- geneities and on the possible four-day atmospheric rotation. The techniques complement one another and give valuable sup- plementary information for the Planetary Explorer missions.

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16 VENUS/MERCURY FLYBY We understand that the Venus/Mercury flyby scheduled for launch in 1913 is intended primarily for the exploration of Mercury. However, the opportunity exists for valuable measurements dur- ing the flyby of Venus. These measurements, particularly high- resolution, high-contrast imaging of the cloud layer, would play an important role in advance planning for the Planetary Explorer program. Specifically, imaging is not contemplated on the early Planetary Explorer missions. Results from the Venus/Mercury flyby could help to optimize the payloads for later Explorer orbiters. For these reasons, we endorse the Venus/Mercury mission for the contribution it can make to Venus science.