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Chapter 1 INTRODUCTION In June 1968 the Space Science Board made a study of Planetary Exploration 1968-1975~ in which the principal conclusion was that lithe planetary exploration program be presented~ not in terms of a single goal, but rather in terms of the contribu- tion that exploration can make to a broad range of scientific disciplines." This view was further expressed in a series of recommen- dations for missions to all targets that seemed practicable at that time, with measurements to forward the objectives of all disciplines involved in the space program. In particular the need for exploration of Venus was brought out. To achieve such a broad and flexible program, the study emphasized the need for modest and relatively low-cost mis- sions. A prime recommendation ,vas "that NASA initiate now a program of Pioneer/IMP-class spinning spacecraft for orbit- ing Venus and Mars at each opportunity~ and for exploratory missions to other targets." While developments since 1968 tend to modify specific points made in the 1968 study, the spirit of the recommenda- tions is as important today as it was then. Nevertheless~ the program of the National Aeronautics and Space Administra- tion contains no significant Venus missions (the Venus/Mercury flyby is essentially a Mercury mission with only a small con- tribution to Venus science) and no small missions to the inner planets in the Pioneer/IMP class (which includes the Planetary Explorer discussed in this report). At the same time a strong Mars program has developed, with a very ambitious program funded for the early 1970's. There is also an ambitious program for Grand Tour missions to the outer planets proposed in the 1969 NAS study The Outer Solar System: A Program for Exploration. The present study was convened to re-examine the ques- tion of Venus exploration in the light of the proposed NASA programs and the scientific developments since 1968. In the event that an imbalance in planetary-exploration strategy should be found, the study had, as a second objective, the task of recommending appropriate Venus missions. 3

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4 The study was concerned with relative priorities within the NASA planetary-exploration program. There are, however, larger issues: the relative priorities between different parts of NASA's unmanned science program (astronomy, applications, lunar exploration, space physics, space biology); the relation- ships between manned and unmanned programs; and the effect of changing national priorities upon both programs. These issues are the concern of an extensive Space Science Board study, also carried out during the summer of 1970, under the chairmanship of Herbert Friedman. The two studies came together because a preliminary version of our recommendations was available to the Friedman study and influenced its conclusions. The reason for the exploration of Venus is related to the justification for exploring the solar system, as well expressed in the NAB studies: Space Research/Directions for the Future; Planetary Exploration 1968-1975; and The Outer Solar System: A Program for Exploration. Nevertheless, it is appropriate briefly to address the question: Why explore Venus now? On the most general level, the answer to the question is tied to the remarkable evolution of planetary studies during the 1960's. Not long ago planetary studies were a part of astronomy, often accorded a different treatment and status from solar physics and astrophysics but not yet related to any other discipline. The advent of the space probe in the 1960's provided the tool and some of the data for the detailed investigations and model building characteristic of, and essen- tial to, terrestrial studies. Very rapidly studies of plane- tary meteorology, planetary aeronomy, planetology, and plane- tary biology emerged which involved, in the main, research workers from the parallel terrestrial disciplines. Earth and planetary studies suddenly merged and simultaneously diverged from astronomy. In some major universities, departmental and research center organization was changed to meet this develop- ment. From an intellectual point of view we have moved to the position in which terrestrial studies have expanded to include the whole of the solar system. Problems such as the origin of the solar system, the origin of life, and the large-scale pro- cesses that control man's environment will in the future be considered in terms of many rather than a single object. The detail will obviously differ from one planet to another: from the immense detail required on earth for environmental pur- poses and made possible by the comparative accessibility to the very small amounts of information on Uranus, Neptune, and Pluto and the difficulty of substantially increasing this information.

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5 Thus the call for a balanced approach to planetary explo- ration has a firm logical basis, although the word "balanced" is not clearly defined in terms of the relative priority of new objectives and the investigation in depth of more familiar problems. In whichever way the situation is viewed, however, the absence of almost any firm information about the solid sur- face and lower atmosphere of Venus is the most striking \.veak- ness in our information about the inner planets. If we consider the need for Venus research in terms of particular examples we find striking examples of how knowledge of this planet can illuminate our knowledge of earth. The sur- prise effect of the unexpected ionospheric data on both Mars and Venus has led to timely re-examination of fixed ideas about the earth's ionosphere. Studies of atmospheric circulations in slowly rotating atmospheres have provided new ideas applicable to tropical meteorology. The extensive cloud systems of Venus have led to investigations of the coupling between clouds and motions not yet undertaken on earth. The question of why Venus has a complete cloud cover and a high surface temperature is beginning to interest those concerned with possible environmen- tal changes on earth. This is not so much in the belief that earth might go the way Venus if the atmosphere is suffi- ciently polluted but rather with the thought that Venus is an observable example of a class of problem that concerns our own environment. One final answer to the question, !I\.Jhy explore Venus now?" is that events have conspired to produce a situation in which exploration of Venus is unusually timely and rewarding. De- spite Venera entry probes and Mariner flybys, we know very little about the lower atmosphere of Venus, and yet we have many competing theories the merits of which can be tested with relatively simple measurements. For example, we do not know how many and how thick are the cloud layers, let alone the composition of the uppermost. We know that lower-atmosphere motions are of central importance to all aspects of the phy- sics and chemistry of the atmosphere, but we know nothing about them. At least three independent hypotheses exist with regard to the high surface temperatures. In each of these major areas rather elaborate theories have been developed, generally of a quantitative nature, so that they can be tested by means of a few well-defined mea- surements with proven instruments. This is a situation that occurs but rarely and should be exploited when it does. It provides a further and powerful reason for a strong program of exploration of Venus in the coming decades.

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6 The Study Group was able to determine at an early stage that the developments in Venus science since 1968 heightened the need for substantial effort to explore the planet. Con- siderable attention was therefore directed to the question of the appropriate exploration strategy. Although since 1968, no programs have been approved or funded involving missions to Venus or small spacecraft, it has been a period of planning activity by NASA, its centers, and its contractors. This planning has taken place in close con- tact with scientific groups from outside the agency, and the result has been a particularly rewarding interaction among scientists, engineers, and planners, which in this group's opinion represents a significant advance in the planning of space missions. This Study Group was, therefore, presented with a series of detailed and careful studies of the explora- tion of Venus by means of Planetary Explorer spacecraft (see Chapter 6). Our study benefited greatly from access to these NASA plans. In general, we found them to be well conceived and almost ideally suited to the requirements of scientific in- vestigation in the 1970's and 1980's. As a consequence, our recommendations are detailed and circumstantial and often presented in terms of this particular spacecraft.