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Chapter 1 INTRODUCTION In the summer of 1965, the Space Science Board undertook a comprehensive review of the scientific opportunities in lunar and planetary exploration.* The purpose of the study was to judge in a broad way the scientific priorities that should be assigned to investigations of the planets and to identify the principal scientific problems that could be elucidated by such exploration. Since 1965, much has happened. As the 1965 study was ending, Mariner 4 encountered Mars and shortly thereafter began to return historic photographs of the Martian surface. In 1967, Mariner 5 and 'the Soviet probe Venus 4 measured the hot, dense atmosphere of Venus and discovered that the solar wind interacts much more directly with the atmosphere of the planet than it does with Earth because of the lack of magnetism on Venus. Surveyors and Lunar Orbiters, to- gether with their Soviet counterparts, have provided a wealth of detailed information concerning the surface and even the interior of the Moon. In addition, ground-based observations, particularly radar studies, have yielded valuable information on the solar system. Long held views with regard to the rate of rotation of Mercury and Venus have been shown to be incorrect and a puzzling set of phenomena involving the coupling of the spin of a planet to its orbital motion discovered. Radar has also given important new information about the major surface features on portions of Venus and Mars. Thus, the combined efforts of Earth-based and space-based observations have greatly increased our knowledge of the planets since 1965. However, it was not only the rapid development in science that motivated the Space Science Board to conduct a reappraisal of the 1965 report with regard to plan- etary exploration. Since 1965, the budget of the National Aeronautics and Space Administration has been cut severely. The level of the present and probable future support that the nation is willing to provide for planetary exploration raises a number of difficult and important issues. What should be the nation's program for planetary exploration in a time of continued budgetary constraints? What portion of the limited resources made available to the space agency should be allocated to plan- etary studies? What information can be gained by further ground-based experimentation or observations from near-Earth satellites, and what information can only result from space vehicles at or in the vicinity of a planet? These are some of the questions that the present study attempts to assess. The 1965 study examined both lunar and planetary investigations, clearly pointing out that the two are closely interrelated and that one cannot adequately plan a plan- etary program without comparable planning for lunar studies. While recognizing the validity of the earlier study's conclusion, the present study examines only opportu- nities for planetary exploration. Scientific investigation of the Moon involves even more complex issues than those in planetary investigations. Exploration of the Moon in part will be a manned venture, and in this venture scientific considerations are intertwined with questions of national prestige. In view of the uncertainties about the future of the manned lunar program, the present group did not consider in depth the development of a total lunar and planetary program. The planning of a program of planetary exploration presents special and complex problems. Voyages to the planets are long and can be undertaken only on limited occasions. While favorable times to visit Mars and Venus occur about once every * Space Research Directions for the Future: Report of a Study by the Space Science Board. Woods Hole Mass., 1965; Part I - Planetary and Lunar Exploration; National Academy of Sciences - National Research Council Publication 1403, Washington, D. C., 1966. -1-

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-2- 19 to 26 months, opportunities to use unusual configurations of the planets to carry out more extensive exploration arise only once in a decade or in some cases only once in a century. For example, in 1970, 1973, and to a lesser degree in 1975, it will be possible to use the gravitational field of Venus to assist an Atlas-Centaur launched spacecraft in a flight to Mercury. Exploration of Mercury would not otherwise be pos- sible without employing a very much larger booster. This opportunity will not be repeated until the 1980's. A once-in-a-century opportunity occurs in 1977-1978 when the planets will be so positioned that their gravitational fields can be exploited for a grand tour of the great planets, Jupiter, Saturn, Uranus, and Neptune -- again without the need for extraordinarily powerful boosters. The rare opportunities for planetary voyages, the length of these voyages, and the long times required for preparing experiments all imply that planning for plan- etary exploration must take place years in advance of the actual missions. Thus, decisions arrived at this year and next will go far in determining the future char- acter and scope of planetary programs. Further, it should be emphasized that the long times involved in planetary missions demand long term commitments on the part of the scientists participating in these investigations. Quite naturally, scien- tists are reluctant to make such commitments without assurance that this nation will indeed have a continuing program for planetary exploration. The nature of the plan- ning process and the importance of involving the scientists in the program make it clear that the decision makers within both the Executive and Legislative branches of the government must understand the very special problems faced by NASA and the scientific community in the planning of a planetary program. For these reasons, Planetary Exploration 1968-1975 is addressed in part to the decision makers. The report consists of seven chapters. Chapters 3 to 7 discuss the individual scientific disciplines relevant to planetary exploration -- Atmospheres; Surfaces; Dynamics and Interiors; Particles, Fields, and Interactions with the Solar Wind; Exobiology -- and suggest the experiments and programs best able to deal with their special questions. These chapters were prepared by small groups of participants in each discipline, making use of position papers prepared and circulated in advance of the study. Using these chapters as the basis for discussion, the study group as a whole agreed upon the priorities and recommendations set forth in Chapter 2.