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2 Scientific Goals and Recommendations GOALS OF PLANETARY E:XPLORATION The scientific goals motivating planetary exploration are: . To understand the origin of the solar system. ~ To understand the evolution and present states of the plan- ets, including Earth. ~ To learn what conditions lead to the origin of life, and how life modifies its environment. To learn how physical laws work in large systems. To understand the origin of the solar system. Research aimed at understanding the origin of the solar system focuses on those objects thought to retain clues about the primordial conditions and processes that contributed to the system's formation. Although critical information is obtained from measurements made of the evolved planets, the most detailed clues come from investigations of those small primitive objects that have changed little since the time of formation in the protoplanetary nebula: comets, asteroids, and certain meteorites. The cold, volatile-rich matter of comets is thought to contain the most faithfully preserved samples of condensed protoplane- tary material remaining in the solar system. The asteroids form an ordered assemblage of protoplanetary fragments, which seem 4

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s to remain near the original locations of their formation. The compositional and structural variations of objects in the asteroid belt are thought to reflect the radial variation of conditions in the protoplanetary nebula. Ongoing laboratory analyses of mete- orites, which are fragments of asteroids and comets, already show the importance of the information that these objects can provide. Detailed study of comets and asteroids is expected to result in fundamental advances in our understanding of the solar system's formation. Because the formation of our planetary system is thought to have been a natural consequence of the Sun's formation, without the intervention of special circumstances, and because the forma- tion of our Sun is thought to have involved processes typical of star formation in general, planetary systems are believed to occur commonly in the universe, although none has yet been detected. Indeed, our present ideas lead so immediately to the conclusion that planetary systems occur frequently around stars that fail- ure to find such systems would force a fundamental revision of our theories about the origin of our planetary system and about star formation. Important advances in our understanding of the formation of the solar system and In our understanding of plan- etary systems as a class are expected to come from studies of star-form~ng regions and from the discovery and study of other planetary systems. To understand the evolution of the planets. Research aimed at understanding the evolution of the planets and the physical pro- cesses that govern their behavior and their environments concen- trates on those bodies that exhibit most clearly the consequences of planetary evolution. Because we live on Earth, a terrestrial planet, the evolution and environment of terrestrial planets is of special interest. Substantial advances in understanding can be re- alized by investigating, as a class, the terrestrial planets and their close analogs. The major targets of comparative terrestrial planet research beyond Earth are Mercury, Venus, Mars, and the Moon. In ad- dition, studies of many of the outer-planet satellites and of the largest asteroids are expected to reveal important information about solid planet evolution. Terrestrial planet research exploits the close relationship be- tween behaviors and physical processes occurring on Earth and

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6 those occurring on many other planets. Indeed, much of what we can understand about the terrestrial planets derives from ideas and concepts that originated in studies of Earth. Conversely, plan- etary investigations of objects that evolved under conditions far different from those on Earth provoke us to achieve a deeper and more general grasp of natural terrestrial phenomena, as wed as a more confident understanding of Earth's history. By exposing circumstances in which concepts based on terrestrial analogs fail, planetary investigations help us define the limits of applicability of these earth-based ideas. To learn what conditions lead to the origin of life. Earth re- ma~ns the only place where we know life has arisen and continued to flourish. Our search to understand the origin of life involves several planetary questions. It is important to know the physical conditions and chemical composition under which biological ac- tivity arose. It is also important to ascertain whether life forms, complex or incipient, have arisen elsewhere where they can be studied. Presumably, life arose out of an organic, prebiotic medium and was preceded by an interval of chemical evolution, which led more or less continuously into biological evolution. By understanding the formation of the planets we will gain knowledge of the circum- stances under which life arose on Earth. Many objects in the solar system seem not to have undergone substantial evolution since their formation. Some Saturn's moon Titan, for example are expected to carry important clues about the early material in which biological activity arose. These and other objects in the solar system including Mars may have supported prebiotic or early biotic evolution, leaving evidence that can be found today. Investigations of the composition of cosmic matter and prim- itive solar system matter show that the basic building blocks of terrestrial life, including amino acids, occur naturally at least in trace amounts. One of the most significant challenges in un- derstanding the origin and distribution of life is to determine the extent to which special terrestrial conditions were involved in pre- biotic chemical evolution. Detailed chemical assays of comets, as- teroids, and other primitive objects will reveal the extent to which life could have arisen directly from preplanetary matter without an interval of special processing to condition the chemical mix.

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7 To learn how physical [awe work in large systems. Various phe- nomena result uniquely from the large scale of natural systems or from the long time scales over which slow processes work. Because these phenomena do not occur under normal laboratory condi- tions, it is only through direct observations in the solar system that we can expect to understand such unportant processes as planetary tecton~m and volcanism, and cosmic plasma processes. In situ studies of physical processes occurring in the solar system strengthen our overall understanding of the behavior of natural systems. Our ideas about observable phenomena through- out the universe are shaped by detailed solar system investigations. There is little prospect that such intense scrutiny will ever be ex- tended to the more distant reaches of the universe. Thus, cletailed investigations within the solar system will continue to be the foun- dation upon which is built most of our understanding of natural phenomena throughout the universe. Investigations of large-scale physical processes encompass vir- tually all of the objects in the solar system. The giant planets provide clues about properties of matter under high pressures; planetary interiors and magnetospheres demonstrate the curious behaviors of magnetized fluids and plasmas; and planetary atmo- spheres and surfaces present puzzles about long-term evolution of the complex interacting systems that constitute planetary envi- ronments and interiors. A BALANCED PLANETARY PROGRAM Altogether, progress toward realizing these goals requires a balanced program of science and exploration encompassing studies of the planets and their satellites, asteroids and comets in our solar system, and star-forming regions and other planetary systems. The history of science shows us that natural objects sometimes behave in astonishing ways. All scientific investigations begin with exploratory studies to establish the basic characteristics of the systems of interest and to discover their physical behaviors. This exploratory phase h" been accomplished for part of the solar system, although significant work remains to be done. Once the basic characteristics and behaviors are discovered, then investigations proceed on to intensive studies aimed at achiev- ing a deeper understanding. Only very limited intensive studies of the solar system will have been accomplished by 1995. These

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8 will involve the Moon, Venus, Mars, and Jupiter. However, the Task Group on Planetary and Lunar Exploration envisions that in the years after 1995, planetary exploration will shift increasingly toward such intensive studies. A variety of scientific techniques can be used to pursue the goals of planetary exploration. These include laboratory experi- mentation and theoretical analysis, astronomical observations, and in situ investigations using spacecraft. During the past 25 years the primary cause of the enormous advance in our understanding of the solar system has been the information obtained from sci- entific spacecraft. The task group envisions that such spacecraft investigations will continue to play the primary role in advancing our understanding of the solar system. A MARS FOCUS Planets and their environments exhibit extraordinary behav- iors that, for fundamental reasons, cannot be predicted from first principles. The complexity and nonlinearity of planetary environ- ments are such that a planet can, in principle, exist in a large variety of states with the same conditions imposed from outside. The possibility of living systems addle further complication to the variety of states in which a planet can exist. One of the major challenges of planetary science is to understand the evolution of terrestrial planets ant] to discover the possible varieties and causes of diverse planetary environments. Achievement of this goal re- quires comparative studies of the various terrestrial planets and satellites, as well as intensive studies of the changes that individual planetary environments undergo. Of particular interest in the comparison of terrestrial planets ~ the puzzle posed by the triad of planets with atmospheres: Venus, Earth, and Mars. These three planets exhibit differences in their present environments and in their styles of evolution that seem large in comparison with the differences in their sizes, locations, ant! overall compositions. Solving this puzzle is important to us because the differences between these planets occur in those aspects of their environments most important for the viability of life. Several ongoing and planned missions in the planetary pro- gram are directed toward the study of terrestrial planets. However, a number of conderations suggest that it may be appropriate to

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9 undertake a special focus on terrestrial planet science within the context of an overall balanced program, with Mars as the center of that focus. Spacecraft investigations of Mars during the past 15 years re- veal that that planet has undergone perplexing changes through- out its history. Although today the planet appears dry and cold, there is clear evidence of abundant flow of water several times in the past. Changes in the martian surface environment directly pertain to concerns about the behavior of Earth's environment. The prior presence of water on Mars raises important questions about its early, if temporary, suitability for life. Photographs returned to Earth from Mariner and Viking spacecraft reveal spectacular geological formations and deep-cut relief. It is evident that detailed study of the martian crust will yield information about the character of the planet's past envi- ronments, their arrangement in time, and perhaps clues regarding the Influences that produced such marked environmental change. These studies may even help us to understand the Earth's ice ages. Of all the planets beyond Earth, Mars Is the one most acces- sible to detailed study. It is relatively easy to reach with scientific spacecraft, and the surface ~ the most conducive to sustained operation of scientific instruments on mobile platforms. Further- more, it is the only planet outside of the Earth-Moon system that we can currently consider for manned exploration and settlement. SUMMARY OF RECOMMENDATIONS In the next 30 years the task group expects to see an increase in our technical infrastructure in space. This increase should in- clude both earth-orbital facilities for scientific investigations, and advanced capabilities for deep-space operation of scientific space- craft. The recommendations put forward here will take advantage of our expanding capabilities in space. If implemented, the recom- mendations will advance -our understanding of the solar system on the broad front that is needed to progress toward answering some of mankind's longest-standing questions about the cosmos. A rec- ommended Mars focus within that broad-based program will fur- ther our understanding of the terrestrial planets, including Earth, and will address pressing questions about planetary environments and their stability. The goals of planetary exploration are achieved primarily

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10 through spacecraft missions, although various earth-based and orbital activities also serve an integral role. Below, the task group has outlined the activities and missions recommended for the pe- riod 1995 to 2015. These are also summarized in Table 2.1. Figures 2.1 through 2.4, which are in time sequence, illustrate the current and projected status in terms of actual missions. This recommended plan is fully consistent with the reports of the Space Science Board's Committee on Planetary and Lu- nar Exploration (COMPLEX). These are A Strategy for the Ex- ploration of the Outer Planets t986-1996, Strategy for the Ex- ploration of Primitive Solar-System Bodies Asteroids, Comets, and Meteoroids: 1980-1990, and Strategy for Exploration of the Inner Planets: 1977-1987, published in 1986, 1980, and 1978, respectively. NASA's own Solar System Exploration Committee (SSEC), formed in 1980, was charged with providing an imple- mentation plan for the COMPLEX strategies and did so in two reports: Planetary Exploration Through Year 2000 A Core Pro- gram (1983) and Planetary Exploration Through Year 2000- An Augmented Program (1986~. The present plan may be regarded as a follow-on, and includes the SSEC plan in its earlier parts. The task group's recommendations fall naturally into two cat- egories: ~ Completion of the reconnaissance and exploration of the solar system. By 1995 all the planets except Pluto will have been visited, but many of them for one brief flyby; encounters with asteroids and comets will be just beginning. Many phenomena will remain to be identified, let alone studied and explored. Intensive exploration of Mare. Mars is the planet most like Earth, and among the most accessible. A sequence of missions, possibly leading to a temporary or permanent base, is recom- mended. Technological initiatives that can enable, or greatly improve, ac- tivities in various areas are discussed later in this chapter. Scientific ~vestigatione The detailed objectives in Table 2.1 can be achieved by the foldowing types of investigation.

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11 TABLE 2.1 Recommended Programs Beyond 1995 Scientific Objectives Implementation Mercury a. geological mapping, Sorority, motion, surface chemistry b. surface and internal properties 2. Venus a. heat flow, internal properties b. detailed characterization 3. Moon a. detailed study 4. Mars a. internal and atmospheric properties b. detailed characterization c. field studies 5. Asteroids and comets a. exploration b. detailed characterization c. field studies (earth-crossing) 6. Jupiter system a. deep atmosphere investigation b. magnetosphere mapping c. surface and interior of satellites, especially Io 7. Saturn system a. atmospheric in~restigation b. satellite in~restigatione c. ring investigation d. Titan atmosphere e. Titan surface (ocean?) 8. Uranus system a. atmospheric investigation b. satellite and ring investigation 9. Neptune system a. atmospheric investigation b. satellite investigations c. Triton investigations 10. Pluto system a. system exploration 11. Other planetary systerne a. search b. investigation orbiter, transponder lander, sensor network sensor network sample return sample return, sensor network, rover sensor network roarer, sample return in situ human investigations multiple rendezvous sample return in situ human in~restigatione deep probe polar orbiter lander, sensor network probe, deep probe orbiter rendezvous probe lander probe, deep probe orbiter probe orbiter to be determined orbiter telescope on Space Station advanced telescopes in space

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16 ~ Composition and internal structure are important for all bodies. Terrestrial planets and large satellites should be sampled at many locations, and networks of seismic and heat-flow stations put in place. Studies of geology and surface geochemistry are im- portant. For bodies with visible surfaces such work can be carried out by imaging from orbiters and landers, and by other remote- sensing techniques, such as infrared and gamma-ray spectroscopy. . Many crucial types of chemical and isotopic analysis can only be made on samples returned to Earth. Such studies bear not only on the present state of crustal material, but also on its origin, age, and history. For Mars and Venus, the samples must be from carefully chosen, well-documented sites. For comets, the main consideration is to preserve the original physical state of the material. . Rendezvous mismons are important for the study of small bodies, comets, and asteroids. The behavior of comets, with their changing distance from the Sun, is of special interest. Although such studies may have begun by 1995, they should be continued in order to explore the diversity of comets and asteroids. A ren- dezvous with Saturn's rings, though technically difficult, would be immensely valuable. ~ Probing of deep atmospheres ~ important for all the Jovian planets, as wed as for Venus and Titan. A start has been made for Venus and Jupiter. However, the Galileo probe to Jupiter will reach only the 12- to Unbar level" not nearly deep enough to penetrate the main cloud layers of planets such as Uranus and Neptune. Analytical instruments on probes offer the only means of measuring noble gases and isotopic abundances, which are almost the only clues to planetary origin and evolution. Such probing of planets whose surfaces are inaccessible can be regarded as the equivalent of sample return for rocky and icy bodies. In addition, the ability of probes to measure winds and other atmospheric ~ - mO[lOnS IS UnlqUe. Planetary environments include magnetospheres, satellites, ring systems, and extended atmospheres. Because of the variety of their interactions, these objects and phenomena are best stud- ied by diverse payloads covering many of the above disciplines on Tong-lived orbiters, perhaps combined with landers or probes. . Other planetary systems are already being sought, and there are a few tantalizing indications of related phenomena,

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17 such as circumsteliar disks. Although there is promise in con- tinued ground-based searches and those made with the Hubble Space Telescope, a dedicated astrometric telescope and a low-light- scattering telescope in orbit wiD be required for a comprehensive search and follow-up studies. ~ The Mars program should be aimed at a deeper under- standing of the entire planet and its history. After the current gen- eration of orbiters, most further studies will require soft-landing automated laboratories, supplemented by networks of stations for seismic and other studies. Intelligent sample return requires use of rovers, which can carry out geological and perhaps geochemical studies as well. further geological work requires a human pres- ence, either literally or at the end of a control and communication link, unless a remarkable amount of intelligence can be built into robotic devices. Technical Developments Many of the recommended investigations will be enabled or enhanced by technical developments beyond those of the mid- 1980s. The power and flexibility of low-thrust propulsion make it the key to serious study, beyond the reconnaissance and explm ration phases, of comets, asteroids, and the solar system beyond the inner planets. For the Jovian planets, the very long trip tunes of ballistic flights create serious problems, and the capability of getting into orbit is limited. Closely related is the concern about power sources. Though solar power is usable only in the inner solar system, much larger arrays would be useful; large amounts of nuclear power seem available only from reactors, and such systems are already seeing some development. Quite a different power problem exists at the surface of Venus, where long-lived landers would be important but are currently in- feasible because of the 750K temperature. Soft-landing technology, as used on Surveyor, Apollo, and Viking, has been very successful, but is also expensive and terrain-sensitive. Much cheaper systems are needed so that arrays of instruments can be deployed on many bodies; payloads could be relatively modest. Penetrator technol- ogy has aroused a great deal of interest for this reason, but other possibilities for hard or semihard landers should be explored, as well as the technology for rovers. The planetary program already

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18 builds a large degree of autonomy into its spacecraft, but very little ability to make independent decisions. Thus, further development in robotics or artificial intelligence would be useful. In addition, some of the most interesting environments in the solar system offer conditions that are intolerable to present-day electronics for more than a few hours. The jovian radiation belts prevent exploration of To, its torus, and the entire inner magnetosphere of Jupiter. The surface of Venus ~ very hot. Developments in radiation-hardened and high-temperature electronics would improve the chances of exploring these regions. On-orbit staging, assembly, and fueling offer new capabilities for the more ambitious missions, especially returning samples from Mars. Such missions could benefit greatly from the ability to use a space station to assemble and fuel larger spacecraft than can be launched fully fueled in a single assembly from the Earth's surface. An integral part of the recommended program is adequate support for analysis and interpretation of the data returned from it, including maintenance of the intellectual base for scientific ac- tivity. Theoretical, laboratory, ground-based, and earth-orbital studies must be supported; this support includes upgrading bay oratory instrumentation and computing equipment, as well as maintaining adequate programs of research and analysis. International Collaboration The matter of international collaboration cuts across the whole of space science, and is therefore considered in the report of the steering group. However, it IS appropriate to note here that planetary exploration is a particularly suitable area for such efforts. Planets are a neutral ground devoid of nationalistic, com- mercial, or military interest. They are analogous to the desert continent of Antarctica, which Is protected by an international treaty for scientific research. Modest international participation in national program has been successful on many occasions, but true cooperative programs, jointly planned by more than one agency, offer great potential benefits.