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4 Future Programs PROPOSED MISSIONS Programs for Planetary Geosciences Types of Missions The goad of planetary exploration are met through observa- tions and missions in which the levels of investigation are generally progressive. Earth-based observations provide limited, but impor- tant data that allow the formulation of first-order questions. As the first level of investigation, reconnaissance by flyby rn~s- sions attempts to reveal the major characteristics of a planet, such as its radius, mass, rotation rate, and the existence of mag- netic fields, an atmosphere, oceans, satellites, and the like. The exploration phase follows and has the goal of describing and un- derstanding the state of a planet and the general processes that have influenced its environment. This phase is carried out by long- lived orbiters equipped with a variety of cameras and other remote sensing instruments and may include entry probes to measure the chemical composition of the planet's atmosphere or surface. Such missions can image the surface of the planet; provide a global map of the distribution of elements and minerals on the planet's surface; 88

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89 determine a planet's global properties including topography, grav- ity, and magnetic fields and density distribution; and characterize the atmospheric and ionospheric structure and dynamics. The intensive phase of investigation addresses the highest- order questions revealed by the earlier phases and involves sophis- ticated and complex missions. Detailed study of the properties of surface materiab, interaction of surface and atmosphere, strati- graphic and depositional history, and biologic questions requires in situ measurements by soft-landed automated laboratories, mobile laboratories (rovers), and networks of instruments. "Network science" is defined as geophysical measurements made over correlatively long time (>1 earth year) at several lo- cations over a planet's or satellite's surface. Although network science can be accomplished with a minimum of 3 to 4 stations, ideally a system would include an array of 6 to 12 stations with instruments to measure seismic events, physical properties of the surface, heat flow, and (where applicable) meteorological parame- ters. The stations must provide simultaneous measurements from the seismic and meteorological experiments, and may also make el- emental chemical and mineralogic measurements with appropriate instruments. Network science can address geophysical questions on local, regional, or global scales. For example, most knowledge of the interior of the Earth has been derived from seismic data. Networks of seismometers on a global scale yield information about the properties of the core, mantle, and lithosphere; arrays of seis- mometers on a regional scale can provide detail about systems of active faults; and local arrays (tens of kilometers or less) can provide information on the presence and configuration of rock lay- ers within the lithosphere. Network stations can be emplaced by penetrators, semihard or soft landers, or a rover. The measurement of heat flow is fundamental for understand- ing the interior characteristics of planets and satellites, yet obtain- ing valid data remains a major technological problem. All present means for emplacing instruments in the subsurface (e.g., penetra- tors, drilled holes) disturb the thermal regime for a period of time that exceeds the lifetime of most missions. These problems must be overcome, or alternative means found for measuring heat flow. Sample return missions can provide fundamental data that can be acquired in no other way about the composition, history, and evolution of other worlds. At the same time, sample return

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go missions present exciting technical challenges; they require so- phisticated robotic spacecraft systems and complex operational capabilities on planetary surfaces. Both terrestrial and extraterrestrial rocks preserve evidence of past events, thereby providing essential clues to understanding the planet's origin, history, and evolution. The fabric of crystals that compose a rock (or a piece of solid ice from a comet) reflects both the original formation and subsequent evolution. One can distinguish between lava flows that cooled quickly at the surface and deep-seated crustal rocks that cooled slowly tens or hundreds of kilometers down. The different minerals in a rock reflect further details of their formation the formation temperature, the cool- ing rate, the nature and abundance of volatiles, and the genetic relations between minerals formed at different times. The detailed chemical character of a rock can thus be the key to identifying global planetary processes core formation, crustal separation, or episodes of widespread volcanism. The measurement of radioac- tive parent and daughter elements in a rock provides independent information about the timing of major planetary events origin, major meteorite impacts, and volcanism. Analyses and studies of samples returned to the Earth are unique in that they: (1) can be performed by a variety of scien- tists with current state-of-the-art technology; (2) permit iterative, imaginative experiments that can be based on prior results, in- cluding unexpected ones; (3) allow effective separation and con- centration of mineral phases, based on the specific properties of the sample; (4) permit many different analyses on the same sample; and (5) permit the deferral of certain experiments, if necessary, until better analytical technology or understanding is available. The flexibility of laboratory sample analyses, and the resulting confidence in results, is largely due to the high analytical precision and to the greater control of experimental parameters possible in a laboratory. Studies of the mineralogy, mineral chemistry, texture, and bulk chemical composition will define the physical and chern~cal history of rocks. Evidence for processes ranging from crustal for- mation to volcanism or chemical weathering at the surface will be addressed through detailed comparison of textural properties, mineral composition, and the distribution of elements and isotopes within the rocks. A wide variety of signatures have been identified among trace

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91 elements as tracers for terrestrial and lunar geochem~cal processes. Analyses of these various types of elements, either in groups or as pairs, will yield evidence on the nature of the bulk starting materi- ab with regard to differentiation, the degree of that differentiation, the internal heat sources, the temperatures and pressures of inter- nal processes, and the nature of meteoritic material impacting the body during the geological past. Experience shows that these anal- yses can be combined with other geological information to unravel the complex evolutionary history of planetary surface materials. Precise isotopic analyses will allow us to solve a wide variety of chronologic and geochemical problems. Long-lived radioactive species and their products (U-Th-Pb, K-Ar, R~Sr, Nd-Sm) pro- vide isotopic ages for rocks and are the only means of establishing an absolute chronology. Stable isotopes (H. O. N. Si, C, S) pro- vide very powerful geochemical tracers that can be used with the chronological data to explore past states of the interior as well as more recent surface processes. Anomalies left by the decay of ex- tinct, short-lived radioactive isotopes can be expected to provide evidence for preaccretion conditions and time scales in the early solar system, before formation of the planets. Analyses for He, Ne, Ar, Kr, Xe, and their isotopes are es- sential for understanding the internal differentiation history, its interaction with cosmic radiation, and the evolution of its atmo- sphere. Such data from surface materials will provide powerful tools for understanding the evolution of the planet's surface and interior. From our experience with lunar samples, meteorites, and terrestrial rocks, Xe isotopes are expected to be the most versa- tile, as the isotopic patterns may reflect several processesextinct short-lived isotopes, fission of long-lived and extinct isotopes of U and Pu, and the mixing effects of various reservoirs of gas. Samples will be examined for evidence of remanent magneti- zation related to any past magnetic fields, as well as for a variety of physical properties, such as grain distribution, density, porosity, thermal conductivity, capacity for retaining volatiles, and seismic wave velocity. These measurements will provide basic data for various physical models. Planned Missions The rn~ssions proposed to study the solar system during the period 1995 to 2015 follow the balanced approach recommended

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92 by the National Research Council's Committee on Planetary and Lunar Exploration (COMPLEX). By 1995, the exploratory and reconnaissance phases should have been completed for the inner planets, with the important exception of Mercury, and the pro- posed missions will involve the intensive study phase. The Voyager spacecraft will have performed the initial reconnaissance study of the satellites of Jupiter in 1979, of Saturn in 198~1981, of Uranus in 1986, and of Neptune in 1989; the Galileo spacecraft will have continued exploration of Jupiter's satellites in the 1990s. During the early part of the study period, the task group rec- ommends that Galileo-like missions explore the Saturn, Uranus, and Neptune systems. These spacecraft, with an extensive array of remote-sensing instruments, would carry out repeated flybys to characterize the major satellites. Because of scientific inter- est in its atmosphere, Titan would receive more intensive study, with radar investigations and a probe to measure the atmospheric composition. Our own Moon remains a body of the highest scientific im- portance. The task group recommends that the global survey by the Lunar Geoscience Orbiter be followed by deployment of rovers and a geophysical network, as well as resumption of sample return from selected locations. The task group also recommends a Mercury orbiter, with a landed transponder if possible, to complete the basic character- ization of the inner planets. Mercury's composition, mass, and magnetic field provide key tests to many theories on the evolution of the solar system. The mission will not only involve planetary science, but will include solar studies and some tests of relativity physics by careful tracking of the planet's motion. The discovery of new trajectories has made this mission possible with existing propulsion systems. The major mission recommended for the initial decade of the study period is a Mars rover and sample return program consist- ing of linked missions launched in 1996 to 1998. This is envisioned as a very comprehensive mission one with a capable rover to collect selected samples for return to Earth and carry out ex- tensive observations over the surface of the planet. The return of unsterilized martian materials could provide unique data on the absolute chronology of martian rock units, on detailed de- tection and characterization of contemporary or fossilized life, on surface-atmospheric interaction processes and rates, and on the

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93 composition and evolution of Mars' crust and mantle. A sample return should, if possible, have the capability to provide rationally chosen samples from a number of carefully selected areas in order to maximize the value of the samples. Study of returned martian samples on Earth provides an ex- cellent opportunity to look for any evidence of martian life, past or present. To protect the geochem~cal and biological integrity of the returned sample, sterilization by any method must be avoided. Because any martian organisms included with the returned sample might be killed by exposure to the high pressure, high water con- tent, and high oxygen content of the terrestrial atmosphere, the most promising life detection Experiments may be those based on chemistry and morphology, rather than on metabolism. Such ex- periments would include ~m~cropaleontology~ examinations, per- haps using stains that are reactive with carbon compounds. If any living systems should be detected in the returned sample- whether viable, dormant, recently dead, or fossilthe direction of our future exploration of Mars would be completely changed and there might be an early reexamination of manned missions to the planet. A rover is necessary as a mobile sampling device in order to ensure that a wide enough variety of samples is collected to meet the mission objectives. The rover should have those capabilities necessary for sample collection, examination, and characteriza- tion. In general, the rover capabilities needed are roughly those of a human geologist collecting samples in the field. Like a geol- ogist, the rover should obtain multispectral, stereoscopic images at a variety of scales and resolutions, process them, and interpret them by comparison to images derived from previous experience. It should lift samples to examine their details closely and to esti- mate their weight and density, thereby evaluating the amount of weathering. It should carry out simple chemical tests analogous to those made with a geologist's traditional Geiger counter and acid bottle. It should provide this information to Earth so that a decision can be made either to collect the sample or to discard it and move on to another. Although the rover's prime objective is to support sample collection, it is important to note that its capabilities, and the data it collects to characterize possible samples, would also be scientifically unportant during an extended traverse to analyze and characterize martian surface materials up to a significant

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94 distance from the landing site. After the rover has completed its sampling traverses (first in the vicinity of the landing site ant! then, if possible, at greater distances), it could carry out an important and exciting surface traverse of Mars, making the same observations over long distances. Such a post-sampling traverse would provide an important regional context for the sample suite and would also help understand better the complex processes that have taken place on the martian surface. Eventually, manned missions will offer the most complete and comprehensive execution of the intensive phase of planetary exploration. During the latter part of the study period the task group proposes to continue geoscience investigations with ongoing inves- tigation of Mars as well as with a number of missions that are now less well defined and of somewhat lower priority, or that will require new technological developments before they can be carried out. It Is important to deploy a network of stations on the surface of Mars to study its interior structure with seismic techniques and to understand the global meteorology. Sensor networks should eventually be emplaced on all the inner planets for continued seis- mic and other studies over a long period of time. Venus has the highest priority, but the high surface temperatures will make this mission very difficult. Detailed study of returned samples will continue to have high priority throughout the study period. These studies will include samples from new locations on Mars, including the polar regions, and on the Moon, including the far side. Eventually, samples should be returned from Mercury and Venus. A mission of special irrportance is the investigation of the resurfacer of Titan, but the nature of this surfacesolid or liquid- will not be known until radar investigations have been made from a spacecraft. A similarly important mission ~ a lander on To to investigate the composition of the materials emitted by volcanoes, both active and inactive. Programs for the Outer Solar System Types of Missions Addressing the goals set forth in Chapter 2 will demand new missions. Required missions to the outer solar system include long-lived orbiters, atmospheric probes, deep atmospheric probes,

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95 and a ring rendezvous spacecraft. These new missions will build on the first look provided by Voyager, which will have observed all the giant planets: Jupiter, Saturn, Uranus, and Neptune. Orbiters provide multiple looks for remote sensing of plane- tary surfaces and atmospheres, and for direct measurements of the magnetosphere at many locations. They also provide a long tune scale to study dynamic phenomena. New discoveries can be investigated in greater depth by changing the later stages of an orbiter mission to respond to new findings. Although remote sensing has important virtues of its own, some kinds of measurement can only be made from within an at- mosphere, and others can be made much more accurately there. Notable examples are the abundances of noble gases the principal clue to the possible presence and nature of a primary atmosphere and the abundance of nitrogen, which can be remotely measured only under favorable circumstances. In general, in situ abun- dance measurements can be much more accurate than those made remotely, and have given us most of our current isotopic data. Further, a descending probe can be tracked to give a vertical wind profile, whereas remote tracking of clouds gives a nearly global view, but only at one or a very few heights. Measurement of cloud properties and radiation balance are also best carried out from a descending probe. Special techniques will be required to study Saturn's rings di- rectly. A proposed ring rendezvous mission would use low-thrust propulsion to orbit Saturn in a plane just above the equatorial (ring) plane. By slowly decreasing the semimajor axis and alti- tude above the ring plane, a spacecraft conic} make multiple, low- relative-velocity encounters with ring particles. This would allow direct measurement of particle physical and chemical properties, and direct observations of inter-particle collisions. Planned Missions As described in the recent publication, A Strategy for Explo- ration of the Outer Planets: 1986-1996 (National Academy Press, 1986), the most immediate priority is intensive study of the Saturn system. To address the scientific questions concerning this system will require a long-lived orbiter and probe investigation of the at- mosphere of Titan, and perhaps Saturn as well. The orbiter would study the planetary atmosphere, the magnetosphere, the rings,

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96 the surfaces of the satellites, and the surface of Titan by radar. The probe would be similar to the Galileo probe, making in situ measurements and perhaps unages. Important scientific objectives of this mission are to determine the composition and structure of the atmospheres of Saturn and Titan; make detailed, long-term studies of Saturn's rings; investigate Saturn's small satellites and the magnetosphere; and measure the physical nature and state of Titan's surface. In addition to the Saturn system, the Uranus and Neptune systems will also become important subjects for study after 1995. Prior to 1995, the Voyager 2 spacecraft will have made a prelim- inary reconnaissance of these two systems. This will provide a basis for planning future intensive studies of these planets. The task group expects that many of the same questions about their atmospheres, magnetospheres, satellites, and rings may arise. This will lead to space missions similar to Galileo and the Saturn orbiter with Titan probe. An orbiter will provide exploration of the sys- tem, multiple looks at the planet and its satellites, and a long time base for study of meteorology, ring dynamics, and magnetospheric interactions. The probe will make direct measurements of the composition, cloudiness, and vertical structure of the planetary atmosphere (or perhaps the atmosphere of a satellite). Future Missions and Progeny for Primitive Bodies and the Origin of the Solar System We can expect that, by 1995, reconnaissance missions to comet nuclei and asteroids will have transformed our view of these bodies from unresolvable points of light into planetary bodies of distinct shape and surface morphology. It is hoped that a good start will have been made in understanding their chemical and mineralogical structure, and their relationship to the small fragments of these bodies sampled on Earth in the form of meteorites and interplane- tary dust. Making full use of the potential information obtainable from these bodies will require initiation of a new phase of detailed study. Multiple flyby and rendezvous missions to asteroids will be needed to address questions of the variety and spatial distribution of asteroidal bodies. In situ data must be related to earth-based spectrophotometric data, and to physical theories of asteroid col- lisional fragmentation and evolution into earth-crossing meteoritic

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97 fragments. Such mussions can also identify the range of mor- phological and structural characteristics of asteroids, and permit addressing the question of separating the record of early solar system history from subsequent collisional processing. With the background and understanding obtained from such studies, it will be possible to wisely select samples for return to laboratories on Earth for detailed chemical, mineralogical, and isotopic investiga- tions of asteroidal material. Measurements of this kind are not only essential to interpretation of early solar system events in a context of planetary geology, but also may provide the key for sim- ilar interpretation of the large quantity of data obtainable from meteorites. The study of comets is in a way easier than that of asteroids, because cometary activity releases large quantities of material from the nucleus into the atmosphere from which it can be sampled without actually landing and collecting samples from the surface. Valuable chemical and isotopic information can be obtained from a returned sample collected in this way, even if some mineralogical structure of the material is destroyed during the collection. A sample return mission of this kind could be of much importance in itself, as well as a valuable forerunner of collection and return to Earth of Pristine samples of a cometary nucleus. The earth-approach~ng Apollo and Amor objects are a special class of primitive bodies. Some of these are likely to be ~l-km- diameter asteroidal fragments derived from main-belt asteroids and transferred to near-Earth orbits by the same resonance mech- anisms responsible for the transfer of smaller meteorite-size frag- ments from the asteroid belt. In fact, objects of this kind are likely to be the more immediate parent bodies of many of the mete- orites in our collections. Understanding the physical structure of this fragmented material should be of much value in understand- ing the evolution of the steady-state collisional size hierarchy and the effects of this collisional history on the natural sampling of asteroidal material in the form of meteorites. Other Apollo-Amor objects exhibit orbits and physical char- acteristics suggesting they are likely to be the devolatilized residue of cometary cores. These can therefore provide an opportunity to sample cometary material that may not be readily available at the surface of an active comet. By displaying the end product

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98 of cometary evolution, these bodies can provide better under- standing of the active processes of comets, and thereby facilitate interpretation of remote observational data on comets. The task of developing the instrumentation and technical means of successfully effecting these future measurements and remote sample returns will be a challenge. The variety and num- ber of rendezvous and sample return opportunities to primitive bodies are now limited by existing ballistic propulsion systems. Full exploitation of the potential offered by such studies will re- quire development of low-thrust propulsion systems that permit selection of targets primarily because of their scientific interest rather than on the basis of their accessibility. As discussed in Chapter 3, the fundamental question of the origin Of the solar system will not be answered simply by future missions, but will require highly disciplined yet imaginative inte- gration of theory and observation. Adequate support of such inves- tigations, including availability of advanced computing systems, will be needed for this work to proceed apace. Investigations of this kind will depend heavily on the understanding obtained from studies of primitive bodies. However, in the long term, evidence for the existence and characteristics of other planetary systems should prove to be of comparable, or even greater importance to resolving this ancient and fundamental subject of human thought. It is essential that we conduct a comprehensive search for other planetary systems. A variety of observational techniques can and should be brought to bear on this problem. Before 1995, ground-based spec- troscopic and astrometric studies will provide a preliminary survey of some of the nearby stars that may harbor planetary systems we can detect. The Hubble Space Telescope can be expected to make important contributions in this area, as can the Space Infrared Telescope Facility. A comprehensive search for and study of other planetary systems will, however, require an astrometric telescope in earth orbit. The need to have a long-term (10 to 20 years) sys- tematic observational program indicates that the telescope should be on the Space Station. If this activity is initiated in 1995, at the beginning of the era under consideration, a full survey should be complete by 2015. Results from this type of survey, which needs to be conducted with a system capable of 5- to lO~arcsec accuracy in terms of its ability to determine the relative positions of stars, would be

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102 polar laminar terrain is a compelling reason to study Mars' climate change in detail. further knowledge would provide models for atmospheric evolution under a distinctly different situation from Earth. Accumulating geological and climatological evidence suggests that early Mars may have been suitable for the development of life during the first billion years of its history. Earth and Mars may have been quite similar during that period when algae flourished on Earth. Although the conditions on Mars currently are very difficult for the survival of life forms as we know them, and although the Viking missions found no evidence for life or organic material, the possibility of finding traces of past life provides a scientific objective for more detailed study. The Mars near-surface rock layers possibly could contain m~crofossils remaining from this early period, or records of chemical change on the earlier surfaces caused by early life. Sedimentary deposits that date back to the first billion years in Mars' history may be exposed in the walls of the great equatorial canyons. Analysm of the geologic history and detailed study of such deposits would have a significant impact on our understanding of the origin of life. Scientific Objectives for a Mare Focus The scientific utility of studying a second terrestrial planet in sufficient depth that the past and present processes can be identified and compared with similar ones on the Earth leads naturally to the recommendation of a major focus on intensive exploration of Mars. Current understanding of the origin, early history, and present state of Mars motivates a set of scientific goals whose accomplishment defines the scope of the recommended campaign to understand Mars: 1. Characterize the internal structure, dynamics, and bulk composition of the planet. 2. Characterize the chemical composition, structural features. and mineralogy of surface materials on a regional and global scale. 3. Determine the chemical composition, mineralogy, and aW solute ages of rocks and soil for the principal geologic provinces. 4. Characterize the processes that have produced the land- forms of the planet. 5. Determine the chemical and isotopic composition, distri- bution, and transport of volatile compounds that relate to the

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103 formation and chemical evolution of the atmosphere, and their incorporation in surface rocks and polar ice. 6. Characterize the planetary magnetic field and its interac- tion with the upper atmosphere, solar radiation, and the solar wind. 7. Determine the extent of organic chemical and possible biological evolution of Mars, and explain how the history of the planet constrains these evolutionary processes. Some of these objectives can be addressed by NASA's planned missions, including the Mars Observer and the Mars Aeronomy Orbiter. These will provide a global map of the distribution of elements and possibly minerals on its surface; determine its global properties including topography, gravity, and magnetic fields; and characterize the atmospheric and ionospheric structure and dy- namics. However, more sophisticated and complex missions are required to answer the higher order questions. The necessity for detailed study of the properties of surface materials, interaction of surface and atmosphere, and stratigraphic and depositional his- tory, and the need for addressing biological questions require in situ measurements by complex, automated laboratories or rovers, and networks of instruments. The measurement of heat flow is fundamental for understand- ing the interior characteristics of planets and satellites; yet obtain- ing valid data remains a major technological problem. All present means for emplacing instruments in the subsurface (e.g., penetra- tors, drilled holes) disturb the thermal regime for a period of time that exceeds the lifetime of most missions. These problems must be overcome, or alternative means found for measuring heat flow. The next major scientific objectives beyond those addressed by current missions could be met with a capable rover to col- lect selected samples for return to Earth and to carry out ex- tensive observations over the surface of the planet. The return and study of pristine martian materials could provide data on the absolute chronology of martian rock units, on detailed detection and characterization of possible contemporary or fossilized life, on surface-atmospheric interaction processes and rates, and on the composition and evolution of Mars' crust and mantle. A sample return mission should provide rationally chosen samples from a number of carefully selected areas in order to maximize the value of the samples.

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104 Detailed study of returned samples will continue to have high priority long after their initial return. These samples should be from many carefully chosen locations on Mars, including the polar regions. The collection of these samples involves increased com- plexity with far-traveling, highly capable rover laboratory/obser- vers and drill stations capable of drilling deep holes. Instruments to measure seismic waves, composition and physical properties of the surface, heat flow, and meteorological parameters should be emplaced in a network of at least 3 to 4 stations; 6 to 12 stations would be preferable. Potential landing sites can be selected from high-resolution Viking orbiter mosaics. Along with previous knowledge, this infor- mation allows the identification of locations where future surface exploration would provide substantial scientific return. Several interesting sites have been identified that contain a rich diversity of geologic ages and rock types. Voicanic deposits are exposed in the southeast portion of the scarp surrounding Olympus Mans. Evidence of tectonic activity is visible in the layered deposits in Candor Chasma and in the mesas at the bottom of the fault trough. Polar layered deposits, which may represent climatic change, are available near the north polar cap, which is, itself, of great interest. A wide variety of geologic features are available in the Mangala Valley, including stream deposits and very young lava flows. For each of these proposed sites, high-resolution geologic and topographic maps are available. Role of Humans in Intensive Mare Melioration The intensive exploration of Mars envisages} here will require substantial technical capabilities on the Mars surface. Long paths must be traversed and explored geologically in detail. The scien- tific goad require samples to be collected, along with preliminary in situ analysis. To meet the sampling requirements, holes must be bored and drill cores extracted. Depths of up to 2 km would be desirable near the poles. The network of seismic and meteoro- logical stations will require maintenance. Achieving a planet-wide understanding of the diverse provinces and processes will require extensive exploration over many parts of the surface. Although autonomous robot vehicles may perform many of the initial ex- ploratory activities, the later, more comprehensive exploration will require capabilities that now are possessed only by humans.

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105 The detailed exploration of Mars and its comparison to Earth, which is the objective of the proposed campaign to understand Mars, can be likened to the Apollo exploration of the Moon. The lunar astronauts successfully landed spacecraft in difficult terrain, selected a wide variety of samples, used simple tools to enhance sampling, and applied ingenuity and strength to overcome opera- tional problems. These technical achievements were accomplished in relatively short stays. The crews traveled up to 25 km at speeds up to 10 km per hour in rovers, and selected and documented a wide variety of samples. Further, the crews have contributed to post-m~ssion data analysis extending over the past 15 years. The ability to use simple took can probably be reproduced by autonomous vehicles. Time pressures would be considerably less for Mars surface exploration: robotic devices would have substantially more time to carry out these tasks. However, the intelligent and interactive selection of appropriate samples, and the concurrent and later provision of contextual information about them is well beyond the capability of current automated systems. Mars shows a much more complex variety of geologic processes than the Moon. Since the martian surface is also weathered, dis- cernible differences based on mineralogy, texture, and resistance to erosion may be important to interpretation. This greater diver- sity requires subtle judgments and interactions among observation, analysis, and interpretation. The total context of any sample will be very important in its scientific interpretation. Two conclusions follow from these considerations. First, ge- olog~sts, properly equipped, learning by observation, improvising as necessary, and advised from Earth, can be fast and effective at exploring the surface of Mars. Second, it is difficult for au- tonomous devices, even remotely linked to Earth by cameras, to achieve the scientific goals set forward above. Ultimately, to re- solve the important questions and to compare Mars in detail with the Earth will require exploration capabilities on the surface of Mars possessed now only by humans. A Phased Approach The task group recommends an intensive study of Mars to be implemented by phases that begin with currently envisioned missions, progress to newly developed robotic and propulsion sys- tems later, and culminate in manned missions to Mars. The Mars

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106 Observer and Mars Aeronomy Observer are missions central to the broad program of solar system exploration. They will provide the global overview from which to mount and target later efforts at the Mars surface. An intensive Mars campaign would begin with a suite of unmanned rn~ssions that would establish surface measure- ment networks and return geologic samples that had been selected and gathered by automated techniques. We know from earth field studies and from the Apollo experience that the scientific utility of these returned samples will be limited by lack of information about their context. The intensive study of Mars must ultimately be extended to incorporate certain capabilities that currently only humans can supply: human intelligence is needed for reasoned sample selection, to provide contextual documentation, and to am sist in the interpretation of sa~nple analyses. In the meantime we should develop robotics and artificial intelligence, but these devel- opments will not obviate the requirement for a human presence on Mars' surface to support the later phases of the Mars intensive study. RECOMMENDATIONS The goals of planetary exploration are achieved primarily through the analysis of data returned from spacecraft missions. Complementary remote observations are obtained using telescopes in the vicinity of Earth. Physical information on planetary ma- terials is acquired from terrestrial prototypes, laboratory investi- gations, and earth orbital observations. In addition, theoretical modeling furthers scientific understanding. A viable planetary program must contain all five elements spacecraft missions, tele- scope observations, field investigations, laboratories, and models- and the task group's recommendations speak to each. Melioration of the Solar System Figures 2.2, 2.3, and 2.4 in Chapter 2 show the status of plane- tary exploration expected by 1995, and the missions recommended for the period 1995 to 2015. The scientific objectives of these m~s- sions are outlined in Table 2.1. It must be emphasized that the near-term and the far-term planetary exploration projects are pro- posed in a logical order, following the sequence of reconnaissance,

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107 exploration, and intensive study. However, no priorities are im- plied by the ordering. Thus, if the status of planetary studies in 1995 is not as projected, then those studies that have not been completed should still have priority over the longer-term projects. The proposed missions follow the phased approach for solar system study. By 1995, the exploratory and reconnaissance phases should have been completed for the inner planets with the impor- tant exception of Mercury. This planet lies deep in the Sun's gravitational well, so that it is difficult for spacecraft to reach, particularly with approach velocities small enough to allow cap- ture into an orbit. Nevertheless, the unique properties of Mercury, especially the high mean density, which is probably indicative of strange internal chemistry and mineralogy, make its study of the greatest interest for comparative planetology. A Mercury orbiter mission wall be needed to complete the reconnaissance stage of that planet's study. The other proposed inner planet mission concepts involve the intensive study phase. For the Moon, the near-term geophysical studies from orbit wall undoubtedly raise questions requiring ad- ditional sample return for their resolution. Some of these samples will be needed from the far side of the Moon, and one contemplated project would select and return such samples. Complementary to the sample return will be global geophysical studies of the planet, for which the establishment of a network of stations will be re- quired. In due course we expect that there will be numerous practical reasons for establishing a permanent base of operations upon the Moon, and the course of lunar research outlined here will prepare the way for such a manned lunar base. For Mars, the stage of intensive study began with the Viking mission and will continue with the Phobos mission. The next ma- jor steps, which are of the greatest scientific importance, will be Mars rovers, the establishment of a global sensor network, and the selection and return of samples for analysis in terrestrial labora- tories. When the sphere of human habitation in space enlarges to encompass Mars, the establishment of a manned base there will become desirable, and the studies outlined here will be necessary precursors. In the case of Venus, a good map is partially in hand; com- pletion is expected with the planned radar mapper mission (Mag- elIan). Current lack of this map inhibits detailed projections for

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108 future mussions. An initial set of geochem~cal and mapping infor- mation has been obtained from Soviet investigations. The hostile environment of the planet requires much more technological devel- opment for future missions than is the case for the other terrestrial planets. Nevertheless, the kind of geophysical and geochemical in- formation desired from Venus is similar to that desired from the other terrestrial planets, and the means needed to acquire this will include probes, the establishment of a global network, and sample returns. Accomplishing these objectives will provide interesting technological challenges. Detailed planning for the intensive study of Mercury must await the unaging of the unseen hemisphere and the geophysical and geochemical mapping that will be done in an orbiter mission. With those data in hand it will be possible to plan the kind of surface investigations a lander will allow and to plan the return of samples for laboratory analysis. It is desirable that these follow-on missions should be done within the contemplated time period of this study. For the four giant planets of the outer solar system the recon- naissance stage of planetary study requires an orbiting spacecraft and an atmospheric entry probe. By the 1995 to 2015 period this should have been accomplished for Jupiter by the Galileo mission. Orbiter and probe missions for Saturn, Uranus, and Neptune are then of the highest priority for the outer solar system. In the case of Jupiter, a different kind of orbiter one in a polar orbit- is needed as a follow-on to Galileo in order to study the rich inner magnetosphere of the planet In detail. The satellites of the giant planets are of great interest, espe- cially the larger ones. The Galileo mission Is expected to complete the reconnaissance of the Galilean satellites of Jupiter. Titan, the large satellite of Saturn, has a substantial obscuring atmosphere and thus should have a dedicated orbiter-probe mission of its own. The Saturn orbiter, after delivery of its probe, may become ded- icated to studies of the rings, and thus would be unavailable for reconnaissance of the more distant regular satellites. In this case, a separate orbiter would be desirable to examine these satellites. The system of Pluto, consisting of an icy planet of low mass with a satellite of high relative mass, is of great intrinsic interest. By 1995 it will not have undergone exploration from space. It is therefore recommended that, owing to the very long flight time to the planet, the exploration and reconnaissance stages of study

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109 be combined, and performed by a dedicated orbiter. Extensive earth-based observations of the system, using the Space Telescope and other major instruments, will be needed to plan this mission. Once the outer planet missions outlined above have been car- ried out, the stage of intensive study can begin. For the gas giant planets deep probes will be needed! in order to study the atmo- sphere to greater depths and to refine the measurements made with the first generation of entry probes. Such deep probes are recommended for Jupiter, Saturn, and Uranus by 2015. Intensive study of the satellites of the giant planets will require surface investigations by means of landers, which should be able to emplace networks. It Is recommended that several selected satellites of the Jupiter and Saturn systems be investigated in this way by 2015. A lander of special design will be needed for Titan in view of its atmosphere; such a lander mission should be a rich source of information both about the atmosphere and whatever kind of surface exists, even if covered by liquid, but the design of such a lander must await results from the Titan orbiter-probe. The reconnaissance of comets should have commenced by 1995 with a comet rendezvous mission. The material constituting comets is likely to be the most primitive form of matter preserved from the environment of the early solar system that will ever be available to us. Thus sample return is vitally important for further studies of comets. It is recommended that a fragmented sample be returned by means of a fast comet flyby spacecraft that can easily return to the Earth. This should be followed by a more advanced mission involving a rendezvous in which special care will be taken to obtain and maintain several samples of the comet in their pristine form, and to continue to maintain them in that state during the return to the vicinity of the Earth. It is likely that a small number of asteroids will have been examined in a flyby mode by 1995. However, these bodies are highly diverse in their properties, and the early glimpses should be followed by a multiple rendezvous mission, in which a variety of surface analyses can be made, including some interactive analyses. These would pave the way for a later mission that would return samples from asteroids. The techniques required for the search for other planetary systems will continually improve. The task group recommends a dedicated telescope associated with the space station for this purpose. Even larger facilities, perhaps space-based and perhaps

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110 lunar-based, will be valuable for this purpose as well as for many other uses that require high-resolution imaging of the solar system and beyond. Earth-based studies are envisioned as a continuous program throughout the 1995 to 2015 period. Included are observations from Earth and near-Earth orbit of solar system objects ~d the search for other planetary systems, laboratory experiments related to planetary processes, and analyses of meteorites. Most of the missions shown in Figures 2.3 and 2.4 can be achieved with existing or near-term technology. The study of Uranus, Neptune, and many of the small bodies will require the d~ velopment of low-thrust propulsion. The intensive study of Venus cannot begin without extending the high-temperature survival of electronics; in addition, the return of Venus samples will require significant developments in propulsion. The task group recom- mends that development efforts in these areas of technology be initiated as soon as possible. Telescopes on Earth and in earth orbit complement space probes by providing observational data that, while usually of lower spatial resolution, can be synoptic in scope and quickly respon- sive to phenomena. The task group recommends support of a vital program of planetary astronomy and particularly encourages preparations to use new generations of astronomical telescopes for planetary observations. An integral part of any rn~ssion is adequate support for analy- sis and interpretation of the data returned from it. Studies in the field, in earth orbit, and in laboratories are required to provide cor- roborating aground truth, calibrations, and fundamental physi- cal constants. The task group recommends continual upgrading of laboratory instrumentation and of computing equipment used for data analysis and theoretical modeling. The missions and activities outlined here would address the objectives of solar system exploration outlined earlier. We have learned a great deal in the past 20 years, and the next 30 can be even more productive. We can also expect to support the exten- sion of the sphere of human habitation into space by improving our knowledge of planetary environments. The task group recom- mends that the program be implemented as vigorously as allowed by economic and national goals.

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111 Melioration of Mare The task group strongly recommends a continuing scientific exploration of the entire solar system, primarily using instruments on automated spacecraft. There Is much to be done to complete even a first-order look at our planetary neighborhood. That in- vestigation must continue. But a major Mars campaign could be carried out concurrently, in the same way that the first decade of planetary exploration was carried out by Mariner spacecraft in parallel with the Apollo Moon program. Mars is the most Earth-like of the other planets, displaying the bull range of terrestrial phenomena (except, possibly, life), although frequently in greatly modified form. Mars is an ideal lo- cation for the study of a long geological history parallel to that of the Earth, including both volcanic and tectonic activity; for exam- ining the chemical evolution of an atmosphere and its interaction with a surface, for investigating a complex meteorology includ- ing cyclic transport of volatiles between surface, atmosphere, and polar caps; and for exploring evidence of climatic cycles. The presence of channels of a variety of ages indicates episodic flow of large amounts of water. Mars may also have preserved evidence of prebiotic chemical evolution, or even possibly of the development and evolution of an indigenous biota. In the extensive exploration of Mars envisaged here, humans will play an essential role. Thousands of kilometers of the sur- face will need to be traversed and explored geologically in detail. Samples must be collected and given preliminary in situ analysis, holes bored and cores extracted, and automatic stations to mon- itor meteorological activity, se~smicity, and heat flow emplaced and maintained. In some areas, particularly on and near the polar caps, extensive drilling and perhaps excavation should be undertaken. In order to reduce costs and increase the efficiency of the operation, equipment for the manufacture of rocket fuel and of essential water and oxygen for personnel must be estate fished. It is difficult to imagine that such an extensive, detailed, planet-wide exploration program conic! be carried out effectively by autonomous robot vehicles alone. The direct application of human knowledge and ingenuity to the detailed exploration of a new world is likely to lead to the maximum return and the deepest level of understanding.

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