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Chapter 2 MAJOR RECOMMENDATIONS A NATIONAL PROGRAM The l965 study of the Space Science Board identified three goals for the nation's planetary program. The total plane- tary program should be designed to provide for progress in our understanding of: (l) the origin and evolution of the solar system, (2) the origin and evolution of life, and (3) the dynamic processes that shape man's terrestrial environ- ment. We believe that these goals remain valid with regard to the study of the outer solar system, and that all three should be recognized in the development of the program for the study of the outer solar system. The major emphasis of the program should be the study of the planets and their near en- vironments. However, spacecraft will spend many years in interplanetary space en route to the planets, and there will be the opportunity for the observation of particles and fields of the interplanetary medium during these times. We wish to emphasize that a study of the interplanetary medium contributes both to an understanding of the origin and evolution of the solar system and to the processes that shape the space environ- ment near the earth. Furthermore, flyby missions offer the possibility of out-of-the-ecliptic and even interstellar tra- jectories, and such flights will greatly enhance the under- standing of how our solar system interacts with the rest of the galaxy. The program for investigation of the outer parts of the solar system should not, in our view, be concentrated on a single goal or a single mission. Rather there should be an emphasis on those experiments and missions that contribute to the understanding of the solar system, the origin of life, and terrestrial processes. The rare opportunities for planetary voyages, the length of these voyages, their cost, and the long times required for preparing spacecraft and experiments all imply that planning for exploration of the outer solar system must take place years in advance of the actual missions. It is imperative that a decision be arrived at determining the character and scope of the program for the exploration of the solar system.
We recommend that NASA in its l97l congressional budgetary presentation bring to the Congress a long-term plan for the ex- ploration of the outer parts of the solar system. In addition, NASA should ask Congress for initial funding for a major Jupiter mission, to be discussed in greater detail below, with a target date of l974. LEVEL OF SUPPORT FOR PLANETARY EXPLORATION The Space Science Board l968 study repeated the recommendation of the l965 study that an increasing fraction of the space pro- grams be devoted to planetary exploration. It is our judgment that the current funding for planetary exploration is totally inadequate to take advantage of the opportunities available to us. Technology to place scientifically meaningful payloads near or on the planets is at hand, and the technology required for the long lifetimes and communication can be developed. The configuration of the planets in the l970's presents a unique opportunity for studying several planets on a single mission, thus substantially reducing the cost of exploration of this part of the solar system. We believe that we must take ad- vantage of this situation. Therefore, we fully endorse the statements with regard to funding of the previous studies and recommend that a substantially increased fraction of the total NASA budget be devoted to planetary exploration. SCIENTIFIC OBJECTIVES IN THE EXPLORATION OF THE OUTER SOLAR SYSTEM In this section we present our views on the prime scientific objectives in the exploration of the outer part of the solar system. These objectives should guide the choice of missions and experiments. In presenting the scientific objectives we recognize that missions to the outer planets provide major values other than scientific. The requirements of the missions on technology will undoubtedly stimulate advances with far- reaching consequences. We are not competent to discuss in detail either the technological consequences of voyages to the distant planets or the problems of national prestige but do note that it is important to take these considerations into account in any over-all decision. We recommend that the prime scientific objectives of the exploration of the outer solar system be:
l. to conduct exploratory investigations of the ap- pearance, size, mass, magnetic properties, and dynamics of each of the outer planets and their major satellites; 2. to determine the chemical and isotopic composi- tion of the atmospheres of the outer planets; 3. to determine whether biologically important organic substances exist in these atmospheres and to character- ize the lower atmospheric environments in terms of biologically significant parameters; 4. to describe the motions of the atmospheres of the major planets and to characterize their temperature-density- composition structure; 5. to make a detailed study for each of the outer planets of the external magnetic field and respective particle population, associated radio emissions, and magnetospheric particle-wave interactions; 6. to determine the mode of interaction of the solar wind with the outer planets including the interaction of the satellites with the planets' magnetospheres; 7. to investigate the properties of the solar wind and the interplanetary magnetic field at great distances from the sun at both low and high solar latitudes, and to search for the outer boundary of the solar-wind flow; 8. to attempt to obtain the composition, energy spectra, and fluxes of cosmic rays in interstellar space, free of the modulating effects of the solar wind. Chemical and Isotopic Composition A major problem facing all theories of the origin of the solar system is the determination of chemical composition of the material out of which the sun and the planets formed. Today the evidence is derived from spectroscopic observations of the composition of the sun, from observations of the rocks on the surface of the earth, and from the determination of the composition of meteorites. The compositions do not agree. Further, it is known that the inner planets have lost their lighter elements during their formation and subsequent evo- lution. Presently available evidence suggests that Jupiter and perhaps Saturn may have retained all elements in the same relative abundance and be much more like the primitive solar system in composition. It would appear that Uranus and Neptune differ since they are deficient in hydrogen with respect to the sun and may be deficient in helium as well. In order to identify the variations in chemical abundance, which must ob- viously be intimately related to the processes involved in
I0 the origin and evolution of the solar system, it is essential to have more quantitative data on the abundances of the ele- ments of those planets that dominate by mass the planetary system. It is important to emphasize that the ratio of hydrogen to helium in Jupiter and possibly Saturn has significance that goes beyond the problems associated with understanding the origin of the solar system. Rival theories for the origin of the universe have suggested that different amounts of helium will be produced. In the "big bang" model, hydrogen and helium are produced almost immediately as part of the initial expan- sion, whereas in other cosmologies, the helium is produced by later nuclear synthesis. There is some question about whether the amount of helium observed in the sun is accurate. The val- ues currently quoted have a large uncertainty but appear to be distinctly higher than the values obtained from the observa- tions of old stars, a result that would be unacceptable to the big bang cosmology. Thus the determination of the hydrogen and helium concentration within the atmosphere of Jupiter would be of great significance to cosmology. Abundance ratios of isotopes are of great value for an understanding of the problem of element formation, a key ques- tion in cosmology. The carbon-l2 to carbon-l3 ratio is one of great interest; but other isotopic ratios less subject to change by neutron capture would be even more revealing in view of the probability that solar surface electromagnetic effects may have modified materials exterior to the sun during the early stages of solar system development. In view of these considerations, we identify as a prime objective for missions to the outer planets the determination of the relative abundance and isotopic ratios of hydrogen, he- lium, carbon, and heavy elements up to mass 40 in the atmos- pheres of all the outer planets. Origin and Evolution of Life It is commonly assumed that conditions on the primitive earth were very different from those presently obtained. There is some disagreement over the details, but it is generally suppos- ed that during this early period the terrestrial atmosphere was highly reducing, consisting primarily of methane and ammonia. Chemical reactions in this atmosphere were stimulated by solar
ultraviolet radiation, electrical discharges, and local sources of planetary thermal energy. The initial result of these re- actions is assumed to be complex organic molecules such as amino acids that are necessary precursors to life itself. Under con- ditions that developed on earth, the progression of complexity continued to the level of formation of limited living organisms and ultimately the wide diffusion of life we now observe. It is already known that conditions in the atmosphere of Jupiter are very similar to this hypothetical model for the primitive earth. A logical first step in the investigation of the outer planets from the biological point of view would be further investigation of the atmospheres to determine how fav- orable conditions are for the abiogenic formation of organic compounds. Are there warm regions in the lower atmospheres? Are electrical discharges present? What solvents are available? What chemical reactions are occurring in the upper atmosphere? The next step in sophistication is a search for the com- plex organic substances themselves. It has already been sug- gested that some of the coloring matter observed in the Jovian atmosphere could be organic polymers dissolved in the cloud material. Laboratory experiments using mixtures of methane and ammonia, subjected to electrical discharges, have produced col- ored substances, thereby lending support to this interpretation. An unequivocal identification has not yet been achieved. To finally resolve these ambiguities it is essential to probe the atmosphere of these planets and make in situ measurements. A prime objective of outer planetary exploration is, therefore, the characterization of lower atmospheric environments in terms of biologically significant parameters and a search for and an identification of organic substances of importance to life. Atmospheric Circulation We would like to understand the terrestrial atmosphere much better than we do. The usual way of acquiring an understand- ing of a physical system is to do experiments on it. We can- not do large-scale experiment on the earth's atmosphere. If it were unique, we would be stuck. But fortunately, there are other planets around, in fact at least three others that are reasonably accessible, so we have at least four examples of atmospheres to work with. While we still cannot do exper- iments, we can do the next best thing, which is to observe several atmospheres of different scale, structure, and compo-
I2 sition, and thus acquire a deeper understanding of atmospheres in general, and ours in particular. Studies of motions in the atmosphere of Jupiter have re- cently begun to achieve a quantitative status and are leading to new ideas about the behavior of rapidly rotating atmospheres and the interaction of clouds and planetary motions. Recent developments in the study of the atmosphere of Venus have led to a new understanding of the circulation originally proposed many years ago by Hadley for the earth; current work on the diurnal circulations on Mars is leading to a fresh appreciation of diurnal effects in terrestrial boundary layers. We may anticipate benefits of a similar nature to meteorology and to other branches of atmospheric physics from the studies of the atmosphere of Jupiter and the other outer planets. An object- ive of the exploration program should be the study of the dy- namics of the atmospheres of the major planets. Magnetic Fields and the Radiation Belts of the Giant Planets The earth's magnetic field is due to motions in an electric- ally conducting core. The energy source driving the motions is unknown. They may be due to internal thermal sources or to the external torques due to the moon. The moon, Venus, and Mars do not possess a magnetic field of internal origin. It is not clear whether the absence of a magnetic field on Venus is due to the lack of rotation or lack of a satellite, while the absence of electrically conducting materials in the in- terior of the moon and Mars is generally thought to explain the lack of a magnetic field on these bodies. Jupiter has a very strong magnetic field as is evidenced by radio emissions from high-energy particles trapped by the field. The source of the field on Jupiter is not known; it could be in the deep interior or in an electrically conducting outer shell. Jupiter is the only planet, other than the earth, known to have belts of electrically charged particles temporarily trapped by the external magnetic field of the planet. Radia- tion belts were discovered by in situ observations with a Geiger-MUller tube flown on the first American satellite, Explorer I. Those of Jupiter were identified shortly there- after by the analysis of the nonthermal decimetric radio noise of that planet. The moon, Venus, and Mars do not have radia- tion belts. It is not known whether the other major planets have magnetic fields or trapped radiation since nonthermal
I3 radio emissions from these planets have not been observed. Detailed observation of the magnetic fields and radiation belts at the planets will provide information vital to the understanding of these planetary phenomena. Because of this, a prime objective of the exploration of the outer solar system should be a detailed study of the external magnetic field and charged-particle populations in the vicinity of the major planets. This information will be most valuable if concomi- tant observations of the nonthermal radio emissions origina- ting through cooperative phenomena are made in situ. Interaction of the Solar Wind with the Outer Planets and of Their Satellites with Their Magnetospheres Three examples of flow of magnetized solar plasma past a dense body in the solar system have been studied. In the case of the earth, the external magnetic field dominates the situation and the solar wind is held off at a great distance from the earth by its magnetic field. In the case of the moon, which has no intrinsic field, .no ionosphere, and a very low conductivity, the solar wind flows unimpeded into the surface leaving a nearly empty cavity behind the moon. In the case of Venus, which has little if any magnetic field but does have a highly conducting ionosphere, the magnetic field of the solar wind cannot quickly penetrate the ionosphere and the solar wind does not flow unimpeded into the atmosphere. The solar wind flows around Venus, and there is evidence for a wake on the downstream side of that planet. It is of great interest to learn whether the solar-wind flow past other planets and their satellites can be classified as one of those three types or whether there are new and sur- prising modes of interaction. Observations of the solar-wind interaction will not only lead to a better understanding of the earth's near-space environment, but they will also shed further light on the behavior of collisionless plasmas, a subject that lies at the heart of many problems in astro- physics. The Nature of the Solar Wind and Magnetic Fields at Great Distances The solar wind cannot continue to flow outward from the sun to indefinite distances. Somewhere it must merge into the inter- stellar gas and the galactic magnetic field. This transition
zone joining the solar system with the rest of the galaxy is of great interest. The zone should not be regular in its plasma properties and magnetic fields, and there may be substantial fluxes of energetic particles covering a great range of energies. In the direction parallel to the sun's pole of rotation (out of the ecliptic) the magnetic field may be more nearly radial, and perhaps interstellar particles of a variety of en- ergies more closely approach the sun than in the plane of the ecliptic. The interaction of the solar wind and the sun's magnetic field with the interstellar particles and fields along this direction will be of great interest. The study of these interactions can lead to major advances in the understanding of plasma physics. Further advances in the understanding of plasma physics can be expected from clarification of the role played by large- scale coherence in natural radio emissions from the planets. Galactic and Solar Cosmic Rays Knowledge of the composition and energy spectrum of galactic cosmic rays is of considerable importance in the understanding of a wide range of cosmological phenomena, including the origin of the elements. The understanding of galactic cosmic rays will be very much enhanced by observation in a location un- affected by the solar magnetic fields and free of energetic solar particles that are so abundant at l AU in the plane of the ecliptic. These kinds of observations are needed if we are to untangle the effects of local magnetic fields on the properties of the cosmic rays. MISSIONS FOR THE EXPLORATION OF THE OUTER SOLAR SYSTEM Pioneer F/G scheduled to fly in l972 and l973 will provide the first information on conditions existing in the interplanetary media out to the distance of Jupiter. We recommend that space- craft of this class and capability be maintained and utilized as appropriate for further missions. Further exploration of the outer solar system will require larger scientific payloads and much more sophisticated instru- mentation. We therefore recommend that a new spacecraft of flexible capability and gross weight of ~ l500 Ib be developed
I5 and used as the principal working vehicle of the outer solar system program for the next decade. We recommend the following missions in order of scientific significance: l. Jupiter deep entry probe and flyby. The target date for this mission would be l974 or l975. Purpose of the mission would be to introduce into the atmosphere of Jupiter a probe capable of sounding to an atmospheric pressure of l0 to l00 bars. After depositing the probe, the remaining spacecraft would be swung by Jupiter's gravitational field out of the ecliptic on such a path that it would pass over a high-latitude region of the sun and have a perihelion of ~ 2.5 astronomical units. The entry probe would carry a mass spectrometer to sample the composition of the atmosphere as well as instruments to determine temperature, density, pressure, and physical and chemical properties of the clouds. The instrumentation remain- ing on the spacecraft would be designed to measure fields and particles over a wide energy range and to detect decametric radio emissions. 2. Jupiter orbiter mission would be designed to place a spacecraft in an orbit having a high inclination (greater than 60°), a perigee of 3 Jupiter radii, and an apogee of about l00 Jupiter radii. The spacecraft would carry instruments to meas- ure the particles, fields, and radio emissions in the near- Jupiter environment and to observe, both visually and in the infrared, motions in the atmosphere of Jupiter. Detailed track- ing of the orbit would provide information regarding the high harmonics in Jupiter's gravitational field. The existence of harmonics other than even zonal harmonics would establish ei- ther the presence of rigid solid material or of vigorous dynam- ic processes capable of maintaining large-scale density inhom- ogeneities. We further recommend that this mission carry a deep-entry probe, similar to that used in mission l, if the capability is available. 3. Earth-Jupiter-Saturn-Pluto missions are proposed to take advantage of the opportunity to visit three of the outer planets with a launch scheduled in l977. Particles and fields in interplanetary space, and in the near vicinity of the plan- ets, would be examined. Thermal and visual imaging equipment would be used to study the surface characteristics of the planets and the nature of Saturn's rings. If possible, a radio occultation beacon would be dropped off at Saturn. In this mission, as in other multiplanetary missions, full use
I6 should be made of the radio occultation to determine the char- acteristics of the upper atmosphere of the planets. The feasibility of small unshielded entry probes should be considered for upper-atmosphere research on all multiple- planet missions, provided they do not endanger the primary mission objective. These missions provide an early opportunity for rapid escape from the solar system and for a variety of important particles and fields observations at the outer fringes of the solar system and beyond. 4. Earth-Jupiter-Uranus-Neptune missions. Two launches are proposed in l979 to take advantage of this multiplanet opportunity. The instrumentation carried aboard the space- craft would be similar to that for the Earth-Jupiter-Saturn- Pluto missions. An investigation of Neptune with an unshielded probe is particularly important on this mission. 5. Earth-Jupiter-Uranus mission scheduled for the early l980?s would be designed to deposit an entry probe at Uranus. In selecting missions we have been guided by the fact that in almost every observable respect the giant planets appear to fall into two fundamentally different classes which we may refer to as Jovian (Jupiter and Saturn) and Uranian (Uranus and Nep- tune) . A knowledge of the composition and structure of both classes is necessary as a preliminary to the formulation of a satisfactory theory of the origin of the solar system. Missions Requiring Further Study The smaller bodies of the outer solar system may contain in- formation vital to the understanding of the history and origin of the solar system. In particular, detailed in situ observa- tions of the asteroids may be valuable in tying together the vast array of information that has been secured by observations on meteorites. A soft landing on an asteroid would tafee the form of a rather simple docking. We do not believe that this kind of mission has been sufficiently studied so that a defin- ite recommendation for it can be made at this time. Similarly, the analysis of cometary material may be extremely valuable in that comets may indeed contain elements not associated with the solar system. The determination of the chemical composition of the comet could be of great cosmological importance. How- ever, we do not believe that there has been sufficient analysis of a cometary mission, in particular with regard to the kind of payloads that would be required to perform a satisfactory
I7 chemical analysis of the varying materials making up a comet. We therefore recommend a detailed analysis of a mission designed to analyze chemically an asteroid or asteroids and a mission designed to determine the physical and chemical prop- erties of a comet. MAJOR ENGINEERING DEVELOPMENTS As we have noted earlier, we believe that the propulsion avail- able in the Titan IIID-Centaur combination is sufficient for the preliminary exploration of the outer planets. However, there are several major engineering developments which are .essential for the effective exploration of the outer parts of the solar system. Entry Probe The determination of the composition of the atmospheres of Jupiter and Uranus, so valuable to the understanding of the solar system, will require entry probes capable of descending to depths equivalent to pressures of l0 to l00 atm. It will require great engineering ingenuity to design and build a lightweight probe capable of withstanding the extraordinary chemical conditions of the atmosphere, the mechanical stresses imposed by the high pressures, and at the same time able to carry out a variety of sophisticated scientific experiments and communicate the results to the mother spacecraft. We recommend an immediate start on detailed design studies of possible probes for the atmospheres of the major planets. Hybrid Spacecraft There has been considerable discussion regarding the relative merits of spacecraft which are spinning or are three-axis sta- bilized. It is recognized that there are advantages to either system both scientifically and technically. In order to con- duct studies of energetic particles, plasmas, magnetic fields, and radio emissions in the interplanetary medium, planetary magnetosphere, and radiation belts, it is essential that the detector systems scan directionally. This is mandatory in order to determine uniquely the anisotropies and pitch-angle distributions of the particle and plasma fluxes. A spinning spacecraft offers distinct advantages over an attitude-stabil-
I8 ized one for both magnetic-field and radio-physics experiments. The intrinsic rotation of the spacecraft permits the accurate determination of two of the three components of the local mag- netic field somewhat independent of the presence of the contam- inating spacecraft magnetic field. Those radio-physics experi- ments utilizing directionally sensitive antenna systems can properly conduct their studies only by scanning directionally past sources (or targets close by) and thus determining the spatial properties of these objects. On the other hand, it is clear that for definitive measurements of the planetary atmos- pheres a stabilized platform is desirable and such a platform may facilitate communication with the earth. The spacecraft would include both a spinning portion suit- able for those experiments requiring such motion and a portion which could be despun and accurately pointed, perhaps only dur- ing planetary encounter. We recommend that NASA develop a hybrid spinning spacecraft, a portion of which could be despun; the total spacecraft would act as an optimum laboratory bench for the broad classes of experiments contemplated in the explor- ation of the outer solar system. Telemetry Data Coverage An important requirement of the scientific studies to be con- ducted during interplanetary cruise is effectively continuous data coverage. During the cruise mode, a relatively modest bit rate of 50 to 200 bits per second is sufficient. Continu- ous coverage can be achieved either by 24-hour ground-antenna coverage scheduled each day or by use of an on-board data storage system to provide for coverage of those time gaps during which ground antenna facilities are not available. The continuous coverage is required because significant transient events such as shock waves and discontinuities in the solar wind occur irregularly and cannot be anticipated. In addition, any space-time correlation between several space probes will require such continuous coverage because of the large time offsets for widely separated probes. During planetary encounter, the data requirements will be quite distinct with the instruments collecting data at a high rate. It is therefore desirable to design an over-all data system that will provide for continuous coverage during the cruise mode and a higher rate of data storage and subsequent transmission during planetary encounter. We recommend that
I9 NASA undertake design studies leading to a data system that can handle the varying requirements for both cruise mode and encoun- ter operation. Magnetically Clean Spacecraft The very weak magnetic field expected beyond 3 astronomical units may well prove to be among the most difficult of the pa- rameters to determine with sufficient accuracy. In order to permit such measurements to be made on satellites and space probes, special nonmagnetic fabrication procedures and long sensor booms are required. The presently developed methods, applicable to satellites at distances less than 3 astronomical units may not be appropriate at larger distances. Indeed, the passage of a spacecraft through the expected very strong field of Jupiter may lead to the magnetization of the spacecraft which will contribute a large uncertainty to the data obtained sub- sequent to the encounter. We recommend that studies be carried out to design a spacecraft that will be magnetically clean even after passing through a strong planetary magnetic field. Design of Instruments for the Encounter at Jupiter The environment near Jupiter is very likely to be hostile to sensitive sensors. The high flux of energetic particles, the strong magnetic field, and the possibility of tenuous dust clouds make it essential that the spacecraft be designed to survive in this environment. This is particularly important for the Jupiter orbiter. The value of this mission depends on long-term survival of the spacecraft. We recommend that spe- cial attention be devoted to the design of an over-all system that can survive the Jovian environment. INSTRUMENTATION The instrumentation for determination of the particles and fields in interplanetary space and near the major planets is well in hand except for the over-all design considerations noted above. On the other hand, the study of the deep atmos- pheres of the large planets will require very substantial in- strument development programs. Mass Spectrometer The only reliable way to measure the abundance of the chemical
20 compounds in the atmosphere is by mass spectrometers with suit- ably chosen sensitivity and sampling inlets; therefore, a mass spectrometer is to be carried aboard the deep-entry probe. The high pressures and possible corrosive chemical atmosphere pre- sent unusual design problems. We recommend that NASA initiate design studies for suitable mass spectrometers. Visual Imaging The study of cloud motions which in turn will lead to critical information regarding the dynamics of the atmospheres of the major planets requires visual imaging over a broad range of horizontal scales. The visual imaging should begin at a dis- tance at which the camera resolution is comparable with that achievable from earth and continue to a similar distance after closest approach. In addition, the imaging system should be adjustable to provide high resolution on selected features. Resolution of as little as a few kilometers at closest approach is needed for observations that will provide information on aerosol distribution and cloud stratification. The images should be acquired in stereo pairs in order to determine the relative heights and motions of clouds. We recommend the in- itiation of design studies of such imaging systems. Thermal Imaging In nonhomogeneous, convective atmospheres, upward mass motions transport heat from hotter to cooler regions. At a sufficient- ly high level, this heat is radiated to the outside, and it can be measured remotely by an infrared sensor. If the re- ceiver is sufficiently sensitive, the surface of the planet can be scanned to provide a two-dimensional representation of the temperature prevailing at a depth in the atmosphere de- termined by its transmission in the wavelength range admitted to the sensor. The temperature field can then be correlated with the visual field to provide further information regarding motions within the atmosphere. We recommend design studies of thermal imaging systems covering the infrared wavelengths likely to be observed in the atmospheres of the major planets. RADIO LINKS, BISTATIC RADAR, AND GROUND-BASED RADAR Radio links between deep space probes and the earth have po- tential for a wide array of scientific experiments. These
2I include possibilities based on measurements on direct links, and over paths that include intermediate reflections from a planet or satellite (bistatic radar). With improved and new ground- based radars, it should be possible to obtain echoes from the Galilean satellites of Jupiter (and possibly from Jupiter, Saturn, Titan, and the rings of Saturn) for experiments that are independent of any spacecraft. In various sections of this report, the scientific possibilities are outlined in terms of studies of: planetary and satellite atmospheres; solar, inter- planetary, and planetary plasmas and magnetic fields; planetary radii, masses, and detailed gravity fields; spin rates, radii, and masses of the larger satellites and Pluto; surfaces and topography of the satellites and Pluto; deep atmospheres and possible surfaces of the Jovian planets; satellite and planet- ary orbits; ring thickness and sizes, reflectivity, velocity distribution, and number density of the particles in the rings of Saturn; the mass of the asteroid belt and possibly the mass, rotations, and reflectivity of a few large asteroids; and sev- eral fundamental relativistic effects. Dramatic increases in knowledge of our planetary system have come in recent years from radar and radio-propagation ex- periments. For ground-based radar, further advances and in- creases in range to Jupiter and Saturn depend on the development of improved and new facilities. Past radio-link and bistatic radar experiments have largely been conducted using the radio system provided for communications and tracking. The experi- ments were in effect afterthoughts, and while important results were obtained, very great improvements are possible for the future. A unified view is needed of the potentialities of the var- ious radio and radar experiments, and the interaction of these experiments with communications and tracking facilities, both on the spacecraft and on the earth. Radio links can be improv- ed for communications in a way that could also greatly increase scientific capabilities. The data needed to improve tracking accuracy for mission-support purposes are the same data that would provide scientific information on space plasmas. The wave polarization generally used for communications makes cer- tain experiments impossible, although the communications could be done just as well with a polarization that would provide scientific information. The large spacecraft antenna needed for communications could be used to provide maximum sensitiv- ity for scientific experiments. The transmitting portion of a very powerful ground-based radar system would provide maxi-
22 mum capability for bistatic radar and radio-link experiments which could make measurements to the limits of the solar system, and even beyond. In order to maximize the scientific return from radio-link and radar studies of the planets: l. We strongly support the l968 recommendation number 2 of the NAS-NRC Space Science Board Panel on Planetary Astronomy,* leading to the upgrading of existing radar/radio astronomy fa- cilities and the construction of a major new facility with capa- bility for planetary radar astronomy. 2. We recommend that spacecraft radio systems be designed to accomplish both mission-support (communications and tracking) and maximum scientific purposes. To this end, we recommend that for the missions to the outer planets, a Radio Science and Cel- estial Mechanics Scientific Team be selected at an early enough date to be able to affect the design of the spacecraft radio systems and the operational capabilities of the ground-based terminals of the Deep Space Net. 3. We recommend that principal investigators and instru- ments for radio-link and bistatic radar experiments, which may share the use of spacecraft antennas or radio system signals, be chosen sufficiently early that these principal investigators can participate with the radio team in helping to determine spacecraft and ground-based radio-system capabilities. GROUND-BASED OBSERVATIONS As has been emphasized in earlier reports, the optimum utili- zation of probes to the planets will require many supporting observations from ground-based, balloon-borne, aircraft, and earth-orbital observatories. The observations from the near- earth environment of the major planets pose special problems, some of which are noted below. Southern Hemisphere Telescope During the next four decades Uranus and Neptune will be at southern declinations, and during the l980's Saturn will also be in the southern sky. We therefore recommend the construction *Planetary Astronomy: An Appraisal of Ground-Based Opportun- ities, NAS Publ. l688, Nat. Acad. Sci., Washington, D. C. (l968), p. 7l.
23 of a fully instrumented high-optical-quality telescope of the l00-in. class in the southern hemisphere for planetary observa- tions. This telescope together with the three large optical telescopes that NASA has already developed for planetary studies will provide continuing coverage of the major planets during the in situ exploration. Fourier Spectrometers The study of the atmospheres of the planets will require a determination of the line profile of the compounds existing in the atmosphere. Information gained from such studies will be of great aid in correlating and enhancing the data secured by a spacecraft. We recommend that NASA support further devel- opment of Fourier spectrometers and the construction of a large light-gathering aperture in the l5-m class for very-high-reso- lution spectroscopy of the planets. Laboratory Programs A great deal of new laboratory data will be required in order to analyze high-resolution infrared spectra of the outer plan- ets. This is particularly true of the near infrared combina- tion bands of CH/ and NH« for which line strengths, pressure- broadening coefficients, and J identifications are (with very few exceptions) unavailable. We therefore recommend that NASA support a comprehensive laboratory program to measure the prop- erties of the bands of CH^, NHo, HU,, and other relevant molecules and a complementary theoretical program to calculate the struc- ture of these bands. Our understanding of the interiors of the major planets is limited by a lack of knowledge of the equations of state of matter at high pressures and the various transport coefficients. Present-day technology of high pressures has progressed suffi.- ciently to promise important results. We, therefore, recommend that NASA support theoretical and experimental studies of the equations of state of hydrogen and of the hydrogen-helium system. Use of a Large Radio Antenna Array Apparently the only ground-based possibility for directly study- ing the deep atmosphere of the major planets is by means of radio and radar observations at wavelengths between l0 and l00 cm. A large antenna array having a resolution of a few seconds
24 of arc could observe structural detail in the Jovian atmos- pheres at pressures greater than l atm. Of particular interest would be structures which could be correlated with visible sur- face features or with magnetic field structure as measured by the flyby spacecraft or orbiters. Although designed primarily for galactic and extragalactic observations, several large antenna arrays now being planned will be capable of making val- uable planetary measurements. We recommend that the designs of large radio antenna arrays include provisions for real- time pencil-beam observations of the planets and that NASA support planetary programs that make use of such arrays. Earth-Orbiting Observations Ultraviolet spectroscopy from earth-orbiting telescopes can be expected to yield much valuable information on the atmos- pheres of major planets. The University of Wisconsin OAO-A2 experiment has already made a substantial number of planetary observations, and we encourage them to obtain more. The poten- tial for doing planetary astronomy with OAO's and other earth- orbital telescopes now being planned is great. We urge that the design of these telescopes, which are primarily for stellar and galactic astronomy, be sufficiently flexible to facilitate planetary observations. We recommend the funding of planetary programs that will make use of the capabilities for high-reso- lution imagery and ultraviolet spectroscopy of the planets from earth-orbiting telescopes. ADVANCED PROPULSION As we have noted, chemical propulsion would appear adequate for the study of the outer reaches of the solar system during those times when Jupiter can provide a gravitational assist to spacecraft traveling beyond that planet. During the l980's the configuration of the planets will not be so advantageous. During the l980's it may be necessary to use low-thrust en- gines continuously in order to decrease trip times. We rec- ommend that NASA proceed with the development of advanced methods of propulsion useful for exploration of the solar system.
