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80 Chapter 7 PLUTO, SATELLITES, ASTEROIDS, AND COMETS The small objects in the region of the outer planets have an interest that is by no means proportional to their size. Some of these objects -- the asteroids, for example, and especially the comets -- may represent extrasolar system material. In addition, some of the outer satellites of the major planets may be captured bodies from either within or without the solar sys- tem. It has even been suggested that Pluto, an outer planet in its own right, although we consider it in this chapter, may not be an original member of the solar system. An additional jus- tification for studying the small celestial objects lies in the possibility that they may represent early undifferentiated so- lar system material, unaffected by the temperature and pressure regimes of large planets and even perhaps some of the light nuclei interactions at the more central locations of a primor- dial solar nebula. SATELLITES OF PLANETS Presently computed mean densities for satellites of the outer planets are almost meaningless, because their diameters are so difficult to measure from the earth and their densities are proportional to the cubes of their diameters, giving a large factor of inaccuracy. The first basic parameter deducible from an imaging experiment is the diameter, and knowledge of the diameter could settle whether some or all of the satellites are essentially composed of rock or of ice. Even without being directed toward a specific satellite flyby, an imaging system directed toward exploring a major planet, and of appropriate resolution and format for that planet, would, for instance, if directed toward Jupiter, automatically come close enough to a satellite to resolve basic geologic structures such as impact craters, possibly volcanic structures, mountain chains (which would be interesting indeed if found -- unless estimates of satellite size and mass are grossly in error), and large basins. None of the things we might expect would predict the huge am- plitude of the observed rotational light curve of lapetus. The
8I determination of synchronous rotation, or spin-orbit coupling of more complicated form, by radar and imagery offers an intriguing problem in celestial dynamics. The question of the possession of atmospheres by the larger satellites is cosmogonically important. On the basis of present knowledge, the satellites form a one-to-one corres- pondence between possession and predicted retentivity, with the exception of Triton, which has no detected atmosphere and yet has a higher retentivity than Mars. Titan, the only sat- ellite known to have an atmosphere, could not have formed it at a significantly higher temperature than it now possesses. The basic question can be answered by radio occultation and perhaps by imagery combined with polarimetry. The fact that lo modulates the decameter-wavelength radio emission from Jupiter is now well established. In fact, lower- amplitude periodic terms in the radio data now lead us to suspect that a mission to this vicinity might succeed in dis- covering additional smaller Jovian satellites. These effects are radical and far-reaching compared to the effect that the moon has on the solar wind. lo's electromagnetic structure needs investigation, particularly the properties of electric conductivity and magnetic permeability. In situ measurements of these are very tempting. In close-up imagery of lo, one might find unique features not existing in other satellites. Observations of the mode in which decametric modulation is created would enable the inference of these properties. Probably the most significant contribution to satellite research in the near future will come when ground-based and spacecraft radar capabilities are advanced to the point where surface reflectivities, roughness, and dielectric constant are measurable for the larger satellites of Jupiter and Saturn. Data from ground-based and spacecraft radar can also greatly contribute to our knowledge of satellite rotation rates and axis orientations, their sizes, densities, and orbits, and the gravity fields of Jupiter and Saturn. Photometric imagery also provides the phase functions of the satellites at angles unachievable from earth. This, together with the gross distribution of brightness over the image, reveals much about the basic nature of the soil struc- ture. The soil of the moon, which possesses a very low heat conductivity, exhibits strong backscattering of visual light and has an unusual photometric function. Ganymede cools during
82 an eclipse at a high rate -- suggesting lunar-like soil. Is this consistent with an icy composition? Evidence of an atmos- phere might appear in such imagery -- especially if time- varying features are found. One important need of space exper- imentation in this area is a technique for in situ chemical analysis of rock material. Saturn's Rings The rings of Saturn are a unique feature of the solar system. Among small bodies, the particulate matter of the rings of Saturn are of as great interest as is their aggregate structure. The rings have unique physical parameters since they are strong backscatterers like the moon only vastly more efficient; i.e., the lunar surface scatters primarily backward but reflects less than l0 percent of the light incident upon it. Saturn's rings scatter backwards with something approaching l00 percent albedo. The geometrical parameters of the rings need determination because the most recent and presumably most accurate values imply that theCassini division is not in resonance with Mimas perturbations. This is not so surprising, perhaps, when one realizes that the rings show no evidence of a gap at the cor- responding Janus resonance which one might expect to be even more potent as a gap ejector on the venerable simple picture hitherto invoked for the Cassini division. It is not impossible that the rings are, astronomically speaking, quite temporary. For example, the Crepe Rings were not discovered by Herschel in spite of his thoroughness as an observer and meticulous attention to detail, and then were discovered virtually simul- taneously by three independent investigators. Struve con- cluded on the basis of previous work that the inner edge of ring B was moving inward. Another suggestion of the transient nature of the rings is the numerous observations of wispy matter outside A, which the original observer has later been unable to confirm. Maxwell showed that simultaneous inward and outward expansion is what one might expect for dying rings. Spacecraft imagery during a flyby might possibly be expected to resolve the largest of the ring particles. At least a new upper bound to size could be made. A first oppor- tunity to measure photometric characteristics at phase angles greater than l3° would be afforded. Such data severely con- strain acceptability of theories of radiative transfer in the rings. Finally, the possibility exists of measuring the
83 absorption features of ices, such as water, in sunlight trans- mitted through the rings. It may be that a very substantial contribution to understanding Saturn's rings will come from ground-based radar. Bistatic radar (using ground-based trans- mitters and receivers in spacecraft near Saturn) or monostatic radar from spacecraft could provide very high sensitivity for direct measurements of the ring thickness, size and velocity distributions, and particle number density. Asteroids The asteroid belt forms a potential hazard to spacecraft passage which requires further investigation. The size dis- tribution of asteroidal particles is unknown. Depending on one's point of interest in this interval, all the discussion above concerning satellites and Saturn's rings applies, except, perhaps, atmospheric considerations. Since the gravitational attraction of these bodies is such that a soft landing is little more than a docking maneuver, the feasibility of surface probe missions and sample return should be given further study. The average mass density of the belt might be measurable from accu- rate spacecraft tracking, and the mass shape, and reflectivity of individual large asteroids could be determined from tracking and bistatic radar. Comets The high velocity necessary for a comet intercept can be acquired by using the gravitational field of Jupiter to accel- erate the spacecraft. In a sense, comets may offer a mission potential far more profound than even their mysterious appear- ance might indicate. Speeding in from the depths of space well beyond Pluto, they may represent matter of more generally galactic origin. Here, perhaps, is the opportunity for the first interstellar space mission, in essence. It would mean a great deal if comets, for instance, were the only celestial bodies which exhibited significantly nonterrestrial isotopic ratios. The masses of comets are such that while considerable maneuvering energy might be involved in equalizing velocities, no significant gravitational potential must be overcome. The deployment of a mass spectrometer would be of paramount value. On a more limited mission, merely the photography of the nucleus, giving especially its diamter would be of value, as we now have no idea of the size of comet nuclei. Ultraviolet and visual spectroscopy near the nucleus would be of value for
84 examining chemical species as they evolve with distance and time into the solar-wind environment. Measuring the absorption spectrum of transmitted sunlight is of benefit. Pluto Atmospheric considerations relating to Pluto are very important. For the last 200 years, Pluto has been moving closer to the sun and is now nearly as close as is Neptune. Pluto's spectral energy distribution shows no ultraviolet brightening character- istic of a Rayleigh scattering atmosphere; however, its rota- tional light curve suggests brightening at the morning termi- nator as though an atmospheric component had formed frost during the night. The possibility of observing He, A, and Ne cannot now be ruled out on theoretical grounds. In addition, the very basic result of obtaining the diameter from radio occultation or flyby imagery and the mass from spacecraft tracking during encounter gives the only clue presently pos- sible for its mean density -- with its attendant cosmogonical significance. If Pluto were to appear rock-like, the nature of experi- ments one would consider would be that of those previously recommended for Martian exploration, exclusive of exobiology. A most interesting result would follow directly from the determination of the mass. Presently, a free parameter deter- mined in the mutual perturbation equations of Uranus, Neptune, and Pluto -- which results in a very inaccurate mass determi- nation (± 200 percent) and is, as a process, an imperfect theory unless the eighteenth-century observations are ignored -- is that the specification of the mass might result in the pre- diction of the existence of a trans-Plutonian planet on the basis of present observations. Even at the distance of Pluto, bistatic radar could pro- vide information on reflectivity, roughness, and topography, as well as measuring the atmosphere by occultation. RECOMMENDATIONS We recommend, on the basis of the scientific importance of small bodies in the region of the outer planets:
85 l. Flyby missions should be undertaken to all the outer planets as a balanced beginning -- none should be ignored at the expense of optimizing the missions to others. Opportuni- ties to view satellites at great increases in resolution should not be sacrificed on the grounds that more images of the primary are of greater value. 2. The flyby missions should carry high-resolution imaging systems of high photometric and photogrammetric integrity. 3. A Jupiter/comet mission should be undertaken in which the experiment design priorities are weighted in favor of the comet, and an imaging system should be involved together with a spectroscopic experiment. In addition, the feasibility of a sufficiently precise rendezvous maneuver that the spacecraft would slowly fall into the comet nucleus after burn-out should be considered, in which case more sophisticated probe-type instruments for chemical analysis and isotopic ratio determi- nation would be strongly urged. 4. We strongly recommend the encouragement of the radar developments now envisioned which will extend to Jupiter, the Galilean satellites, and Saturn and perhaps its rings the capabilities now being exercised on Mars and Venus. 5. We recommend the development of bistatic radar tech- niques for study of the small solid bodies in the outer reaches of the solar system and the improvement of radio tracking accuracy for determining their masses and orbits.
