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Chapter 6 PARTICLES, FIELDS, AND PLANETARY INTERACTIONS WITH THE SOLAR WIND Introduction A study of the near-space environment of the planets requires investigations in situ of planetary magnetic fields, ionospheres, exospheres, radiation belts, and interactions with the interplanetary medium. Such studies contribute both directly and indirectly to satisfying the three main objectives for the exploration of the solar system identified by the 1965 Woods Hole Study. Most of the volume of space and a substantial fraction of the mass of the uni- verse is composed of a magnetized, collisionless plasma containing electrons, pro- tons, and ionized atoms of heavier elements. A dramatic example of this plasma is given by the Earth's radiation belts. The state of plasma, the nature of shock waves and of the instabilities occurring within it, and the large-scale processes that accelerate and produce energetic charged particles can very rarely be studied in the laboratory. The space program has presented scientists with unique opportunities for performing in situ measurements of plasma within the solar system; our increas- ing knowledge of basic plasma and high energy particle physics is useful in both astrophysics and in laboratory plasma studies such as controlled fusion. It also can directly answer questions fundamental to an understanding of the origin and evolution of our solar system. The origin of the solar system by condensation from a solar nebula requires understanding the basic magnetohydromagnetic processes of the interaction of ionized gases and magnetic fields. Studies of the near-space environment of the planets provide us with an understanding and knowledge of these basic processes. Theoretical extrapolation to the past can then be accomplished with a degree of certainty hereto- fore impossible. Recent Developments Significant progress has been made in this general field since 1965 by the United States with the small Earth- and lunar-orbiting IMP satellites, the helio- centric Pioneer space probes and the Mariner 5 Venus fly-by in 1967. Considerable advances have been made in studying the solar wind -- natural plasma from the Sun -- which represents the evaporation of the solar atmosphere supersonically into inter- planetary space. The interaction of this plasma with the Earth's magnetic field generates a detached bow shock wave as the solar wind flow is deflected around the Earth's magnetic field, forming an extended magnetic tail of the Earth quite similar to cometary tails. Acceleration of charged particles appears to take place in the vicinity of the bow shock wave and also in the neutral sheet imbedded in the Earth's magnetic tail. Both processes are of direct interest to plasma physics and con- trolled thermonuclear reactions in the laboratory. The studies from Lunar-Explorer 35 (IMP 6) reveal the direct impact of solar plasma on the Moon and the very rapid diffusion of the imbedded interplanetary magnetic field through the lunar body. It indicates that the outer layers of the Moon are at relatively low temperatures and that the composition and internal temperatures cannot correspond to an origin similar to chondritic meteorites. Mariner 5 data indicate that Venus has no appreciable magnetic field although its ionosphere leads to an interaction similar to the Earth's whereby a collision- less bow shock wave develops. These results were also obtained by the Soviet Venus probe which performed plasma and magnetic field measurements to within 200 km of -42-
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-43- the planetary surface. In summary, these studies reveal the existence of two additional classes of solar wind interaction with the planets and suggest a new and powerful method for indirectly studying the planetary interior by observing the solar wind interaction with those planets that do not possess an appreciable intrinsic magnetic field or an ionosphere. Relation to Prime Objectives The study of the history and origin of the solar system is not a short term proj- ect but one that demands successive increments of understanding as progress toward the ultimate goal is achieved. A knowledge of the present state of the solar system is an important element of this understanding. As additional information on the near-space environment of the planets is obtained, a better understanding of the significance of differences in their environment will result. The sister planets Venus and Earth are now known to be radically different: although Venus has approximately the same size and average density as Earth, it has no magnetic field nor radiation belts. An understanding of the origin of planetary atmospheres requires an evaluation of the long term effects of the contributions, both positive and negative, of the solar wind to the atmosphere. The nature of ancient planetary atmospheres requires estimates of the strength of the solar wind interaction with the planets. As knowledge is gained concerning the near-space environment of the planets, it will be possible to evaluate the merits of specific theoretical models of the Earth's environment. One example is the strong support that the negative results obtained on a Venus magnetic field gives to theories that require rapid planetary rotation to produce planetary magnetic fields. The existence of the Earth's field prevents the solar wind from interacting directly with the terrestrial atmosphere. Solar disturb- ances do affect the atmosphere and ionosphere indirectly, in ways that affect basic day-to-day living on Earth, such as in communications and perhaps in long term weather cycles. The response of the Earth's atmosphere to charged particles from the Sun has been studied by observing the varying atmospheric effects on satellite motions. The contributions that the study of near-space environments can make to an under- standing of the origin and nature of life are at best indirect. The existence of a planetary magnetic field partially shields the surface from bombardment by biolog- ically harmful radiation. More important, it facilitates the formation and retention of a planetary atmosphere which is indispensible to life and which also plays a larger role in blocking harmful radiation. The long term effects of exposure to low level ionizing radiation are not well known. However, the establishment of quantitative values for cosmic ray background flux and for periodic outbursts of solar flare particles will provide estimates for comparison with dose levels to which present-day forms of life are exposed. Indeed, one of the results of satellite studies of the near-Earth environment has been to secure data on the radiation hazards of manned space flight. At the same time, space- craft have provided quantitative information on the levels of radiation directly incident on those planets possessing no magnetic field and only a small atmosphere. Measurements Required and Scientific Priorities Detailed measurements of the near-space environment of the planets do not re- quire large and expensive three-axis oriented spacecraft of the Mariner class or larger. It is possible to conduct highly accurate and precise measurements of magnetic fields, plasmas, energetic particle fluxes, and subsidiary measurements of the exospheric plasma in the planetary environment by ion and mass spectrometers from small and relatively inexpensive spacecraft. This was pointed out by the Space
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-44- Science Board in a report issued in 1967.* The present study strongly endorses the concept of using small spacecraft in the planetary exploration program as discussed in detail in that report. Lack of Near-Space Environment Studies in Present Program The present NASA program for planetary exploration does not include any direct, in situ measurements of the near-space environment. In fact, the Mariner-class space- craft that have flown thus far have not provided sufficiently clean magnetic back- grounds for accurate studies of the interplanetary magnetic field or planetary magnetic fields. As the exploration of the solar system progresses and exploratory studies are replaced by refined measurements, it will be essential that magnetic field meas- urements of the planetary environments be performed on spacecraft that are magnet- ically clean along the lines of the IMF's, anchored IMF's (i.e., planetary or lunar orbiters), or Pioneers. Such measurements can be best performed from spacecraft placed into moderately eccentric orbits and for which the possibility of decaying periapsis through the use of onboard propulsion will permit radial profiling of the exospheric and upper atmos- pheric composition. An important requirement of these orbital studies is that they extend for periods of approximately half a planetary year so that seasonal variations can be effectively studied and a synoptic monitoring both of the environment and its dynamic response to solar disturbances be carried out. Anchored Monitoring Platforms Thus far, measurements of the interplanetary medium have been made on space probes from the orbits of Venus to Mars, but not beyond. The extension of these measurements to the outer regions of the solar system beyond Jupiter and closer to the Sun than the planet Mercury will contribute significantly to our understanding of the present-day solar system and our development of appropriate theoretical models. Simultaneous observations from anchored monitoring platforms will permit detailed studies of the propagation of solar disturbances into interplanetary space and the response of the planetary environment, the ionospheres and atmospheres, to such phenom- ena. Adequate provision for availability and scheduling of ground-based antenna facil- ities in support of the monitoring programs must be considered in any long range plans for systematic study of the solar system. Planetary Priorities The planet having the highest priority by far in terms of studying its near- space environment is Jupiter. In addition to its radio emission, which demonstrates the existence of a large planetary magnetic field and a huge radiation belt, the study of its near-space environment is significant with respect to a study of the depth of penetration of the solar wind into deep space. Jupiter at 5 AU may be near the bound- ary separating our own solar-dominated environment from that dominated by the galaxy. An early exploratory probe to Jupiter would provide a definitive study of the radial gradients of the physical properties of the interplanetary medium as well as estab- lishing the quantitative nature of the Jovian environment. *"Report of a Study on Explorations in Space with Sub-Voyager Systems," Space Science Board, NAS-NRC, Washington, D C., 1967
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-45- Measurements beyond 5 AU present practical difficulties with respect to power sources, and radioisotope thermonuclear generators may be necessary to furnish suffic- ient thrust and effective communications. As previously discussed, the Moon, Venus, and Earth represent a suite of near- space environments that becomes increasingly complex as interaction with the inter- planetary environment progresses from minimum to maximum. Although Venus has been identified as an intermediate case between Moon and Earth with respect to its near- space environment, no synoptic studies have yet been performed. Thus, a small space- craft to monitor and study synoptically the Venus environment, to define more pre- cisely the effects of the solar wind sweeping on the high atmosphere and to investi- gate the formation of the ionosphere, are of high priority in the study of near-space environments. A distant fly-by of Mars by the Mariner 4 spacecraft has established only a con- servative upper limit to the presence of a planetary magnetic field. While it is expected that the solar wind interaction with this planet and its magnetic field is more analogous to Venus than to Earth or Moon no positive conclusions can be drawn without definitive measurements _in situ of the magnetic field, plasma, and charged particle environments. The higher rotation rate of Mars, once every 24 hours, may induce the formation of an intrinsic planetary field. The absence of nonthermal emission from Mars, however, does not encourage this point of view. Little is known about the possible magnetic field of Mercury. The planet's high density and probable lack of an ionosphere suggest that a study of solar wind inter- action with the planet can provide information concerning the electrical conductivity and thermal regime of the interior, as in the case of the Moon. In summary, balancing scientific interest and technical feasibility, the follow- ing ordering in priority of the planets is suggested: 1. Jupiter 2. Mars 3. Venus 4. Mercury We strongly endorse present plans for Pioneer F and G fly-bys of Jupiter in 1973 and 1974. A series of small planetary orbiters as recommended in this report (see Chapter 2) will provide the necessary measurements of the Mars and Venus near- space environments in the early 1970's at relatively low cost. Finally, a Mercury- Venus swing-by mission in the mid-1970's will present an opportunity for detailed studies of Mercury's environment.