Figure 8. Cosmic ray flux. Smoothed composite curve of Applied Physics Laboratory single-counter counting rates above White Sands, New Mexico, geomagnetic latitude 41°N. Gangnes, Jenkins, and Van Allen in Physical Review 75 (1949): 57-69; courtesy of J. A. Van Allen and Physical Review. SIGNIFICANCE The foregoing nontechnical description is merely illustrative. A review of a length appropriate to this book cannot cover in detail 12 years of work by hundreds of scientists. Nor can the description convey to the reader the many subtleties and innumerable interrelationships with which both experimenters and theorists concerned themselves. Nevertheless, brief though it is, the summary shows how the rocket sounding work contributed to atmospheric and cosmic ray research. The new tool, the high-altitude research rocket, had indeed made it possible to obtain data hitherto unobtainable and to solve problems hitherto intractable-as anticipated. The rocket results enabled ground-based observers to improve their techniques and to obtain better results from their measurements-that is, to calibrate their experiments. Whereas at the start some had expressed grave doubts as to the wisdom of using rockets for high-altitude research, a decade later the importance of the sounding rocket to the field was universally recognized. It is natural, then, to ask whether sounding rockets had revolutionized the field of upper atmospheric research. In the excitement of new discoveries amid a continuing flow of important data from a long list of topics-solar physics; atmospheric pressure, temperature, density, composition, and winds; the ionosphere; magnetic fields; the airglow; the auroras; and cosmic rays—the rocket experimenters liked to think and speak of their work as revolutionizing the field. But it is clear in retrospect that the first decade of high-altitude rocket research was normal science, not revolution. Put otherwise, the results from those years of research elaborated and expanded upon the already accepted paradigm, but did not force any fundamental changes in it. Were one to compose a schematic diagram of the upper atmosphere based on what was known immediately following the launching of Sputnik, the picture would probably look much like the drawing of figure 9. A comparison with figure 1 drawn from information set forth in Mitra's book of 10 years earlier shows a striking similarity in overall concepts. In both, the atmosphere is visualized as consisting of a number of characteristic layers-troposphere, stratosphere, ozonosphere, ionosphere, and exosphereat essentially the same altitude levels. In both, temperatures vary markedly with altitude, and these variations are associated with the different atmospheric layers. Solar radiations are considered to be the cause of photochemical processes going on in the atmosphere, affecting composition, giving rise to the night airglow, and forming the ozonosphere and ionosphere. Heating of different levels in the atmosphere is ascribed to incoming solar energy, which in a series of stages ultimately degenerates into heat. There is little doubt that the auroras are caused by charged particles from the sun, and that in some way such particles are also responsible for changes in the earth's magnetic field during magnetic storms. Clearly the two paradigms, before and after, are essentially the same. The expert will, of course, see a new richness of detail in the later picture, but nothing that the earlier paradigm could not accommodate once the facts were known. Thus, space science's first decade, the sounding rocket period, must be characterized as extremely fruitful normal science. Nevertheless, in that early harvest were the elements of some remarkable discoveries. The soft radiation that Van Allen had detected in the auroral regions presaged the discovery of a largely unsuspected aspect of the earth's environment. Following up his interest in these soft radiations, Van Allen instrumented the first American satellite, Explorer 1, with counters to probe further the incoming cosmic rays. Unexpectedly high radiation intensities were found above the atmosphere and, after additional measurements in Explorer 3, Van Allen on 1 May 1958 announced the discovery of a belt of Figure 9. Upper atmosphere as visualized in 1958. The general features are similar to those of figure 1, corresponding to the mid-1940s, but there is much more detail. Height (kilometers) radiation surrounding the earth, which at once became known as the Van Allen Radiation Belt.52 The discovery set in motion a long chain of investigations that in the course of the next several years forced a revision of the picture scientists had developed of how the sun's particle radiations affect the earth's atmosphere. The new features of the geophysical paradigm will be presented in chapter 11. The second discovery came from the rocket investigations of the sun's short-wavelength spectrum. The discovery that x-rays were an important variable in the solar spectrum suggested that x-rays might also be important in other stars and celestial objects, which later proved to be correct.53 When the experimenters in the Naval Research Laboratory group turned their ultraviolet and x-ray detectors toward the stars, they initiated a new field of rocket astronomy, which will be described more fully in chapter 20. In the meantime, the early harvest from rocket sounding of the upper atmosphere was convincing evidence of the rich returns that could be expected from a program of scientific research in space. In this aspect of space, at least, the United States could consider itself fully competitive with any rival. |