Bold hypothesis

Abstract: My interest in the subject of meteors was aroused by the visit of Ernst Opik to the Harvard College Observatory in 1931-32, during my first year as a newly graduated Ph.D., at the start of my professional career. The Director, Harlow Shapley, supported the study of meteors, probably because of the tradition of photographic meteor studies at Harvard, the occurrence of many meteor trails on the plates of the Harvard sky-patrol cameras, and perhaps the enthusiasm of that “professional” amateur Willard J. Fisher. In 1932, Peter M. Millman,’ for example, was writing his thesis on the subject of meteor spectra, based largely on objective-prism spectra photographed during the large Harvard program of stellar spectroscopy for the Henry Draper Catalogue and related researches. Elsewhere, professional astronomical activity in the study of meteors had largely fallen into the doldrums, because of the intense impetus being given to stellar spectroscopy and astrophysics by the rapid strides that had taken place during the preceding decades in laboratory and theoretical spectroscopy. Unfortunately, too, the purely visual methods of observing meteors were basically inadequate to provide data that could sustain enthusiasm for theory. The spectacular Leonid shower of 1833, nearly a century earlier, had first focused serious scientific interest on the subject of meteors. Charles P. Olivier, in his book Meteors,2 gives references to the major research on the subject through the nineteenth century, so that I need not repeat that history. By the 1930s a few specific meteor showers such as the Perseids, Leonids, and Bielids were definitely associated with comets. Opinion was divided as to the sources of sporadic meteors, fireballs, and meteorites. Theories that they had hyperbolic orbits and originated outside the solar system were quite popular, but both scientific and public interest in the subject waned after the highly publicized but disappointing Leonid shower of 1899. Furthermore, W. F. Denning3 of Bristol, England, concluded from his extensive compilations of visual observations of meteors that stationary radiants were numerous among stream members. That is, Denning insisted strongly that the radiants of major meteor streams remained fixed in their apparent direction in the sky over several days. More precise photographic observations have since disproved this conclusion, but the demand on theoreticians to develop models of meteor-stream characteristics that provide stationary radiants was overwhelmingly difficult and discouraged further effort. In the meantime, however, F. A. Lindemann and G. M. B. Dobson4 realized that meteors could be used as a tool for studying the upper atmosphere. From an inadequate theory and a poor statistical treatment of meteor and fireball end heights, they correctly concluded that atmospheric densities at great heights were much greater than had generally been assumed and that a temperature rise above the stratosphere reached a maximum in the neighborhood of 60 km altitude. Naturally, most investigators were interested in ascertaining the origin of meteors, fireballs, and meteorites. Are their orbits hyperbolic or closed about the sun? This effort was consolidated in 1925 when G. von Niessl and C. Hoffmeister published their famous catalog5 of 611 fireball orbits. They determined atmos-