At the beginning of this project, a bibliography of radar astronomy literature, consisting of 384 items arranged chronologically by year of publication and alphabetically by author within each year, was constructed from a search of the NASA STI Database (aeronautics and space) and a published bibliography, Jean E. Britton and Paul E. Green, Jr., Radar Astronomy (Cambridge: MIT Lincoln Laboratory Library, 1962), which Mr. Green generously made available. The NASA STI Database search alone resulted in a printout of 589 items published since 1963. To this initial bibliography were added additional publications uncovered in the researching and writing of this book.
The initial bibliography, with fewer than 400 entries, illustrated the diminutive character of planetary radar astronomy. In comparison, the radio astronomy literature of just the past two decades measures in the thousands. Because the extent, as well as the development, of the literature might help to characterize the progress of planetary radar astronomy over several decades, the bibliography was pruned and grafted in such a way as to reflect the published literature. Dissertations were missing from the bibliography, while publications by foreign researchers and abstracts abounded.
A number of rules were followed in including and excluding publications. Internal reports were omitted; these are not intended for consumption by the general public or the scientific community. Only works by American practitioners were included; British and Soviet titles were excluded. Planetary radar astronomy was defined more strictly than in the text; solar, lunar, meteor, auroral, and Earth radar studies; and synthetic radar aperture research were left out, because they are specializations unto themselves. Also excluded were items dealing with hardware, instruments, or techniques and those providing interpretations of radar results by individuals outside the field. For example, an article on the interpretation of radar topographic data, whose first author was a planetary geologist, was left out; however, if the first author was a radar astronomer, the article was added. Finally, abstracts were excluded, dissertations included.
The resulting planetary radar literature, spanning the period from 1958 to 1994 inclusively, amounted to 272 entries, or an annual average of about seven. Only twice did 15 or more items appear in a single year. A line chart (Table 9) showing the annual distribution of planetary radar publications indicates the explosion of radar astronomy activity during the 1960s. The remainder of the chart suggests the technological dependence of radar astronomy. A second spurt of growth appears following 1975, when the Arecibo Observatory S-band radar first became available, and a third spurt occurred around 1990, just after the Voyager upgrade of the Goldstone radar.
When the annual publication numbers are grouped by 5-year intervals, the sharp peaks and valleys of the annual chart are smoothed out and a new trend emerges (Table 10). The volatile growth of the 1960s remains, but what appeared to be seesaw-like growth around 1975 and 1990 disappears. Instead, a dip replaces the growth following 1975, and the literature reaches a plateau of activity. This plateau suggests that since 1980 the field has reached the limits to its growth.