[ix] The motivation behind Project Orion can perhaps be best understood by answering the question "Why is a search for other planetary systems important?" In large measure, the answer to that question is contained in two topics: the origin of the solar system and the existence of extraterrestrial intelligence topics that seem at first glance unrelated.
Speculation concerning the origin of the solar system is as old as man himself. The "modern" era of such speculation is generally considered to have begun with Copernicus, who showed that Earth is not the center of the universe. The major cosmogonic hypotheses of this early period were those of Laplace and Kant. Although differing from each other in detail, their hypotheses were similar in that both men envisioned the planets as having formed from material that was spun off the Sun. These hypotheses were discarded a little over a century after they were advanced when it was realized that the Sun. which has about 99 percent of the mass in the solar system has only about 2 percent of the angular momentum of the solar system. The principal reason these hypotheses fell into disfavor was that, at the time of this observational discovery, there was no known mechanism that would remove angular momentum from the Sun thereby making it hard to understand why the planets have so much angular momentum relative to the Sun if they were originally part of the Sun.
The next generation of cosmogonic hypotheses was directed specifically at the problem of the angular momentum distribution in the solar system. Again, there were variations among the hypotheses, but they all tended to rely on singular or catastrophic events such as a supernova or a stellar collision, to account for the solar system. Most of these "catastrophic" hypotheses which were popular in the first few decades of this century, have subsequently been shown to be physically untenable or, at the very least, highly unlikely.
Most current hypotheses concerning the origin of the solar system are in many ways similar to the early hypotheses of Laplace and Kant. They envision the planets forming from a nebula of gas and dust which is thought to have surrounded the Sun early in its [x] history. This return to a "nebular" hypothesis has come about primarily for two reasons. First, unlike our counterparts of the previous century, we now know of several reasonable physical mechanisms that could have removed angular momentum from the Sun. Combining observational studies of the rotational velocities of stars on the main sequence (stars of various masses which, like the Sun, are deriving their luminous energy mainly from conversion of hydrogen into helium) with theoretical models of stellar structure, a consistent picture emerges which indicates that the Sun has very little angular momentum because most of it was removed by the solar wind, a plasma that continuously flows off the Sun's surface. The second reason for a return to "nebular" hypotheses is that the advent of high-speed electronic computers permits us to perform numerical experiments or modeling of the collapse of an interstellar cloud under the influence of its own gravity. It is generally felt that stars form by virtue of such a collapse process. Although these numerical experiments are not definitive, they strongly suggest that when a star is born it is likely to have a circumstellar nebula, and that conditions in such a nebula would be highly conducive to the formation of planetary bodies. An equivalent way of expressing present cosmogonic hypotheses is to say that planetary system formation seems to be a natural, if not inevitable, aspect of the star formation process. The important distinction between the catastrophic and nebular cosmogonic hypotheses is that, if the former is correct, planetary systems are the exception rather than the rule, whereas if the latter is correct, planetary systems are the rule. A systematic study of the frequency of occurrence of planetary systems would thus provide a valuable observational check on present theories of star formation.
The possible existence of extraterrestrial intelligence (ETI) has long piqued man's curiosity. This curiosity has been the basis for a number of science fiction efforts. A graphic demonstration of this fascination with ETI is Orson Welles' radio dramatization in 1939 of the work by H. G. Wells entitled, "War of the Worlds." Until relatively recently, ETI had been the plaything of science fiction, largely ignored as a subject of scientific inquiry. However, Cocconi and Morrison in 1959 took the first major step in changing the attitude of the scientific community toward the question of ETI. They pointed out that there is a natural signpost in the electromagnetic spectrum that would be known to any advanced civilization, and that [xi] such civilizations might send radio signals at or near the frequency of this natural marker. This signpost is the 21-cm-wavelength radiation arising from a hyperfine transition in atomic hydrogen the most abundant element in the universe. Shortly after the paper by Cocconi and Morrison, Frank Drake conducted a search, known as Project Ozma, for such signals. Drake's search was unsuccessful, but its importance cannot be overlooked as it was the first serious attempt at detecting ETI signals. The relevance of a SETI (search for ETI) effort to a search for other planetary systems lies in the fact that the only known intelligent life form, namely ourselves, developed and was nurtured on a planet. If planets are required for the existence of ETI, knowledge of the frequency of occurrence of planetary systems is clearly highly desirable.
A systematic search for other planetary systems would thus reveal whether there is justification in arguing that a natural. perhaps even causal, relation exists between the phenomenon of star formation, which has occurred some 1011 times in the galaxy, and the existence of ETI. The detection of other planetary systems is difficult; present observational techniques and instrumentation are at best marginal in terms of their ability to carry out such a search. The purpose of this design study was to apply modem technology to the problem in the form of specific design concepts for systems that could successfully mount a search for other planetary systems. The Earth is an object of exquisite beauty, and to the extent that the material on the ensuing pages is instrumental in the discovery of another such object, it will have rendered mankind an invaluable service.
