There are so many things to say about Les Shepherd, who died on Saturday, February 18, that I scarcely know where to begin. Born in 1918, Leslie Robert Shepherd was a key player in the creation of the International Astronautical Federation (IAF), becoming its third president in 1957 — this was at the 8th Congress in Barcelona just a week after the launch of Sputnik — and in 1962 he would be called upon to serve as its president for a second time. A specialist in nuclear fission who became deeply involved in nuclear reactor technology, Shepherd was one of the founding members of the International Academy of Astronautics (IAA), and served as chairman of the Interstellar Space Exploration Committee, which met for the first time at the 1984 IAF Congress in Lausanne, Switzerland.

The IAF Congress in Stockholm the following year was the scene of the first ISEC symposium on interstellar flight, one whose papers were subsequently collected in one of the famous ‘red cover’ issues of the Journal of the British Interplanetary Society. But let’s go back a bit. Those ‘red cover’ issues might never have occurred were it not for the labors of Shepherd, who was an early member and, in 1954, the successor to Arthur C. Clarke as chairman of the organization. It was in 1952 that Shepherd’s paper “Interstellar Flight” first appeared in JBIS, a wide-ranging look at the potential for deep space journeys and their enabling technologies. He served again as BIS chairman (later president) between 1957 and 1960 and from 1965 to 1967.

I first learned about Shepherd’s career through conversations with Giancarlo Genta at the Aosta interstellar conference several years ago, a fitting place given his long association and friendship with Italian interstellar luminaries Giovanni Vulpetti, Claudio Maccone and Genta himself. It was only later that I learned that Shepherd was the organizer and first chairman of the Aosta sessions, which continue today. When I heard of Shepherd’s death, I wrote to both Vulpetti and Maccone for their thoughts, because from everything I could determine, the “Interstellar Flight” paper was one of the earliest scientific studies on how we might reach the stars. I considered it a driver for future investigations and suspected that it had a powerful influence on the next generation of scientists.

Dr. Vulpetti was able to confirm the importance of the work, which looked at nuclear fission and fusion as well as ion propulsion and went on to ponder the possibilities of antimatter. The latter is significant given how little antimatter propulsion had been studied at the time. Vulpetti goes on to say:

It may be interesting to consider the state of particle physics in which the paper of 1952 was written… For understanding some key aspects, we have to remember (a) that Dr. Shepherd was born in 1918, (b) the positron was discovered in 1932 by C. D. Anderson (USA), and (c) the antiproton was found in 1955 by E. Segré (Italy). Thus, [Shepherd] was aware of the prediction of antiproton existence made by P.A.M. Dirac in the late 1920s and 1930, but – when he wrote the paper – there existed no experimental evidence about the antiproton. Nevertheless, Dr. Shepherd realized that the matter-antimatter annihilation might have the capability to give a spaceship a high enough speed to reach nearby stars. In other words, the concept of interstellar flight (by/for human beings) may go out from pure fantasy and (slowly) come into Science, simply because the Laws of Physics would, in principle, allow it! This fundamental concept of Astronautics was accepted by investigators in the subsequent three decades, and extended/generalized just before the end of the 2nd millennium.

That the 1952 paper was ground-breaking should not minimize the contribution Shepherd made in other papers, including his 1949 and later collaborations with rocket engineer Val Cleaver on the uses of atomic energy in rocket technologies that not only examined nuclear-thermal propulsion but looked as well at the kind of nuclear-electric schemes we are now seeing actively used in operations like the Dawn mission to Vesta and Ceres. But it may be that his thoughts on antimatter in “Interstellar Flight” were his most provocative at the time — he published that paper a year before Eugen Sänger’s famous paper on photon rockets. Vulpetti adds that his work revealed the “important relationship between the mass of an annihilation-based rocket spaceship and its payload mass. This was confirmed in the 1970s, and generalized in the 1980s.”

Reading through the pages of “Interstellar Flight” is a fascinating exercise. Shepherd must confront not only the immense distances involved but the fact that at the time, we had no knowledge of any planetary systems other than our own. Yet even in this very early era of astronautics he is thinking through the implications of future technologies, and I suspect that a few science fiction stories may also have crossed his path as he pondered the likelihood of ‘generation ships’ that could take thousands of people on such journeys. From the paper:

It is obvious that a vehicle carrying a colony of men to a new system should be a veritable Noah’s Ark. Many other creatures besides man might be needed to colonize the other world. Similarly, a wide range of flora would need to be carried. A very careful control of population would be required, particularly in view of the large number of generations involved. This would apply alike to humankind and all creatures transported. Life would go on in the vehicle in a closed cycle, it would be a completely self-contained world. For this and many other obvious reasons the vehicle would assume huge proportions: it would, in fact, be a very small planetoid, weighing perhaps a million tons excluding the dead weight of propellants and fuel. Even this would be pitifully small, but clever design might make it a sufficiently varied world to make living bearable.

So wide-ranging is the “Interstellar Flight” paper that it also takes in relativistic flight (here there is no option, he believes, but antimatter for propulsion) and time dilation as experienced by the crew, and goes from there to an examination of the interstellar medium and the problems it could present to such a fast-moving vehicle. Shepherd saw early on that collisions with dust particles and interstellar gas had to be considered if a vehicle were moving at a substantial percentage of the speed of light, working out that at velocities of 200,000 kilometers per second or more, the oncoming flux would be about 10¹¹ times as intense as that found at the top of the Earth’s atmosphere. He saw that a considerable mass of material would have to shield the living quarters of any spacecraft moving at these velocities. Project Daedalus would, in the 1970s, re-examine the problem and consider various mechanisms for shielding its unmanned probe.

Leslie Shepherd had many collaborators and, as Dr. Vulpetti told me, encouraged wide studies in propulsion systems for deep space exploration (Vulpetti himself was one of Shepherd’s collaborators — I give the citation below). I thank Giovanni Vulpetti and Claudio Maccone for their thoughts on Shepherd. Thanks also to Kelvin Long and Robert Parkinson of the BIS for helpful background information. But we’re not done: Dr. Maccone was kind enough to send along some personal recollections of Shepherd and his work that I want to run tomorrow — I had originally intended to publish them today but they give such a good sense of the man that I want to run them in their entirety as a tribute to a great figure in astronautics whose loss is deeply felt.

Les Shepherd’s ground-breaking paper on interstellar propulsion is “Interstellar Flight,” JBIS, Vol. 11, 149-167, July 1952. His 1994 paper with Giovanni Vulpetti is “Operation of Low-Thrust Nuclear-Powered Propulsion Systems from Deep Gravitational Energy Wells,” IAA-94-A.4.1.654, IAF Congress, Jerusalem, October 1994.