The serious study of flight to the stars is a comparatively recent phenomenon. One of the early papers to take interstellar travel to a new level — and to my knowledge the first technical article on manned interstellar missions — was Leslie Shepherd’s ‘Interstellar Flight,’ which appeared in the Journal of the British Interplanetary Society in 1952. These days we all tinker with sociology and psychology, musing about what drives a society spaceward, but Shepherd, a British physicist and one of the godfathers of today’s interstellar work, thought the reasons were obvious. We’ll go to the stars out of scientific curiosity and the pure love of adventure.

Thus the view from a somewhat more optimistic 1952, at least where space was concerned. It was an era when what seemed possible far outweighed the budgetary and political concerns that would silence efforts like Project Orion and, eventually, Apollo itself. But Shepherd, who at the time he wrote the paper was technical director for the BIS, recognized other motives as well, including the need to communicate with the other species he assumed must inhabit other star systems, and the imperative to disperse humankind over many worlds to ensure its survival.

Image: The April, 1953 issue of Hugo Gernsback’s Science Fiction Plus, which contained a version of Leslie Shepherd’s ‘Interstellar Flight’ for a broad audience. The image shows what appears to be a hollowed out asteroid being used as a massive generation ship.

For all these reasons, the view from 1952 was startling to many who read Shepherd in JBIS or, in more popular form, in Hugo Gernsback’s Science Fiction Plus, where he wrote on the subject in 1953. Shepherd wasn’t much interested in probes in that era. He wanted to get not just a few humans but entire colonies of them across the interstellar gulfs. And after going to great pains to address the problems of distance and energy, and to explain them with the relevant mathematics, he went on to take a science fictional concept into the realm of the possible:

…the explorer or colonist setting out for some distant system may do so in the knowledge, not only that he will never again see his native planet, but that he will not even see the planet of his destination – a privilege reserved for his descendants. Thus the philosophy of the explorer may be that of the soldier or airman setting out on a suicide raid, doing so in the knowledge that for him there can be no personal gain, only the dying knowledge that some will survive to benefit from his action. Indeed, interstellar colonization may call for the sacrifice of whole generations in the lonely reaches of space. Colonies once established may have to exist for generations in a state of complete isolation and such communications as exists between systems may be a very tenuous and precarious matter.

When Starflight Becomes Possible

This is bracing talk because it assumes, as Robert Forward would do a decade later, that despite its incredible hardships, an interstellar journey is possible within the bounds of known physics. When would we begin to take the idea seriously? In Shepherd’s view, a journey to Alpha Centauri would start to resonate when we could reach velocities of up to 10,000 kilometers per second, a speed that gets you to the Centauri stars in a bit less than 130 years. The figures and mathematics he provides in his paper show how severe the power requirements would be with conventional rocketry, and why systems with low thrust and high exhaust velocity (an ‘ion rocket’) would be optimal.

Remember, this is in the pre-laser days, when the idea of a solar sail was just beginning to go from a theoretical fancy to a real possibility for space missions — here we think back to Carl Wiley (writing as ‘Russell Saunders’), who published ‘Clipper Ships of Space’ in the May, 1951 issue of Astounding Science Fiction. So sail configurations and in particular Forward’s ‘lightsails,’ with their laser beam push, have no place in Shepherd’s thinking. Nor did he have the benefit of our catalog of over 500 exoplanets to work with, though he did note the necessity of such observations before any interstellar departure.

And, of course, he captured the essence of the drama that various science fiction authors, including most recently Greg Bear in his novel Hull Zero Three (2010), have worked with. The ‘slowship’ to the stars becomes a sociological experiment on a grand scale, particularly when the destinations are so distant that travel times of a thousand years can be anticipated:

In the normal way, some thirty generations would be born and would die upon the ship. It would be as though the vessel had set out for its final destination under the command of King Canute and arrived with President Truman in control. The original crew would be legendary figures in the minds of those who finally came to the new world. Between them would lie the drama of perhaps ten thousand souls who had been born and had lived and died in an alien world without knowing a natural home.

Shepherd saw this ship as a kind of Noah’s Ark, one carrying everything, including the vast variety of Earthly life forms, that colonists would need to set up shop on another world. The vehicle would of course be gigantic, a small planetoid in its own right weighing at least a million tons excluding the weight of propellants and fuel. He recognized that on a journey of many generations even a million tons makes for a tiny living space, but assumed that sufficient care to design could make it bearable. All along the route, his crew would work to preserve order:

The community would be subjected to a degree of discipline not maintained in any existing community. This isolated group would need to preserve its civilization, hand on precious knowledge and culture from generation to generation and even add to the store of science and art, since stagnation would probably be the first step to degradation.

The Hope for Relativistic Flight

And if we could reach much faster speeds? Shepherd’s section on relativistic flight works through the benefits of time dilation and ponders the possibilities of antimatter to generate the energies required. But he also introduces, in one of the earliest discussions of the question, the consequences of the ship’s interactions with interstellar gas and dust, noting what the designers of Project Daedalus would wrestle with much more extensively 25 years later, that any spacecraft moving at a significant fraction of lightspeed would have to be well shielded.

Shepherd’s conclusion is straightforward, like his entire paper:

There does not appear to be any fundamental reason why human communities should not be transported to planets around neighbouring stars, always assuming that such planets can be discovered. However, it may transpire that the time of transit from one system to another is so great that many generations must live and die in space, in order that a group may eventually reach the given destination. There is no reason why interstellar exploration should not proceed along such lines, though it is quite natural that we should hope for something better. To achieve a more satisfactory performance, however, we should need sources of energy far more powerful than any utilized or known today.

Shepherd remains a seminal figure in the development of interstellar studies, and I notice that he is honorary symposium chairman for the 2011 interstellar sessions in Aosta, to be held this July. His sober analysis of generational starflight foreshadows Robert Forward’s attempts to ramp up velocities while remaining, as Shepherd did, within the realm of known physics and technologies that could be extrapolated from what we use today. His paper is well worth reading for its historical value in placing our dreams of the stars on a solidly scientific foundation.

Thanks to Kelvin Long for passing along a copy of this paper. The reference is Shepherd, “Interstellar Flight,” JBIS Vol. 11 (1952), pp. 149-167.