This, then, was the final culmination of a succession of dreams that had emerged progressively in 11 steps or stages that had begun in antiquity. In logical order, the several steps were from: (1) the birth of ancient Greek and other myths of flight, to (2) proposals for machines that would make flight possible by mimicking the flapping wings of birds, to (3) actual attempts at human flight, to (4) successful human flight through the air by means of balloons, to (5) powered, controlled, sustained human flight through the atmosphere by winged vehicles, to (6) fictional accounts of flying to the Moon, to (7) the invention of rockets leading to an understanding of the principles of space flight, to (8) the Apollo Project Moon landings, to (9) fictional accounts of traveling to Mars, to (10) actual landings on Mars by rockets and robotic rovers, to (11) the idea of leaving Earth and colonizing the universe.

DARPA and NASA had jointly realized that nobody in their right mind formulated plans and undertook projects on anything like the 100-year time horizon that they thought was needed to design, build, outfit, and launch a crewed interstellar vehicle. So they wanted to seed-fund some private organization to do so, and for an essentially backdoor reason: namely to reap whatever possible spinoff technologies might accrue from such an endeavor. “DARPA also anticipates that the advancements achieved by such technologies will have substantial relevance to Department of Defense (DoD) mission areas including propulsion, energy storage, biology/life support, computing, structures, navigation, and others.”

The techniques of celestial dynamics are useful within the solar system and other planetary systems. However, techniques that are useful in a system containing a few mutually gravitating objects are not as useful in a system containing a hundred thousand million objects.

The noun “dynamics” entered the English language in the eighteenth century, when natural philosophers, following the lead of Isaac Newton, began thinking of motion in terms of applied forces and the resulting accelerations. In 1788, the New Royal Encyclopaedia contained the definition, “Dynamics is the science of moving powers; more particularly of the motion of bodies that mutually act on one another.” This is still a useful definition. For the purposes of this book, we can define dynamics as the study of objects that move while interacting through mutual forces.

While the fate of a multigenerational interstellar population cannot be predicted with anything approaching certainty, the many dangers presented by the instantaneously lethal environment of space, plus the interpersonal pressures and conflicts that might result in social breakdown, make it doubtful that a successful transit to another star system with all the successive onboard generations remaining safe, healthy, and happy across time, is a realistic possibility. It is far more likely that the crew would suffer one or another kind of irremediable catastrophe en route than that everyone aboard would survive, and that the final, arriving generation would get there intact. But if that is true, then the question arises whether it would be morally justifiable to launch such an expedition to begin with, given its immense costs, high probability of failure, and lack of any benefit accruing to the sponsors back on Earth who had paid for it all.

A gravitationally bound two-body system (if the two bodies are spheres of constant mass) shows simple periodic motion. We have seen that a three-body system, even if we install restrictions for computational simplicity, can show a rich variety of behaviors. Tadpole orbits, horseshoe orbits, and ZLK oscillations are just a sampling of what can happen.

Beyond the task of developing a realistic and workable propulsion system that would make interstellar travel possible and practical, there is the prior challenge of identifying an extrasolar planet that would be suitable for long-term human habitation. Any planet that is a candidate for human colonization has to satisfy a surprisingly large number of requirements stemming from the fact that human biology has evolved on Earth and nowhere else, and is therefore fit to survive only in an environment that is substantially similar to our own. As Daniel Deudney has said in his book Dark Skies, “Humans are sprung from the Earth, have never lived anywhere but on Earth, and the features of this planet have shaped every aspect of human life .… Life is not on Earth, it is of Earth.” And for that reason, a planet fit for human colonization elsewhere must be earthlike in several important respects.

Researchers proposed ever larger and yet more implausible designs for interstellar vehicles. And so in 1996, writing in the journal Nanotechnology, one Thomas L. McKendree discussed what would be possible if materials provided by molecular nanotechnology were used to build spacecraft in place of then current structural building materials such as aluminum, steel, and titanium. Molecular nanotechnology was the theoretical ability to design and build products to atomic precision. Such a technology, which does not exist as yet and might never, would allow the use of diamondoid materials that had much higher strength-to-density ratios than those that are now used to build structures. In his paper “Implications of Molecular Nanotechnology Technical Performance Parameters on Previously Defined Space System Architectures,” McKendree argued that the use of diamondoid structural materials would make possible extremely large space colonies. The classic cylindrical colony, for example, if made of diamondoid structural elements could have a radius of 461 kilometers and a length of 4,610 kilometers, or 2,865 miles.

The prospect of human travel to the stars faces such an exceptionally wide and diverse assortment of obstacles, improbabilities, multiple risks, and inestimable costs, as to make any attempt to traverse the final frontier far more likely to end in tragedy than to succeed in getting human beings safely lodged on the surface of an extrasolar planet that is in all respects suitable for continued and sustained human life. There are, in general, seven separate categories of problems facing starflight: physical, biological, psychological, social, financial, ethical, and motivational. Starting with the physics of the enterprise, we have seen that none of the three icons of star travel embodies a realistic, practical, proven design that would be likely to work as advertised. Not the nuclear-powered Bernal sphere, nor the Bussard Interstellar Ramjet, nor the Project Daedalus rocket, which in any case was not even intended to carry passengers. Project Orion represented the high-watermark of deep space craziness, as many project members themselves realized afterward. As Freeman Dyson acknowledged much later, “We really were a bit insane, thinking that all these things would work.”