The obligation to support space exploration can be defended in at least three ways: (1) the ‘argument from resources,’ that space exploration is useful for amplifying our available resources; (2) the ‘argument from asteroids,’ that space exploration is necessary for protecting the environment and its inhabitants from extraterrestrial threats such as meteorite impacts; and (3) the ‘argument from solar burnout,’ that we are obligated to pursue interstellar colonization in order to ensure long-term human survival. However, even if we accept all three propositions, that space exploration will give us access to asteroidal and other resources; will allow us to defend ourselves against meteorites (by intercepting or destroying them); and finally that interstellar colonization might be useful in saving us from solar burnout, it does not follow that we have an obligation to do any of those things. What follows is that we have reasons to take those actions as practical measures that will bring about the ends in question. But no obligation per se arises from the fact that those measures will be helpful in attaining those ends.

The Project Orion spacecraft is by common consent the craziest interstellar flight concept ever devised. Ironically, it was also the spacecraft design that received the widest support by scientists, the military and other branches of the US government, as well as by private industry. It was as if all of these people had collectively lost their minds. The basic idea was utterly simple and so intuitively obvious that it could be understood by a child. This was a craft whose propulsion system was built upon the Newtonian principle of action and reaction. The central notion was that of placing a bomb under a rocket and then detonating it to loft the rocket up and away – exactly the same process as putting a firecracker under a tin can and watching it blow sky high. To keep it going up, of course, a series of bombs detonated in sequence would be required. And so the Orion rocket would be propelled through space by a stream of bombs, in fact nuclear bombs, exploding one after another behind it, thereby continuously accelerating the craft. That was the project’s key concept, and as such it was simultaneously perfect and insane.

The chapter describes three iconic interstellar travel vehicles: the Bernal sphere, the Bussard Interstellar Ramjet, and Project Daedalus. Nobody took the Bernal sphere seriously. The Bussard vehicle would not work as intended, and the Daedalus vehicle lacked a credible propulsion system. The principal difficulty with star travel is that the stars are very far away, at distances measured in light years.

Let us optimistically assume that sooner or later a workable interstellar propulsion system will be found, and also be built and successfully tested in space. While this would be a great advance toward making interstellar travel possible, it nevertheless does not automatically follow that a voyage to the stars will in fact be attempted. There are a few other issues that must also be settled first: for example, a habitable exoplanet must be identified. It must be suitable for human colonization and ought to be a reachable distance away from Earth within a reasonable period of travel time. Second, engineers must provide a plausible space vehicle design architecture, and a spacecraft of that design must then be constructed, and tested successfully. Such a craft does not exist as yet, one among many reasons being that the specifications for it depend in turn upon the size and makeup of the likely boarding population. But both of those factors are still unknown. In addition, and perhaps most important of all, an unprecedented level of funding and resources must be allocated to the project.

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.”

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.

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.”

Many propulsion systems designed for interstellar travel are last-ditch, desperation schemes with very small chances of a payoff. The decidedly iffy status of some of the propulsion concepts so far discussed – the Alcubierre Drive, Sonny White’s warp drive – have led some star travel proponents to conceive of other exotic, “alternative,” or overly imaginative propulsion methodologies: flying through wormholes, for example, or crackpot faster-than-light schemes such as tachyon drives. But those concepts are so far-out and unlikely as to be well beyond even Hail Mary desperation status. There are some further theoretically possible systems, however, that just might work. The least implausible of them all is the controlled nuclear fusion drive. It was this type of engine that would supposedly propel the otherwise unworkable Bussard Interstellar Ramjet as well as the second stage of the Project Daedalus starship. In its favor is the fact that nuclear fusion is the single Hail Mary propulsion technology that is currently under active development.

The obligation to support space exploration can be defended in at least three ways: (1) the ‘argument from resources,’ that space exploration is useful for amplifying our available resources; (2) the ‘argument from asteroids,’ that space exploration is necessary for protecting the environment and its inhabitants from extraterrestrial threats such as meteorite impacts; and (3) the ‘argument from solar burnout,’ that we are obligated to pursue interstellar colonization in order to ensure long-term human survival. However, even if we accept all three propositions, that space exploration will give us access to asteroidal and other resources; will allow us to defend ourselves against meteorites (by intercepting or destroying them); a+L16nd finally that interstellar colonization might be useful in saving us from solar burnout, it does not follow that we have an obligation to do any of those things. What follows is that we have reasons to take those actions as practical measures that will bring about the ends in question. But no obligation per se arises from the fact that those measures will be helpful in attaining those ends.