Nick Nielsen’s new essay follows up his speculations on interstellar infrastructure with a look at the kind of starships we might one day build. The consequences are profound. What if we master interstellar technologies without needing the Solar System-wide infrastructure many of us assume will precede them? A civilization’s interstellar ‘footprint’ would be radically altered if this is the case, and evidence of mega-engineering among the stars sharply constrained. Then too, how we view what is possible could be transformed by breakthroughs in biology and longevity, all part of the mix as we look at what Nick calls ‘undetermined nodes in future history.”
by J. N. Nielsen
In my previous Centauri Dreams post, The Infrastructure Problem, I sought to make a distinction between fundamentally different forms that a spacefaring civilization might take, one tending toward primarily planetary-based infrastructure, and another tending toward primarily space-based infrastructure. I am always pleased by the insightful comments I receive from Centauri Dreams readers, which never fail to spur me on to further (hopefully improved) formulations.  This last post was no exception. I was particularly interested in a comment by William Blight:
“A lot of ifs in this author’s presentation. Large scale industrialization of the moon for power and materials using automation and robotics for rapid bootstrapping is probably the best method for developing a powerful space infrastructure. Colonizing Mars will accelerate the development of propulsion systems. I don’t see how speculation in regard [to] Alcubierre drives has real connection to the development of near-term, space-based industry.”
This comment has helped me to understand the limitations of my exposition. There were a lot of “ifs” in my presentation. Of course what I wrote was highly speculative, as all contemporary writing on interstellar travel must be, but it was speculation with a purpose, and I am concerned that my purpose was not sufficiently clear.
We cannot see the future in detail, but we can distinguish broad patterns of development, just as we can see broad patterns of development in the past, if we look to the past for its overall lessons and not for the ideographic detail that fascinates biographers. Every “if” represents an undetermined node in future history, where under conditions of constraint we may be forced to choose between mutually exclusive alternatives, while given an open future somewhat less subject to constraint (e.g., a future in space where energy and materials are cheaply available, if only we can keep ourselves alive in space long enough to exploit them), an undetermined node represents a point of bifurcation where different communities will take different directions. These are the patterns I am trying to explicate.
Interstellar travel represents an undetermined node in future history, and we do not yet know all the constraints that will bear upon starships once we build them. It would be a mistake to think of interstellar travel in all-or-nothing terms, i.e., either we have the technological capacity or we don’t, because this technological capacity will be developed little-by-little, step-by-step. When an interstellar voyage comes at great personal cost (in time, money, opportunity cost, inconvenience, and discomfort), only a trickle of individuals will possess both the resources and the overwhelming desire to go. As the journey declines in the personal costs it demands, it will appeal to greater numbers of individuals, until the trickle eventually becomes a flood. The relative ease or lack thereof in interstellar travel will be a function of the technologies employed, so that the technologies we will eventually use to travel to the stars will shape the historical structure of that travel, and of the spacefaring civilization that undertakes interstellar travel. In other words, how we get there matters.
There is no more compelling argument for the fact that how we get there matters than the present dependency of the transportation network, and indeed of the whole of industrial-technological civilization, on fossil fuels. We all know that the geopolitics of fossil fuels has decisively shaped the world we live in, and that if some other technology, a non-fossil fuel technology, were the basis of global energy markets, the world today would be a different place.
Future technologies of interstellar flight will shape spacefaring civilization as profoundly as fossil fuels shape our world. Until particular technologies are developed and put into practice, we cannot know which will prove practicable and which will be mere curiosities of little utility, yet by reducing the possibilities for starships down to a few broadly-defined classes, we can sharpen the focus of how we think about the potential niches for spacefaring civilization. Consider this division of potential interstellar transit technologies into four classes:
- Class 0: Very long term interstellar travel, beyond the practicability of generational starships. Another way to think of this would be in terms of interstellar travel on geological time scales.
- Class 1: Generational starships, i.e., starships that would require from one to several generations (measured in ordinary human life spans) in order to reach their destination.
- Class 2: Interstellar transit within the life-span of an individual, measured in months, years, or decades.
- Class 3: Rapid interstellar transit on the order of global transportation today, measurable in hours or days.
This is a very rough and provisional division, and the reader should place no emphasis on the particular divisions I have made or the particular technologies that I cite, but only on the idea that we can divide potential interstellar transit technologies into broadly distinct classes. (The possibilities for interstellar drives are parallel to the possibility of some other industrial-technological civilization in the galaxy, not identical to us, but differing in terms of countless contingencies. The important point is not the identity of a particular technology or civilization, but the capacity it has to serve a particular role.) In each of these classes we can identify a series of technological developments that could shorten the voyage, but the voyage on the whole would remain roughly within the parameters of the classes sketched above, so that the upper edge of class 0 touches the lower edge of class 1, and so forth.
Image: Categories of starships. Credit: Nick Nielsen.
The other variables that enter into the equation of interstellar travel—longevity and destination choice among them—also admit of many possible solutions. Human life might be extended by many different technological means (incremental improvements in the life sciences, regenerative medicine, suspended animation, etc.), or even someday by the simplest of biological means.  And once having met the minimum interstellar threshold for a destination, interstellar travelers will have a wide choice of destinations that will affect the length of the trip. A Class 1 starship that would be a generational ship for human beings with an average life-span of today could be considered a Class 2 starship if life spans were considerably lengthened or if suspended animation technology proved to be practicable. The point here is that there is more than one way to approach the problem, and how we solve the problem matters to the kind of spacefaring civilization we eventually build.
The gravitational slingshot technology employed to send the Voyager spacecraft on interstellar trajectories could be further extrapolated with gravitational slingshots around other star systems, which might raise the velocity of a spacecraft to one percent of the speed of light.  This could be much faster than the Voyager spacecraft are traveling at present, but still clearly constituting class 0 interstellar transit. Were we to develop biological reconstitution technology that could remain functional for thousands of years, and we launched this on a class 0 starship (like Voyager, i.e., something that we could build with known technologies), we would then begin the era of human interstellar travel.
Image: The Daedalus starship. Credit: Adrian Mann.
A light sail might be at the upper edge of class 0 or the lower edge of class 1 interstellar travel, while a light sail further propelled by a laser might approach the upper edge of class 1. The Daedalus starship design should be considered a class 1 starship, though incremental improvements in fusion technology might boost it to the lower edge of class 2 starships. More exotic drives such as matter-antimatter reaction might qualify as class 2 starships, perhaps attaining the status of a 1G starship (such as I discussed in Stepping Stones Across the Cosmos), which would allow travel throughout our galaxy within an ordinary human lifespan, though relativistic effects would mean that accelerated communities would be temporally disjointed from non-accelerated communities. Even more exotic propulsion systems – whether the Alcubierre drive, the technology to manufacture wormholes at will, or other possibilities not imagined today – would qualify as class 3 starships that would convey us between stars as readily as jet aircraft convey us between continents today.
Image: The Bussard ramjet design. Credit: Adrian Mann.
The technological developments that could shorten the voyage of a particular class of interstellar travel represent technological succession, just as does the sequence of classes itself (which constitutes technological succession on a greater order of magnitude). In many historical cases of technological succession we see the gradual development of improved technologies, as with automobiles or integrated circuits. When technological succession happens in this way it is largely predictable, but technological succession is sometimes disruptive rather than a smooth progression. In the middle of the twentieth century many assumed that human spaceflight would be attained by the gradual improvement in supersonic flight. However, hypersonic flight has proved to be a difficult engineering challenge, and we have not yet mastered it, but chemical rocket technology leapfrogged supersonic flight and put human beings in orbit and on the moon before the gradual technological succession of improving supersonic to hypersonic to escape velocity technology could catch up. It still hasn’t caught up.
Image: Conceptualizing the Alcubierre drive. Credit: Anderson Institute.
Gradual technological succession would take place within classes of starships; disruptive technological succession would occur when one class of starship supersedes another. If we launched a class 0 starship with reconstitution technology on board, and a hundred years later (or even a thousand years later) developed class 2 starship technology, the class 2 starships would overtake the class 0 starship in a way not unlike how jet aircraft overtook propeller-driven aircraft, and chemical rockets overtook jet aircraft. If class 2 starship technology disruptively precedes practicable class 0 or class 1 starship technology, the entire era of generational starships, class 0 and class 1, will be bypassed.
We are not in a position to judge the relative success of technologies only now imagined, but once we have in place a way to differentiate between entirely different classes of starships, we can speak in terms of the kind of spacefaring civilization emergent from any technology capable of building a class x starship. What the particular technology will be is indifferent to our problem; any class x starship will do. With these considerations in mind, I can return to the point of my previous post, The Infrastructure Problem.
To restate the infrastructure problem, any sufficiently advanced class 2 starship, or any class 3 starship, that can be constructed exclusively with terrestrial infrastructure would yield a spacefaring civilization that possessed only a minimal space-based infrastructure. A spacefaring civilization with minimal space-based infrastructure would be unlikely to engage in megastructure engineering and would thus have a much more modest “footprint” in the cosmos than a Kardashevian supercivilization.
If contemporary terrestrial industrial-technological civilization continues in its present development (i.e., if it does not stagnate), and if it is not destroyed, our sophistication in science and technology will likely improve to the point at which we can build at least an advanced class 2 starship (if not a class 3 starship) and fly directly from the surface of Earth to other worlds – an SSTS spacecraft (single-stage to stellar), if you will.
Such a trajectory of development creates its own great filter, as the ongoing existential viability of a terrestrial-based industrial-technological civilization is contingent upon passing through an extended window of vulnerability when we have the technological capacity to destroy ourselves (intentionally through warfare or unintentionally through the toxic byproducts of industrialism) without bothering to exploit the technology we also possess to establish a rudimentary spacefaring civilization with multiple independent centers of civilization tolerant of local extinction, where “local” means “terrestrial.”
Ever since the advent of the Space Age in the middle of the twentieth century there have been ambitious plans to rapidly expand the human presence in space, from the “Collier’s” space program (Man Will Conquer Space Soon!) to O’Neill colonies. To date, none of these ambitious plans have come to fruition, although our technology is considerably more advanced than when humanity first entered space. Only superpower competition has proved to be a sufficient spur to a major space effort. It does not appear, then, that humanity is an “early adopter” of existential risk mitigation by way of space settlement; we are not moving in the direction of creating a spacefaring civilization predicated upon a robust space-based infrastructure.
The trajectory of development that humanity has not taken represents a possibility, a niche for spacefaring civilization, that some other intelligent species might have taken, or might yet take, and the result of taking this space-based infrastructure path of development would be a spacefaring civilization of a structure disjoint from that characterizing spacefaring civilization of a primarily Earth-based infrastructure. 
If none of the technologies that would make possible advanced class 2 or class 3 starships could be made sufficiently compact that they could be built on Earth and boosted into space, then a civilization would be forced into a choice between remaining stranded within its solar system or eventually building a space-based infrastructure in order to build a starship (this is an instance of “conditions of constraint” resulting in mutually exclusive alternatives mentioned above). For example, a class 1 starship like Daedalus could not be constructed without space-based infrastructure.
I am not an engineer. I will not be designing any starships. Others will design starships, and others will formulate the ideas that are eventually translated into technologies and designs for interstellar flight. As I see it, these technologies are variables in the equation of the large scale structure of any spacefaring civilization. If there is no solution to the equation of spacefaring civilization, given some particular value for the variable of feasible interstellar travel, then we try to solve it again using a different variable. If there are no solutions at all, then we are stuck in our own solar system and the same is true of any other spacefaring civilization that emerges on any other world. 
What interests me is the large scale structure of civilization of any possible spacefaring civilization. I assume if a spacefaring civilization emerges more than once in our universe, these multiple spacefaring civilizations may take multiple paths of development (cf. note ), or they may converge upon some particular path of development to spaceflight if the parameters of possible spacefaring technologies are quite narrow. Different solutions to the equation for spacefaring civilizations yield different large scale structures of that civilization. If there is only one solution to the problem, i.e., only one technology for practicable interstellar travel, then this will exercise a strongly convergent force on the structure of any spacefaring civilization and is an equally strong condition of constraint.
From these considerations another typology begins to emerge:
1. There is no solution to the problem of interstellar travel. (Cf. note )
2. There is a single solution to the problem of interstellar travel, where “single solution” means only one practicable class of starships. A single class of practicable starships still admits of the possibility of technological succession within this class, so that interstellar civilizations might admit of different stages of development in their mastery of the single practicable interstellar technology.
3. There are multiple solutions to the problem of interstellar travel, so that multiple classes of starships are technologically practicable.
In the first case, all spacefaring civilizations are confined to their star system of origin. We already know this to be false, because the Voyager spacecraft are in interstellar space at this moment. However, if one redefines interstellar travel as to exclude class 0 starships, then humanity remains confined within our solar system in this first case. In the second case, spacefaring civilizations are constrained by technology to the choice of becoming an interstellar civilization or not, but all interstellar civilizations will be constrained by the parameters of the single practicable interstellar technology. In the third case, if a spacefaring civilization achieves interstellar travel, it may do so by multiple means, and interstellar civilizations will be differently constrained according to the technology or technologies they develop (in addition to other factors). 
 All of the comments I have received are greatly appreciated, and I regret that I have not responded to each comment individually, but when the reader sees the extent to which this response to a comment runs, it may perhaps be understandable.
 The point I am trying to make in this present argument, how we get there matters, applies equally to the technologies of transhumanism, which will not be separate from interstellar travel but will interact with the human exploration of space. Whether human beings are able to travel to distant stars because of greatly extended life-spans, or suspended animation, or reconstitution, how we get to an extended life-span matters, because each technology interacts differently with the individual life and the socioeconomic structures within which the individual finds a place. Similarly, each interstellar propulsion technology interacts differently with the individual life, making use of such propulsion technologies and the socioeconomic structure within which the individual finds a place.
 In a post titled, “Galactic Grand Tours, and strengthening Fermi’s Paradox” on the Well-Bred Insolence blog, Duncan Forgan writes, “…if a probe carries out a series of slingshots as it tours the Galaxy, the probe can be accelerated to approximately 1% of the speed of light without shipping enormous amounts of fuel (bear in mind Voyager 1 is travelling at 0.003% of lightspeed).”
 An alien civilization might take a different technological path due to different intellectual endowments. It may be that a science and technology, which remains opaque to the kind of minds that we have, will be readily mastered by an intelligent species with a different kind of mind, and vice versa. Bertrand Russell provided an imaginative example that serves as a kind of thought experiment in this respect:
We are certainly stimulated by our experience to the creation of the concept of number – the connection of the decimal system with our ten fingers is enough to prove this. If one could imagine intelligent beings living on the sun, where everything is gaseous, they would presumably have no concept of number, any more than of “things.” They might have mathematics, but the most elementary branch would be topology. Some solar Einstein might invent arithmetic, and imagine a world to which it would be applicable, but the subject would be considered too difficult for schoolboys. (Bertrand Russell, The Philosophy of Bertrand Russell, edited by Paul Arthur Schilpp, Evanston and Chicago: Northwestern University, 1944, p. 697.)
These considerations apply both to the large-scale structure of a spacefaring civilization as well as the particular technologies any such civilization pursues in the attempt to master interstellar flight.
 This is the position of Peter D. Ward and Donald Brownlee (best known for their book Rare Earth: Why Complex Life is Uncommon in the Universe): “The starships of TV, movies, and novels are products of wishful thinking. Interstellar travel will likely never happen, meaning we are stranded in this solar system forever. We are also likely to be permanently stuck on Earth. It is our oasis in space, and the present is our very special place in time. Humans should enjoy and cherish their day in the Sun on a very special planet… Our experience on Earth is probably repeated endlessly in the cosmos. Life develops on planets but it is ultimately destroyed by the light of a slowly brightening star. It is a cruel fact of nature that life-giving stars always go bad.” (The Life and Death of Planet Earth: How the New Science of Astrobiology Charts the Ultimate Fate of Our World, New York: Henry Holt and Company, 2002, pp. 207-208). In this case, the possibility of a large scale spacefaring civilization does not disappear (though Ward and Brownlee explicitly exclude this possibility also), but it takes on a different form, and any communication between advanced industrial-technological civilizations would have to come about by way of SETI and METI. The impossibility of interstellar travel is entirely compatible with megascale engineering within our own solar system, which megastructures could include the building of vast EM spectrum communications antennae capable of communicating across interstellar distances.
 A further distinction could be made in the third case between “more than one solution to the problem of interstellar travel exists” (i.e., at least two solutions exist to the problem of interstellar travel), and, “all classes of interstellar travel are technologically practicable.”