by Paul Gilster | May 30, 2014 | Culture and Society
Cameron Smith last joined us just over a year ago with an essay on human interstellar migration in the context of biological evolution. Here he turns to issues of culture and change over time. An anthropologist and prehistorian at Portland State University in Oregon, Dr. Smith brings insights he has gained in the study of the early human experience on Earth to the manifold problems confronting us as we head for the stars. His current work on interstellar issues is part of his engagement with Project Hyperion, an attempt by Icarus Interstellar to develop parameters and reference studies for a multi-generational worldship. Be aware of Dr. Smith’s excellent recent volume Emigrating Beyond Earth: Human Adaptation and Space Colonization (Springer-Praxis, 2013), and ponder the synergies that occur between the study of past human migrations and the ongoing cultural and biological evolution of a species aspiring to leave the world that gave it birth.
by Cameron M. Smith, PhD
1. Biological and Cultural Evolution
Any change in the heritable properties of a population over generations is evolution, and we can be sure that the human genome will evolve on multigenerational interstellar voyages, just as it continues to evolve here on Earth today. Even in relatively short, 5-generation voyages, a relatively small gene pool for humanity would have to be carefully composed to prevent collapse in the form of a demographic ‘extinction vortex’ following, for example, a catastrophic plague. This is all clear from elementary population genetics, as I pointed out recently in a technical article in Acta Astronautica1—and it’s important to note that not only will we have to be careful to ensure the genetic health of humanity, but also that of all our plant and animal domesticates and symbionts.2
And, we must remember that genetic information is, in the human lineage, only one channel or stream of adaptive, evolving information; another is culture.3 Culture evolves as surely as biology; like genes, information is replicated (culturally, in the form of ideas), varies in its expression, and those variable expressions of ideas do not all survive to be transmitted to the next generation, but some are more-often copied than the rest; culture, then, does not merely change by a process analogous to biological evolution, it does indeed evolve, albeit in its own way.4 While natural selection has not been entirely buffered out with technology, in human cultural evolution it has largely been replaced by ‘cultural selection’; two examples on opposite ends of a spectrum of selection are propaganda, the deliberate spread of certain information in a population, and censorship, deliberate prevention of the spread of such information. While these are driven by conscious motives, a significant difference from natural selection, which has no intent or purpose, these remain selection in that they alter the frequency of ideas in a culture, and shape its history.
2. How Will Human Culture Evolve Beyond Earth?
We can be sure that our genome will be shaped by new conditions, and some such adaptations, to new gravity environments for example, may be predicted based on what we know of human genetic diversity and adaptability. Can we say anything similar about how human culture might evolve? We can, but we must remember that culture change is much more rapid than biological change. With mass media, an idea can be presented to, and affect the behavior of, billions of people in a matter of seconds. For this reason, culture change is more historically contingent and less orderly, we might say, than biological change. But human behavior is not random, and there are patterns.
For example, in all human cultures a number of basic issues must be solved for sheer survival; an individual needs certain quantities of calories, fresh water and nutrients daily, and the way these are obtained can be strongly conditioned by the immediate resource environment. This has led to humanity, worldwide, developing or evolving cultural traits—“human universals”—that reflect basic problem-solving for the kinds of life form that we are, biologically and socially. An example of this is our subsistence mode. While this differs widely in detail worldwide, all humans have solved basic subsistence needs in one of four main modes of subsistence; foraging (hunting and gathering), pastoralism (raising domesticated animals), horticulture (low-intensity farming and animal domestication) or agriculture (intense farming with complex irrigation and animal domestication). These work for humanity, as opposed to, for example, photosynthesis (not available to [directly] process energy in most animal life). And, the mode of subsistence strongly conditions many cultural variables; for instance, foraging people are normally rather egalitarian, with few instances of social ranking, while among horticulturalists and agriculturalists, social ranking (inequal access to resources by population members) is common. In this case, elementary issues such as social ranking ethos are strongly conditioned by subsistence mode. There are plenty of other factors that play into the shape of a culture, and responsible anthropologists are careful not to be ‘ecological determinists’. But we can learn from such cases.
3. Where to Begin? ‘Human Universals’
Where to begin? It’s easy to be overwhelmed when we begin to think about how human culture will change on multigenerational voyages. To order my thoughts, I have begun to tackle this issue by researching the above-mentioned “human universals”. Again, these are domains of behavioral regulation common to all human cultures. For example, all cultures, in one way or another, regulate sexual behavior, and incest in particular. All human cultures also have basic moral systems for dealing with truth-telling, property theft, and killing. And all human cultures have distinctive languages, styles of bodily decoration, family structure and so on. Remember, the ways that these issues are solved differs by culture (leading to tremendous cultural diversity worldwide), but a set of universal cultural facets is a good place to start when we consider what we might expect to evolve beyond Earth. Table 1 displays some universal cultural domains (over 50 have been identified by anthropologists5), explains the concept of the domain, and provides an example of some alternatives.
Table 1. Common Cultural Domains
|Language||Specific spoken and gestural (bodily) systems of communication, including vocabularies and grammars.||Some languages assign gender to nouns, while others do not.
|Ethics||Concepts of right and wrong, justice, and fairness.||Some cultures execute murderers, while others do not.
|Social Roles||Rights and responsibilities differ by categories such as age (child, adult), gender (man, woman), and status (peasant, King).||Cultures differ in the ages at which people take on certain rights and responsibilities, and specifically what those rights and responsibilities are.
|The Supernatural||Concepts regarding a universe considered fundamentally different from daily experience.||Different cultures worship different gods, goddesses, and other supernatural entities.
|Styles of Bodily Decoration||Human identity is often communicated by bodily decoration, either directly on the body or with clothing.||Some cultures heavily tattoo the body while others communicate identity more with clothing styles
|Family Structure||Concepts of kinship or relations between kin, and associated ideas such as inheritance||Some cultures are polygynous, where males have several wives, and some are polyandrous, where females have several husbands.
|Sexual Behavior||Regulation of sexual behavior, including incest rules.||Cultures differ in the age at which sexual activity is permitted.
|Food Preferences||Concepts of what are appropriate food and drink in certain situations.||Some cultures eat certain animals while others consider them unfit to eat.
|Aesthetics||Concepts of ideals, beauty, and their opposites.||Some cultures value visual arts more than song, and vice versa.
|Ultimate Sacred Postulates||Central, unquestionable concepts about the nature of reality.||Some cultures consider time to be cyclic while others consider it linear.
Rather than generally cast about, then, grasping for what might change over generations of human cultural evolution during future voyages to distant worlds, each of us guided by our own specific ideas and interests, we can systematically use the tools of anthropology and the principles of evolutionary theory to make useful predictions and recommendations to care for the cultural health of interstellar migrant populations.
Such recommendations are often considered ‘meddling’ or ‘social engineering’ by critics, but of course every new law we pass, every tradition we begin to follow, every marriage we sanction or disallow is ‘social engineering’. I take the view that rather than interfering with ‘natural’ culture change, such informed regulation of culture would make for a better future (especially for future human voyagers) than if we left things to ‘fate’. That could too easily go wrong, as we see in the last century of global warfare, where civilization is a thin veneer, nothing more than a set of agreements that, once and easily breached, result in conflict. Thinking clearly about what might befall future generations is not social engineering, it is taking care of the future, as we do by purchasing insurance individually, or working to ensure universal human rights by the passage of international regulations on certain behaviors.6
4. Two Guidelines for Multigenerational Cultural Health
As mentioned, currently I have no clear recommendations, but I am working on many in my research for a forthcoming book, (tentatively titled Principles of Space Anthropology). For example, while we cannot be certain how the regulation of sexual behavior, a cultural universal, will play out in detail off-Earth, we can be certain that it will be adjusted to accommodate a lower population growth rate than we have in many areas of Earth today. If interstellar voyagers are to emphasize no, or very low population growth for some generations, for instance, social institutions and sanctions might well be used to delay impregnation until somewhat later in life than we see, for example, in developed countries today. Marriages themselves might be carried out at a substantially later age than we are accustomed to, and out-of-wedlock births might be significantly discouraged by social means. In one way or another, the finite size and resources of the interstellar vehicle or vehicles would demand a low-or-no population growth ethos for some centuries, and this would surely affect reproductive rights and result in some kind of regulation of sexual behavior. How do we decide what recommendations to make for planning interstellar voyaging? I have at least two guiding principles that will help condition my own recommendations.
First, human culture, and evolution in general, do not typically tolerate rapid and significant structural change. There are many reasons for this, including the fact that bodies and cultural values are somewhat tailored to modern or even ancient conditions, and radical change on a short scale might well dissolve the close connection between the anatomical or cultural feature and the environmental variable. For this reason, I tend not to recommend new and radical social structures or other arrangements for interstellar voyages. Rather, I think it will be best to use social and cultural arrangements that have been familiar to humans for thousands of years; people should not be arranged, then, in new kinds of family structure, or unfamiliar housing such as ships’s barracks, but as families living in familiar environments including villages and towns connected by travel routes that pass through uninhabited areas.
Second, concerning the populations of interstellar vessels, I strongly believe that we should go in large numbers rather than small, barely-sufficient numbers. While some human populations have been able to survive for centuries in the low hundreds, even cut off from other breeding populations, such cases are exceptions to the rule that human populations are interlinked, yielding genetic populations not of just hundreds of individuals, but multiple thousands. This accords with what we see in vertebrates in general, and mammals and primates in particular, where natural and genetically healthy populations rarely drop below 5,000. I also recommend doubling or tripling the outsetting population to guard against the possibility of catastrophe; plague is a particularly terrifying spectre for enclosed populations, and just as passenger aircraft wings are engineered to take not 3 g’s but 10 or more g’s before failure, safety margins should be built into such plans. Since populations alone can significantly affect the shape of cultural universal adaptions, these must be considered together.
The prospect of interstellar voyaging to spread and preserve humanity and civilization is too great a leap for some to make; they call it ‘pie in the sky’, ‘irresponsible dreaming’, ‘escapism’ and plenty else. My anthropological perspective suggests these are all pessimistic critiques from people lacking in creativity or foresight—or even the hindsight that reveals the ruins of each of the ancient civilizations. I am thrilled to be working with Icarus Interstellar to slowly assemble the puzzle pieces required to provide the breathtaking option of long-term space settlement for humanity over the next 100 years.
1. Smith, C.M. 2014. Estimation of a Genetically Viable Population for Multigenerational Interstellar Voyaging: Review and Data for Project Hyperion. Acta Astronautica 97(2014):16–29 (abstract).
2. Xu, J. and J.I. Gordon. 2003. Honor thy Symbionts. Proceedings of the National Academy of Sciences 100(18):10452-10459.
3. Whiten, A., R.A. Hinde, K.N. Laland and C.B. Stringer. 2011. Culture Evolves. Philosophical Transactions of the Royal Society B (366):938-948.
4. Gabora, L. 2013. An evolutionary framework for culture: Selectionism versus communal exchange. Physics of Life Reviews 10(2): 117-145.
5. Brown, D.E. 1991. Human Universals. Philadelpha, Temple University Press.
6. For example, see the United Nations’ Universal Declaration of Human Rights, and its use of sanction in many cases to ensure them. Though the UN can be ineffective, its very existence at least demonstrates the will to work in union for the betterment of all.
by Paul Gilster | May 29, 2014 | Exoplanetary Science
Given the high quality imagery returned by Cassini on an almost routine basis, it’s interesting to remember how little we knew about Saturn’s moon Titan back in November of 1980, when Voyager 1 made its closest approach to the planet. Think of the options the Voyager 1 team had in front of it. The craft could have been sent on to Uranus and Neptune, a trek Voyager 2 would later accomplish. It could have preempted New Horizons if, on a different trajectory, it had been sent to Pluto. But Titan had the allure of a thick atmosphere, making it an irresistible target.
Deflecting Voyager 1 past Titan meant taking it out of the plane of the ecliptic, canceling the other two options, and the frustration of the Titan images the spacecraft returned is summed up in the view we see at the right, a moon whose surface is completely obscured. The visually impenetrable atmosphere was also found to be topped by a thick layer of haze. Learning about that atmosphere was hugely important for planetary science — the spacecraft detected methane, ethane, nitrogen and numerous organic compounds — but I can recall the sense of letdown in the general public upon seeing a featureless orange ball instead of a detailed surface like those of other gas giant moons Voyager 1 had seen.
Image: Titan as seen by Voyager 1’s cameras, which could not penetrate the deep haze of organic aerosols. Credit: NASA.
Titan’s murk would eventually be penetrated by Cassini’s instruments and, of course, the Huygens lander that descended into the atmosphere for its long parachute drop to the surface. But the haze continues to be a major part of Titan’s story, and as this JPL news release points out, its effects can be useful even in terms of how we learn about exoplanets. For Cassini is in position to witness Titan’s sunsets, solar occultations that mimic what we observe when collecting spectra of an exoplanet’s atmosphere as its star’s light passes through it.
The technique is called transmission spectroscopy. As the star’s light passes through the atmosphere of an exoplanet in transit, some of that light is absorbed by the atmosphere, giving us a spectrum that can tell us about the atoms, molecules and grains found there. Knowing that clouds and high-altitude hazes like those on Titan are not uncommon in our own system, we can assume that many exoplanets will have them as well. That means that understanding their effects is key to describing the limits of this transit technique.
A team led by Tyler Robinson (NASA Ames) used four Cassini observations of Titan made between 2006 and 2011 using the spacecraft’s visual and infrared mapping spectrometer instrument (VIMS). The work, described in Proceedings of the National Academy of Sciences, then compared the complex effect of hazes to exoplanet models and observations. The result: Hazes like those on Titan provide a severe check on what we can learn from transmission spectroscopy, and may give us information only about the planet’s upper atmosphere. That’s a level that on Titan would be 150 to 300 kilometers above the surface.
Image: Using data collected by Cassini’s Visual and Infrared Mapping Spectrometer, or VIMS, while observing Titan’s sunsets, researchers created simulated spectra of Titan as if it were a planet transiting across the face of a distant star. The research helps scientists to better understand observations of exoplanets with hazy atmospheres. Credit: NASA/JPL.
We also learn that Titan’s hazes affect shorter, bluer wavelengths of light more strongly than other colors, a result not anticipated by previous studies of exoplanet spectra, which assumed that hazes affected all colors of light in more or less the same way.
“People had dreamed up rules for how planets would behave when seen in transit, but Titan didn’t get the memo,” said Mark Marley, a co-author of the study at NASA Ames. “It looks nothing like some of the previous suggestions, and it’s because of the haze.”
Using Titan as a close-up stand-in for a distant exoplanet helps us see how much an effect haze can have. In Titan’s case, we would be looking at layers far above the densest and most complex layers of its atmosphere. All of these complicated effects have to be worked out as we extract the signature of an exoplanet’s atmosphere, a challenge that is clearly formidable. The good news is that techniques like these can be applied to the atmospheres of planets in our own Solar System just as they were at Titan to help us develop more accurate, workable models.
by Paul Gilster | May 28, 2014 | Outer Solar System
I love a long journey by car or rail, but not by airplane. Back in my flight instructing days, I used to love to take a Cessna 182 on a long jaunt, but these days the flying I do means sitting in the cheap seats in the back of a gigantic jet and suffering the various indignities of security checks, long lines and tightly packed quarters. Hence my 1000 mile rule: If the trip is less than that distance, I’ll drive it or look for a rail connection. My recent trip back to the Midwest reminded me how much I enjoy seeing the scenery at my own pace and having plenty of time to think.
One of the things I thought about was how to extract maximum value from spacecraft. A decade or so ago, JPL’s James Lesh explained to me how the signal from a distant probe passing behind a planet would be affected by that planet’s atmosphere. An elementary way to do atmospheric science! I’ve mused ever since about how to do complicated things with existing resources and how to put technology in the right place for bonus information returns. All that led to thoughts about our prime astrobiology targets in the outer system: Europa and Enceladus.
Earlier this month I wrote about Lee Billings’ Aeon Magazine essay Onward to Europa, in which he speculated about the the possibility of exploring what is beneath the Europan crust. A mission like this could be done without actually descending to the surface and penetrating the ice. Billings noted that the Hubble Space Telescope has detected water vapor, estimated at about 7000 kg of water per second, being blown into space from the surface. Europa’s high-radiation environment is challenging even for a robotic lander, but maybe we could fly through the Europan plume to sample the moon’s chemistry and possibly even detect signs of biological activity. The essay Ship of Dreams in Astrobiology Magazine speculates on the proposed Europa Clipper mission flying through the plumes, but budget issues could make the $2.1 billion Clipper too expensive.
Image: A prime target for astrobiology, Europa (as imaged here by the Galileo spacecraft) is the subject of multiple mission concepts. Credit: NASA/JPL/Ted Stryk.
Similar ideas have surfaced about Enceladus, with an eye toward flying a complicated mission on a tight budget indeed. Consider the Life Investigation for Enceladus mission concept, championed by Peter Tsou (Sample Exploration Systems). I read more about this one in an essay by Andrew LePage on his Drew ex machina website. LePage, a physicist and writer who serves as senior project scientist for Visidyne Inc. in Boston, notes that the LIFE mission would use an aerogel collector like the one NASA used in the Stardust sample return mission to return cometary dust in 2006. Some concepts also call for sample return from Saturn’s E-ring, thought to be made up of particles originally from Enceladus’ geysers.
All this came into the public eye last summer at the Low-Cost Planetary Missions Conference (LCPM-10) at Caltech, where Tsou laid out a 15-year mission that would launch in the early 2020s, reaching Saturn in May of 2030 after a series of gravity assists past Venus and the Earth. LIFE would use close passes by Titan to alter its orbit, allowing multiple low-speed approaches through the Enceladus geyser region above the moon’s south pole. At speeds slower than Stardust’s encounter with Comet Wild 2, the Enceladan material should be better preserved when captured. LIFE would then use Titan for further gravity assists followed by a return to Earth in 2036.
I love the concept as much as I love extracting atmospheric science from communications signals. The cost excluding launch might be kept as low as $425 million. The potential gain is high. LePage likes it, too, and goes on to suggest not just improvements to the Enceladus idea, but a different sample return mission that would bring back materials from both Europa and Io. The mission could launch as early as 2021, with rendezvous with Jupiter in October of 2025. LePage lays out the basics: An elongated orbit to avoid the worst of Jupiter’s radiation belts, gravity assists from Europa, Ganymede and Callisto, multiple low-velocity encounters with the Europan polar plumes and gathering of plume materials with the aerogel collector.
Then figure several months of observation to scope out a target on Io, then a close pass by the moon to sample one of its volcanic plumes, a scary swing through the most intense regions of Jupiter’s radiation belts, but one of only two passes through the worst of them (the other being at insertion into orbit around Jupiter). Re-entry to Earth’s atmosphere would occur in 2030. It’s a mission concept with intriguing resonance with the LIFE mission and builds on the same technologies. Says LePage:
While a lot more work is required to flesh out the details of a Europa-Io sample return mission (especially more information on the nature of Europa’s purported plumes), at first blush it does appear to be feasible using the same hardware proposed for the LIFE mission to Enceladus employing readily available launch vehicles. This proposed mission also nicely complements the investigations of the LIFE mission by returning samples from yet another set of plumes on a potentially life-bearing moon with the added bonus of sampling volcanic material from a second target of keen interest to planetary scientists – Io, the solar system’s most volcanically active world.
The nine-year mission LePage envisions is substantially shorter than the 15-year LIFE mission, and could be completed about the time that LIFE arrived at Saturn. This would have been a great concept to mull over on my trip, and I wish I could have read about it before I left. I’d love to see follow-up work, particularly on that white-knuckle pass by Io. The essay continues:
For minimal additional costs (i.e. a second spacecraft and launch vehicle along with the incremental cost increase of running two missions in parallel), this scientifically interesting mission could be flown in parallel with LIFE and greatly increase its total science return. And it could probably do so within the Administration’s proposed billion dollar price cap for a Europa mission.
Given that we’re now dealing with budget proposals that confine a NASA Europa mission to under a billion dollars, the sample return mission to Europa even without the Io component offers a profoundly interesting science return, and I like the synergies with the more fully developed LIFE concept. In any case, we have two highly intriguing astrobiology targets that are conveniently venting material into nearby space, making landing on the surface — much less trying to penetrate fissures or drill through thick ice — unnecessary at this stage of our investigations. What we learn from such missions could well determine how we press ahead with later, more complex missions that would demand operations on or below the ice.
by Paul Gilster | May 27, 2014 | Missions
Of the many interesting questions Nick Nielsen raised in last Friday’s post, the one that may be most familiar to the interstellar community is the question of potential breakthroughs. What happens if an unexpected discovery in propulsion makes all the intervening stages — building up a Solar System-wide infrastructure step by step — unnecessary? If we had the kind of disruptive breakthrough that enabled starflight tomorrow, wouldn’t the society that grew out of that capability be fundamentally different than one in which starflight took centuries to achieve?
I was mulling this over yesterday when I read Pluto-bound Probe Faces Crisis, a short article in Nature that several readers had passed along. With the New Horizons probe pressing on for a close-pass of Pluto/Charon next year, the assumption all along has been that it would make a course correction after the encounter to set up a flyby of a Kuiper Belt Object (KBO). The trick there is that the New Horizons team is running out of time to find the right KBO. The sense of urgency is revealed in the fact that mission scientists have asked for 160 orbits of observing time on the Hubble instrument, which the article calls a ‘rare request’ for an already operational mission.
Alexandra Witze sums up the reason for the delay in identifying a target in the Nature story:
In theory, project scientists should have identified a suitable KBO long ago. But they postponed their main search until 2011, waiting for all the possible KBO targets to begin converging on a narrow cone of space that New Horizons should be able to reach after its Pluto encounter. Starting to look for them before 2011 would have been impossible, says [mission co-investigator Will] Grundy, because they would have been spread over too much of the sky.
The Voyagers, Galileo, New Horizons and their ilk represent a familiar evolutionary model of our expansion into the outer Solar System as opposed to the kind of disruptive breakthrough Nick was speculating about. In this model, we learn from mission to mission, making each more capable, adding technologies that can get instruments to their destinations at a faster clip. We can’t predict disruptive technologies, but we can see a rational line of development of current tech as we tune up our deep space craft, one in which the ongoing New Horizons issues play a major role.
Image: Artist’s impression of the New Horizons spacecraft encountering a Kuiper Belt object. The Sun, more than 4.1 billion miles (6.7 billion kilometers) away, shines as a bright star embedded in the glow of the zodiacal dust cloud. Jupiter and Neptune are visible as orange and blue “stars” to the right of the Sun. Although you would not actually see the myriad other objects that make up the Kuiper Belt because they are so far apart, they are shown here to give the impression of an extensive disk of icy worlds beyond Neptune. Credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute (JHUAPL/SwRI)
The next year is going to be filled with New Horizons news and, let’s hope, a resolution of the KBO issue. Fifty new KBOs have thus far been identified in the hunt, which has used the resources of the 8.2-metre Subaru Telescope in Hawaii and the 6.5-metre Magellan Telescopes in Chile. None, as it turns out, is close enough to New Horizons’ trajectory to make it feasible given the constraints on the spacecraft’s ability to maneuver. And as I’ve mentioned in these pages before, the search field is tricky, looking directly out along the plane of the galaxy, which means the faint signature of a KBO is readily lost in the starfield. The good news is that by adding Hubble into the mix — and a decision on this won’t be reached until June 13 — the chances of a detection soar over what they would be using ground-based telescopes alone.
Make no mistake, even a long-distance observation of a KBO from New Horizons’ 21-centimeter telescope would trump what we can see from Earth orbit, but obviously a much closer look at a primordial survivor from the Solar System’s early history would be preferable. We wait and hope for the best. Meanwhile, we in the interstellar community should be tracking this mission with great interest. New Horizons is pushing into terra incognita with instruments designed for the job, and represents, as Michael Michaud recently commented to me, a more relevant transition to our deep space future than the Voyager spacecraft. It should energize our designs for future craft that will push further into the Kuiper Belt and beyond. This incremental model works, and if along the way a disruptive breakthrough occurs, then so much the better.
by Paul Gilster | May 23, 2014 | Culture and Society
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.”