Even with some combination of these implementations, however, some problems remain. We don’t yet know how much AG—what level and for how long—is needed to counteract the deconditioning effects. With research this can be determined along with the engineering solutions. The benefits would appear to be significant. AG would be almost ideal for bone and muscle effects, and most especially for the problems attendant to headward fluid shifts, which cannot be countered by exercise. For some other areas, there are unfortunately undesired side effects. The vestibular system, for example—the balance organs in the inner ear—can adapt well to a zero-gee environment but is used to orient within inertial reference frames. In other words, rotating environments are not kind to the balance system. Anyone who has spun around a few times on a bar stool (sober or otherwise) and then tried to stand and walk understands the problem viscerally. Several playground devices have similar effects. Humans can adapt to these challenging settings (Lackner and DiZio 2003), but the transition to and from an AG situation could be difficult.
But wait—are we missing something here? Is it possible that there are salutary effects of zero-gee itself—effects that would be missing if AG were implemented? Consider that spaceflight itself and the spacecraft environment with its close quarters, strenuous workload, and imminent danger are demanding enough in and of themselves. Is it possible—just possible—that the experience of weightlessness in this setting provides one of the few pleasures that helps make it tolerable? Would the imposition of AG in such an already stressful situation actually make things worse? We do not know, and the idea might be far-fetched, but it is worth considering. There are pleasurable effects from unusual patterns of vestibular stimulation—witness the popularity of roller coasters and other semi-nauseating escapades. Likewise, the experience of short periods of zero-gee in parabolic flight has been described as the most fun one can have in public—at least short of actual spaceflight. The Apollo astronauts remarked that they could each find their little bit of personal space in the cramped spacecraft, since they were free to explore all three dimensions in zero-gee and could inhabit nooks that were otherwise inaccessible. How much of the famous “overview effect” is due to seeing Earth from above, how much from the experience of 0g, and how much from their interaction? (The overview effect is the vivid realization reported by many astronauts of the fragility and isolation of the Earth in space, the invisibility of political boundaries, and the sense of union among the people on the planet (White 1998).) These are open questions and although it might be small it is too soon to dismiss the possible beneficial effects of zero-gee itself on psychological well-being. Without it, a spaceflight journey of decades might become mere drudgery rather than an adventure—one may need a reminder that the environment, and hence the journey, is unique, and the constant reminder of zero-gee could do that very effectively (despite the problems that it also creates).
Let us return to the more practical aspects of artificial gravity. As noted, AG can address in a natural manner many of the physiological concerns of extended spaceflight. However, even with AG there will be problems. Currently the biggest risks identified by NASA are related to radiation exposure and adverse effects of spaceflight stressors on cognitive function and behavior. AG will not help these. Radiation might be addressed with clever shielding and maybe even radio-protectant pharmaceuticals. But make no mistake: Psychological maladjustment is the potential killer and this problem will not be solved with AG.
PSYCHO KILLER
Consider the psychological issues arising during a decades-long journey (Manzey 2004). Some will be inherently mitigated relative to even the first Mars missions, because of a larger spacecraft with more people on board. Thus, the problems related to confinement might be lessened, although it’s still likely that the minimum tolerable amount of volume per person will be implemented—it is just that that volume will be used to better effect: It will be reconfigurable for the changing needs of the crew, allow group interactions or individual solitude as needed, and provide for some individual psychological needs. The presence of many people can reduce the problem of small groups becoming tired of each other and getting on each other’s nerves; of course, other problems are raised through the greater diversity of skills, motivations, and dedication. Teamwork might be aided by the ability to rearrange teams for some tasks through cross-training of individuals so that the same small group does not have to work together all the time.
However, remoteness and isolation will continue to be significant issues. There were dire predictions in the early days of human spaceflight, to the effect that astronauts might experience a dramatic detachment or breakaway phenomenon, being physically separated from Earth to an extent never before experienced—beyond the atmosphere. This concern turned out to be misplaced, even overblown. Instead, it turned out approximately six decades later that astronauts take great pleasure in viewing Earth from the cupola of the International Space Station. Many have commented over the years, even before the ISS, that watching Earth was a great treat, possibly even therapeutic (not in so many words). This consoling aspect will not be available to those on long-distance missions. Even in going to Mars, the image presented of Earth will be that of just another star in the heavens. The psychological impact of this is impossible to determine. Add to that the realization that there is no return if the journey is one of long-term exploration or colonization, and some form of separation anxiety might well occur. There will be the need to change one’s loyalties and sense of identity to a wholly new place.
Until this mental transition fully takes place, there may be some erosion of resilience—a relative deficit in performance and in dealing with problems—at least in the middle term of the voyage as memories and longing for Earth and what it represents remain strong. Certainly, in a generation or two this reminiscence will remain but perhaps it can be modified and infused with feelings of attachment for the new celestial home. This is not a “mere” psychological factor—it has an impact on several factors which, as we have seen, are tightly connected. Thus, in the mid-term, it may be necessary to have Earthlike reminders that gradually shift in a weaning process. It has been proposed, for example, that deep-space missions now on the horizon include a virtual-reality arrangement that mimics the traveler’s home—his or her favorite room, settings, people, plants, animals, and so on. One cannot help wondering if this is wise. Would not a “clean break” be better? For early missions there might be no alternative, but later it might be better to break this connection with Earth as soon as possible. (NASA and other space agencies use a variety of Earth-based analog facilities in an attempt to mimic and mitigate these psychological concerns. They all, however, fall short in some way from reproducing the full range of effects: The missions are not long enough, the setting is not inherently dangerous, and the facility is not truly remote, or it is so large that the crew is not confined in the same sense as in a spacecraft. In this critical issue we will not truly know until we go.)
How will we track if these measures are effective, even the psychological measures? These aspects have implications for physiology and performance through stress pathways. So, just as proposed above for shorter missions, the continuous monitoring of physiology and performance, including psychology and interpersonal relations, will be one key to tracking the effectiveness of breaking one’s loyalties from one planet to a new one.
INTERACTIONS FOR THE FAR FUTURE
The psychological issues are bad enough. But all this discussion does not even touch the real issue. The likely worst case is that there will be a confluence of circumstances that is unforeseen: a set of events in which several risks come under attack, each of which was considered to be adequately mitigated. Examples have been given previously in the context of a mission to Mars, but this will take on increased importance in the case of longer and farther missions in which the crew will be on its own. Reliance on Earth might be realistic for the first few weeks, after which it will be a rueful wish and then nothing b
ut a quaint memory. Crews and colonists will need the ability to maintain resilience—mission success in the face of unknown perturbations—apart from the assistance of anyone on Earth. And as we have seen, this will be in large part a consequence of paying due attention to interactions among factors (Shelhamer 2016; Mindock et al. 2017).
This takes on a whole new meaning in the context of colonization, when the travelers will truly separate themselves from Earth and its support mechanisms (Bell and Morris 2009).
Faced with the prospect of never returning to Earth, psychological issues will loom large, especially on the first of such missions. As noted, this has widespread effects since stress is a major factor with many connections. There will also be physiological and psychological changes that occur once ensconced on another planet or moon. These might impair the ability to return to Earth, which would be another reminder that that is no longer a viable option. The question then becomes whether to allow these adaptive alterations to proceed or to attempt to slow or inhibit them. The answer is not a simple one. Adaptive evolutionary changes take place on Earth over long timescales, partly because the environment is relatively stable (certainly as far as gravity is concerned). Faced with a dramatically different environment—altered gravity level, unfamiliar atmospheric pressure and composition, different magnetic field, to name a few—evolutionary processes in the human organism might be accelerated. Under such circumstances, epigenetic alterations might take on a larger role in the heritability of acquired traits. Whatever the mechanism, settlers will likely be faced with the problems inherent in rapid change—only this will involve changes to the humans themselves. The possibility that some of these changes will be undesired—and could interact with other changes to the overall detriment of the person—should not be ignored. To the extent that these adaptive alterations proceed, some monitoring might be in order so that undesired and unanticipated interactions can be identified.
It is almost certain that some adaptive changes will prevent the person from ever returning to Earth, or even to a planet with a different gravity. Consider landing, settling, and evolving on a planet with considerably less than the one-gee of Earth: This might eventually lead to taller and thinner humans, since maintenance of blood flow to the head would not be as challenging. Cardiac capacity would change for the same reason. The long, weight-bearing bones would also become thinner and weaker, as would the supporting musculature. In short, organisms are good at shedding unnecessary metabolic costs to make efficient use of resources such as nutrients, and these are all appropriately adaptive changes for the environment in question. But these people would then not be able to function normally on Earth, where they would be highly prone to injuries. It would be unrealistic and unethical to even consider sending them (or their offspring) to Earth or other locations where the gravity level is significantly higher. Thus, humans might indeed become a multi-planetary species in this way, but each subpopulation of humans would be forever tied to just one or a small subset of planetary bodies: No single individual would be truly multi-planetary.
But also consider less dramatic effects. Taller people could be problematic in habitats designed with small dimensions to preserve resources. It would be well to track and predict these types of interactions between physiology and engineering.
On the other end of the spectrum, there might be a temptation to enhance or accelerate adaptations to specific environments. Or to simply make immediate alterations for expediency. This raises the specter of genetic modifications, or surgical ones. One might argue that many space settings would call for the shortening or removal of the legs. In a constant zero-gee setting such as a permanently orbiting station, legs are unnecessary for locomotion. In fact, they can be a hindrance by banging into things, especially as proprioceptive sense of leg position decays from lack of use. The resulting reduction in body mass and fluid reservoir would be beneficial in reducing radiation exposure and in combatting headward fluid shift. On the other extreme, on a planet with a very high gravity, metabolic costs could be diminished by reducing the hydrostatic gradient—that is, by making people significantly shorter.
As intriguing and tempting as these concepts might be, society best tread lightly in any such endeavor at the risk of introducing new complications or overlooking crucial interactions. The introduction of new species to an isolated environmental niche is a historical example: Where there is no natural predator the new species overtakes the available resources. This is one simple form of unintended consequence. There are many others, and in a space-settler setting where there is precious little backup capability (you can’t go home again), even subtle second-order effects can take on outsized significance.
CONCLUSION
Thus, the key in all these situations of long journeys of settlement or colonization is to recognize that we might—just might—be smart enough to mitigate the major known risks for long-duration spaceflight on an individual basis for a relatively short Mars exploration mission (three years), but we are unlikely to be smart enough to determine in advance the countermeasures that will be needed on journeys of colonization. It is almost a certainty that unexpected and unanticipated problems will arise—perhaps a new form of psychological syndrome caused by an unusually strong attachment to an artificial habitat that takes on undue importance in the absence of a familiar Earth and a viable atmosphere, displacing emotions and bonding with other humans. This is pure conjecture of course. More likely, perhaps, would be a novel combination of radiation that interacts with an organism in the soil and revives a dormant species (we see viral shedding on the ISS) for which the weakened immune system is no match, while at the same time the medical supplies have been depleted in treating more conventional problems.
So, how do we give crews the tools to deal with these larger problems—the unknown unknowns? What would these tools be? Some are tangible and, while not trivial, are at least easy to delineate in principle: 3-D printers, DNA sequencers, medical instrumentation and diagnostic equipment, a vast database of information and the ability to acquire updates (not a trivial matter when communication with Earth is challenged by time lag), and information systems that provide the crew useful and important information in a timely manner without saturating them.
But more fundamentally we need to provide the conceptual tools for dealing with the unknown. These are the same as described previously but at a higher level of complexity. This essentially entails a mathematical model that encompasses a deep understanding of the many factors that impact survival and mission success. These are, as noted, not only medical, physiological, and psychological, but also encompass interpersonal interactions, habitat configuration, task planning and design, scheduling, and many others. Sensors for these key variables can track the most important of these factors, continuously feeding data to the model, which would monitor each individual parameter for problematic deviations but also track interactions between parameters and compare them to what is expected from its stored database. Adding to this complexity is the fact that this model must adapt as the people adapt to their new setting and understand when significant changes are part of a beneficial adaptation process versus a detrimental maladaptation or dysfunction.
Thus, we must provide crews of the future the tools for solving problems and not the answers to the problems per se. We can teach a crew to fish and it may survive for a year, but if we give the crew the tools to make rods and reels and find fish and adapt in order to metabolize other types of fish, then they can survive and thrive for generations.
EPILOGUERISK AND RESILIENCE AS A SPACEFARING SOCIETY
Many of the issues raised here transcend those of engineering and operations, the typical realm of advanced spaceflight discussions. They become issues that the larger society should address. As a society, are we willing to do what it takes to enable these voyages of colonization and settlement? The concern is not just the financial cost or the opportunity cost, but the larger cost to society in terms of resource allocation and even mo
re so in terms of perspective. Being audacious enough to give people a fighting chance of surviving and thriving on other planets might mean, as indicated here, that changes in human psychology, and perhaps even physiology, might be needed. New structures of governance and civic cohesion might also be needed. All these changes might in fact occur on their own as a natural adaptive response to a new environment, whether we like it or not. Yet we retain the right to decide whether to put our fellow humans into that situation. How might the attendant physiological and psychological changes in turn change our view of what it means to be human, when it no longer explicitly means “Earth-dweller”? Will we recognize governing structures that are designed to accommodate small populations and environmental stressors on an alien world, and will we be comfortable with them as representative of the civic decisions that have guided our institutions on Earth? Are we ready for such a change in how we see ourselves?
It is one thing for government space agencies, or private companies like Blue Origin and SpaceX, or futurists, to ponder these issues—even to make pronouncements as to preferred policies (as we do in this volume). However, if humans are truly to colonize and settle elsewhere in the solar system as a species and not just as a small group of rugged individualists, then society must ponder these issues in an open forum and attempt to reach some consensus. To not do this will leave the decisions to those organizations—public or private—that first have the means to undertake the journeys.
Consider a microcosm of this larger issue. Spaceflight is a risky endeavor. It often results in the loss of life, and there is a general acceptance of this fact as a society. We have decided to accept that risk. It is important to recognize that this is not just a set of individual choices made by individual astronauts (Kahn et al. 2014). Astronauts might, for example, be willing to accept a large potential increase in lifetime cancer likelihood in exchange for trips into deep space. As a society, on the other hand, we might not accept this risk (through the decisions made by the space agency as dictated by law and regulation).
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