SpaceX chief Elon Musk laid out a long-term vision for regular interplanetary transport and colonization in a 27 September presentation at the International Astronautical Congress. Details and vision alike were further steps along the path SpaceX has been pursuing for years, as it repeatedly counters naysayers by taking up the so-called impossible—and getting it done.
Yet while Musk concentrated on engineering, propulsion, efficiency, and finance, the toughest limiters on long-term Mars habitation may well turn out to be biological. Could life evolved on Earth, especially more complex organisms such as ourselves, thrive there indefinitely and across generations?
Musk’s aim is to make humanity a multiplanetary species. He envisions a city of a million people on Mars that could become “self-sustaining.” In other words, if Earth becomes uninhabitable, humanity would have a second home, and avoid extinction.
Most of the technical issues with Mars habitation can be addressed with technical means. Radiation can be shielded against. Water, air, and regulated temperatures can be produced, and chemical plants such as for ship propellant can be built. Psychological and other factors in long-term, small-scale hab confinement have already been under study both in space and in remote desert sims.
The gravity of the situation
However, the harshest sticking point for a colonization plan could be something that Musk mentioned, but characterized only as a source of fun—38% Earth gravity on Mars. He presented images of jumping high and lifting heavy things with ease.
The possible problems would only appear, as they so often do, over the longer term. Research on the health effects of low gravity has already begun to suggest a quite unfavorable pattern. Much of this research as been done in zero g, but long-term exposure to 38% Earth gravity—Mars g—could well produce many similar effects along the same spectrum, just more slowly.
Zero g has been found to produce not only the expected muscle atrophy in astronauts, but a host of other health issues, which isometrics and exercise bikes can only partially limit. Research on both astronauts and lab animals point to falling bone mineral density and circulatory issues, including impaired heart health.
Limited research to date thus already suggests negative effects on three major physical systems. Yet muscular, skeletal, and circulatory systems are hardly footnotes to transporting brains; they are most of what a complex organism consists. Moreover, there is no reason to expect nervous and reproductive systems to get free passes either, especially over years and decades.
Studies of zero-g animal embryonic development raise even greater concerns for long-term Mars colonization. Reproduction among spacefaring rodents has gone quite badly. Experiments with mice on a Space Shuttle mission resulted in normal embryos for the earthside controls and no growing embryos in zero g. Rat groups sent into orbit produced some weightless pregnancies, but with no resulting births. The pregnancies spontaneously terminated—all of them.
Evolutionary and developmental processes could always assume 1g
Simple organisms such as bacteria are the least likely to be bothered by gravity changes. The more complex the developmental process, however, the more likely that aspects of this process will be fine-tuned to happen in 1g. That said, Mars g could well be better for development than zero g because it would at least supply developmental processes with some vertical orientation, an up and a down, albeit with a much weaker signal.
The plans encoded in DNA for growing an organism are completely unlike engineering plans. They are decentralized developmental instructions. Each cell responds to its immediate environment. It takes cues from the type of cell it has become, from the types of cells around it, and from the specific chemistry and hormones in its blood supply. The so-far unquestioned constant has been that all earthly life has evolved in 1g (with very tiny variations) and every embryonic developmental process has evolved to take place in this 1g.
What about adaptation? As powerful a force as evolution by natural selection is, it tends to require extremely long time scales, on the order of thousands and more generations, especially for larger-scale adaptations. Too great a change—or an entirely unprecedented type of change—and a species will simply not make it.
Adaptations to something so pervasive and otherwise constant as gravity would have to proceed in steps. If a hypothetical planet’s gravity were to (somehow) shift to 38% of its former level, but do so over several million years or more, then life there would have a decent chance of adapting because any given generation would only be subject to minute changes. However, by the time gravity reached 95% of its former level, organisms then would already tend to be optimally adapted to that new 95% level. Checking in again a thousand generations later, organisms would tend to be well adapted to the newly current 90% gravity, and so on as gravity crept down. In contrast, evolution copes far less well with sudden large jumps, which tend to be associated with mass extinctions.
Temperature variation is a variable to which earthly life is widely adapted, both across species and to a lesser degree within each organism. Temperature has changed remarkably and continuously throughout Earth’s 4.5 billion year history and it also varies starkly with season and geography. Temperature adaptation therefore has a vast range of evolutionary precedent. Atmospheric composition, pressure, and radiation levels have also changed back and forth over geologic history.
What earthly life has never had to do, not even once, is what a Mars relocation would ask of it. Low g is something that evolution has had no opportunity to tackle. One of the few rough constants throughout the 3 billion or more years of earthly life has been 1g.
This still does not make some degree of individual gravity adaptation impossible now, but it does suggest that this could be a very serious issue for colonization and a potential deal-breaker for both indefinite stays on Mars and natural reproduction of future generations there.
The probably need for artificial gravity and how to produce it
For long-term extra-terrestrial colonization, artificial structures capable of producing artificial gravity that approximate 1g seem more promising. One concept involves large cylindrical spacecraft on axial rotations. The interior surface of the cylinder can be built to a size and given a rotation to approximate 1g over a large habitable interior surface area. That would be another huge engineering challenge. Yet SpaceX’s work in interplanetary transport, along with advancements in asteroid mining, would help lead to a future in which this too could become more feasible.
Given the grave potential health and reproductive risks of long-term exposure to zero g and/or Mars g for Earth-evolved organisms, those interested in space colonization ought to assign a high priority, alongside ongoing engineering work, to low- and zero-g health research. Critical for colonization are three research areas: effects of Mars g on the health of Earth-leavers, likely health of long-term Mars residents upon potential return to Earth, and effects of low and no g on embryonic and childhood development.
Getting people to Mars is an engineering challenge. Musk, SpaceX, and collaborators are up to the task and well on their way. But the length of time that hopeful new Martian arrivals can expect to live there, in what state of health, and with what likelihood of producing healthy offspring, are critical questions in need of serious research and consideration in relation to any developing colonization plans. Early animal and astronaut studies combined with an evolutionary perspective suggest that shorter-term Mars visits are likely to be far more feasible from a health perspective, that natural reproduction among colonists might well be out of the question, and that the development of spacecraft and stations with artificial gravity is likely to be a biological priority for any future long-term extra-terrestrial residents.
This provides a more realistic base scenario from which to refine the engineering details of an early Mars transport and habitation system. It may well be that 1g environments would have to be available at least part of the time to support health longer term. The most realistic approach to creating artificial gravity is a rotating habitat, but this could well prove easier to achieve in space than on a planet with gravitational and atmospheric resistance, albeit both much lower than Earth’s.
At minimum, it should be clear that lab mice and rats ought to be the first serious colonists on Mars—and this for quite some time. Their mission: to live where no earthly creature has lived before. Godspeed to those pioneering rodents; I suspect they’ll need it.