The need for artificial gravity simulation on the surface
“Since its inception in late-nineteenth century, astronautics has been viewed as a historical outcome of human evolution as well as a future driver thereof.” — Tolkowsky G.
Space settlements are the next, and perhaps the inevitable, epoch in the evolution of the human race. Ever since man perceived the existence of a universe beyond Earth, he has been looking up at the night sky in awe seeking his place among the stars. Over the course of human history this pondering has manifested itself in novels and in the imagination of each individual who has ever looked up at the night sky, while in the present it drives government policy, cultural change, billion dollar economies, and technological advancement. In the last decade, questions such as can people live permanently in space, and can large human settlements be built and sustained in space have become increasingly commonplace. This prevalence is not only due to human curiosity, but also due to the realisation that space colonisation offers large potential benefits to an increasingly confined and limited — both socially and economically — mankind.
The Need for Gravity
“Gravity is a habit that is hard to shake off.” — Terry Pratchett
A mission to another planet, say Mars, is fraught with a plethora of technical difficulties, but the exposure to a reduced-gravity or a microgravity environment has far more permanent and far less discernible consequences to the human body. Physiological abnormalities accompany such prolonged exposure, as the force of gravity is unbalanced, or virtually lacking, and net force on the human body becomes so weak that virtual weightlessness results.
Firstly, without an ever existing unbalanced force on the body, equivalent to a force pushing you down towards the earth, the vestibular system, which controls motion, equilibrium, orientation and gravity, is made redundant, and loses its sensitivity. Furthermore, the position and movement changes in the human body — as common as standing up — are correlated with the maintenance of stable blood pressure. Consequently, a dysfunctional vestibular system can result in a drop in blood pressure leading to dizziness or even fainting. Astronauts aboard the International Space Station have been known to experience similar disorientation and dizziness during the first few days in microgravity. Moreover, upon return back to Earth astronauts frequently have trouble standing upright, stabilising their gaze and walking in a coordinated manner. These symptoms are often grouped under a single condition known as space adaptation syndrome. Although these abnormalities correct themselves on reintroducing earth-normal conditions, but there have been very few astronauts who have been exposed to microgravity long enough to show any permanent damage, and thus don’t set a tangible trend.
Secondly, the effects of microgravity on the brain and the central nervous system have received considerable attention. A recent study published in the New England Journal of Medicine, observed an upward shift of the brain, narrowing of the central sulcus, and narrowing of CSF spaces at the vertex in most of the astronauts in the study who had had long-duration flights. The upward shift of the brain is correlated with increased pressure especially on the frontal and parietal lobes, which control executive functions in the body, including but not limited to voluntary movement, language and numeracy skills, and integrating sensory information from various parts of the body and visuospatial processing respectively. Furthermore, the increased pressure due to the narrowing of CSF spaces has led to blurry vision and optic disc oedema, obscuring sight in astronauts.
Thirdly, because muscles provide the strength and support to the body for moving against Earth’s gravity, the absence of sufficient gravity removes a major stimulus to maintain normal body strength and endurance in reduced gravity environments. The problem with losing muscle mass is not that it hampers productivity levels in reduced gravity environments, but rather that it seems to hamper safe return and normal productivity at earth-normal conditions after prolonged exposure to such environments. Lack of sufficient gravity removes the forces which causes bodily fluids to gravitate towards the lower part of the body; hence the tension on the weight bearing parts of the musculoskeletal system are reduced dramatically. Due to this, the body performs a reductive remodelling of the bone and muscle structure to adapt to the less demanding environment. With prolonged exposure, this remodelling can become severe and astronauts can lose up to 1–2% of their bone density every month, and up to 20% within six months. Furthermore, muscle loss of 10- 20 % has been observed on short missions and, if no countermeasures were applied, this could go up to 50% on long duration missions.
Lastly, mammalian reproduction has evolved within earth-normal conditions of 1g; therefore, deviations from normal gravity conditions may compromise humans’ ability to reproduce. Research suggests that spaceflight leads to transient, but dramatic decrease in testosterone levels in human and rats. Furthermore, the male reproductive system shows a unique susceptibility to deviations from 1g, with humans under induced hypo-gravity showing altered sperm morphology and a reduction in the number of active sperms. Moreover, rats exposed to simulated hypo-gravity showed 85% loss of spermatogenic cells, indicating that both spermatogenesis and steroidogenesis were impaired in them. On the other hand, female reproduction is much less affected by deviations in the gravity environments, but much more research into the effects of spaceflight on the female reproductive system is required before making the assertion that the female reproductive system is entirely unaffected.
In conclusion, the human body is uniquely suited to a earth-normal gravity environment. Almost all major organ systems of the human body, from the cardiovascular system to the lymphatic system, are negatively affected by a lack of earth-normal gravity.
Countermeasures to Physiological change
Several countermeasures to prevent or to partially offset the negative effects caused due to prolonged exposure to a reduced-gravity environment have been developed and deployed aboard the International Space Station. The first of these includes resistive exercise devices such as the Advanced Resistive Exercise Device (ARED), Cycle Ergometer with Vibration Isolation System (CEVIS) and VELO Ergometer Bike (VB-3). These are used to carry out exercise regimes to prevent muscle mass and bone density loss. In spite of administering heavy exercise routines on ISS crewmen, on average exercising two hours per day, exercise has shown little success in mitigating bone loss from long‐duration spaceflight. A 2012 article published in the Journal of Bone and Mineral Research, emphasises that only those astronauts who strictly adhere to both exercise and nutritional regiments show evidence that nutrition and exercise may be able to mitigate bone loss and reduce risk for spaceflight-induced osteoporosis. Data from the Mir missions from 1990 paint an entirely different picture, consistently showing no effect of spaceflight on bone formation markers, or if anything, a slight decrease. Furthermore, the study states that, “the importance of both exercise and nutrition for successful bone and muscle outcomes is obvious to most researchers, controlling these factors has been challenging in spaceflight and related ground analog studies.” These conflicting conclusions have led to unreliable generalisations regarding the role of exercise in counteracting physiological changes brought on by a reduced-gravity environment.
The Need for a better solution
Conclusions drawn from the above observations and assertions point towards the need for a better solution for counteracting the detrimental effects of a reduced-gravity environment. Evidence suggests that using nutritional restrictions and following strict exercise regimes as a countermeasure are irregular at best, and ineffectual at worse. In the present scenario, the effects of spaceflight and a reduced-gravity environment are corrected by a return to earth-normal conditions, with astronauts having access to world class facilities, here on earth, to help them transition back to the 1g environment, but as humanity moves farther away from Earth this solution will render itself infeasible, if not completely impossible.
Even a reconnaissance mission to the Martian surface will, in the very least, be 26 months long. This is due to the interval between launch windows and Earth-Mars orbit alignment. In other words, the delta-v needed to transfer between the planetary orbits is lowest at 26 month intervals for Earth and Mars. Furthermore, the lowest energy transfer to the Martian orbit is a Hohmann transfer orbit which involves on approximate a travel time of 9 months. Therefore, the shortest mission to Mars will involve the crew to be exposed to 9 months of microgravity, and then an additional 500 days of Martian gravity, which is one-third of Earth gravity, and finally exposure to microgravity for an additional 9 months on the return trip home. Cumulatively, the crew members will be exposed to a reduced gravity environment for over 2 full years, more than double the duration Scott Kelly spent aboard the International Space Station.
In the long term, Martian colonisation will depend on routine trips, to and fro, from the Martian surface every 26 months transporting supplies and settlers. In this scenario, the settlers on Mars will be returning to Earth after durations much longer than just those 26 months endured by the first settlers, and, consequently, the first long term Martian settlement will need to have provisions to seriously counteract the negative effects of the reduced gravity environment on Mars, not just for a crew of few, but for an entire colony.
On Orbit Settlements for the Surface
Space settlement research is largely one-sided, with much of the focus directed towards on-orbit settlements. This inclination is justifiable as the first settlements beyond earth will be on-orbit, and this is due to several reasons. First, on-orbit settlements are much easier to rotate, and hence optimal for generating artificial gravity. Secondly, on-orbit settlements receive uninterrupted sunlight, and hence can generate constant power unlike habitats on the Moon or Mars which receive sunlight only for half of the time. Lastly, transportation of people and resources is much more difficult for surface settlements due to the considerable gravity wells of the planets in question. Therefore, it is hard to deny the superiority of on-orbit settlements over surface settlements, but surface settlements are essential for large scale resource utilisation from any planet or asteroid.
In conclusion, instead of simply conforming, the industry should attempt to incorporate the gravity generating capability of the on-orbit approach with the convenience and accessibility of the surface settlement.
Because after all, the future is red.
The effect of spaceflight on the lymphatic and the cardiovascular system, in addition to a number of psychological and additional physiological effects, has been omitted for the brevity of this article. This article is only to support the argument that humans are uniquely suited to a earth-normal gravity environment.
Saksham Arora is the Co-Founder & President of the New Delhi Space Society, and has worked on orbital debris, space policy and settlement design.
