Our success at establishing a permanent Moon colony or brushing gloved fingertips through Martian soil is intimately tied with how our bodies handle extended periods of living in non-Earth gravitational environments. If the human body can’t adapt to these conditions, it will be impossible to further explore the heavens until our spaceships can support sustained artificial gravity.

Imagine the gravity wheels of Space Station V in 2001: A Space Odyssey by Arthur C. Clarke or the Hermes Spacecraft in The Martian by Andy Weir. There are the gravity wells of the world-sized starships in The Stars are Legion by Kameron Hurley and the classic halo superstructures that rotate around Earth in Seveneves (Neal Stephenson) or around a star in Ringworld (Larry Niven).

Whether these structures are practical is a different question, but each is meant to protect the fragile bodies of human travelers living in conditions of lower gravity than Earth (referred from here on as microgravity).

Microgravity environments change the human body and understanding and preventing them are essential for safe space exploration. NASA, the European Space Agency, and the China National Space Administration are working to better understand these changes and how the human body adapts to microgravity so we can better prepare ourselves for sustained travel outside of low-Earth orbit.

The shortest round-trip journey to Mars would take at least two years depending on how long humans stay on the surface. The current record for the longest continuous trip living in microgravity is held by cosmonaut Valeri Polyakov, who stayed 438 days onboard the Mir space station in the mid-90’s. Even then, he didn’t experience complete weightlessness all the time.

When he returned to Earth, Polyakov was able to walk away from his landing capsule, suggesting even a year at space doesn’t detrimentally impact the function of the human body.

A battery of mental and psychological tests before, during, and after his flight showed he maintained a relatively stable mental and emotional state during his mission. The only exception to this were changes in his mood during the first weeks of his mission and subsequent return to Earth. This was attributed to the stress of adjusting to his new environment¹.

By all metrics investigated, Polyakov eventually returned to his preflight baselines and his mission was taken as evidence that humans are capable of extended trips into space and for possible landings onto Mars and beyond.

We now know that space affects the human body in some very surprising ways and at a very deep level. It is important to note that not every individual experiences all of these changes together, but rather may experience one to several at any given time.

For example, within minutes of entering microgravity, systemic changes in blood flow and pressure can occur as the lack of gravity alters the homeostasis of the circulatory system.

International Space Station (ISS) crews have reported chronic swelling and bloating in the head and extremities as blood and other fluids pool in those areas and are slow to recirculate back out. Since the start of space exploration, astronauts, cosmonauts, and spationauts (French astronauts) have experienced headaches, nausea, fluctuations in heart rate, and even bouts of anorexia².

Many of these symptoms are temporary and stabilize over a period of a few days to weeks as the body adjusts to microgravity. However, chronic changes to the body can also develop. The core body temperature often increases during long trips into space and is slow to recover upon return to Earth³.

No one is yet sure why this occurs, but changes in body temperature at rest and during exercise in space are related to increased inflammatory states and may impair immune function. This could harm astronauts traveling long distances to Mars and put them at risk for inflammatory-related diseases such as cardiovascular disease or immune dysfunctions, all while still in space, down on the surface, or when returning to Earth.

One of the best studied microgravity-related conditions is Space Flight-Associated Neuro-ocular Syndrome (SANS), which can develop within a few months of living in space. SANS causes physical changes to the eye and impairs vision, including flattening of the eyeball, damage to the optic nerve, swelling in the optic disc (where the optic nerve meets the retina), and other structural fluctuations.

Almost half of astronauts experience a decrease in close-range visual clarity and others have reported changes in far-sighted clarity, even on short flights to the ISS⁴. Unfortunately, there is no single cause yet-identified that is responsible for SANS. It is hypothesized that increased fluid pressure in the head is a contributing factor. Identifying the mechanism of these conditions may help pinpoint preventative strategies for humans on deep space missions and most, but not all, of these effects return to normal once back on Earth.

Recent MRI studies have shown that brain structure can change in microgravity. Gray matter decreases were observed in the frontal lobes of the brain in crew members of both short shuttle missions and longer-duration ISS stays and gray matter increases in sensorimotor regions were also seen⁵. Gray matter tissue is where many of our neurons and their supporting cells are located and this may provide insight into neurological changes seen in microgravity. How these influence our ability to maneuver in space, or whether there is a functional impact on cognition or behavior are not known.

To peer into whether cognitive changes are an inherent part of space exploration, recent research studies such as MARS-500 and NASA’s HI-SEAS replicated long journeys into space by placing groups of research volunteers into a small environment for simulated trips to Mars. Behavior and cognitive tests were performed to examine how crew members interacted over time and to provide preliminary data on how to best deal with conflicts that could develop during long missions. However, these projects do not account for any changes in gray matter as they were not conducted in space.

Interpreting these findings and designing preventative strategies to slow any microgravity-assisted adjustments in the brain may indeed help future crews, but more studies will need to be performed on the ISS or other spacecraft.

In science fiction, Meg Howrey’s The Wanderers is partially inspired by the MARS-500 project and explores the emotional aspects of long-duration space travel and how this may impact passengers on a mission to Mars.

In other works of fiction, the best-known and most-often discussed changes to the human body relate with loss of bone and muscle density. Many science fiction novels use artificial gravity as a way to combat these changes, as mentioned above. But living on the Moon, which has only 1/6th of our gravity, or on Mars (2/5ths), might also have long-term detrimental effects compared to life on Earth. In Ian McDonald’s Luna Trilogy, visitors to our Moon have a set time limit before permanent changes to muscle and bone strength prevent their return to Earth.

Likewise, those born on the Moon can have serious physiological and health consequences upon visiting Earth and experiencing its gravity for the first time.

Crew members of the ISS exercise regularly to prevent bone and muscle loss and protect against permanent damage. They are strapped down to exercise equipment, such as the COLBERT treadmill, and routinely run up to two hours a day⁶. It remains to be seen how much muscle and bone loss will actually occur on a years-long adventure to Mars where there is no system of sustained artificial gravity, but it is prudent to anticipate challenges.

Additional questions include how pregnancies will be affected in microgravity. Previous experiments in rats suggest a role for gravity in helping pups change body position in the womb, as well as aiding the branching of neurons in the vestibular nuclei and lateral nucleus—regions of the brain involved with posture after birth.

Rat pups in space during gestation had slower-developing neurons in these regions compared to Earth controls, but when the mothers returned back to Earth before delivery the pups who were in space had their neuronal growth rates return to normal levels after a few weeks⁷. This study doesn’t consider pregnancies that occur and develop completely in microgravity, nor the birth and postpartum process for both the pups and the mother. Understanding these processes will be important for permanent colony settlement off of Earth.

In order to address in deeper detail what we already know of what happens to the human body in space, NASA began the Human Research Program (HRP) in 2005⁸. The HRP is the most thorough examination yet of how the human body withstands microgravity and long-duration space flights. Prospective, hypothesis-driven research studies are currently looking at changes in human physiology, emotional states, and a wide range of cellular and genetic markers after space travel.

The Twins Study, a major initiative of the HRP, is examining how astronaut Scott Kelly’s body changed during a full year in space. The control on Earth is his identical twin brother Mark, who is also an astronaut. It’s a fantastic looking glass into how space changes an individual with the perfect genetic and environmental control back on Earth.

Preliminary reports out of NASA indicate that microgravity caused Scott Kelly’s telomeres in his immune cells to lengthen. Telomeres are the caps of our chromosomes and protect the integrity of the chromosomal structure and the genetic information contained on each one. This was a surprising and exciting finding because telomeres shorten as we age and this change contributes to the onset of age-related diseases. It could be that space has an anti-aging effect or even reverses the process in some ways. Remarkably, Scott Kelly’s telomeres shortened back to his preflight baseline within days of his return⁹.

Similar to Star Trek’s medical tricorder, NASA has developed new tools to track the change in biological states of human cells in microgravity and all in realtime. This could aid in health-care decision making and contribute to preventive strategies against SANS or infection. For example, the WetLab-2 is a toolbox that will allow ISS crew members to monitor how their own gene expression changes throughout their mission in their own immune cells or track the onset of fungal or bacterial infections onboard the ISS¹⁰. The Bioculture System is a handheld engineering marvel where cells can be grown in tissue culture and experimented upon, aiding our discovery of how our individual cells function in space. The Biomolecule Sequencer will allow crew members to track if radiation from space is mutating their own genomes by performing on-board, direct gene sequencing of their own cells.

Together, these and other tools could help crew members of deep space flights to Mars and beyond respond to acute health changes during the mission, while also monitoring for signs of chronic disease or illness.

Until now, little else was known about how our cells function in microgravity. In simulated microgravity environments, evidence suggested that our immune cells can respond and adapt to changes in gravity within minutes¹¹. Analysis of gene expression in macrophages (those white blood cells that directly attack pathogens in our bodies) showed that genes involved with the structural shape of the cell membrane, including genes related to the cytoskeleton (the internal skeletal structure that gives cells their shape and anchor points during growth and development) do not change expression in microgravity.

However, several surface proteins involved with cell-to-cell communication are affected by microgravity and this may help explain why immune cells are less functional in space and have decreased inflammatory responses¹².

The oxidative burst reaction, the initial reaction of an immune cell to a potential pathogen or threat, is also decreased in microgravity¹³. Immune cells may not have the capability to ward off infections due to this slowed reaction. This could limit the protective capacity of the immune system on other worlds, or for astronauts living long-term in space. Longer research projects are needed to confirm these findings and tools like the WetLab-2 and Bioculture System will play a pivotal role.

While microgravity alters the functioning of immune cells and our physiology, the same is also true for bacteria that have hitched a ride along with the astronauts. Recent analysis of bacterial gene expression has shown that Salmonella typhimurium, a pathogenic bacterium that can harm the GI tract, adapts to microgravity by expressing genes that enable it to better interact and latch onto its environment and form dense biofilms. These changes may influence pathogenicity and harm crew efforts in eradicating an outbreak.

The Salmonella cells flown in space were then introduced into mice back on Earth and were more virulent compared with ground controls, suggesting that microgravity-altered bacteria could be difficult to treat if conditions are not monitored more effectively¹⁴.

NASA’s HRP is also studying the changes in human skin and gut microbiomes to determine if there are changes in the diversity and function of the bacteria naturally found on our bodies. Analysis of twin astronauts Scott and Mark Kelly’s microbiomes are currently underway. A recent study assessed the changes in another astronaut’s skin microbiome in moist regions of the skin while wearing a SkinSuit, a compression suit designed to protect astronauts while in space¹⁵.

Differences in bacterial diversity on the skin pre- and postflight were observed, with a distinct microbiome signature associated with the ISS that disappeared upon return to Earth. There were no changes in the number of pathogenic bacteria observed but with tools such as the Biomolecular Sequencer, these changes can be tracked in real time.

These research studies will aid in our understanding of how to prepare for life in space, as well as lift the veil on disease and aging mechanisms back on Earth. We may also be able to understand how space could influence the evolution of the human species.

It’s important to understand that while it is likely humans can individually adapt to space and microgravity conditions, these adaptations are not passed along to the next generation. It will take hundreds of generations of birth and life within a microgravity environment for space to act as a natural selector of human traits.

This process could begin as soon as a permanent colony is formed and if the development of a fetus in the womb requires certain traits for full-term development that aren’t necessarily selected against on Earth.

What is certain is that our bodies will change to our new environments and understanding these changes will aid in our ability to extend our footprint to other worlds of the Solar System, and maybe one day, further beyond.

References

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6. NASA, www.nasa.gov/audience/foreducators/stem-on-station/ditl_exercising, (Accessed January 30, 2018).

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8. J. S. Alwood et al., NPJ Microgravity 3, 5 (2017).

9. NASA, www.nasa.gov/feature/nasa-twins-study-confirms-preliminary-findings, (Accessed February 10, 2018).

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11. C. S. Thiel et al., Sci Rep 7, 5204 (2017).

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14. J. W. Wilson et al., Proc Natl Acad Sci U S A 104, 16299-16304 (2007).

15. R. A. Stabler et al., NPJ Microgravity 3, 23 (2017).

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