https://orcid.org/0000-0003-2249-5203
Current space missions and the future ahead
The International Space Station (ISS), the largest man-made object in space, is a collaboration between space agencies of the United States, Canada, Russia, Europe and Japan. This research laboratory circles around the Earth at about 400 km above the Earth’s surface and houses international crews around the clock to perform experiments ranging from the effects of microgravity on the physiology of humans and other organisms, the cultivation of plants and food crops in space, to astronomy and physics observations. Since its inception, more than 200 men and women have inhabited the ISS for different lengths of time. Individual crew members stay in the ISS for missions of a total of about 3 months to a year, while some people have completed multiple missions.
Nearing the end of its life, the ISS is expected to fulfill its duties until about the year 2030. With the ending of the ISS approaching quickly, plans are made for manned missions deeper into our solar system, such as to the moon, other near-Earth objects such as asteroids, and even the planet Mars. Currently, the National Aeronautics and Space Administration (NASA) aims to begin operating in the cis lunar space in the 2020s and to build the Space or Lunar Gateway, a space station orbiting the Moon and allowing missions deeper into space. Then, NASA is tasked to complete manned missions orbiting Mars in the 2030s, with the final goal of crew expeditions to the surface of Mars.
During missions into deep space, men and women will be exposed to a combination of stressors related to the nature of the space environment. Moreover, missions may be much longer than the current stays of astonauts at the ISS. In order to make future manned missions into deep space possible, these stressors need to be well understood and controlled or minimized by physical and/or medical means. Complicating the matter, during deep space missions, crew members cannot return to Earth for emergency medical attention. Therefore, health risks need to be well understood and appropriate medical facilities should be incorporated into mission planning.
Modeling the environmental stressors of deep space
Some effects of space travel can be studied on human subjects onboard the ISS or other habitats in space. Moreover, small experimental animals can be housed onboard the ISS and studied during and after flight to provide additional insight into the physiology of space travel. However, as described below, in several aspects the environment in deep space will be different from the ISS. Such environmental factors need to be modeled in ground-based experiments. To make matters more complicated, these factors do not act alone but coexist to influence each other in their biological effects.
This special issue contains both review/opinion articles and original research reports discussing currently available models of some of the main stressors of the environment in space and how the research outcomes could assist in understanding the risks and identifying countermeasures to reduce them.
Environmental stressors of manned missions into deep space
Around the world, research is being performed to improve the safety of space missions. The NASA Human Research Roadmap (https://humanresearchroadmap.nasa.gov) provides an outline of the main risks of manned space travel (currently, 24 risks are identified) and the gaps in our understanding of each of these risks. NASA identifies each of the risks as green (well understood and can be managed with physical or medical countermeasures or other methods), yellow (somewhat understood) or red (not yet understood and no approved countermeasures available). This roadmap continues to be updated as research progresses, forming a guide for additional studies that need to be performed to make manned space missions safer and missions into deep space possible. The paragraphs below provide a brief introduction into some of the main risks and environmental factors specific to space that can have adverse effects on human health, with a reference to articles addressing these topics in the current special issue.
Ionizing radiation
The presence of ionizing radiation in our solar system is ubiquitous. A large portion of this radiation is in the form of high-energy charged particles that can be damaging to equipment as well as the human body.
During largely unpredictable solar particle events, the sun erupts large amounts of high-energy protons. While the regular hull of a space craft may shield the crew from most of these protons, one is not protected from proton exposures during extravehicular activities. Some solar particle events can lead to high dose rates of protons that can potentially lead to severe acute radiation sickness.
Galactic cosmic rays (GCR) consist for a large part of high-energy ions, from the size of helium up to iron ions that originate from outside our solar system and form a constant presence in space. While the ISS is largely protected from GCR exposures because it circles within the magnetic fields around the Earth, exposures to GCR will be higher in missions beyond low-Earth orbit. Moreover, it is much more difficult to physically shield crew members and equipment from these forms of ionizing radiation, at least with the currently available materials. Although the dose rates of GCR exposures are low, there are concerns about adverse effects on health, including increased risk of cancer and long-term adverse remodeling in various organ and systems, referred to as degenerative tissue effects.
NASA has set permissible exposure limits (PELs) for its crew members, which are currently largely based on epidemiological evidence of radiation carcinogenesis and some degenerative tissue effects of scenarios of exposure to ionizing radiation on Earth. Since radiation damage can be carried lifelong, PELs are based on the accumulative radiation exposures in each astronaut’s lifetime. Future long-distance space travel will require experienced crew members. Therefore, if PELs are set unreasonably low, many of the astronauts with the most experience may not be allowed to take part in deep space missions. Moreover, women are currently bound to lower PELs because the additional risk of breast, ovarian or uterine cancer compared to men.
In order to adjust the PELs based on relevant information concerning risk, research is needed to understand the biological effects of space-relevant radiation exposures, determine which clinical outcomes seem most relevant, and identify potential countermeasures against such effects of space radiation. In this issue, Chancellor et alFootnote1 provide an insight in the characteristics of ionizing radiation in space and the challenges of modeling these radiation exposures in ground-based studies. Davis, Allen and BowlesFootnote2 describe current animal models of degenerative tissue effects of space radiation on the cardiovascular and central nervous systems.
Microgravity
At the altitude at which the ISS circles the Earth, gravity is still about 90% of that experienced on the Earth’s surface. However, because the ISS is essentially in a state of continuous freefall, gravity experience onboard the ISS is minimal and referred to as microgravity. Microgravity will also be experienced by crew members in space crafts that go beyond the gravity pull of the Earth. Although research is performed to build artificial gravity onboard space crafts, this technology is not yet available. Missions to the surface of other space objects will also be associated with different levels of gravity compared to the Earth. Gravity on the lunar surface is only about one sixth that of the Earth, and gravity on the surface of Mars is roughly one third.
Because the human circulatory system is designed to counter the level of gravity experienced on Earth, prolonged human existence in microgravity leads to a shift of fluids to the thorax and head. Well-known physiological effects of microgravity are changes in cardiac morphology and the Spaceflight Associated Neuro-ocular Syndrome (SANS), caused in large part by an increased pressure on the eyes. SANS can potentially lead to permanently reduced vision. Prolonged exposure to microgravity may also lead to muscle atrophy and bone demineralization, which in turn increases the risk of kidney stones due to increased levels of calcium in the circulation. Research onboard the ISS has shown that these last adverse physiological effects may be counteracted at least in part with regular exercise on equipment designed to provide the proper resistance when onboard a space craft. However, other adverse effects of long-term microgravity are not yet well understood or controlled.
In this issue, Mao and WilleyFootnote3 established a team of authors to discuss the biological effects of microgravity and how microgravity may interact with radiation exposures to change physiology.
“Omics” in the assessment and prediction of health consequences from environmental stressors
An early assessment of adverse health effects of environmental factors can be beneficial in designing early treatments or administering appropriate countermeasures. Therefore, studies are underway to develop biomarker assays that may identify an individual’s risks. Circulating extracellular vesicles, RNA molecules, proteins and metabolites may all be indicators of cellular physiology and could be used as a read-out of risk and disease progression. Since metabolomics-based assays are robust, rapid, cost-effective, and require minimal sample processing and small sample volumes, some of the articles in this special issue address metabolomics-based assays in the assessment of radiation effects.
This issue provides two research articles by Cheema and colleaguesFootnote4 as examples of the application of animal models of exposure to modeled space radiation to identify metabolic biomarkers that may predict early and late adverse biological effects.
Additional aspects of deep space missions
Manned missions into deep space carry several more risks than can be addressed in this special issue. For instance, our understanding of the complexity of the human diet, and macro- as well micronutrients that are required for long-term health, is rapidly increasing. It is important to incorporate that understanding in the careful design of a flight crew’s diet. However, diet in space is limited by the volume and weight that can be carried on any mission. As future missions will likely increase in duration, shelf-life and payload limits may become a significant impediment in the types of nutrition that can be given to the crew. Moreover, in longer duration missions, any prolonged dietary deficiencies may become more apparent and detrimental. Research is being performed to develop novel strategies for crew diets, in addition to developing strategies to grow crops in space.
It has become apparent that in time, space crafts become inhabited by a microbiome that is different in composition and may contain microorganisms with a different virulence compared to common human habitats on Earth. The exact causes of these microbial changes in the space craft environment and their effects on human health and infectious diseases are largely unknown.
Lastly, a concern of deep space missions is the psychological stress of being far from friends and family, the absence of a day-night cycle as experienced on Earth, and having to co-exist with only a few other human beings in a confined space for prolonged periods of time. Human interactions and behavioral changes are extensively studied in space analog to research facilities on Earth.
Altogether, these are exciting times as we discover more and more of every corner of our solar system. While gains in knowledge and technological advancements make manned missions of longer duration and deeper space possible, research into understanding and reducing health risk factors is still required. Eventually, we hope that some of these research findings will also contribute to our understanding of physiology and health risks on Earth.
Division of Radiation Health, University of Arkansas for Medical Sciences, Little Rock, AR, USA
* [email protected]
Igor Koturbash
Department of Environmental and Occupational Health, University of Arkansas for Medical Sciences, Little Rock, AR, USA
Notes
1 Request production to hyperlink to JESHC-2020-0059
2 Request production to hyperlink to JESHC-2020-0067
3 Request production to hyperlink to JESHC-2020-0053
4 Request production to hyperlink to JESHC-2020-0051 and JESHC-2020-0052