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Editor's choice: International Space Station science

Is colonisation of space feasible? The International Space Station (ISS) is the collaborative hub of research which aims to answer this question. Here, Editors from two open-access Nature Research journals, npj Microgravity and Scientific Reports, have brought together more than 30 free-to-access papers describing data generated aboard the ISS. This Collection spans materials science, genetics, plant science, biology of fish and rodents, and human physiology.

Human

Prolonged exposure to microgravity has a long-term effect on the perception of upright. On earth we use visual, body, and gravity cues to help us determine the orientation of ourselves relative to the world which affects many perceptual tasks including reading, recognizing faces, and navigating. Laurence R. Harris and colleagues at York University assessed how seven astronauts who spent 168 days on average on the International Space Station perceived their orientation before, during and after flight. Although no changes were observed during their missions, astronauts’ judgements in the absence of visual cues were worse upon return to earth compared with ground-based controls. Harris and his team found that the effect persisted for up to four months after the astronauts returned to earth. These findings could help develop countermeasures to avoid perceptual mistakes during space travel, and contribute to facilitating safer, long-duration journeys without gravity.

Article | Open Access | | npj Microgravity

Long-duration spaceflight increases the reactivation of latent herpes viruses in astronauts and is accompanied by a rise in stress hormone levels. This study shows that the frequency and viral loads of reactivation of Epstein-Barr virus, varicella-zoster virus, and cytomegalovirus were even greater in blood, urine, and saliva samples from astronauts staying 60 to 180 days onboard the International Space Station than has previously been observed for short-duration (10–16 days) missions. Changes in viral reactivation were also found to be associated with changes in the daily trajectory of salivary cortisol during these long-duration missions. These results indicate that the effects of the microgravity environment on the immune system are increased with prolonged exposure and highlight the potential increased risk of infection among crewmembers.

Article | Open Access | | npj Microgravity

Long space voyages could pose health risks resulting from changes to astronauts' immune systems, warn NASA scientists.A team led by Clarence Sams of NASA's Johnson Space Center in Houston analyzed blood samples taken from astronauts before, during, and after six-month stays on the International Space Station.They detected a variety of persistent changes in immune cell numbers and functions, and altered production of signaling molecules that mediate the immune response.During extended space flights, such as missions to Mars, these changes could cause medical problems such as increased susceptibility to infectious disease, allergies, altered wound healing. It is also possible that the risk for cancer or the development of autoimmune disease would also be elevated.The characterization of immune changes during flight represents the first step in determining the need for immune-specific countermeasures that would improve the health and safety of astronauts on extended space missions.

Article | Open Access | | npj Microgravity

A compression garment that applies gravity-like pressure to the skin alters the composition of skin microbes, but not in a dangerous manner. A team led by Peter Taylor from University College London, UK, characterised the bacterial skin communities at dry and moist body sites of five Earth-bound volunteers before and after wearing the Mk VI SkinSuit, which creates a pressure loading system that simulates gravity’s effects. 8 h in the SkinSuit changed the skin microbiota at the genus level but had little to no impact in community structure. The researchers observed more dramatic changes in one astronaut who wore the garment on the International Space Station. However, the microbial makeup reverted back to pre-flight profiles upon the astronaut’s return to Earth. The findings suggest that short-term SkinSuit wear is unlikely to compromise bacterial skin health.

Article | Open Access | | npj Microgravity

Astronauts on long-duration space missions lose sleep and experience misaligned circadian rhythm, say US scientists. Erin Flynn-Evans at NASA’s Ames Research Center in California and co-workers conducted the largest-ever study of sleep cycles during spaceflight, monitoring 21 astronauts both on Earth and during long stays on the International Space Station. They found that the body’s circadian rhythm—which normally responds to night and day through naturally restful and active phases—was commonly disrupted both immediately before and during spaceflight, with one out of every five sleep episodes disrupted. The team found that sleep disruption was particularly prevalent during critical mission operations, a finding that holds important safety implications. Shift work and a lack of natural light may be two factors influencing sleep disruption in space. Countermeasures such as improved lighting on spacecraft should be prioritized.

Article | Open Access | | npj Microgravity

Fish & rodent

Shuttle-era ‘mousetronaut’ homes tolerate longer space trips than expected, allowing animal experiments on the International Space Station. Eric Moyer at the NASA Ames Research Center in California and colleagues tested whether existing Shuttle-era animal enclosures, which facilitate rat and mouse experiments in space, could accommodate longer stays than their rated 20-day flight window. They found that with minor modifications to feeding hardware, the enclosures handled 35-day rat and mouse experiments, with no ill effects for animal health (e.g, carbon dioxide buildup) or astronauts (e.g, smell confinement). Animals showed slight differences in organ mass compared with controls. This work opens up the possibility of extended animal experiments on International Space Station trips, experiments which examine topics like bone and muscle degradation in astronauts undergoing extended spaceflight.

Article | Open Access | | npj Microgravity

An experiment conducted in mice offers support to the claim that long space missions may have adverse effects on the skin. Such effects have been documented by astronauts, who often complain of skin irritation and dryness after spending lengthy periods in zero gravity. Betty Nusgens and co-workers at the University of Liège, Belgium, together with scientists in Italy, sent mice to spend three months on the International Space Station. Upon the animals' return to Earth, the scientists found a 15% reduction in the thickness of the dermis – the layer of skin tissue directly beneath the skin surface – compared with mice kept on the ground. This skin thinning may result from defective collagen synthesis. The “astromice” also exhibited disrupted hair follicle growth and abnormal gene expression in the skin's muscle layer.

Article | Open Access | | npj Microgravity

Other biology

A four-year study that monitored bacteria aboard the International Space Station (ISS) has discovered multiple species, mostly of human origin, living aboard. The research, by Masao Nasu and colleagues from Osaka University and the Japan Aerospace Exploration Agency, will help space agencies assess risks to astronauts during long-term spaceflight. The team found multiple types of bacteria lived on the Japanese Experiment Module of the ISS, named Kibo, despite it being disinfected weekly. Most bacteria were of human origin (e.g. gut microbes), which were likely transferred by the astronauts. A small proportion of bacteria was of non-human origin, such as Legionella, which the authors speculate arrived on resupply materials. Since microgravity can affect bacteria in unpredictable ways, such as increasing their virulence, tracking bacterial habitation in the ISS will provide insights for future space travel.

Article | Open Access | | npj Microgravity

Researchers in Japan have uncovered the molecular genetic changes that lead to the loss of muscle mass after lengthy periods in space. Atsushi Higashitani at Tohoku University and colleagues in Japan and the UK sent nematode worms into orbit on the International Space Station and observed changes in their gene and protein expression. Compared with worms grown in a centrifuge in space that kept them at normal Earth gravity, worms grown in microgravity showed down-regulation of genes coding for proteins that make up internal cell scaffolding and attach muscle cells to the cuticle. In addition, genes controlling mitochondrial metabolism were switched to “energysaving” mode. Both changes likely contribute to the loss of muscle mass in orbit, and the same proteins are believed to be associated with muscle atrophy in human astronauts.

Article | Open Access | | npj Microgravity

A tiny worm has helped scientists to isolate potential genetic pathways linked to muscle loss suffered by astronauts during spaceflight. An international team including Atsushi Higashitani from Tohoku University in Japan subjected juvenile nematode worms (Caenorhabditis elegans) to different environments - including spaceflight and submersion in water - which taxed their growth, altering gene expression. To determine which of these genes were critical for muscular development, researchers knocked out isolated genes in fresh worms, then measured their adult body length. Two neuronal proteins, DOP-4 and UNC-8, proved to be key controllers of body size and muscular composition. Study of C. elegans has underpinned key breakthroughs in medical research, from Alzheimer's disease to diabetes; researchers thus hope that identifying key growth proteins will provide drug targets to combat muscular wasting in astronauts during space missions.

Article | Open Access | | npj Microgravity

Experiments on the International Space Station (ISS) have shown how key plant developmental hormones respond to microgravity, or not. Robert Ferl and Anna-Lisa Paul of the University of Florida studied the reporter-gene expression patterns of several genes related to auxin and cytokinin in the roots of plants grown on the ISS and on Earth’s surface. Although they saw a difference in the cytokinin response pattern in the ground control compared to the space-grown plants, there were no changes in the auxin-related genes. While auxin is central to the root’s gravity response on Earth, these experiments demonstrate that the distribution of auxin is established by developmental processes independent of gravity. Similar studies of other genes and other contexts will further clarify the role of gravity in plant development.

Article | Open Access | | npj Microgravity

Simulated gravity may help to enable proper growth of food crops in space, say Japanese researchers. Cucumbers normally develop a small, specialized protuberance at the transition between the plant’s root and stem, with gravity acting as an important environmental cue for the formation of this so-called “peg”. Hideyuki Takahashi from Tohoku University, with colleagues across Japan, investigated the morphology and distribution of a protein called CsPIN1, which is involved in mediating peg development through the plant hormone auxin. The team looked at cucumber seedlings grown on the International Space Station under either microgravity or simulated gravity via centrifugation. Centrifugation directed crosswise to the axis of the seedling led to the correct localization of CsPIN1 in the plant's cells, which could facilitate proper peg formation. The findings could help future astronauts grow food in space.

Article | Open Access | | npj Microgravity

Physical science

Recent International Space Station (ISS) chemistry experiments may improve the reliability of satellites recording Earth's climate data. Orbital monitoring of infrared radiance requires probes with extraordinarily low drift rates of less than 0.1 kelvin per decade. To achieve such precision, materials such as gallium (Ga) that undergo liquid/solid phase changes at precise temperatures are included onboard the satellite and then calibrated against ground-based standards. Shane Topham from the Space Dynamics Laboratory in Utah and co-workers have now resolved uncertainty surrounding the effects of microgravity on the temperature of Ga phase transitions. Experiments on the ISS tested how hermetically sealed Ga samples melted and froze during repeated 6 hour cycles. Only single millikelvin differences were detected between orbiting samples and those on Earth — an accuracy well-suited for remote sensing thermal reference standards.

Article | Open Access | | npj Microgravity

New research conducted aboard the International Space Station highlights the possibilities of materials processing in space. A team led by Yasuhiro Hayakawa at Shizuoka University in Japan sought a new way to process indium–gallium–antimonide (InGaSb) alloys, which are gaining attention as heat-to-electricity convertors and infrared sensors but can be impractically brittle when made using conventional methods. In a series of long-term crystal growth experiments performed under low-gravity conditions, Hayakawa's team was able to predictably control InGaSb composition. This is because, in outer space, GaSb starting materials dissolve at rates that depend on crystal orientations, producing concentration gradients. These gradients, in combination with favorable interfaces for uniform distributions of indium, offer ways to tune and manipulate InGaSb materials that are quite convoluted on Earth.

Article | Open Access | | npj Microgravity

New research shows that minimizing the effects of gravity can improve the fabrication of high-tech semiconductors. Yuko Inatomi from the Japan Aerospace Exploration Agency and co-workers investigated the growth of indium gallium antimonide (InGaSb) alloys on the International Space Station (ISS) and under standard terrestrial conditions. The team placed ‘sandwich’ samples, where a thin InSb layer sits between thicker chunks of GaSb, into a high-temperature furnace and characterized the alloy crystals formed in the mixing zone. The researchers found significant differences in crystal qualities and growth rates. Whereas samples grown on Earth had mostly curved growth interfaces, those formed on the ISS were nearly flat-a change that produced smoother distributions of atoms with a higher growth rate than typical conditions. The authors attribute the improved kinetics in microgravity to a reduction in convection forces at growth interfaces.

Article | Open Access | | npj Microgravity