Since its establishment in 1958, the United States National Aeronautics and Space Administration (NASA) has been 
on the forefront of space exploration. Spaceflight programs launched mankind into frontiers previously unknown – from the historic Moon landing to the creation of the International Space Station (ISS) in Earth’s orbit. As NASA plans future manned missions to Mars, expeditions that could last two to three years, it is critical that we understand how to best maintain the health and safety of astronauts navigating the extreme space environment.
NASA’s landmark Twins Study observed the identical twin astronauts, Scott and Mark Kelly over the course of two years. Scott spent a year in low-Earth orbit aboard the ISS, while Mark remained on Earth. The twins’ identical genetic profiles controlled for many of the physical variations that occur between people, enabling scientists to explore the effects of the extreme environment of space on the human body. More about the initiative and its aims can be found in our previous article (NASA’s Twins Study) in Volume 5, Issue 1. Results from the Twins Study were recently published in the peer-reviewed journal Science. The integrated paper encompassed work from ten research teams, providing a comprehensive and multidimensional description of physiological, molecular, and cognitive changes brought on by a long-duration space mission.
Key results included alterations in at least ten physiological processes. Changes in gene expression occurred during 
Scott’s time in space, as his body reacted to the varying surroundings. Previously dormant genes were activated, including genes involved in processes related to DNA repair, collagen production, and the immune response. Some DNA damage also occurred, likely attributable to increased space-related radiation exposure. 91.3% of the genes, however, returned to pre-spaceflight expression levels within 6 months of landing. This observation demonstrated the body’s resilience and adaptability to conditions in space and will help researchers understand the role of environmental stressors on gene activity.
Spaceflight similarly manipulated microbiome composition; the collection of bacteria inhabiting the gut underwent taxonomic and functional shifts during the mission. The gut microbiota plays a role in maintaining health and preventing disease, and microbial community imbalances may leave individuals more susceptible to illness. Space-specific factors such as microgravity, radiation exposure, and an altered diet may have induced the changes, but it is difficult to identify the exact cause. Scott’s microbial diversity remained consistent throughout the study and his return to Earth restored normal bacterial relative abundances. These results were promising for future long-duration flights, suggesting that microbiome health could be sustained over prolonged missions.
To test the immune system, the flu vaccine was administered. Notably, this was the first time an astronaut had ever received one in space. The vaccination rendered the desired effect – it worked in providing influenza immunity. This 
finding is critical because it offers proof of a properly functioning immune system. Should preventative or therapeutic measures need to be taken, doctors can protect astronaut health during space operations.
“The Twins Study will serve as the guide for future biomedical space research to ultimately prepare us for further space exploration.”
Unexpectedly, the telomeres in Scott’s cells lengthened over the course of his time in space. Telomeres protect our chromosomes, which contain the genetic instructions for the cell. Human telomeres are biomarkers of ageing and usually shorten with time, the rate of which is influenced by lifestyle and environmental factors. Scott’s telomere behavior, therefore, was the opposite of what scientists had initially predicted. Upon Scott’s return to Earth, his telomeres quickly compressed to about pre-flight lengths, but follow-up assessments revealed many were critically shortened or lost. This could be concerning, considering the association with age-related diseases, and warrants further investigation to identify potential long-term risks of prolonged space exposure.
The Twins Study’s integrative approach was unprecedented and launched NASA into the “omics” realm of space 
travel research. By combining multiple biological disciplines, it produced a thorough analysis of how the human body responds to long-duration spaceflight. The study revealed that human health can be sustained over one year in space. It is important to note that the research encompassed observations of a single participant aboard the ISS (test subject) and single participant on Earth (control subject). The opportunity to study identical twin astronauts was extraordinary. While we may not be able to replicate the experimental conditions, the study’s findings indicate areas for prospective focused research and provide the basis for developing interventions to potentially hazardous physiological changes. The Twins Study will serve as the guide for future biomedical space research to ultimately prepare us for further space exploration.
Noelle Yee
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