Astronaut Scott Kelly's year in space did not result in any significant health problems but the length of his chromosomes changed during and after the flight, and his cognitive performance declined slightly when he returned to Earth.
These changes, along with others, were identified during the NASA Twins Study, the first of its kind, which compared Scott, who spent nearly a year aboard the International Space Station (ISS) and Mark, his earthbound twin brother and a former astronaut.
The study, which detailed the health impacts of NASA's longest-duration spaceflight aimed to fill in the gaps about the potential health consequences for astronauts who remain in space for extended periods of time, particularly as the prospect for multi-year missions to nearby planets like Mars are considered. The results of the landmark study were published in the April 12 issue of Science.
"The Twins Study demonstrated on the molecular level the resilience and robustness of how one human body adapted to the spaceflight environment," said Jenn Fogarty, chief scientist of NASA's human research program. "This study was a stepping stone to future biological space research focusing on molecular changes and how they may predict health and performance of astronauts."
Before, during and after Scott Kelly's one-year residence in low Earth orbit, an interdisciplinary team of researchers closely monitored Scott and Mark's physical and mental health. Because the brothers share the same genetic makeup, the researchers were able to better determine how the human body responds to the unique environment of space.
According to Weill Cornell Medical College researcher Francine Garrett-Bakelman, the study's lead author, and her colleagues, there is almost no human experience with space flights lasting longer than six months, and as a result the potential perils of long-duration space travel, such as those from microgravity or radiation exposure, remain unclear.
"The Twins Study is the most comprehensive view of the response of the human body to space flight ever conducted," said study co-author Susan Bailey, director of cancer biology at Colorado State University.
Throughout the duration of the study, biological samples were collected from the Kelly brothers. Samples obtained from Scott while aboard the ISS were rapidly shipped back to Earth via Soyuz resupply rockets.
Overall, the results of the Twins Study demonstrate the remarkable adaptability of the human body to spaceflight, according to Bailey. However, the analysis identified several post-flight changes that occurred in Scott, including some at the genetic and cognitive levels.
One of the most surprising results was the discovery that Scott's telomeres (the tail-ends of his chromosomes) grew longer while in space. As we grow older, our telomeres grow shorter and can be used as a biomarker for accelerated aging as well as the risk of developing age-associated diseases like dementia or cardiovascular disease.
"Certainly we imagined going into the study that the unique kinds of stresses and extreme environmental exposures like space radiation and microgravity would act to accelerate telomere loss," said Bailey.
However, Scott's telomeres were longer during the space flight than they were before or after — exactly the opposite of what was expected. Furthermore, upon returning to Earth, Scott's telomeres rapidly decreased in length. According to Bailey, the link between telomere length and spaceflight remains an open question.
The study also revealed a persistent decline in Scott's post-flight cognitive performance. University of Pennsylvania researcher Mathias Baser and his colleagues developed a suite of cognition tests specific to the spatial, emotional and decision-making functions required by astronauts. These tests were given to both brothers over the course of the study. While Scott's cognitive performance showed no change during the extent of the spaceflight, a decline was noted after he returned to Earth.
"The decline in cognitive performance after returning to Earth is a potential red flag, as it suggests that astronauts on a mission to Mars may struggle during mission-critical tasks of descending to the Martian surface and setting up camp," said Basner.
How this decline might be related to the spaceflight remains unknown. "Re-exposure to Earth's gravity after such a long time in space as well as a hectic science and media schedule after his return could have contributed to this finding," said Basner.
Due to the limited sample size, the authors emphasize that the variations identified in the study may not be attributed to space flight alone.
"We need additional astronauts on missions of various lengths so that we can validate initial findings and establish health effect baselines on the ISS, which will be of value as humans venture longer and deeper into outer space," said Bailey.
"I love being a part of the adventure and journey of space exploration, with my feet firmly planted on the ground," she added.