A study of US astronaut Scott Kelly and his identical twin suggests human health is not significantly affected by longer missions to space.
Scott spent nearly a year in orbit on board the International Space Station between 2015 and 2016 while his brother Mark remained on Earth.
Using data collected from both men before, during and after the 340-day mission, US scientists carried out 10 separate investigations.
Their findings, published in journal Science, detail the human health impact of Nasa’s longest human spaceflight, with several changes identified in Scott which were not seen in his brother.
Some of these effects persisted after his time in space but many subsided in the following six months.
“Given that the majority of the biological and human health variables remained stable, or returned to baseline, after a 340-day space mission, these data suggest that human health can be mostly sustained over this duration of spaceflight,” the authors wrote.
Studying the identical twins, who were 50 years old at the start of the study, gave scientists a unique opportunity to compare individuals with the same genetic material subjected to different environments.
Exposure to radiation, lower gravity, less exercise, a restricted diet and disrupted sleep are among the possible risks of spending time in space.
Mark is a fellow astronaut and has also been to space before, but not for almost four years prior to Scott’s mission.
“The twins study is certainly the most comprehensive view that we’ve ever had of the response of the human body to space flight,” Professor Susan Bailey, from Colorado State University, said.
Blood samples, physiological data and cognitive measurements were taken from the brothers at regular intervals over 25 months.
While Scott was in space, his samples were sent back to Earth with shipments that had delivered supplies by rocket to the ISS.
Changes to the shape of Scott’s eyeball, including a thicker retinal nerve, were among those detected, as well as a decline in some of his cognitive abilities.
The scientists also found that the astronaut’s telomeres – the protective caps on the ends of chromosomes – got longer while he was in space.
Telomeres typically shorten as someone becomes older and are associated with cardiovascular diseases and some cancers.
However, many rapidly decreased in length after the flight, and he now has more short telomeres than long ones.
There were also changes in the expression of some of Scott’s genes, particularly those related to the immune system.
However, more than 90% of these had returned to normal six months after the flight.
Persistent changes were observed in a few other areas, including some cognitive function.
“There are dramatic changes, there are thousands of genes and molecular changes, that occur as someone goes in space,” Dr Mike Snyder, from Stanford University School of Medicine, said.
“I think it’s reassuring to know that when you come back things will largely be back to the same.”
Scientists believe the findings could help predict possible health complications during long-term missions. “If we know what to expect, we can anticipate health problems astronauts may encounter and ensure that medicines and other remedies are at hand during a mission,” Dr Andy Feinberg, at Johns Hopkins University, said.