For decades, scientists have studied twins and triplets to measure how human characteristics are influenced by heredity and to what degree they can be altered by the surrounding environment.
But never have they tested an environment so unusual as the one described in a new study published Thursday: 340 days aboard the International Space Station.
Astronaut Scott Kelly was subjected to a battery of biological tests before, during, and after his voyage while his identical twin, Mark, a retired astronaut, underwent the same tests on Earth.
Ten teams of researchers detected a variety of subtle changes in the physiology of the space-bound twin, they reported in the journal Science. Most of Scott Kelly's test results returned to the level of his twin brother after the mission ended in March 2016, and he remains in good health -- offering hope that astronauts someday could endure a much longer mission to Mars.
But Scott Kelly's performance on one of the 10 areas evaluated revealed a possible area for concern. On several cognitive tests designed at the University of Pennsylvania, his accuracy slipped slightly during the mission, then dipped even more upon his return, said Mathias Basner, an associate professor at Penn's Perelman School of Medicine.
With just two participants, the NASA-sponsored study was not designed to reveal any grand statistical truths. The effort was largely about working out the complex logistics for future such studies with larger groups of astronauts. Still, authors said the results were intriguing. In addition to the cognitive changes, Scott Kelly also developed a thicker wall in his carotid artery, an altered population of bacteria in the gut, and longer telomeres -- the "caps" on the ends of each strand of DNA.
That was a surprise, as longer telomeres are thought to be connected to a longer lifespan, and they typically grow shorter with the passage of time, not longer.
Not that space travel should be viewed as a youth tonic, as Scott Kelly's telomeres shrank back to their previous lengths after his return, said Colorado State University professor Susan M. Bailey, one of the study's senior authors along with Basner. Still, the results were provocative.
"If we can figure out something that is helping to maintain telomere length, that could help all of us here on Earth," Bailey said in a Colorado State video interview.
As for the cognitive tests, they were challenging, created specifically for astronauts, and Scott Kelly's level of decline was nowhere near enough to matter in everyday life, said Basner, who collaborated with Penn colleagues Ruben Gur and David Dinges.