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For decades, scientists have studied twins to measure how human characteristics are influenced by heredity and to what degree they can be altered by the environment.

But never have they tested an environment so unusual as the one published in the journal Science: 340 days aboard the International Space Station. Astronaut Scott Kelly was subjected to a battery of 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. Most of Scott Kelly’s test results returned to the level of his twin after the mission ended in March 2016, and he remains in good health — offering hope that astronauts could endure a longer mission to Mars.

But his performance on one of the 10 areas evaluated revealed a possible area for concern. On several cognitive tests, 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. However, his decline was nowhere near enough to matter in everyday life, Basner said.

With just two participants, the NASA-sponsored study was not designed to reveal grand truths. Still, authors said the results were intriguing. Scott Kelly also developed a thicker wall in his carotid artery and an altered population of bacteria in the gut.

One of the most dramatic findings concerned epigenetics — how genes are turned on or off to produce proteins. Gene expression changed in both Kellys but in significantly different ways. The study found that more than 90 percent of Scott Kelly’s gene expression changes reverted to normal when he returned to the surface.

His telomeres — structures that protect the ends of chromosomes and erode as part of the natural aging process — lengthened in space. But the telomeres shortened when he returned to Earth. And months later, he still showed a slightly elevated number of cells with shortened telomeres, possibly an effect of radiation exposure. “He might be at some increased risk for cardiovascular disease or some types of cancer,” said Susan Bailey, a biologist at Colorado State University. “If we can figure out something that is helping to maintain telomere length, that could help all of us here on Earth.”

Mark Kelly is six minutes older. But Scott Kelly said he’s a few milliseconds younger still, due to having spent 500 more days in space than his astronaut brother. Einstein’s special theory of relativity leads to a “twins paradox” in which someone moving at a high velocity, such as 17,500 mph in low Earth orbit, ages more slowly than a twin on Earth.

“I look younger than he does,” Scott Kelly said. “He’s busy running for office. His telomeres are going to be way worse than mine. I’m not worried. I’ll be on a beach in the Bahamas, and he’s going to be hopefully in the U.S. Senate. He’s going to age much faster than me.”

The Washington Post contributed to this report.