Spending 340 days aboard the International Space Station between 2015 and 2016 caused changes in astronaut Scott Kelly's body, from his weight down to his genes, according to the results of the NASA Twins Study, released Thursday.

The majority of changes that occurred in Kelly's body, compared with his identical brother, Mark, on Earth, returned to normal once he came back from the space station. The study results suggest that human health can be "mostly sustained" for a year in space, the researchers said.
 
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