NASA’s Twin Study Reveals the Effects of a Year in Space

Humans evolved on Earth — everything about us expects the familiar tug of gravity, and the body can behave in unexpected ways when sent into space. Scientists have been studying the effects of long-term space habitation for decades, but astronaut Scott Kelly’s one-year stint aboard the International Space Station (ISS) afforded them a unique opportunity to run a controlled experiment. Scott’s brother Mark is also an astronaut, but he was on Earth. The team has just published its long-awaited study on how space affected Scott Kelly.

Scott Kelly spent almost a year on the ISS in 2015 and 2016. His identical twin brother Mark, however, was on Earth the whole time. In science, you’d call Mark the “control.” Since the brothers have identical genomes, it’s easier to determine what effects living in space for a year had on Scott Kelly.

Some of the changes were expected based on past studies on astronauts. Living in space causes muscles to shrink and bones to thin, and the eyes tend to deform in subtle ways that can affect vision. NASA has astronauts exercise under simulated gravity to mitigate some of these effects, but there’s only so much you can do.

In addition to the changes in Scott Kelly’s eye, his retinal nerve thickened. It’s unclear if that will have long-term effects on his vision. The composition of his gut bacteria also shifted dramatically in space, but it returned to normal when he came home. Scientists are still unsure of how to explain that. Long-term space habitation also affects the brain. Kelly’s cognitive abilities declined when he returned to Earth, rebounding over time. However, they never returned to preflight levels. It’s unclear if this is a physical consequence of returning to Earth or a psychological one caused by the stress of constant cognitive performance tests and media events.

The researchers were particularly interested in the way Scott Kelly’s genome changed in space. After all, Mark Kelly had an identical set of DNA that didn’t spend a year in space. According to the study, Scott’s chromosomes underwent structural changes that shortened his telomeres. These “caps” at the end of chromosomes protect genes when cells divide. Telomeres get shorter as you age, eventually causing cells to stop dividing. Most of Scott’s telomeres bounced back after he returned to Earth, but they’re still shorter than Mark’s. That suggests his cells are functionally “older” than they would have been if he’d stayed on Earth.

Scott Kelly’s DNA sequence didn’t change in space, obviously, but the way the DNA is expressed by his cells certainly did. Scientists found a huge number of genes behaving differently in Scott’s immune system and mitochondria. About 90 percent of his genes returned to normal when he came back to Earth, but 10 percent are still in an altered state. We don’t currently know if these changes are good or bad — they might have no long-term effects at all.

The researchers concluded that humans can survive a year in space with no major ill effects. However, there’s still a lot to learn before people spend longer stretches off-planet. Scott Kelly is now retired from NASA, but researchers will probably be poking and prodding him on occasion for many years to come.

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