After 340 days aboard the International Space Station, American astronaut Scott Kelly returned to Earth in March 2016.
Nasa has since undertaken tests to study the effects living in orbit had on Kelly using his identical twin brother Mark – who remained on Earth – as a control subject.
The Kelly brothers have nearly identical genomes, allowing for an unprecedented look at the physical effects of long-term spaceflight.
Blood and other biological samples were collected from the pair before, during, and after Scott Kelly’s mission.
The agency found that Kelly came home 5 cm (2 inches) taller than his twin – a change had resolved itself within two days of his return.
The height difference was caused by the ISS’ microgravity conditions which elongate the spine – but the effect was only temporary.
Nasa found that while 93 per cent of Kelly’s genes returned to normal shortly after returning home, seven per cent were permanently altered.
These long-term changes hit genes related to the immune system, DNA repair, bone formation and the ways his tissues take up oxygen and carbon dioxide.
Kelly’s telomeres – the caps at the end of each chromosome – lengthened while in space.
Telomeres are key to protecting DNA from damage and tend to shorten with age. Kelly’s telomeres shortened again once he was back on Earth.
Nasa says that Kelly’s lengthening telomeres are linked with his diet and exercise routine on the station.
The ratio of two groups of gut bacteria shifted while Kelly was in space, likely due to his change in diet. This also returned to normal shortly after his return.
Nasa research has spotted hundreds of diverging genetic mutations in Kelly and Mark’s genomes.
The research team speculate that a ‘space gene’ could have been activated while Kelly was in orbit.
‘When he went up into space it was like fireworks of gene expression,’ Christopher Mason, a principal investigator on the NASA twins study and an associate professor at Weill Cornell Medical College, told Business Insider.
‘But the changes that seem to have stuck around include changes in immune system function and retinal function related to his eye health.’
Kelly said he was surprised by that change in a recent interview with Marketplace.