"I've never felt completely normal in space," the now-retired Kelly said in an email to AP, citing the usual congestion from shifting fluid, headaches and difficulty concentrating from extra carbon dioxide, and digestive complaints from microgravity.
But this study was a unique dive into the molecular level, with former astronaut Mark Kelly, Scott's twin, on the ground for comparison. Full results haven't yet been published, but researchers presented some findings at a meeting of the American Association for the Advancement of Science.
A number of genes connected to the immune system became hyperactive, Mason said. Mason also spotted a spike in the bloodstream of another marker that primed the immune system. Yet Kelly's blood showed fewer of another cell type that was an early defence against viruses.
It's no surprise gene activity changes in space — it does in response to all kinds of stress.
"You can see the body adapting to the change in its environment," Mason said.
The good news: Most everything returned to normal shortly after Kelly got back on Earth in March 2016. Those immune-related genes, however, "seemed to have this memory or this need to almost be on high alert" even six months later, Mason said.
"On the whole it's encouraging," said Craig Kundrot, who heads space life and science research for Nasa. "There are no major new warning signs. We are seeing changes that we didn't necessarily anticipate but don't know if those changes matter."
From four Russians living in space for more than a year, Nasa already knew prolonged time off Earth was possible, Kundrot said, adding, "We also aim for more than just possible. We want our astronauts to do more than just survive."
Immune issues sound familiar to Jerry Linenger, a US astronaut who spent more than four months on the Russian space station Mir. He said he was never sick in orbit, but once back on Earth, "I was probably more sick than I was in my life".
Astronauts got exposed to their crewmates' germs, then after a week with nothing else new in the sterile environment of a space station, "your immune system is really not challenged," Linenger said.
- AP