Scott landed in March 2016, and it appears his body has yet to return to normal. Some of the genes that seem to have changed permanently involved DNA fix, bone formation and how the cells use oxygen.
Still twins? Astronauts Mark, left, and Scott Kelly.
The long-term effects of space habitation are still unknown and the space agency said the experiment was a stepping stone for its mission to Mars. While Scott was away, experts monitored the brothers' DNA. Researchers say 93 percent of Kelly's genes returned to normal after he came back to Earth two years ago, but the remaining 7 percent suggests long-term changes related to his immune system, DNA fix, bone formation networks, hypoxia (deficiency in the amount of oxygen that reaches tissue) and hypercapnia (excessive carbon dioxide in the bloodstream), according to preliminary results from NASA's Twins Study.
"We've seen thousands of genes change. This happens as soon as an astronaut gets into space, and the activity persists temporarily upon return to Earth".
- After a year in space, an astronaut's genes were transformed, and no longer matches his twin, according to a NASA study.
"The change related to only 7 percent of the gene expression that changed during spaceflight that had not returned to preflight after six months on Earth".
The study showed no cognitive difference between Scott and Mark after being on the space station.
Scientists found that Scott Kelly, who set the record for most consecutive days spent in orbit, underwent an "unexpected" genetic change.
NASA is expected to release an integrated summary later this year, including the possible impact the findings will have on future space travel.