A study of Kelly and his identical twin brother found that spending nearly a year in space significantly changed the astronaut's DNA.

Kelly spent 340 straight days aboard the International Space Station from 2015 to 2016. When the NASA veteran returned to Earth, researchers immediately noted that he had grown two inches in height. A new study comparing Scott to his identical twin, Mark — who is also a NASA astronaut and stayed on Earth during the 340-day trip — has revealed that long-term space travel alters more than just your height.

"Scott's telomeres (endcaps of chromosomes that shorten as one ages) actually became significantly longer in space," NASA researchers wrote in a statement. The space agency added that Kelly had hundreds of "space genes" activated by the year-long flight which reportedly altered the astronaut's "immune system, DNA repair, bone formation networks, hypoxia, and hypercapnia."