Studies of twins are a favorite tool of scientists to understand environmental influences, and having the only identical twins to have flown in space as human guinea pigs opens physiological and psychological research opportunities for NASA. The space agency has budgeted $1.5 million for the 10 three-year studies, which were first proposed by the Kellys.

Dr. Feinberg said he would perform a full genome analysis of both men to study any epigenetic effects — that is, how the environment changes the genes and their function. Mark Kelly, who like his brother is a retired United States Navy captain, has been to space four times, as a shuttle pilot and commander, ending his last mission in 2011. He is married to former Congresswoman Gabrielle Giffords; after an attempt on her life in 2011 and her resignation from Congress the next year, they formed Americans for Responsible Solutions, an advocacy organization working to keep guns out of the hands of the mentally ill, criminals and terrorists.

On a recent visit to The New York Times, the Kelly brothers, who are 50, showed that whatever your accomplishments in this world or off it, you’re still kind of a knucklehead when your brother is around. When the question of primacy was raised, Mark Kelly noted that he was six minutes older than his brother. But Scott Kelly was quick to point out that he was the first to space, “and I’ll be the last.” When a photographer asked one of them to stand slightly in front of the other for a portrait, they jokingly elbowed each other to be the one out front.

Scott Kelly said that an extended expedition aboard the space station is easier now than in the past, with decent access to email and telephone communications as well as entertainment programming, exercise equipment and good air quality. (The old Russian space station Mir apparently was smelly, humid and moldy.) Scott Kelly said that on his previous mission to the station, “I had a little bit of degradation” physically, “which is good.”

Knowing that his body is likely to change during this longer stay is promising, he said.

Their enthusiasm for the experiment has startled NASA scientists. “I volunteered to have them put a pressure probe in my skull,” Scott Kelly said. “I thought it would be kind of cool to have a little bolt,” he said, tapping a spot on his head. The scientists decided that it might cause problems, and demurred.