(Photo courtesy of NASA)

NASA astronauts Mark (left) and Scott Kelly.

(Photo courtesy of @StationCDRKelly - Twitter)

Astronaut Scott Kelly prepares for a spacewalk in October 2015.

Astronaut Scott Kelly is scheduled to return to Earth on Tuesday night after spending 340 days in space. He and his identical twin brother, Mark Kelly of Tucson, are participating in a study to determine the effects of long-duration space travel on the human body.

Mark Kelly spent a total of 54 days in space as an astronaut. When NASA asked if he would be part of a twins study during his brother’s near year in space, he didn’t hesitate.

"I agreed to it because NASA asked. So they asked, and I said yes. I mean I’m a big fan of science, research, and I’m incredibly grateful to NASA and the U.S. government to give me the opportunity to have an incredible career there for 15 years, so it didn’t take much," said Mark.

What Kelly signed up for was being a research subject, along with his brother, to be scrutinized by scientists from a dozen universities from the United States and abroad. They’re looking for physiological and psychological changes that nearly a year in space might have caused.

"In general, there’s like 10 major experiments. A lot of them are molecular and genetic with regards to Scott and I and our genomes and our genes and our mitochondria and molecular things like as an example one of them looks at the length of our telomeres, and telomeres are I think it’s part of our DNA," said Mark. "I’m not even sure, I don’t have to be the scientist here, I just have to give the samples. And it’s looking at the effect of being in space on the age of the human body. You know, there’s your age according to the calendar but then there’s your physical age. And those are not always the same. And the length of your telomeres is indicative of your physical age."

Telomeres are the caps at the end of each strand of DNA that protect chromosomes, like the plastic tips at the end of shoelaces. Because the Kellys are identical twins, their DNA, including genes, mitochondria and telomeres, are mostly identical. Or, they were nearly identical before Scott’s time in space. The research will tell if that’s still true.

Scott Kelly spoke about the research in a press conference from the International Space Station last week.

"One part of it is kind of a new area that NASA is getting into with this study is the effects of space flight on a genetic level. And so that’s something I’m pretty excited about for personal reasons but also for the research to try to have a better understanding how this microgravity environment and the radiation environment affects us genetically," said Scott. "There’s a lot to learn and, as we’ve seen over the last 10 years, that discipline of science has really just taken off, so it’s great to see us starting to focus on that in space as well.

Mark said he is interested in learning about vision problems that long-mission space men have experienced. And “men” is the operative word here.

"Other experiments have to do with astronauts’ vision. It’s interesting, I think it’s about 30 percent of male astronauts that spend a long time in space have a serious degradation in their vision due to a change in their optic nerve," said Mark. "For women, not the same amount. It’s zero percent. So it hasn’t happened to a single woman yet. So maybe when we send astronauts to Mars for the first time it will be a crew of women. Who knows."

Mark said these 12 months are not the longest time he has been away from his younger brother — younger by 6 minutes.

"This is not the longest I’ve gone without seeing him, and he’s been in space for nearly a year. I was stationed in Japan for 2 1/2 years I think at that point, we probably went a stretch of about maybe a year and a half," said Mark.

The current separation has been softened by regular communication.

"People are like, well, 'What is it like for him to be off the planet for a year? This must be really hard on you,'" said Mark. "Nah, it’s not hard on me. I talk to him on the phone about every day."

Mark said he and a number of family members are in Houston on Tuesday as the Soyuz spacecraft will return Scott and his Russian crewmates to Kazakhstan. They’ll be reunited Wednesday in Texas.

"He’ll land in Kazakhstan, he’ll go take a shower, he’ll do some medical tests, he’ll get something to eat and then he’ll be on a NASA airplane coming home," said Mark.

Mark said he misses space travel and would have liked to have had a long flight — just not one lasting for a year.