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Story highlights

Although Scott and Mark Kelly both retired from NASA, they miss spaceflight

Their groundbreaking Twins Study is lending new material to the field of genomics

(CNN) —  

Although they never flew in space together, twins and retired astronauts Scott and Mark Kelly both understand why the other one misses viewing Earth as a big round ball in space from the window of a shuttle or the cupola of the International Space Station.

Arriving in Los Angeles the other day, Mark Kelly saw the Space Shuttle Endeavour again on display at the California Science Center. It’s the shuttle that took him on his first mission into space, the one he also commanded on its last flight in 2011.

It was a bittersweet and nostalgic moment for the retired astronaut. “I just wanted to roll it to the launchpad and go up for one last time,” he said, speaking on a panel with his brother and CNN Chief Medical Correspondent Dr. Sanjay Gupta at the International Space Station Research and Development Conference in San Diego.

“I won’t go to space again with NASA, but I would never rule it out,” Scott said, referring to commercial spaceflight. “The best part of being an astronaut wasn’t just the launch or the landing or looking out the window – those things are great, but it’s really hard working on something so challenging and doing it safely and successfully. That’s what motivated me.”

Mark Kelly spent more than 50 days in space, while his twin, Scott, logged more than 520 days, the most for any astronaut in the history of U.S. spaceflight. Mark retired from NASA in 2011 and Scott retired in April after completing his year in space.

Scott Kelly answers questions about life in space

Because Scott spent so much more time in space than his twin, who remained on Earth for the 340-day space mission as a control for their Twins Study, Scott joked about the fact that he came back temporarily 2 inches taller and 5 milliseconds younger, with a smaller heart.

Space was a dream that was a long time coming for the Kelly brothers, especially Mark. “As a kid, I wanted to be the first person to walk on Mars,” he said. “And as a Navy test pilot, it’s like the next step in aviation. There is no job that I can think of that is more exciting than being an astronaut. Both of our parents were police officers and public servants, so I wanted to serve, and have a job that I just wanted to go to work every single day.”

The twins, who both served as test pilots in the Navy, applied for the astronaut program without telling each other at first. It’s a general assumption when test pilots reach a certain point in their career that they will apply, Scott said, so it wasn’t something they discussed.

“I didn’t want any extra competition,” Mark joked.

That is, until Mark received the call for an interview. He asked if he could borrow Scott’s suit and wore it for his interview and week of testing. Months later, when Scott received the same call for an interview for the astronaut program, they had a similar phone call.

“I told him he had to buy me a new suit because I couldn’t go down there in the same suit,” Scott said. “He refused, although I wore his shoes. So I have the only suit that has been worn by two people who became astronauts.”

Scott worried that he might mess up both of their chances because he interviewed after his brother, but both were accepted and they are the only siblings who have both traveled in space, although never at the same time.

In the brothers’ groundbreaking Twins Study, researchers at 10 universities are using samples from Earth and the ISS to study where Scott and Mark genetically diverge after Scott’s year in space. It has led to what Twins Study researcher Dr. Andrew Feinberg refers to as the “inaugurated year of space genomics.” Genomics is the sequencing and analysis of a genome, or as Feinberg describes it, “DNA are the words of your genome and chemical changes are the grammar that tells DNA what to do.”

Because chemical changes in DNA are affected by your environment, it’s particularly eye-opening for researchers looking at how microgravity might have changed Scott’s DNA. Observing those changes could not only help improve the environment of spacecraft for long-term flight missions to deep space, like a Mars mission, but also help reduce or mitigate cancer risks for people on Earth. Many of the experiments on the ISS have resulted in impacts for medicine and technology that can be used on Earth.

Scott’s year in space wasn’t without risk to his health, the effects of which may not be known for years. But psychologically, spending 340 days away from loved ones and in such a tight space, while always working, can have lasting effects as well.

NASA flight surgeon Dr. Stevan Gilmore, who worked with Scott on both his 6-month and yearlong missions to the ISS, commended his ability to keep a positive attitude throughout. Scott was only given 4½ “days off” apart from the breaks NASA builds into his day, but unlike a day off work on Earth, he was still technically on the job.

“Scott was a great example of taking things day by day,” Gilmore said. “He started this journey of hundreds of millions of miles around the Earth with one step, and even when we would talk about planning things that were months out, he would refocus back to that day.”

It helped Scott maintain his enthusiasm for duration of the mission.

Besides the obvious fun they have joking with one another, Scott and Mark’s bond as twins was also apparent in the way they’ve supported each other during their respective missions in space, no matter the duration of the flight. They understand each other, what the other one is experiencing during spaceflight and their worries about what they’re missing on Earth.

And they understand that spaceflight is risk versus reward.

“We often think about the risk of flying in space as the liftoff and the landing, especially with what happened to the Challenger and Columbia missions, but when you analyze a space shuttle flight, statistically, the riskiest time is actually being in space,” Mark said. “To be in space for a year on the ISS, you also run the risk of being hit by orbital debris or a meteorite or dealing with an ammonia leak. But I also understand the risks of landing. There probably aren’t many people Scott can talk to who understand what’s happening on the space station, or even just the numbers, like I do.”

But for the brothers, not being able to come home if something happens to their loved ones was the greatest challenge.

“What I thought about more than anything was the case where something happens to somebody on the ground – your kids, my sister-in-law Gabby Giffords – and you can’t come home,” Scott said.

In 2011, Scott was on the ISS when Mark’s wife, former Rep. Gabrielle Giffords, was shot during an assassination attempt. Although he couldn’t come home, he was on the phone with his brother and their family every day for weeks afterward.

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During Scott’s year in space, he spoke with his brother on the phone almost every day. They talked about everything from the space program to football to politics. One conversation remains amusingly memorable for both of them.

“I was on the phone with him about three months after he went up and I was walking down the street in New York, telling him I was on a trip and wouldn’t be home for three weeks and that it was such a long time,” Mark said. “I forgot who I was talking to for a minute. He said two words, and the first one started with the letter F.”