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When astronaut Scott Kelly's space capsule 

, it was a familiar scene to Mark Kelly, who is a retired astronaut and Scott's identical twin.


NASA is conducting a "twin study" on the brothers to explore what spaceflight does to the body. Multiple universities are involved in the research.


"We're going to need to understand this really well before we decide to send people to Mars on an extended mission," Mark Kelly tells NPR's Renee Montagne on 

. For the study, he acts as a control to compare to his twin, who broke the U.S. record for longest space journey.


Kelly says one part of the study will look into the impact of space on aging. "One of the universities is studying a genetic indicator of how people age, and that's the length of your telomeres. So the length of your telomeres is indicative of your physical age," he says.

Researchers are looking into whether that physical age is linked to the significant amount of radiation that astronauts are exposed to in space, Kelly says. He adds: "I'm not the scientist here, but I would presume the radiation would have a negative effect."

, "The most dangerous aspect of traveling to Mars is space radiation. On the space station, astronauts receive over 10 times the radiation than what's naturally occurring on Earth."


While his brother was in orbit, Kelly was providing NASA with his data for the study, including MRIs, ultrasounds, and blood and urine samples.


One effect of the extended time in space has already come and gone, Kelly says. His brother was temporarily 2 inches taller:


Kelly also talked about how his brother is feeling now that he's back on Earth, beyond the shifting height and the unseen effects of radiation:

Science

Kelly says his twin brother, Scott Kelly, who just returned to Earth after 340 days in space, was temporarily 2 inches taller. NASA is studying the pair to explore what spaceflight does to the body.

'Everybody Stretches' without gravity: Mark Kelly talks about NASA's twins study

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