When astronaut Scott Kelly returned to Earth after nearly a year on the International Space Station, he was 2 inches taller than his twin brother, astronaut Mark Kelly. When Scott left for the mission, he and his earthbound brother were the same height.

What happened? Scientists at NASA are poring over the data compiled from the Twins Study, information that benchmarks Scott and Mark Kellys genomic and physical markers before and after Scotts yearlong mission on the space station.

So far weve learned that being in space for prolonged periods does have an effect on the chromosomes, bone structure and even the content of the culture in an astronauts gut. Scotts DNA and RNA underwent hundreds of mutations in space that gradually returned to normal once he was back on Earth.

Scott Kelly exhibited declining bone density, but a healing hormone kicked in during his exercise regimen on the space station. Still, his cognitive abilities and muscle dexterity showed signs of having slowed a bit once he returned to Earth.

That will be a factor to consider when astronauts land on Mars after six months traveling through space. Setting up their living environment on the planet will present both physical and mental challenges.

NASAs plans for manned missions to Mars in the 2030s are still in the early stages. The rival civilian space programs that have popped up are shooting for a Mars mission in the early-to-mid-2020s. Whoever gets to Mars first will be better prepared for the challenges of space travel thanks to the Twins Study.

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Another View: NASA's Twins Study offers vital sign for space travel - Press Herald