U.S. astronauts Scott Kelly and Mark Kelly are the first and only identical twins to have travelled into outer space. Scott spent a year on the International Space Station (ISS), the longest mission in outer space by any NASA astronaut. Mark, who retired from NASA in 2011, stayed on Earth. Because the Kellys have virtually the same genetic material, NASA can study how long-duration space flight affects the body and the mind, using Mark as the control. One of the results? Scott is now two inches taller than Mark! On earth, the discs in the spinal column are slightly compressed due to gravity. In space, that compression is no longer present causing the discs to expand. The result: the spine lengthens, and the astronaut is taller.
Human bodies did not evolve to float in microgravity or to thrive under the radiation levels in space. When NASA astronaut Scott Kelly spent nearly a year on the International Space Station, in a mission launching in 2015, his body was put under incredible stress: Fluids swelled his upper body and head, his genes activated in different ways, and his immune system jumped into overdrive compared to that of his identical twin, Mark Kelly. Mark has also flown in space, but he remained on the ground during that long-duration mission. Over time, Scott experienced decreased body mass, instability in his genome, swelling in major blood vessels, changes in eye shape, metabolism shifts, inflammation and alterations in his micro biome — as well as a strange lengthening of his telomeres, the protective structures at the ends of chromosomes. (They shortened again after he landed.)
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