Scott and Mark Kelly are identical twin brothers. Though that alone does not make them unique, what does is the fact that they are also both astronauts. In order to take advantage of the Kellys’ unique situation, NASA scientists decided to conduct a detailed study on the twins, aimed at unraveling how nature versus nurture plays out in space.

As part of NASA’s Twins Study, researchers collected biological samples from each of the Kellys before sending Scott to the International Space Stat