Identical twins allow scientists to study how our environment affects the human body, even how well we adapt to space. And for that we can thank identical twin astronauts Mark and Scott Kelly. They're the only twins to both go to space and serve on multiple missions.

The last was Scott Kelly's one year stint on the International Space Station from two thousand fifteen to two thousand sixteen. This allowed researchers to compare alterations in Scott's genes and basic body functions with his Earth-bound brother, Mark. It was an unparalleled opportunity and critical to astronauts who will make the two and a half year trip to Mars.

To see what changes would occur, both men's genomes were sequenced. When Scott's genome was sequenced again upon his return, thousands of new mutations were found. Another interesting difference between the twins is that more than two hundred thousand RNA molecules were produced in different quantities.

RNA makes proteins which define how our cells function, so maybe space acts as a switch telling our cells to make different proteins in that environment. Another change involved Scott's telomeres 'structures at the ends of chromosomes that protect them from damage. Aging is associated with shorter telomeres and Scott's actually became longer in space and began shortening once he returned.

His body made less bone during the last half of his year in space. And back on earth, there was a brief decrease in his cognitive ability. But some important bodily systems, such as the immune system, showed very little change.

The bottom line is that nothing here suggests yet that a trip to Mars is unachievable.