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 Kelly’s genomic and physical markers before and after Scott’s yearlong mission on the space station.

So far we’ve 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 astronaut’s gut. Scott’s DNA and RNA underwent hundreds of mutations in space that gradually returned to normal once he was back on Earth.

NASA’s 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.