Mark and Scott Kelly are the only twins that have ever traveled to space — and their experience will be invaluable if we want to get to Mars one day. The brothers are taking part in what NASA calls the Twins Study — a genetic experiment to see how our bodies change in zero gravity in the long term. That’s important to understand before we put humans on a spaceship and send them on a round trip to the Red Planet. Between 2015 and 2016, Scott spent 340 days on the International Space Station, while his genetically identical twin Mark stayed on Earth to function as a control subject. Before, during, and after Scott’s trip, the brothers have been giving NASA numerous biological samples — blood, saliva, poop, you name it. By comparing Scott’s samples with Mark’s, NASA is trying to understand what long-term space travel does to our bodies. Some preliminary findings have already come out. One study showed that Scott’s DNA changed while he was in space: his telomeres — the protective caps on the end of DNA strands — were unexpectedly longer than Mark’s. (Telomere length can affect aging and age-associated diseases.) Another study showed that there were major fluctuations in Scott’s gut bacteria while he lived in zero G compared to his twin. But we’re still waiting for the bulk of the results, and we might not see those for another year or two. While we wait, The Verge spoke on the phone with Mark and Scott to talk about the Twins Study, whether they’d fly to Mars or the Moon next, and what it feels like to be guinea pigs for the sake of space.