Well, consider the counterfactual of what would have happened to the astronaut's DNA if he hadn't gone to space. We don't know how it would have changed, so we

Well, consider the counterfactual of what would have happened to the astronaut's DNA if he hadn't gone to space. We don't know how it would have changed, so we can't compare it directly. Since we needed to do the experiment I'm guessing our knowledge DNA was not sufficient to predict that, so it makes sense to use a twin to get the closest proxy to the counterfactual that we can.