The NPR News article and podcast Scientists Share Results From NASA's Twins Study says:
SCOTT KELLY (NASA Astronaut): You know, the symptomatic stuff is fine. I don't have any long-term negative feelings, physically, from being in space. Now, there's the things you can't feel. And hopefully, I will never learn that those are a problem.
GREENE (Host): Those things you can't feel - well, it turns out they are as small as the protective structures at the ends of his chromosomes.
MARTIN (Host): Yeah. These are called telomeres, and normally, they get shorter with age. But what about in space?
SUSAN BAILEY (Principle Investigator): What we wanted to do was evaluate telomere length in both of the twins before and after so that we could see, you know, where they started and then where they ended.
MARTIN: Susan Bailey was one of the scientists who answered this question. She expected the stresses of space to shorten telomeres quicker.
BAILEY: And, in fact, we saw exactly the opposite thing - that during spaceflight, he had many more long telomeres than he did before he went up. So that really couldn't have been more of a surprise to us.
Question: Why did investigators initially believe that Scott Kelley's year in space would accelerate the rate of telomere loss, relative to his baseline rate and the rate of his identical twin brother on the ground? What would be the postulated mechanisms?