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.

This interview has been edited for clarity and brevity.

Loren Grush: I was doing an interview a while back with one of the researchers for the Twins Study, and she mentioned that you guys actually approached NASA with the idea for the study. Is that true? What sparked that idea?

Scott Kelly: When I was assigned to this year-long mission, I had a briefing with the scientists to prepare for the press release and the press conference about sending two guys into space for a year. And during that briefing, I asked the scientists if anyone had any interest in doing any comparative studies on Mark and I, considering he’s also an astronaut and they had a lot of data on him for a really long time. And they went back over the next couple of weeks and talked about it and decided that there was in fact an interest and asked us if we would be participants.

LG: Mark, what did you think when he suggested that idea?

Mark Kelly: After talking to NASA about this, Scott came to me and said, “Would you be willing to do this?” The science that NASA does is incredibly important and I’m so appreciative of everything that NASA’s done for me in allowing me to be part of the space program for 15 years. So I said, “absolutely,” to do whatever they’d like. They don’t even have to pay me. So even though I didn’t work there and I told NASA they don’t have to pay me. Then they came back a little bit later and they said well, it turns out we actually have to pay you. I got paid minimum wage.

SK: Wait a minute. You get paid minimum wage? I don’t get paid anything.

MK: Yeah, I still do. You’re getting ripped off. I get like $10.50 an hour or whatever it is whenever I deal with one of those NASA experiments.

SK: Wow.

Alessandra Potenza: How much work do you guys still do for the Twins Study, now that you’re almost one year from landing?

SK: Well for me, I have my one-year medical test. We get an annual physical every year, but part of this will be data collection for the Twins Study. I’m not really sure how much of it. It’s probably like a full day’s worth of stuff. And then I think I have another one at a year and a half. And then after that, it gets more and more irregular and spread out longer. But most astronauts are part of a different study called the Longitudinal Study of Astronaut Health, that just tracks their health throughout the course of their lives to try to get an understanding of the effects of spaceflight on us from a long-term perspective, but also to try to understand just a group of people that have pretty good health care and are monitored closely. What and how does that change things?

LG: Now that some of the preliminary results are coming in, were you guys surprised at some of the genetic changes that they’ve been finding, for instance?

SK: Yeah, I was really surprised.

MK: The one big surprising thing was his telomeres got longer while in space, and that’s kind of the opposite of what they thought would happen. The presumption was that in the radiation — there’s a lot of radiation in space — and the stress of being on the space station, they thought that those things would result in the shortening of his telomeres. They’re like the structure on your genes that’s indicative of how old you are. But the opposite happened. And there were some people out there in the media that were speculating that maybe NASA discovered the fountain of youth, which is going into space, which is not true and that’s wild speculation. But I think the interesting thing is, here’s an experiment that the scientists have their hypothesis and they wound up with the opposite result.

LG: Didn’t Scott get a little bit younger because of time dilation? Maybe not because of the telomeres, but something like a couple milliseconds or something like that?

SK: Yeah, by like three milliseconds.

MK: No, no, Scott, I think if you actually add up your 520 days, because I did this, I saw what people say is that per day, at 17,500 miles an hour, I think where I used to be six minutes older, I think I am now — if you did the math correctly — it’s six minutes and 13 milliseconds.

LG: Another study showed that your gut bacteria changed while in space. Did that manifest in any crazy ways while you were on the station?

SK: It’s interesting that our microbiome consists of all these cells that aren’t us, so there’s actually more of them than there are our own human cells. And your microbiome is affected by a lot of things. It’s affected by what you eat and it’s affected by where you live, your environment. It’s interesting that Mark and I, our microbiomes are very different. So I guess my point is that our microbiome is affected by our environment and the space environment is an extreme environment, it’s a unique environment. So it doesn’t surprise me that they saw changes while I was in space.

AP: Both of you have been in the spotlight a lot with the Twins Study. How does it feel to be guinea pigs?

SK: It’s part of it. As an astronaut, from day one, when you agree to take that job, you’re also agreeing to be a human subject. So it’s something that we’ve been used to for the last 20 years. I just think it’s part of the responsibility when you’re in this very unique position, to participate in these studies and be a human subject. So it’s not even something I think about, actually.

MK: Well, then they also try to make sure that we don’t do anything really stupid. I mean it’s not like the mice I carried on my first space flight, or the mice and rodents that my brother dealt with in space. As subjects of science experiments, they’ve got a much more difficult job than we do. The outcome is much better for us, at least in the short term.

LG: I know there’s a protocol, when they get these results from the Twins Study, they’re supposed to share it with you beforehand. What is that process like and do you guys have veto power if you don’t want something shared?

SK: Yeah, they’ll generally send us the research paper and sometimes preliminary material and ask us how we feel about it, because it is our medical data and it is protected under the law. So far we haven’t declined release on anything in any studies. As time goes on and we see how the process works, I anticipate that there’s not going to be much stuff that we would not want people to see.

AP: What kind of follow-ups to the Twins Study would you like to see from NASA?

MK: Well I’d like them to let me go back in space for just like a couple weeks. That would be nice.

SK: I’d like a follow-up for them to send Mark to that new solar system we discovered.

LG: Can you take me with you?

SK: I’m just kidding, obviously. It’s 40 light-years away. How long do you think it would take to get there, me and my brother and the two of you ladies, at the speed that we could travel at to go 40 light-years?

LG: Well what is the deal? Voyager 1 is still like 0.05 percent from Alpha Centauri, which is like, four light-years away?

SK: Exactly! So you should be able to figure this out. Give me a number.

LG: I’ll do the math and get back to you.

SK: No, you’ve got to do it in your head right now! How many years?

LG: I’m working on it I’ll get it to you.

SK: Come on. About 10 minutes ago I tweeted my guess. So what is your guess?

LG: I’m going to guess 200,000 years.

SK: No, more like 800,000 years. At 35,000 miles an hour, it would take about 800,000 years.

LG: Oh, gosh. We could have multiple generations later get there.

SK: That’s what I was thinking.

MK: Multiple? It could take all of the generations of humans to get there.

SK: The people that go there, if you did that, you get on this spaceship and when you got off 800,000 years later, it would be a new species.

LG: Yeah, we would be the aliens once we got there.

SK: We wouldn’t even recognize our descendants.

LG: Well, speaking of traveling through space, Scott, you said that being in space changed you and gave you a better appreciation for our planet. Mark, does that resonate with you? And I guess both of you, how did you guys feel you changed when you were in space?

SK: For me, it’s just looking down at the Earth for a long period of time, and not just on this mission, but between my first mission in 1999 and then my last, which I landed in 2016, you see significant changes on the Earth, especially the rainforests in South America. It’s just really heartbreaking to see how they’ve been decimated. Pollution that is almost constantly over certain parts of our planet. The fragility of the atmosphere that you notice.

But also in space, you do have this orbital perspective where you feel detached from all of the people and everything that has happened to the point of the planet while you’re in space, especially for long periods of time. And the news we get from Earth, by and large, it’s all bad stuff. You know, it’s mostly bad news. And when you look out the window and you consider how lucky we are to have this planet that, until recently, we figured it was pretty unique in the universe. I think we should take better care of it, we should take better care of each other. We need to be better teammates on our shared spaceship that we’re flying around the universe in. Spaceship Earth.

AP: Mark, do you want to answer the question on how being in space changed you?

MK: What he said. Same answer.

AP: What do you guys miss the most about being in space and what don’t you miss?

SK: I miss the work and the technical challenge of it. I miss the teamwork, the working on something that is technically challenging and doing it with a group of highly professional and inspiring people, both your crewmates and the folks on the ground. Doing something you believe is important. I miss that a lot more than floating around in zero-g and looking at the sites out the window.

MK: Yeah. Having a mission and trying to do something really complicated and being successful at it is very rewarding. So I really miss that part of trying to do a really good job at a very difficult thing.

LG: Now you both talk so fondly about your time in space, and I know Scott, you’ve mentioned not closing the book on that. Are you both interested in making a trip with a private company in the future?

MK: So I have co-founded a company called World View, which part of our business is space tourism, but with a helium balloon to the edge of space. I’m excited about the fact that other people are going to get to have these kinds of experiences, whether it’s with us or launching with Virgin Galactic or Blue Origin in a rocket ship. That’s a great thing for a country for a lot of different reasons.  

SK: You know, under the right circumstances, I’d be all for it. We will get there someday, where people are flying into space for different reasons, some of which is just for pleasure. And we have a little bit of that now, of course, but we’re talking on a much bigger scale. So yeah, under the right circumstances, I would go. I’d never rule out never flying in space again. I think that’s definitely something that would interest me.

AP: What’s the life of a retired astronaut like for both of you?

SK: You know, I’m busier now than I think I’ve ever been in my life. I’m trying to write several books and doing some public speaking that takes a lot of my time. And starting out on this new part of my life after getting back from space, being there for a year has definitely been a challenge. But it was a welcome challenge and I’m enjoying it.

MK: I’ve got a lot of stuff that I’m involved in right now. I was on the road about 80 percent of last year, and probably the year before that as well. I serve on a few boards, I’ve got this company I’m the co-founder of in Tucson, public speaking, some book project stuff. So I’ve been probably as busy as I’ve ever been. I’ve certainly spent more time away from home than I ever did as an astronaut. So it’s a challenge. But I do a lot of things that I enjoy, and it’s nice to be able to control your own schedule.

LG: What about your personal preferences when it comes to the future of human space exploration? Is there anything in particular you guys want to see from NASA or from other companies?

SK: I’ve always been a fan of going back to the Moon. I think there’s a lot we can learn from the Moon that’s going to help us go to Mars some day and it’s something that always excites me.

MK: We should just go straight to Mars. Forget about the Moon. We’ve been there. We’ve already done that.

SK: But more importantly, when we transition from one administration to another, we need to kind of let NASA continue along the path they’re on and not change direction because it just wastes time and effort and money and it doesn’t help us get anywhere at all.

MK: I do agree with you, Scott, that it’s incredibly important for our long-term success as a government agency and to have consistency. And that’s something that’s lacking. We often get these wild changes in direction from either the White House or often from Congress as well. So consistency from year after year is really important to our space program. So I hope as the new administration in the White House, our new president, starts to formulate a plan for NASA, I hope they keep in mind that having a goal and a strategic plan and predictable funding, if we do that, then NASA can accomplish great things. But predictability and being able to execute a long-term plan over time is really important to our success.

AP: Is there anything else you guys would like to add about the Twins Study or your experience?

SK: Well one more thing I want to say is, one of the things that also keeps us busy is our relationship with Breitling, the watch company who I think set up this interview for us, right?

AP and LG: Right.

SK: So I was going to put in a good Breitling plug. Your readers probably like stuff and pilots and astronauts like watches. And Breitling makes the best watches, especially for aviators and astronauts.

LG: Did you use Breitling watches when you were on the station?

SK: Yeah I did, and you need to put that in the article, because this is what I was told this article was going to be about.