Following is a transcript of the video.

Narrator: This is astronaut Scott Kelly back in 2015. For 340 days, he circled Earth inside the International Space Station, the longest any American has ever spent in space at once. Meanwhile, someone with the same face, body, and DNA was back on Earth: his identical twin, Mark Kelly. Scott and Mark are the only twin NASA astronauts in history. Separated by 250 miles of space and sky, the twins participated in a groundbreaking study unlike anything NASA had ever tried before. The mission: discover long-term effects of space on the human body. And now, four years after Scott Kelly launched into space, we have the full results, which confirm that, yes, a crewed mission to Mars is possible.

Bakelman: To me this is very reassuring and really suggests that... longer-term travel is a possibility.

Narrator: NASA is trying to put humans on Mars in the 2030s, but even if it had the rocket technology to visit today, there's still one major obstacle standing in the way: our bodies. Because traveling to Mars will be the longest crewed space mission in history, lasting for over a year, and while NASA has tested in-depth how the isolation will affect our minds with missions like HI-SEAS, it has little information about what space travel will do to our bodies. That's where NASA's twin study comes in.

News Anchor: Astronaut Scott Kelly will try something no American has ever done before. This spring, he will leave for a mission and spend a year in space.

Narrator: Over the course of that year, Scott gathered all kinds of data about himself. He drew blood, collected urine, and took cognitive tests. And back on Earth, his twin brother, the perfect human control, did the exact same thing. Then, in 2016, Scott returned home to Earth where scientists studied him for another six to nine months. And now, finally, four years after Scott's launch, researchers have published the full results, and here's what they found. For one, there were signs of radiation damage in Scott's chromosomes. Those are the structures in our cells that contain DNA.

Bailey: We know that radiation causes or will induce these chromosome aberrations, or rearrangement of the chromosomes. So essentially it breaks the DNA.

Narrator: And on a longer mission, say to Mars, more of those DNA breaks could build up.

Bailey: And that can contribute to genomic instability.

Narrator: Which could lead to diseases like cancer. The year in space also activated thousands of genes in Scott's genetic code, something similar to what can only happen in extreme environments on Earth. That's because many of these genes are linked to our immune system.

Bailey: When we get into stressful situations, or we have an injury, or we get sick, the immune response kicks in. And the way it does that is it turns the genes on that it needs.

Narrator: Bailey was part of a group that also looked at structures in the twins' cells called telomeres. Telomeres are caps at the end of chromosomes that help protect our DNA from damage, and as we age or are stressed, they shrink. That's why Bailey figured that Scott's telomeres would shorten during his year in space. He wasn't just aging, but he was exposed to all kinds of stressors from cosmic radiation to zero gravity, and pretty much everything in between. But she was wrong. From what researchers could tell, Scott's telomeres weren't shorter in space. They were significantly longer.

Bailey: I don't know what's going on in space. We imagine things like, "Oh, they're just having such a grand time." You see 'em floating around. They all look real happy while they're up there. A dream come true. But it's just, I just don't think it's the fountain of youth.

Narrator: One explanation is that Scott had a healthy regimen on the ISS replete with exercise and nutritious astronaut food. But Bailey says longer telomeres are more likely yet another signal of stress or even injury. Radiation, for example, might have triggered the production of stem cells, which can renew damaged structures in the body, and those cells inherently have longer telomeres. So one major takeaway from the study is that space is, well, stressful. But all that stress was short-lived for Scott, and for the most part, his body actually returned to normal after landing back on Earth. Within 48 hours, his telomeres were short once again, and after six months, more than 90% of those activated genes turned back off, and nine months out, there was less damage in his chromosomes, which means that none of the changes the researchers observed was immediately life-threatening.

Bakelman: To me what that means is that people can be in space. They can be there for a year, maybe longer, we don't know. Even though we're not born in space, our bodies figure it out, and they're able to function.

Narrator: Taken altogether, that's great news for NASA or for anyone else hoping to send humans to Mars. Swanton: I definitely think they're gonna be possible.

Narrator: That's Dr. Charles Swanton, a cancer biologist at the Francis Crick Institute in Britain, who wasn't involved in the study.

Swanton: What I can't predict at the moment is the impact on human health and reproductive potential.

Narrator: He acknowledges that a yearlong trip to Mars isn't the same as a year on the ISS. There'd be more radiation, for example. But, following the twins study, he and the researchers have far more confidence than ever before that we're going to get there.