Each of astronaut Scott Kelly's four space flights has been longer than the last - and if he wants to keep up the trend, he jokes, he'll have to make Mars the destination for his fifth trip.

"Flying in space is a privilege whether it's the first time or the fourth time," he told CBS' "Face the Nation" in an interview from the International Space Station, where he is in command. Kelly is spending 342 days in space, the longest flight ever attempted by an American.

"It's a much different experience now with this International Space Station and the international cooperation and all the research we have that go with it than what we were doing back in the 1990s and previously," he said.

A major part of Kelly's mission is to generate the data that will allow NASA to learn more about the long-term physiological effects of weightlessness and space radiation. He'll be compared to his brother, fellow astronaut Mark Kelly, who will remain on Earth.

"A lot of the data we collect is stuff that has to be analyzed on the ground. For instance we can't see, you know, bone loss. Our cells, you know, that's something that we'll have to notice with imaging technology when I get back," Kelly said.

One thing he can see: Loss of muscle mass.

"The amount of muscle I've lost in my calf muscle, because we don't walk up here is pretty significant," he said, although he has seen some muscle gain from the exercise he does in space.

He also said his vision was initially affected, but that has "leveled off" and become consistent with previous space flights.

The medical research fascinates him, and Kelly also said some of the experiments with rodents has been interesting. But he said the coolest part of the experience so far has been the two space walks he did.

"I had never done those before. And, you know, that's a very challenging experience," he said.

Kelly also shared some tips on what it takes to be an astronaut. Earlier this month, NASA opened its application process for the next class of space travelers - some of whom will participate in a human mission to Mars in the 2030s.

"What we look for are people that are technically competent. You need a background in a scientific field. Whether it's as a scientist, an engineer, medical doctor, or you know a person that's in the military with some kind of technical background," Kelly said. "And we want those people to have proven themselves in their current profession, being very high performers, but also people that get along well as part of a team because this is a huge team effort, not just your crew members here on board but also with all the folks you have to work with on the ground."

And if that isn't enough, astronauts also have to have a more general set of skills.

"We have a lot of systems here on board the space station and we can't call a repair man when one of them breaks," Kelly said.

Despite the work and research to be done in space, there's still been time to follow the news back home. Kelly told "Face the Nation" moderator John Dickerson that the news is on "pretty much all the time" and that he follows the presidential election "very closely."

"I have to say, it's been very interesting," he said.

"I also heard you watched 'Game of Thrones' while you were up there," Dickerson asked, referencing the notoriously violent HBO fantasy series. "How long do you think you'd last up there if you worked with your colleagues the way they do on 'Game of Thrones'?"

"I'd like to think I would last a long time, but you never know," Kelly replied. "But the good news is we don't work that way. We actually get along very well. You know, it's an International Space Station. We have crew members from both the U.S. and Russia and now the United Kingdom with Tim Peake. ... It's great to see that, on this space station, that we can work across cultures in a very cooperative way."