No NASA astronaut has ever spent a year in space. Scott Kelly's about to change that.

No NASA astronaut has ever spent more than 215 days in space.

But on March 27, when Scott Kelly takes off for the International Space Station, it'll be for a year-long mission in orbit. Along with Russia's Mikhail Kornienko, Kelly will be participating in NASA's One Year Mission, an effort to better understand how the human body copes with a year in microgravity.

it's part of an effort to prepare for an eventual mission to mars

Astronauts typically spend six months on the space station, though three Russian cosmonauts did spend more than a year on the now-defunct Mir in 1980s and '90s. But Kelly and Kornienko's health will be monitored in ways that weren't possible back then: with MRI imaging, close tracking of their cellular metabolism, and study of the millions of bacteria that live inside the astronauts' digestive tracts. As a control, NASA will even compare Kelly with his identical twin Mark Kelly, a former astronaut who'll stay back on Earth.

It's all part of an effort to better understand how the human body holds up over the course of a full year in space — in order to better prepare for a potential mission to Mars in the 2030s.

On a recent press call, Kelly spoke with me, Miriam Kramer of Space.com, and Travis Kircher of WDRB Louisville. Here's what he said about the mission.

Preparing for a year-long mission

Kelly undergoes spacewalk training prior to the mission. (NASA)

"On a six-month mission, your mindset is that you're going to go up, be there for a period of time, then come home," Kelly said. "When it's a whole year, I don't have that same perspective. It's almost like I feel like I'm moving there, and I'm not coming back. Or that it's going to be so long that when I come back, it's almost like I never lived here."

"So I feel some pressure to get certain things in order on Earth, things I won't be able to take care of when I get to space," Kelly says. That includes mundane things like paying his taxes and making sure his electricity bill will keep getting paid, and more important things, like making sure his twenty-year-old daughter is settled at college.

One of the difficult things about living in space: going to sleep

Piers Sellers sleeps aboard Space Shuttle Atlantis. (NASA)

"Going to sleep in space is not easy," Kelly says. "I think it's because on Earth, when you get into bed, it's more comfortable than sitting up or standing, so it gives you some relief at the end of the day to crawl into bed. But in space, when you go to sleep, you might as well just shut your eyes wherever you are, because you're floating, and you're going to float when you're asleep. You don't get a relaxing feeling."

Kelly is one of many astronauts to note this difficulty. In fact, a 2014 study found that a high percentage of astronauts aboard the ISS suffer from sleep deprivation, and three quarters end up taking sleeping pills to help. Given that astronauts on a Mars mission would need to be well-rested to carry out their tasks during the journey there, this is a pretty big challenge that NASA needs to solve.

How the year-long mission will help us get to Mars

A rendering of a NASA spacecraft orbiting Mars, with the Orion capsule. (NASA)

"[NASA hasn't] had a person live in space for longer than about 220 days," Kelly says. "The Russians have had a few guys in space a year or longer, so they have some experience doing it, but that was a long time ago, and that was their program, their science. And none of us — Russia, the US, or our international partners — had the capability to do the type of science we can do today... it's changed dramatically in the last 20 years."

"Flying for a year is going to help us see whether or not we're on the right track to understanding human physiology so we can go to Mars, which will take even longer," he says.

it'll help us better understand how long-term space travel affects the human body

This mission will include a number of research programs that work toward this goal. During Kelly and Kornienko's year in space, NASA scientists will study their cognition (with both MRI imaging and mental testing) and mental health (by monitoring their sleep and their daily journals).

Regular blood and saliva sampling will also allow for analysis of the astronauts' cell metabolism, looking for markers of inflammation, as well as the microbes living in their bodies. Scientists on Earth will also monitor the astronauts' bone and muscle strength — which typically decay during long space missions because of the reduced amount of gravity acting on astronauts' bodies — and eyesight, which can deteriorate in space due to pressure building up inside the skull.

Then, after Kornienko and Kelly return, scientists will study how long it takes them to regain full motor performance and strength — something it'd be important to recover quickly upon landing on Mars.

Why crewed space exploration is worth it

Michael Gernhardt, during a 2001 spacewalk. (NASA)

There's a school of thought that, given the added costs and risks of sending humans into space, it makes a lot more sense to simply send robots instead — a strategy that NASA and other space agencies have already had lots of success with, including on Mars.

But Kelly thinks there are substantial benefits to sending humans. "You can learn a lot more when you send humans," he says. "With the Martian rovers Spirit and Opportunity, for instance, the science that it took those rovers years to do, a crew of a few people could have done in a matter of days. People can just do more."

"Certainly, it's more expensive to send people, because you need to have more redundant and safer systems to protect them," he says. "But there's also the human element to it. We are explorers, and we need to continue to expand our experience base, and push beyond our limits. We've always done that, and I think we need to continue to."

Further reading: The Strange, Deadly Effects Mars Would Have on Your Body

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