After 340 days in orbit, astronaut Scott Kelly and cosmonaut Mikhail Kornienko are coming home. The mission began March 27, 2015, and will end at about 11:27 P.M. Eastern time, March 1, when the pair lands on the steppes of Kazakhstan onboard a Russian Soyuz capsule. For just under a year Kelly and Kornienko lived in the close quarters of the International Space Station with four other rotating crew members, who worked standard six-month missions. Kelly and Kornienko’s mission examined how a long-term stay in a microgravity environment impacts the human body and mind, with an eye toward planning for future multiyear missions that will be necessary to take crews to Mars or other solar system sites far beyond Earth orbit.

Kelly has now officially spent more continuous time in space than any American astronaut. (A handful of Russians still have him beat.) Soon after Kelly and Kornienko land, a medical team will thoroughly evaluate them. These tests will be the first of many hours of investigation into the physical and mental state of the yearlong crew members as research teams determine what impact their stays in orbit had on their bodies and minds.

On a daily basis over the course of the mission NASA researchers tracked data relating to Kelly’s caloric intake, temperature, exposure to radiation and bone density—the loss of which is a major concern in microgravity. From a scientific standpoint, using Kelly provides an additional unique advantage in terms of studying his health and how he responds to a long-term space mission: His identical twin, Mark Kelly, remained on Earth and is serving as a type of control.

The Twins Study, as it came to be called, was too good an opportunity to pass up. “NASA was going to do this mission anyway, and when they selected Scott for it [that choice] came with the nice bonus of Mark, his brother, a former astronaut who was willing to help in any way and was very happy to provide any data he could from his biomedical measurements,” says Michael Massimino, a former astronaut who flew two space shuttle missions to repair Hubble. “They are identical twins, so they collected a bunch of information from them before and they’ll compare it afterwards. It’s only two guys, but it’s one of those added bonuses.”

With only two data points, NASA’s Twins Study is hardly a traditional one, and one of its two participants was permitted more freedom than scientific rigor would typically allow. “We keep saying we would’ve liked to lock [Mark] up in a chamber and give him the same diet and exercises [as Scott]. That is one of the limitations of the studies,” concedes Scott Smith, a nutritionist with NASA and the principal investigator on one of the 10 investigations in the Twins Study. Smith’s research focuses on how nutrition can help stave off some of the negative effects of living in space, such as bone density loss. He argues, however, that the unprecedented nature of the Twins Study somewhat compensates for the lack of control over Mark’s Earthbound diet. “We’re looking at them with detail that is almost unimaginable in all other studies” dealing with astronauts. Smith’s team can compare everything from their genes to their proteins to how their blood flows.

Perfecting the art of astro-cuisine will be essential for long-term space missions, and Scott Kelly’s year in space will help reveal what an astronaut’s dietary needs are over the span of years rather than the days or weeks of previous U.S. missions. “When we looked at shuttle flights, we always looked at those like camping trips. When you start to fly people for one to three months [half the typical stay on the space station], it gets a little more serious in that your risks of not having everything you need in that food system go up,” Smith explains. For yearlong missions, perfecting space diets becomes essential both for astronauts’ physical health and their mental states. “The psychology of food becomes a big deal when you’re a long way from home with people you may or may not know very well,” Smith says. He hopes to provide astronauts with better, more personalized foods that retain the satisfying textures Earthbound eaters take for granted, such as the crunch of broccoli—something that Kelly said he missed during his mission.

Kelly’s perspective on all things space travel, not just the menu, will be tapped for future missions. “When Scott gets back, he’s going to share his experience of what it was like to be up there for a year—what made it good, what could be better and what we need to think about for keeping people up there even longer,” Massimino says.

In a press conference preceding his return to Earth, Kelly floated one necessary ingredient for long-term space travel: privacy. The International Space Station offers phone booth-size individual living spaces in its crew quarters, where Kelly slept, used the computer and spoke with friends and family back on Earth. For the rest of the year, Kelly was with his crew members in various workstations such as the greenhouse that grew space lettuce or exercising in the gym. That degree of privacy could be luxurious in future deep-space faring vehicles, which will be much smaller than the football field–size station. “Making that private area as perfect as possible will go a long way towards reducing fatigue, stress and helping for a successful mission,” Kelly said.

Kelly’s mission turned out to be slightly shy of a full year but that hardly diminishes the achievement. “If I launched in March and land in March, it’s pretty close to a year, so I’m not disappointed at all,” he said.

While Kelly was in orbit NASA posted a rare job opening: astronaut. A record-breaking 18,300 would-be space travelers applied for eight to 14 possible slots. The next class of astronauts chosen from this overwhelming pool will likely be sent on long-duration missions, and their experiences will be strongly shaped by the data and experience gleaned from Kelly and Kornienko’s year in space.

Editor's Note (3/1/16): The headline of this story was changed to reflect the fact that Scott Kelly, not his twin brother Mark, is returning for nearly a year in space. We regret the error.