Scott Kelly and Mikhail Kornienko have done a lot this past year.

In the 340 days they spent on the International Space Station, the American astronaut and Russian cosmonaut orbited Earth 5,440 times, conducted hundreds of experiments, and floated out in space in bulky suits, secured only by a tether, to maintain one of humanity’s most sophisticated pieces of engineering.

There are also some things they didn’t do, like walk or shower, or feel the wind against their skin, or sleep in a bed that didn’t hang from a wall, or drink water that wasn’t recycled urine and sweat.

“Even after I’ve been here nearly a year, you don’t feel perfectly normal,” Kelly said in a press conference last week. “There’s always a lingering something you feel. It’s not necessarily uncomfortable, but it is a harsh environment. For instance, having no running water. It’s kind of like I’ve been in the woods camping for a year.”

Kelly and Kornienko returned to Earth late Tuesday night (or very early Wednesday morning in Kazakhstan, where their Soyuz capsule landed). Their stay was twice the usual tour of duty on the ISS, a joint operation of the two countries. For the United States, it was record-breaking: Kelly’s mission marks the longest spaceflight of an American astronaut. The Russians hold the world record; Valery Polyakov spent nearly 438 days on the Mir space station in the 1990s. The last time anyone spent a year in space was in 1999.

The goal of the yearlong expedition is to better understand how the human body reacts to microgravity for long durations. Researchers say they hope the data acquired in this mission will help them figure out how to send humans on even longer missions, like one to Mars, which would take two-and-a-half years, roundtrip.

These days, scientists know generally what astronauts should expect when they leave Earth’s atmosphere. The most common physiological changes result from the lack of gravity. When astronauts first experience weightlessness, their sensorimotor system becomes immediately disrupted.

“Your inner ear thinks you’re tumbling: the balance system in there is going all over the place … Meanwhile your eyes are telling you you’re not tumbling; you’re upright,” Leroy Chiao, a retired NASA astronaut who flew three shuttle missions and spent six months on the ISS in the mid-2000s, told Charles Fishman in The Atlantic last year. “The two systems are sending all this contradictory information to your brain.” Cue nausea that takes a few days to subside.

Without the forces of gravity to help circulate air inside the orbital laboratory, the carbon dioxide its residents exhale can form an invisible cloud around their head, which can lead to headaches. In weightlessness, the fluids in the human body float upward and clog the sinuses, making astronauts’ heads feel congested and their faces appear puffy. Their skeletons become useless; bones don’t need to support muscles in microgravity, so they start losing minerals and regenerating cells at a slower pace. Astronauts can lose 1 percent of their bone density a month. Back on Earth, it takes a year for aging men and women to lose the same amount of bone mass. In a environment that requires little strength to move around and work, muscles atrophy, their fibers shrinking.

These effects can be remedied. Astronauts wear compression cuffs on their thighs to keep the blood in their lower body from pooling upward, and take vitamin D supplements. They maintain muscle and bone strength by exercising for two-and-a-half hours a day, six days a week, guided by strength coaches. (In 2014, Mike Hopkins posted on YouTube some workouts that put Crossfit to shame.) The station’s fans help spread the exhaled carbon dioxide around.

But scientists are still learning. Astronauts have complained of vision problems since the first missions in the 1970s, but it was only in the last decade that scientists discovered such problems were an occupational hazard. In 2009, two NASA astronauts noticed they started having trouble seeing things close up. Eye exams and high-tech cameras revealed their eyeballs had become a bit squashed and their optic nerves had swelled, leading to farsightedness that persisted post-mission. Researchers suspect the change in vision is caused by cerebrospinal fluid in the skull, free from gravity, pushing on the back of the eyeballs, but they don’t know for sure. NASA keeps the ISS stocked with glasses just in case.

Still, scientists have managed to figure out how to keep humans alive and relatively well for months at a time, a remarkable feat that now appears routine to those watching from the ground. But on a trip to Mars, it’s distance from Earth, not duration of spaceflight, that becomes the bigger enemy. The ISS orbits about 200 miles away, just within Earth’s protective magnetic field. There, astronauts receive 10 times the usual amount of radiation, high-speed particles from the sun or other parts of the galaxy that tear through DNA molecules, that increase their risk of dying from cancer. Farther out, the exposure would get much worse.

Human bodies were not made for outer space. Neither were their minds, which is why NASA astronauts talk to psychologists once every two weeks, and write in their personal journals at least three times a week. The living and working quarters of the ISS are about the size of a six-bedroom house—spacious by manned satellite standards, but certainly very, very cozy (see for yourself in this panoramic tour). On a voyage to Mars, no amount of Instagramming could stave off the potentially harmful effects of months of confinement with only a handful of people, in an environment so isolated from the rest of humankind it would make Sartre cringe.

Scientists have until the 2030s—when NASA wants to send humans to Mars—to figure out how to keep long-distance spacefarers alive. Kelly and Kornienko spent a year undergoing a barrage of cognitive, visual, and medical tests before they launched to the ISS last year. They conducted more tests during their 340 days aboard, and will receive even more in the year after they return—all in the name of space exploration’s latest moonshot. When the last test is over, Kelly said he’s going to go home and jump into his pool, where water doesn’t float around in big globs.