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.