People literally can’t be in space for prolonged periods of time.

Human bodies really can't handle space. Spaceflight damages DNA, changes the microbiome, disrupts circadian rhythms, impairs vision, increases the risk of cancer, causes muscle and bone loss, inhibits the immune system, weakens the heart, and shifts fluids toward the head, which may be pathological for the brain over the long term—among other things. At the University of California, San Francisco, medical researcher Sonja Schrepfer has dug into two of the conditions that afflict space explorers. Her research, using mice floating within the International Space Station, has revealed that blood vessels leading to the brain get stiffer in microgravity. It's part of why today's astronauts can't simply walk out of their capsules once they return to Earth, and it would play out the same way on Mars—where there's no one to wheel them to their new habitat on arrival. Schrepfer and her colleagues did, however, uncover a molecular pathway that might prevent those cardiovascular changes. "But now the question I try to understand is, 'Do we want that?'" she says. Maybe the vessels' stiffening is a protective mechanism, Schrepfer suggests, and limbering them up might cause other problems. She also wants to figure out how to help astronauts' faltering immune systems, which look older and have a harder time repairing tissue damage than they should after spending time in space. "The immune system is aging quite fast in microgravity," Schrepfer says. She sends biological samples from young, healthy people on Earth up to orbit on tissue chips and tracks how they degrade. Vision and bone problems are also among the more serious side effects. When astronauts spend a month or more in space, their eyeballs flatten, one aspect of a condition called spaceflight-associated neuro-ocular syndrome, which can cause long-lasting damage to eyesight. Bones and muscles are built for life on Earth, which involves the ever present pull of gravity. The work the body does against gravity to stay upright and move around keeps muscles from atrophying and stimulates bone growth. In space, without a force to push against, astronauts can experience bone loss that outpaces bone growth, and their muscles shrink. That's why they must do hours of exercise every day, using specialized equipment that helps to simulate some of the forces their anatomy would feel on the ground—and even this training doesn't fully alleviate the loss. Perhaps the most significant concern about bodies in space, though, is radiation, something that is manageable for today's astronauts flying in low-Earth orbit but would be a bigger deal for people traveling farther and for longer. Some of it comes from the sun, which spews naked protons that can damage DNA, particularly during solar storms. "[That] could make you very, very sick and give you acute radiation syndrome," says Dorit Donoviel, a professor at the Baylor College of Medicine and director of the Translational Research Institute for Space Health (TRISH).