Bad things happen to a human body in zero gravity. Just look at what happens to astronauts who spend time in orbit: Bones disintegrate. Muscles weaken. So does immunity. “When you go up into space,” says Saïd Mekari, who studies exercise physiology at the University of Sherbrooke, in Canada, “it’s an accelerated model of aging.” Earthbound experiments mimicking weightlessness have revealed similar effects. In the 1970s, Russian scientists immersed volunteers in bathtubs covered in a large sheet of waterproof fabric, enabling them to float without being wet. In some of these studies, which lasted up to 56 days, subjects developed serious heart problems and struggled to control their posture and leg movements.
Weightlessness hurts us because our bodies are fine-tuned to gravity as we experience it here on Earth. It tugs at us from birth to death, and still our intestines stay firmly coiled in their stack, blood flows upward, and our spine is capable of holding up our head. Unnatural contortions can throw things off: People have died from hanging upside down for too long. But as a general rule, the constant push of g-force on our body is a part of life that we rarely notice.