The US has big plans for putting humans in space. In doing so, it will be building on our experience at the International Space Station, which is approaching the 20th anniversary of the arrival of its first occupants. Some of that experience comes in the form of knowledge of habitation and operations in low gravity. But a large chunk of it is in the form of understanding what time in space does to the human body.
Today, NASA is releasing detailed results of its most audacious experiment yet: sending one half of a pair of identical twin astronauts into space and carefully monitoring both of them for a year. Three years after astronaut Scott Kelly returned to Earth, and a year after some horrifically speculative press coverage, a paper in today's issue of Science provides excruciating detail on the changes Kelly's body experienced over a year in orbit. While many of the problems highlighted in the new paper had been identified previously, the results aren't exactly good news for the US' long-term exploration plan.
Over the course of many long-term stays in space, the US and Russia have identified a number of health issues caused by extended stays in minimal gravity. Some of them are pretty simple to correct. Without gravity's constant pull, bones and muscles don't experience the resistance that helps keep them robust. A careful exercise plan, however, can minimize these issues. Less easy to minimize is the fact that the body's internal water content shifts upwards—about two liters of fluid move to the upper body over the short term. Among the consequences are eye problems that can persist after the astronaut returns to Earth.