section_path: Safety concerns/Environmental hazards/Medical issues/Microgravity
section_name: Microgravity
paragraph_text: Microgravity
  
  Medical data from astronauts in low Earth orbits for long periods, dating back to the 1970s, show several adverse effects of a microgravity environment: loss of bone density, decreased muscle strength and endurance, postural instability, and reductions in aerobic capacity. Over time these deconditioning effects can impair astronauts' performance or increase their risk of injury.
  
  In a weightless environment, astronauts put almost no weight on the back muscles or leg muscles used for standing up, which causes the muscles to weaken and get smaller. Astronauts can lose up to twenty per cent of their muscle mass on spaceflights lasting five to eleven days. The consequent loss of strength could be a serious problem in case of a landing emergency. Upon returning to Earth from long-duration flights, astronauts are considerably weakened and are not allowed to drive a car for twenty-one days.
  
  Astronauts experiencing weightlessness will often lose their orientation, get motion sickness, and lose their sense of direction as their bodies try to get used to a weightless environment. When they get back to Earth, they have to readjust and may have problems standing up, focusing their gaze, walking, and turning. Importantly, those motor disturbances only get worse the longer the exposure to weightlessness. These changes can affect the ability to perform tasks required for approach and landing, docking, remote manipulation, and emergencies that may occur while landing.
  
  In addition, after long space flight missions, male astronauts may experience severe eyesight problems, which may be a major concern for future deep space flight missions, including a crewed mission to the planet Mars. Long space flights can also alter a space traveler's eye movements.