Humans are physiologically well-adapted to life on Earth. Consequently, spaceflight has many negative effects on the body.
The most significant adverse effects of long-term weightlessness are muscle atrophy and deterioration of the skeleton (spaceflight osteopenia). Other significant effects include a slowing of cardiovascular system functions, decreased production of red blood cells, balance disorders, and a weakening of the immune system. Lesser symptoms include fluid redistribution (causing the "moon-face" appearance typical in pictures of astronauts experiencing weightlessness), loss of body mass, nasal congestion, sleep disturbance, and excess flatulence. Most of these effects begin to reverse quickly upon return to Earth. The engineering problems associated with leaving Earth and developing space propulsion systems have been examined for over a century, and millions of man-hours of research have been spent on them. In recent years there has been an increase in research on the issue of how humans can survive and work in space for extended and possibly indefinite periods of time. This question requires input from the physical and biological sciences and has now become the greatest challenge (other than funding) facing human space exploration. A fundamental step in overcoming this challenge is trying to understand the effects and impact of long-term space travel on the human body.