Spaceflight poses both challenges and opportunities for human health.
Exposure to the microgravity environment causes a number of changes within the human body itself. However, a study of these changes may help scientists better understand the human body as a whole. Adaptations in the body to spaceflight can have adverse effects on astronauts when they return to earth. Many of these effects, such as bone and muscle loss, are similar to symptoms of diseases experienced here. The International Space Station, which is expected to be completed in 2004, will enable unprecedented study of the effects of long-duration space travel on the human body.