The effects of spaceflight on astronauts have been an important area of study since the start of manned spaceflight. The effects of weightlessness on the body are believed to be the key problem astronaut's face in medium to long term space missions. Short term weightlessness can lead to motion sickness, lethargy and nausea. Long term weightlessness may lead to the loss of bone and muscle mass, disrupted vision and weakened immune system. More research is necessary to limit the effects of weightlessness on astronauts if manned space flight is to continue.