Humans have evolved to work best in the atmosphere and gravity that exist on the Earth’s surface. To survive in space, astronauts have to take an Earth-like environment with them such as fresh oxygen, which is circulated around the craft for them to breathe. The main difference in space is the weightlessness causing astronauts to float around. As soon as astronauts go into space, their bodies start adapting to this weightlessness. Muscles, bones, heart and blood all undergo changes. At least half of all astronauts suffer an unpleasant reaction to weightlessness. On the Earth, gravity exerts a force on our bodies, which gives us weight and keeps us rooted to the ground. It also pulls body fluid downwards. In space, astronauts lose their sense of balance. They can feel sick, and go off their food. It can take two weeks for the digestive system to fully adjust. Therefore, NASA plans no spacewalks during the first three days of a mission, because an astronaut who vomits inside a spacesuit risks suffocation.