In addition to space motion sickness that some astronauts experience in the first day or two of flight, there is an acute shift of body fluids in the direction of the head. Faces become puffy and legs thin out. As a consequence of the fluid shift, many body systems adapt by altering their normal functions. For a time, the heart enlarges. Red blood cell production diminishes, and kidney and gland functions increase.
Weightlessness also leads to deconditioning of muscles and calcium loss in bones.
The experiments on the SLS-1 mission are aimed at trying to answer many important questions regarding the functioning of the human body in weightlessness and its readaptation upon return to the normal environment on Earth. Spacelab Spacelab is an international resource for scientific investigations in space. Built by the European Space Agency for NASA, Spacelab is mounted in the Space Shuttle's payload bay. For SLS-1, the Spacelab long module configuration, seven meters long by five meters wide, will be used. It is a pressurized cylindrical module that is connected to the airlock in the orbiter's middeck by a tunnel.