When the Space Shuttle Columbia lifts off for the STS-40 mission in late spring 1991, it will be carrying the first Spacelab payload to fly into space in six years. When astronauts are launched into orbit about Earth, they experience a condition that is equivalent to the absence of gravity. This condition, called by various names including weightlessness, zero gravity, and microgravity, produces fascinating changes in the human body. 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.