ABSTRACT
Gravity is an invisible force that has a constant effect upon the terrestrial environment, causing rivers to empty into oceans and keeping objects on the surface of the planet. Weightlessness—that is, the absence of gravity—is one of the most striking aspects of a space flight. It alters an astronaut’s motor performance and lifestyle and gives rise to a whole series of physiological stress responses. These are usually discrete though some can be more striking. A quick summary of the physics of weightlessness and how it is generated in space will lead to a better understanding of these effects.
