weightlessness The condition in which a mass possesses no weight, as in the absence of gravitational or accelerative forces, or when the vector sum of opposing forces or fields acting upon it is zero. The state is produced during space flight when the accelerative force due to gravity is exactly balanced by the tangential and inertial forces associated with the motion of the spacecraft through space.

Weightlessness — or microgravity, as it is frequently termed — has major effects on the movement of man in space and upon his physiology. The condition can be produced for only very short periods of time on earth. It exists during the initial stages of free fall through the atmosphere and can be generated for 12–40 second during parabolic flight in an aircraft.

John Ernsting

weightlessness Condition experienced by an object on which the force due to gravitation is neutralized. Such an object is said to have zero gravity and no weight; it floats and cannot fall. Weightlessness can be experienced in space and during free fall. The adverse effects on the human body of prolonged weightlessness (called hydrogravics) include decreased circulation of blood, less water retention in tissues and the bloodstream, and loss of muscle tone.