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Weightlessness: Condition in which no acceleration, whether due to gravity or any other force, can be detected by an observer within a system. It also means the absence of weight or the absence of the force of gravity acting on a body. Microgravity, gravitational force between 0 and 10 -6 g, is included here. (From NASA Thesaurus, 1988)Weightlessness Simulation: Condition under normal Earth gravity where the force of gravity itself is not actually altered but its influence or effect may be modified and studied. (From ASGSB Bull 1992;5(2):27)Space Flight: Travel beyond the earth's atmosphere.Astronauts: Members of spacecraft crew including those who travel in space, and those in training for space flight. (From Webster, 10th ed; Jane's Aerospace Dictionary, 3d ed)Hypergravity: Condition wherein the force of gravity is greater than or is increased above that on the surface of the earth. This is expressed as being greater than 1 g.Hindlimb Suspension: Technique for limiting use, activity, or movement by immobilizing or restraining animal by suspending from hindlimbs or tails. This immobilization is used to simulate some effects of reduced gravity and study weightlessness physiology.Gravity, Altered: A change in, or manipulation of, gravitational force. This may be a natural or artificial effect.Gravitation: Acceleration produced by the mutual attraction of two masses, and of magnitude inversely proportional to the square of the distance between the two centers of mass. It is also the force imparted by the earth, moon, or a planet to an object near its surface. (From NASA Thesaurus, 1988)Weightlessness Countermeasures: Techniques and routines designed to prevent or reverse unwanted effects of weightlessness experienced during actual and simulated space flight, including physiologic changes related to removal of gravitational loading. Specific measures include creation of artificial gravity, exercise, low-level lower body negative pressure, and use of anti-deconditioning devices. (From Nicogossian, Space Physiology and Medicine, 2d ed, pp294-297)