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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)Space Flight: Travel beyond the earth's atmosphere.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)Cosmic Radiation: High-energy radiation or particles from extraterrestrial space that strike the earth, its atmosphere, or spacecraft and may create secondary radiation as a result of collisions with the atmosphere or spacecraft.Space Suits: Pressure suits for wear in space or at very low ambient pressures within the atmosphere, designed to permit the wearer to leave the protection of a pressurized cabin. (NASA Thesaurus, 1994)Space Simulation: An environment simulating one or more parameters of the space environment, applied in testing space systems or components. Often, a closed chamber is used, capable of approximating the vacuum and normal environments of space. (From NASA Thesaurus, 1988) This also includes simulated EXTRAVEHICULAR ACTIVITY studies in atmosphere exposure chambers or water tanks.Aerospace Medicine: That branch of medicine dealing with the studies and effects of flight through the atmosphere or in space upon the human body and with the prevention or cure of physiological or psychological malfunctions arising from these effects. (from NASA Thesaurus)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)United States National Aeronautics and Space Administration: An independent Federal agency established in 1958. It conducts research for the solution of problems of flight within and outside the Earth's atmosphere and develops, constructs, tests, and operates aeronautical and space vehicles. (From U.S. Government Manual, 1993)Solar Activity: Any type of variation in the appearance of energy output of the sun. (NASA Thesaurus, 1994)Extravehicular Activity: Activities by crew members conducted outside the pressurized hull of a spacecraft.Spacecraft: Devices, manned and unmanned, which are designed to be placed into an orbit about the Earth or into a trajectory to another celestial body. (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)Visible Human Projects: Digital image data sets, consisting of complete, anatomically detailed, three-dimensional representations of the normal male and female human bodies.Linear Energy Transfer: Rate of energy dissipation along the path of charged particles. In radiobiology and health physics, exposure is measured in kiloelectron volts per micrometer of tissue (keV/micrometer T).Bed Rest: Confinement of an individual to bed for therapeutic or experimental reasons.Orthostatic Intolerance: Symptoms of cerebral hypoperfusion or autonomic overaction which develop while the subject is standing, but are relieved on recumbency. Types of this include NEUROCARDIOGENIC SYNCOPE; POSTURAL ORTHOSTATIC TACHYCARDIA SYNDROME; and neurogenic ORTHOSTATIC HYPOTENSION. (From Noseworthy, JH., Neurological Therapeutics Principles and Practice, 2007, p2575-2576)Blood Circulation Time: Determination of the shortest time interval between the injection of a substance in the vein and its arrival at some distant site in sufficient concentration to produce a recognizable end result. It represents approximately the inverse of the average velocity of blood flow between two points.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.Hypotension, Orthostatic: A significant drop in BLOOD PRESSURE after assuming a standing position. Orthostatic hypotension is a finding, and defined as a 20-mm Hg decrease in systolic pressure or a 10-mm Hg decrease in diastolic pressure 3 minutes after the person has risen from supine to standing. Symptoms generally include DIZZINESS, blurred vision, and SYNCOPE.Head-Down Tilt: Posture while lying with the head lower than the rest of the body. Extended time in this position is associated with temporary physiologic disturbances.Heavy Ions: Positively-charged atomic nuclei that have been stripped of their electrons. These particles have one or more units of electric charge and a mass exceeding that of the Helium-4 nucleus (alpha particle).