TARGET 090909 Zero Gravity

Weightlessness ---- is a phenomenon experienced by people during free-fall. Although the term zero gravity is often used as a synonym, weightlessness in orbit is not the result of the force of gravity being eliminated or even significantly reduced (in fact, the force of the Earth's gravity at an altitude of 100 km is only 3% less than at the Earth’s surface). Weightlessness typically occurs when an object or person is falling freely, in orbit, in deep space (far from a planet, star, or other massive body), in an airplane following a particular parabolic flight path (e.g., the “Vomit Comet”), or in one of several other more unusual situations.

Weightlessness occurs whenever all forces applied to a person or object are uniformly distributed across the object's mass (as in a uniform gravitational field), or when the object is not acted upon by any force. This is in contrast with typical human experiences in which a non-uniform force is acting, such as: * standing on the ground, sitting in a chair on the ground, etc., where gravity is countered by the reaction force of the ground * flying in a plane, where a reaction force is transmitted from the lift the wings provide (special trajectories which form an exception are described below) * during atmospheric reentry, or during the use of a parachute, when atmospheric drag decelerates a vehicle * during an orbital maneuver in a spacecraft, or during the launch phase, when rocket engines provide thrust

In cases where an object is not weightless, as in the above examples, a force acts non-uniformly on the person or object in question. Aerodynamic lift, drag, and thrust are all non-uniform forces (they are applied at a point or surface, rather than acting on the entire mass of an object), and thus prevent the phenomenon of weightlessness. This non-uniform force may also be transmitted to an object at the point of contact with a second object, such as the contact between the surface of the Earth and one's feet, or between a parachute harness and one's body.

Gravity is a field force which can usually be considered to act uniformly on the mass of all people and objects in the frame of reference. This assumption is valid when the size of the region being considered is small relative to its distance from the center of mass of the gravitational attractor. The small size of a person relative to the radius of Earth is one such example. In contrast, objects near a black hole are subject to a highly non-uniform gravitational field.

Often, the terms zero gravity or reduced gravity are used to mean weightlessness as it is experienced by orbiting spacecraft, but this is not technically accurate. Spacecraft are held in orbit by the gravity of the planet which they are orbiting. In Newtonian physics, the sensation of weightlessness experienced by astronauts is not the result of there being zero gravitational acceleration (as seen from the Earth), but of there being zero difference between the acceleration of the spacecraft and the acceleration of the astronaut. Space journalist James Oberg explains the phenomenon this way:

The myth that satellites remain in orbit because they have "escaped Earth's gravity" is perpetuated further (and falsely) by almost universal use of the zingy but physically nonsensical phrase "zero gravity" (and its techweenie cousin, "microgravity") to describe the free-falling conditions aboard orbiting space vehicles. Of course, this isn't true; gravity still exists in space. It keeps satellites from flying straight off into interstellar emptiness. What's missing is "weight", the resistance of gravitational attraction by an anchored structure or a counterforce. Satellites stay in space because of their tremendous horizontal speed, which allows them — while being unavoidably pulled toward Earth by gravity — to fall "over the horizon." The ground's curved withdrawal along the Earth's round surface offsets the satellites' fall toward the ground. Speed, not position or lack of gravity, keeps satellites up, and the failure to understand this fundamental concept means that many other things people "know" just ain't so.

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To a modern physicist working with Einstein's general theory of relativity, the situation is even more complicated than is suggested above. Einstein's theory suggests that it actually is valid to consider that objects in inertial motion (such as falling in an elevator, or in a parabola in an airplane, or orbiting a planet) can indeed be considered to experience a local loss of the gravitational field responsible for their general motion. Thus, in the point of view (or frame) of the astronaut or orbiting ship, there actually is nearly-zero acceleration, just as would be the case far out in space, away from any mass. It is thus valid to consider that most of the gravitational field in such situations is actually absent from the point of view of the falling observer, just as the colloquial view suggests (see equivalence principle for a fuller explanation of this point).

However, this loss of gravity, in Einstein's theory, is for a different reason that is popularly supposed: the loss of gravity in orbit, or in a falling elevator, is due to the falling motion itself, and not due to increased distance from the Earth. However, the gravity nevertheless is considered to be absent. In the theory of general relativity, the only gravity which remains for the observer following a falling path or "inertial" path, is that which is due to non-uniformities in the gravitational field. This non-uniformity, which is a tidal effect, constitutes part of the "microgravity" which is felt by all spacially-extended objects falling in any natural gravitational field originating from a mass, since such a field will have its origin in a centralized place (the compact mass), and thus will vary slightly in strength, according to distance from the mass.

Many thanks to Ray McClure for suggesting and programming this target.