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The orbital speed of a body, generally a planet, a natural satellite, an artificial satellite, or a multiple star, is the speed at which it orbits around the barycenter of a system, usually around a more massive body. It can be used to refer to either the mean orbital speed, i.e. the average speed as it completes an orbit, or the speed at a particular point in its orbit such as perihelia. The orbital speed at any position in the orbit can be computed from the distance to the central body at that position, and the specific orbital energy, which is independent of position: the kinetic energy is the total energy minus the potential energy. ==Radial trajectories== In the case of radial motion: * If the specific orbital energy is positive, the body's kinetic energy is greater than its potential energy: The orbit is thus open, following a hyperbola with focus at the other body. See radial hyperbolic trajectory * For the zero-energy case, the body's kinetic energy is exactly equal to its potential energy: the orbit is thus a parabola with focus at the other body. See radial parabolic trajectory. * If the energy is negative, the body's potential energy is greater than its kinetic energy: The orbit is thus closed. The motion is on an ellipse with one focus at the other body. See radial elliptic trajectory, free-fall time. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「orbital speed」の詳細全文を読む スポンサード リンク
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