In planning certain types of trajectories of spacecraft within the solar system, engineers rely on a technique called gravitational assist, or gravity assist. This technique underlies the feasibility of effecting a net change in both the speed and direction of motion of a spacecraft by passage through the gravitational field of a planet or a planetary satellite. The resulting increase, or decrease, in the kinetic energy of the spacecraft appears to contradict the casual expectation that in such an encounter the kinetic energy of the spacecraft after the encounter would be the same as that before the encounter. This paper describes the December 1973 encounter of the Pioneer 10 spacecraft with the planet Jupiter as a real-life example of gravitational assist. It then discusses the physical principles involved in understanding the dynamics of the encounter and concludes with remarks on the important role of gravitational assist in space exploration with artificial spacecraft and in understanding the motion of comets within the solar system.

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