The gravitational slingshot effect can increase the speed of a spacecraft, but not decrease it.

Question

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The gravitational slingshot effect can both increase and decrease the speed of a spacecraft. The direction and relative speed of the spacecraft and celestial body being used for the slingshot determine the effect on the speed. The gravitational slingshot effect can both increase and decrease the speed of a spacecraft by using the gravity of a planet or other celestial body to alter the spacecraft's trajectory.

The Gravitational Slingshot Effect Can Increase the Speed of a Spacecraft