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gravity assist

gravity assist A technique used to accelerate or slow spacecraft without consuming fuel by transferring angular momentum from a planet to the spacecraft. The craft approaches the planet, is accelerated towards it by gravitational force, and as it leaves decelerates to its original speed relative to the planet, but not relative to the Sun. If a spacecraft approaches from behind, travelling in the same direction as the planet, it will accelerate relative to the Sun by acquiring angular momentum; if it approaches from ahead, travelling in the opposite direction to the planet, it will slow relative to the Sun by losing angular momentum to the planet.

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