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A spacecraft cannot simply fly straight to a distant planet, because the planet is a moving target. Instead, engineers aim the craft at the spot in space where the planet will be many months or years later, when the craft finally arrives. They also use the gravity of planets along the way to gain speed for free, swinging past a world so that its pull flings the craft faster toward its goal. Careful timing makes these long journeys possible.

Engineers do not aim a spacecraft at a planet's current position but at where it —willdidnever be on arrival, and they use a planet's —gravitycolornoise to gain speed without using extra fuel.