Gravity assist

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The manoeuvre that lets a spacecraft gain speed relative to the Sun without burning fuel, by passing close to a moving planet. In the planet's own frame the spacecraft arrives and leaves at the same speed, merely turned through an angle; in the Sun's frame that turn adds the planet's orbital velocity vector to the spacecraft's, and the spacecraft leaves faster. Momentum is conserved because the planet is slowed by an exactly compensating amount — an amount so small against Jupiter's mass that it is unmeasurable. The technique was worked out in the early 1960s, notably by Michael Minovitch, and applied at Venus by Mariner 10 before Voyager used it four times in sequence. It is what made an outer-planet tour affordable: reaching Neptune directly with a 1970s launch vehicle would have taken about thirty years, and Voyager 2 did it in twelve. Not a place.

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