How a gravity assist changes a spacecraft's path
A spacecraft passing a moving planet can exchange energy with the planet's orbital motion. The encounter may increase or decrease the spacecraft's speed relative to the Sun while changing its direction. The planet's corresponding change is tiny because its mass is so much greater than the spacecraft's.
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A moving planet provides the crucial ingredient
During a flyby, a planet's gravity bends a spacecraft's trajectory. In a simplified view centred on the planet, the spacecraft approaches and departs with approximately the same speed far from the encounter, but in different directions.
The planet itself is moving around the Sun. Combining that motion with the redirected spacecraft velocity can change the spacecraft's speed and energy in the Sun-centred view.
The result depends on the approach geometry. A flyby can speed the spacecraft up, slow it down or mainly redirect it.
- Planetary gravity turns the relative trajectory.
- The planet's orbital motion enables an energy exchange.
- Mission planners choose the encounter geometry.
Energy is conserved across the full interaction. The planet experiences an opposite change, but its enormous mass makes that change negligible for ordinary mission planning.