STEAL A LITTLE MOMENTUM
A spacecraft can change speed by flying past a moving planet. Shape the encounter and see how the same flyby looks from two different frames.
TWO WAYS TO LOOK AT ONE FLYBY
The spacecraft swings around the planet. In the planet’s frame, it leaves with about the same speed it arrived with—just pointed in a new direction.
The planet is moving. Its motion combines with the spacecraft’s changed direction: one flyby can add speed, while the other can take some away.
A closer pass bends the path more. A faster approach bends it less. Choose a planet and try both sides of its orbit.
This is a simplified two-body model: it ignores atmospheres, moons, and the gravity of other bodies. Planet speeds and sizes are rounded example values.
NASA’s gravity-assist primer explains how spacecraft can gain or lose heliocentric speed during a planetary flyby.
NASA · Basics of Spaceflight: A Gravity Assist Primer ↗