Swingby

Meet Swingby, a space probe who steals a little speed from a giant planet without using fuel.

Seen from the planet, a probe that swings past leaves with the same speed it arrived with, only in a new direction. But the planet is moving around the Sun. Seen from the Sun, the probe can pick up some of the planet's speed.

In the best case, flying head-on into a planet moving at U, the probe gains up to 2U. Jupiter orbits at about 13 km/s, so in principle a probe could gain up to about 26 km/s. Real flybys gain less, because the path bends by less than 180°.

The planet slows down by an amount far too tiny to measure, because it is vastly heavier than the probe.

In 1977 the two Voyager probes used a rare line-up of the outer planets to hop from one to the next, and they are now travelling between the stars.