How are slingshot mechanisms used to accelerate interplanetary spacecraft? - Answers

Good question. Imagine a spacecraft is approaching a planet. The planet is moving around the sun. The spacecraft path is adjusted to approach the trailing limb of the planet -- the rear edge of the planet when you look at its orbit around the sun, not its dark side. The planet pulls on the spacecraft as it goes by (and actually the spacecraft pulls on the planet, too). If the spacecraft were close enough to the planet, and traveling slowly enough, it would be captured by the planet. But it is possible to put the space craft in a path so that will not be captured--it can be pulled by the planet so that the spacecraft gains velocity. The planet loses velocity, but since planets are huge and spacecraft small, the planet's velocity is barely affected. It is hard to visualize this, but imagine a ping pong ball being struck by a soccerball in mid-air (this would make a good science class demonstration)--the ping pong ball will pick up tremendous speed by being struck by a heavier ball. The heavy ball will hardly notice it. You can do this by dropping the soccer ball with the ping pong ball on top of it. Slingshotting a spacecraft (also called gravity assist) works in a similar way except the spacecraft would be pulled by the planet's gravity instead of being pushed (as with the two-ball demonstration).