How do gravitational sligshots work?

Dave - This is a really neat trick which NASA and all the space agencies use called the gravitational slingshot. The way it works is that, because the planets are orbiting they're moving. They're not stationary so if you imagine firing yourself if you're a space probe, going outwards towards a planet. Imagine going just behind the planet. The planet's gravity is going to pull you towards it and because the planet is moving away from you all the time it's going to pull you towards it more than it would do if it were stationary because you've got to catch up with it as keeps moving away from you. You don't move towards it as quickly as you would have done so you gain energy. Once you have left the planet you actually gain a load of energy from the planet and you slow the planet down a bit. Chris - Do you have to get the angle of attack or approach to the planet so that as the planet moves away from you it doesn't pull you off course again. Dave - The whole point is that it's using the planet to pull you off course in a way that helps you. You've got to have a very careful approach. If you want to speed up you're behind the planet . If you want to slow down you go in front of it. Overall this will give you more energy or less energy and get you to where you want to be.