Gravitational slingshot How does this work exactly? Where does my analysis go wrong? I know it is used to speed satellites up, but its name alludes to a permanent improvement in speed. The way I see it, you would get a slight benefit in journey time as long as the satellite is within the (significant) gravitational field of an object. If we assume it is launched at ~infinity, if the target destination is also ~infinity, that means the end speed ~ start speed. The gains come from the acceleration as the satellite moves towards the mass, and away as it would be decelerating but still have a velocity greater than it would have had otherwise traversing empty space. The only other possible source of kinetic energy for the probe I can think of would be through angular momentum transfer from the body to the probe (the body slows down in rotation speed)...but for this to happen you must have an asymmetric mass. Sort of like a planet shaped like an obelisk.