Dynamical friction

Dynamical friction is a term in astronomy related to loss of kinetic energy of a moving object through a gravitational interaction with surrounding matter in space. It is sometimes reffered to as gravitational drag. When the dynamical friction happens to be negative (gain of kinetic energy of an object) when a moving object passes behind a planet and a part of the kinetic energy of a planet is transfered through a gravitational interaction to the object under consideration, it is called sling effect instead and it might be used to accelerate space probes.