Editing Orbital mechanics (section)

===Gravity assist and the Oberth effect===
In a [[gravity assist]], a spacecraft swings by a planet and leaves in a different direction, at a different speed.  This is useful to speed or slow a spacecraft instead of carrying more fuel.

This maneuver can be approximated by an [[elastic collision]] at large distances, though the flyby does not involve any physical contact.  Due to Newton's third law (equal and opposite reaction), any momentum gained by a spacecraft must be lost by the planet, or vice versa.  However, because the planet is much, much more massive than the spacecraft, the effect on the planet's orbit is negligible.

The [[Oberth effect]] can be employed, particularly during a gravity assist operation. This effect is that use of a propulsion system works better at high speeds, and hence course changes are best done when close to a gravitating body; this can multiply the effective [[delta-v]].