The encounter between a moving spacecraft and a moving planet in which the spacecraft experiences an elastic collision affecting its motion. Depending on the nature of the encounter, the spacecraft can gain or lose appreciable amounts of energy, and experience changes in the direction of its motion.

Swing-by manoeuvres with the moon have been used to reach the L1 Lagrangian point and fly-by manoeuvres with the planets have played an essential role in space missions exploring the solar system.