A gravitational slingshot , gravity assist maneuver or swing-by is the use of the relative movement and gravity of a planet to alter the path and speed of a spacecraft , typically in order to save fuel , time , and expense . In orbital mechanics and aerospace engineering , a gravitational slingshot , gravity assist maneuver , or swing-by is the use of the relative movement and gravity of a planet or other celestial body to alter the path and speed of a spacecraft , typically in order to save propellant , time , and expense .