Editing Interplanetary spaceflight (section)

===Gravitational slingshot===
{{main|Gravity assist}}

[[File:Voyager 2 velocity vs distance from sun.svg|thumb|right|Plot of ''Voyager 2''{{'}}s heliocentric velocity against its distance from the Sun, illustrating the use of gravity assist to accelerate the spacecraft by Jupiter, Saturn and Uranus. To observe [[Triton (moon)|Triton]], ''Voyager 2'' passed over Neptune's north pole resulting in an acceleration out of the plane of the ecliptic and reduced velocity away from the Sun.<ref>{{cite web|author=Dave Doody|url=http://www2.jpl.nasa.gov/basics/bsf4-1.php |title=Basics of Space Flight Section I. The Environment of Space |publisher=.jpl.nasa.gov |date=2004-09-15 |access-date=2016-06-26}}</ref>]]
The gravitational slingshot technique uses the [[gravity]] of planets and moons to change the speed and direction of a spacecraft without using fuel. In typical example, a spacecraft is sent to a distant planet on a path that is much faster than what the Hohmann transfer would call for. This would typically mean that it would arrive at the planet's orbit and continue past it. However, if there is a planet between the departure point and the target, it can be used to bend the path toward the target, and in many cases the overall travel time is greatly reduced. A prime example of this are the two crafts of the [[Voyager program]], which used slingshot effects to change trajectories several times in the outer Solar System. It is difficult to use this method for journeys in the inner part of the Solar System, although it is possible to use other nearby planets such as Venus or even the [[Moon]] as slingshots in journeys to the outer planets.

This maneuver can only change an object's velocity relative to a third, uninvolved object, – possibly the “centre of mass” or the Sun. There is no change in the velocities of the two objects involved in the maneuver relative to each other.  The Sun cannot be used in a gravitational slingshot because it is stationary compared to rest of the Solar System, which orbits the Sun.  It may be used to send a spaceship or probe into the galaxy because the Sun revolves around the center of the [[Milky Way]].