# What is a 'Gravitational Slingshot'?

## Main Question or Discussion Point

I was wondering, since i've heard it said so many times. They space probes use gravitational slingshots. Okay, I get it, a probe goes around a planet or something, and when it goes around the other side, its faster right? Well...how exactly does it work?
1: How does a Gravitational Slingshot work?
2: How much faster does the slingshot make the object?
3: Does the object go on at the speed the slingshot gives it forever? (In space, i thought you never slow down since there is no friction.)
4: Is there some mathematical formula for this? If so, what is it? thanks!
5: anything else you can tell me about it helpful? ANYTHING about gravitational slingshots really will help!

## Answers and Replies

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DaveC426913
Gold Member
1. As a probe swings past a planet, they exchange some of their momentum. If the trajectory is planned well, the probe comes away with a little more momentum and the planet coems away with a little less. Slingshot 100 million probes past Jupiter, and it'll start falling sunward.

It can work in reverse too. You can slow probes the same way. You might want to do this if going to an inner planet and you want to slow to go into orbit (it will have too much velocity from falling sunward).

2. http://en.wikipedia.org/wiki/File:Cassini's_speed_related_to_Sun.png

3. Well, it gives it a kick. What happens to it after that is up to the gravity of any other objects it's around (the sun will work on it as long as it is in or near the solar system).

4. It's a pretty complex maneuver.

5. http://en.wikipedia.org/wiki/Gravity_assist

Last edited:
Filip Larsen
Gold Member
2. According to [1] the maximum change in speed for an unpowered planetary flyby is

3.01 km/s Mercury
7.33 km/s Venus
7.91 km/s Earh
3.55 km/s Mars
42.73 km/s Jupiter
25.62 km/s Saturn
15.18 km/s Uranus
16.75 km/s Neptune
1.10 km/s Pluto (still counted as a planet back in 1998 :)

Since change in speed very much depends on how close a probe passes the planet, the maximum values above comes from assuming the probe just grazes the planet surface during flyby (any closer and the probe will impact on the surface). Since probes in practice obviously has to stay out of any planetary atmosphere that might be present, the actual maximum change obtainable in a practical mission may be somewhat lower. There may also be other constraints that "lowers" the maximum on a particular mission, such as geometrically constraints requiring the probe to originate from one planet, flyby a second and then head off to a third.

[1] Multiple Gravity Assist Interplanetary Trajectories, Labunsky, Papkov, and Sukhanov. Gordon and Breach Science Publishers, 1998.