# Gravity Slingshot

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#### trisco

##### Guest
Im wondering just how the process of using the gravity of a planet to slingshot a probe to a higher speed works. I have the basic concept but I cant understand one thing... I though that conservation of energy would cause the probe to leave at the same speed it came. If gravity causes it to speed up upon approach then wouldnt it do the opposite when leaving??? arg!

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#### chriscdc

##### Guest
The reason that you can get extra speed via the slingshot maneuver is due to the motion of the center of gravity in relation to the craft you are trying to accelerate.<br /><br />It is imprtant to remember that the direction that you are accelerated in is not the same direction that you leave in. Your velocity in the original direction is unchanged, but you gain vel in the direction that the planet was moving in. However because you time it right you pass behind the planet. You fall toward the planet, but never reach the same speed as the planet, in the direction it is travelling. Thus when you start to move away from the planet, the planet is also moving away from you and so the force on you diminishes faster and so you retain more velocity.<br /><br />You actually steal momentum from the planet, so you don't break any conservation laws. <br /><br />Of course pop Sci-fi (star trek) has completely misrepresented this maneuver.

K

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#### trisco

##### Guest
thanks kmarinas86, this cartoon really helped explain it well, plus the explanation of angular momentum and trajectory bending being the keys.

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#### nexium

##### Guest
If the space craft is traveling twice as fast as the planet orbits the sun, can a gravity assist increase the speed of the space craft to 2.1 times the speed of the planet? The space craft might be 3 times faster than the planet at closest approach. Sometimes the direction change is more valuable than the speed increase? Neil

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#### thalion

##### Guest
IIRC, I don't think a gravity assist can give more than 2x the orbital velocity of the planet.

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#### vogon13

##### Guest
An example:<br /><br />As one approaches Jupiter, one accelerates towards it. Also, after passing Jupiter, one would symetrically decelerate as one moves away from it.<br /><br />--However--<br /><br />You pass Jupiter to the right (or left) and the pull perpendicular to your path is never negated. This will manifest as a change in direction and if set up correctly, a net increase in momentum.<br /><br /><br />(note: when Voyager II passed Neptune in 1989, the geometry of the encounter to align the Triton flyby resulted in a net slight decel of the craft)<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

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##### Guest
A pull perpendicular to your path can't change your speed but as you say changes direction, which is an acceleration <i>transferred</i> via the planets gravity as momentum. Gravity is the means not the power just as rolling downhill. <br />The planet has been slowed but no gravity lost which is a rest product of mass.<br />You can't slingshot around the sun which is at rest but could 'burn' at the best time as shown above.

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#### trisco

##### Guest
really cool sim but hard to get used to if youre an <font color="yellow">id10T</font>like myself, LOL

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