Planetary "Slingshot" Flybys ```Name: Glenn Status: other Grade: other Location: NV Country: N/A Date: N/A ``` Question: What is the slingshot effect for planetary flybys, and how is it achieved? Replies: Glenn, The slingshot effect occurs when a spacecraft flying by a planet uses the gravitational pull of the planet to get a "boost" in the spacecraft's heliocentric speed. (This assumes we are talking about an Earth spacecraft flying to an outer planet. An Earth spacecraft flying to an inner planet actually experiences a decrease in its heliocentric speed. For our discussion, we will assume the spacecraft is flying to an outer planet.) When a spacecraft flies to an outer planet, upon its arrival, it is traveling slower than the planet (i.e. its heliocentric speed is less than the planet's heliocentric speed. This is predicted by the vis-viva equation, a famous equation in orbital mechanics.) Now, the spacecraft's heliocentric velocity (its velocity relative to the Sun) can be broken down into the vector sum of the spacecraft's velocity relative to the planet + the planet's velocity relative to the Sun. When the spacecraft flies by the planet, the planet pulls on the spacecraft, which has the effect of "turning" the spacecraft's velocity vector relative to the planet, (i.e. changing its direction) but not changing its magnitude. As the spacecraft leaves the planet, its new heliocentric velocity is the vector sum of the spacecraft's new velocity vector relative to the planet + the planet's velocity relative to the sun (which is unchanged by the flyby). A quick vector addition diagram shows us that the magnitude of the spacecraft's new heliocentric velocity is always larger than the magnitude of the spacecraft's old heliocentric velocity. And the magnitude of any velocity vector is equal to the speed. Thus we see that a spacecraft flying by an outer planet experiences an increase in its heliocentric speed. Take care, Aaron Brown Click here to return to the Astronomy Archives

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