Planetary "Slingshot" Flybys
What is the slingshot effect for planetary
flybys, and how is it achieved?
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.
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Update: June 2012