
Gravity assist (2013) - betolink
http://www.planetary.org/blogs/guest-blogs/2013/20130926-gravity-assist.html
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jessriedel
Honestly, the easiest way to see how this is consistent with conservation of
energy is to just imagine that the craft is physically striking the planet and
bouncing elastically from it. It's very obvious that if you lightly toss a
whiffle ball into the path of a speeding bowling ball then the whiffle ball
will exit the interaction with much more speed than it entered it (and, in
fact, with as much as twice the speed of the bowling ball).

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ridgeguy
I don't think that's right. Modeling Voyager's interaction with Jupiter as an
elastic collision with the planet gives an incorrect answer. The spacecraft's
speed with respect to Jupiter doesn't change.

As the post discusses, Voyager's speed (scalar) _in the Jupiter frame_ is
essentially unchanged. Its velocity (vector) of course changes quite a lot
because the spacecraft's direction changes as a result of the encounter, but
its speed with respect to Jupiter (and therefore its kinetic energy) isn't
significantly different. It approached Jupiter with enough energy that it
couldn't be captured and left with pretty much the same energy _with respect
to Jupiter_.

What matters is that Voyager's speed _with respect to the sun_ is greatly
increased during its interaction with Jupiter's gravitational field. And since
the point of the mission is to escape the sun, what matters is the
spacecraft's energy state with respect to the sun, not to Jupiter.

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jessriedel
A whiffle ball's speed with respect to the bowling ball doesn't change in the
bowling ball's rest frame.

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PaulHoule
But if NASA does this too much, won't Jupiter spiral into the sun?

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pdonis
For an appropriate value of "too much", yes. As a rough order of magnitude,
"too much" would be about 10^24 such encounters. For comparison, that is about
the same as the number of microseconds since the Big Bang.

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grogers
For another perspective, 10^24 voyagers is about 100 times the mass of earth.
That isn't even including the propellant and rockets needed to launch them
all.

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pdonis
_> 10^24 voyagers is about 100 times the mass of earth._

Yes, or about the mass of Jupiter, which is where the number came from--the
article's comparison of the two.

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m_mueller
I'm thinking "slingshot" might actually be a better term than "Gravity
assist". Gravity is just the force through which energy from an object's orbit
is transferred to another object. Putting it first probably makes many people
wonder why it doesn't cancel out.

