How does gravity assist with interplanetary satellites?

The idea behind a gravity assist is to use a planet's motion to accelerate a satellite. Check out our article on HowStuffWorks to learn more about the effects of gravity on satellites.

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Episode Transcript

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(00:28):
I'm Marshall Brain with today's question, how does gravity assist
work with interplanetary satellites? When NASA sends satellites out to
the outer reaches of the Solar System, it will often
send the satellites past the planet for a gravity assist.
The satellite speeds up in the process. The idea behind

(00:50):
a gravity assist is to use the planet's motion to
accelerate the satellite. For example, as satellite heads towards Jupiter
in the process, it accelerates because it's falling towards Jupiter
through Jupiter's gravity. Then it passes fairly close to the
planet and starts speeding away from it. However, at that point,

(01:10):
the satellites starts slowing down because gravity is pulling it
back towards the planet. From that description, it would seem
like the net effect of gravity assist is zero. The
satellite gains speed as it falls towards the planet, but
then it loses it as it heads away. The thing
that makes the assist work is the fact that the
planet is in motion in its orbit. Jupiter, for example,

(01:33):
is about five hundred million miles away from the Sun,
which means that the circumference of its orbit is about
three billion miles. Jupiter travels that distance in about twelve years,
so it's moving through space at about thirty thousand miles
per hour. If the satellite is moving in the same
direction as Jupiter in its orbit, then it can actually

(01:54):
increase its speed by thirty thousand miles per hour. That's
a huge speed increase, and it's completely free. The problem
with the assist is that you have to wait for
the planets to line up correctly for it to work.
That's why missions have to fly within certain time windows.
For more on this and thousands of other topics, is

(02:15):
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