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Gravity assist. A gravity
assist is a planetary

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flyby technique that's 
used by spacecraft to get
to where they're going.

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It's often used to speed up
the spacecraft's velocity

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and propel it outward 
in the solar system, much
farther away from the Sun

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than the rocket it 
launched on would have
been capable of doing

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so on its own.

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The technique has been used

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for multiple missions,
from the Voyagers to
the Parker Solar Probe.

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So how does it work?

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A gravity assist can change
the velocity of a spacecraft

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by entering the gravitational
influence of a large object,
like a planet.

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The orbit of a planet
around the Sun is used
as a source of energy.

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The spacecraft falls in
toward the planet under
the influence of gravity,

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and then it flies back away.

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A gravity assist can either
increase or decrease

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the energy of the spacecraft's orbit.

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If the spacecraft moves
around the planet

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in the same direction
as the planet's 
orbit around the Sun,

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it'll gain a bit of energy,
as done with the Voyager
missions around Jupiter.

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The Lucy spacecraft will need

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two gravity assists to
get on trajectory to
the Trojan asteroids.

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The second assist will
happen in December 2024,
giving Lucy the energy

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it needs to cross to the 
main asteroid belt, where 
it will observe the asteroid

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Donaldjohanson, then 
travel into the leading
Trojan asteroid swarm.