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[music throughout]

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The Trojan asteroids
 are found in Lagrange points,

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which are these special places that lead
 or follow a planet in its orbit

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by 60 degrees, and it's sort of where
 the gravitational force of the planet

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and the gravitational force
 of the Sun all cancel out.

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So if you put an object there,
 it will stay there for a long period

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of time, basically forever. 

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So when we see objects there,
 these are objects

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that we know were in place
 a very long time ago.

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So if you just take 

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a random asteroid
 and just put it in the outer Solar System,

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the gravitational force 

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of the four outer planets will just clear
 them out in a very short period of time.

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So they'll basically be gone. 

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So the only places
 it turns out in the outer Solar System

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where you can find stable regions
 are these Lagrange points.

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Lucy in particular,
 is going to go after the Jupiter Trojans.

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We're trying to see a type of object

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that represents and constrains
 the formation of the outer planets.

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And you need to go to these Lagrange
 points in order to see that kind of thing.

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One of the really groovy 

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aspects of our mission is its trajectory,
 because we're visiting

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a record number of objects
 like these Trojans.

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And we do that with this very complicated
 dance, particularly in the beginning,

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where we are using the earth
 actually as a gravitational slingshot.

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So Lucy will start off in an orbit
 very similar to the Earth,

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and then gravitational encounters
 with the Earth will actually pump it up.

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So it gets out to the Lagrange points
 near Jupiter's orbit.

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So if you're going to understand
 how planets like the Earth formed,

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you have to understand
 how the bigger planets

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which sort of dominated
 the whole process came to be.

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And that's what Lucy is going to do.