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- If you had to guess,

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where would you think
our solar system starts,

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and where do you think it ends?

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If you think our solar system
starts at the Sun,

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then you would be right.

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How about where it ends?

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Does it end at A,
just past Neptune;

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B, at the Kuiper Belt;

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or C, past the Oort Cloud?

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The answer is...

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If you guessed C,
past the Oort Cloud,

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you're correct.

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- The Oort Cloud
is a giant cloud

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or collection of comets
that surrounds the Sun.

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It's huge,
and all of these comets

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are--are bound to the Sun
gravitationally,

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which means they are
in orbit around the Sun.

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They're not escaping
into interstellar space.

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So they really belong
to our solar system.

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The limits of our solar system
are really defined

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by the limits
of the Sun's gravity,

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and that extends, basically,

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to the edges
of the Oort Cloud.

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The solar system begins
at the Sun.

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That's the main body in the
solar system, gravitationally.

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The solar system ends where

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the gravitational influence
of the Sun stops or fades out.

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- I get it!

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So if you thought
our solar system ended

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with our planets,
you're wrong.

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The solar system begins
with our Sun

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and extends way beyond
the planets.

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But how does NASA know that?

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- One way to tell
where the solar system ends

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is by actually going there
with a spacecraft.

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NASA studies the solar system
through missions

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that go out and investigate
planets.

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For example, Voyager
was launched in the 1970s

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to go to Jupiter, Saturn,
Uranus, and Neptune,

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to study those planets
up close, and their moons.

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And Voyager went to the edge
of the Heliosphere,

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and it passed through a--
kind of a boundary

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into interstellar space,

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and that's one definition of
where the solar system ends.

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Another way is to track
the orbits of comets,

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and we can see
that they come from this--

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these extremities
of the Oort Cloud

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that are so far away
from the Sun.

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So we get a feeling for how
large the Oort Cloud is

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by looking at the orbits
of these comets.

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- And that's what is
so exciting about science.

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New technologies continue
to help us learn more

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about our vast solar system.

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How vast is our solar system?

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Tune into part two,
"Scaling the Solar System,"

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to learn more.

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[exciting music]

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♪ ♪