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? [music] ?

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KIDS: Our World!

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EVAN: Hey Globey, have you ever
wondered what keeps satellites,

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like the International Space
Station and the space shuttle

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in orbit? Once they get to a
certain place in space, they

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just keep going round and round
Earth. But I also know about

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inertia - moving objects have a
tendency to keep

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moving in a straight line.
So what’s to stop them from

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just flying off into outer
space? [crash]

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Whoops...

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Well, it turns out that there
is a balance between inertia

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and gravity acting on these
satellites. Now we all know

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that gravity is the attraction
between objects. How big that

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attraction is depends on the
mass of the objects (or how big

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they are) and how far away the
objects are from each other.

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Objects on or near Earth are
caught in this attraction and

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are pulled down toward Earth’s
surface.

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So, even if you throw a ball up
in the air,

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gravity pulls the ball back
down to the ground.

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But in space, all that seems to
change.

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I’m sure you’ve seen pictures
of astronauts floating around

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inside the space shuttle and we
know the astronauts don’t float

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around here on Earth. So does
that mean there’s no gravity

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once the astronauts get in
orbit? And what about way out

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there in outer space? Is
gravity still

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important out there?

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Actually, gravity is still at
work, even as far away as outer

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space. So what do you think
keeps the space station or

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space shuttle from floating
away into deep space?

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You got it, gravity! But if
there’s gravity in space,

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why do the astronauts
experience weightlessness?

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And why do they appear to be
floating around? I know.

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It’s a bit complicated. So
let’s ask a friend to help

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explain it.

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Dr. John Bacon is an engineer
at the

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NASA Johnson Space Center in
Houston, Texas.

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JOHN BACON: Hi Evan. A lot of
people ask me about why gravity

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seems to go away when you get
to orbit.

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I can explain that with my keys
here. I have a little elastic

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on the keys, and I’m going to
show what happens when I drop

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this whole system. The elastic
is all stretched out and

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gravity is the most important
in setting up how my keys

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behave at the moment. But the
instant that I let it fall,

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gravity is not going to be
important anymore.

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It’s not going to fit against
my fingers. You’ll see this

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whole thing change in 3-2-1.

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You’ll see that even before the
keys hit my hand this little

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elastic has immediately
relaxed. I’ve changed the ways

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the keys behave when I let it
fall.

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The same thing is true with
water on the surface of the

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table. It has a nice flat
surface for days and days and

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days until it finds the edge of
the table and is allowed to

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trickle off. Instantly when it
falls we get surface tension

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taking the water into a nice
little cylinder and then air

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forces start sheering it into
drops and you see it do

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anything but stay flat. When we
take gravity away by letting

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things fall we get a whole new
behavior.

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The problem is if we get high
above the planet Earth,

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things start falling towards
it. No matter where I go around

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the equator, I will fall to the
middle of the Earth.

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Then I’m going to hit the Earth
rather hard. So if I want to

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get this wonderful experience
of falling forever, what I need

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to do is go sideways very fast.

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If I can go sideways fast
enough, I’ll actually fall

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right over the horizon.

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I’ll just keep going sideways
over the center of the Earth

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pulling me to the middle. If
I’m on board the shuttle or a

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space station up here in the
vacuum, I’m going to feel like

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I’m falling forever. Everybody
on the station and all the

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experiments with them appear to
float in the middle of the

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cabin forever. Notice there’s
actually lots of gravity there.

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It’s what’s holding the space
station right next to the

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Earth. Gravity is all the way
out as far as the Moon.

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It’s what holds the Moon in its
orbit around us. But when we

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get going sideways fast enough,
we just keep falling.

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EVAN: Just like skydivers,
astronauts experience

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weightlessness as they fall
towards Earth. So even though

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gravity is constantly pulling
them down, they can’t feel

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their own weight as they are
falling. And if there are two

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skydivers falling together, it
seems as if they’re floating

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toward each other. Hey, don’t I
know you?

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But what keeps the space
shuttle from falling to the

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ground like the skydivers?
Well, like Dr. Bacon said, the

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space shuttle would indeed fall
if it wasn’t traveling

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so fast around Earth.

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How fast does the shuttle
actually have to travel to stay

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in orbit? The shuttle is flying
approximately 28,000 kilometers

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per hour. That’s 17,500 miles
per hour!

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That’s so fast the shuttle
makes a complete orbit around

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Earth in just 90 minutes. Can
you imagine that?

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So, the astronauts are being
carried with the shuttle as it

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orbits Earth. At the same time,
Earth is pulling on the

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shuttle and everything inside,
but because the shuttle is

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moving sideways so fast, the
astronauts, and anything else

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that isn’t fastened down, are
getting pulled to the middle of

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the shuttle, making it look as
if they are floating.

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And that same sideways speed
keeps the shuttle

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itself from falling back to
Earth.

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I bet you’re wondering how the
shuttle ever comes back to

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Earth if it’s traveling too
fast to fall to the surface.

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The answer is simple. They have
to slow the shuttle down.

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The shuttle commander actually
fires the shuttle’s thrusters

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in reverse. This slows the
space shuttle down, and because

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it’s no longer traveling fast
enough to stay in orbit, the

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shuttle falls into the
atmosphere and begins its

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journey back to the surface of
the Earth.

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Gravity sure is complex. And
every time astronauts travel

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into space, NASA learns more
about this special attraction

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that keeps all kinds of things
in their places, including us!

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Do you think you’d like the
feeling of floating forever?

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I don’t know. I think it might
make me a little sick to my

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stomach. I think I ought to
rethink that whole

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skydiving thing!

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? [music] ?

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KIDS: Our World!