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MISSION CONTROL: Main engine
ignition. 4,3,2,1…

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Zero and liftoff.

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JOSH: Ever notice shuttle
launches happen at all hours?

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Some are during the day, some
at dusk,

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some in the middle of the
night. Ever wonder why?

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Well, there’s a lot of math
that happens in order to figure

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out what time the shuttle takes
off. So get out your calculator

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and we’ll show you how to come
up with a good time

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for lift off, next on Real
World.

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? [music] ?

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JOSH: One of the most important
missions of the space shuttle

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is to deliver astronauts and
material to the International

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Space Station. Seems pretty
easy, right.

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But it’s a lot more complicated
than typing ISS into a

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GPS and taking off. The
International Space Station

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or ISS, is in orbit, around the
earth, and for the space

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shuttle to catch up and dock…
well, it’s a lot trickier than

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it sounds. The lesson starts
with gravity.

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DR. JOHN BACON: If we go around
the earth and any other planet

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or heavenly body, there is
gravity that is going to pull

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us to the middle of it wherever
we go. And in any orbit all

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that we end up doing is we go
fast enough sideways that we

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just truly fall over the
horizon.

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JOSH: Dr John Bacon is a
systems integration engineer

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for ISS at NASA’s Johnson Space
Center.

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DR. BACON: And near Earth we
have to do it about

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8 kilometers a second or 5
miles a second.

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We get above the atmosphere and
we just stay in a continuous

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circle going around the center
of the plane.

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Imagine the plane, a flat sheet

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that goes right through the
planet. And draw a circle on

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that and that represents our
orbit. And that sheet is

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called the orbit plane. Now we
don’t always go right around

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the equator, and in fact it’s
quite hard to do that from the

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Kennedy Space Center, here.
What we have to do is go around

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the center of the planet. So we
have to get into an orbit

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that inclines around the planet
so that it goes over the

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Kennedy Space Center. That
plane has to be centered on

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earth’s center and has to go
over the

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Kennedy Space Center somehow.

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JOSH: So the Space Station is
traveling along its orbit,

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on its orbital plane. We know
that an object’s orbit is fixed

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in space. It follows the same
path over and over.

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At the same time the ISS is
orbiting at nearly 28,800 km

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per hour, Earth is rotating at
a speed of 1,665 km per hour.

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If the ISS passes over the
Kennedy Space Center during one

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orbit, the Earth will have
rotated about 22.5 degrees in

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the 90 minutes it takes the
station to make another orbit,

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putting the path of the ISS
about 1600 kilometers

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west of Kennedy. To find out
how long it will take to pass

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over KSC again, we need to
divide the circumference of

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Earth, approximately 40,000 km,
by the speed of Earth’s

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rotation, approximately 1,665
km/hour and we see that the ISS

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will pass in the correct spot
only once every 24 hours.

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DR. BACON: We have pretty open
ocean heading up to the north

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and so we always like to head
out into orbit on the pass that

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sends us over the Kennedy Space
Center,

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heading from south to north.
This happens once a day.

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JOSH: So if it’s launch day,

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that is the moment the shuttle
would launch.

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DR. BACON: So we get this very
narrow window of time, when the

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Kennedy Space Center is under
the disc that is being made by

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the Space Station. And that’s
called our launch window.

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JOSH: But once launched, and in
orbit,

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it still has to catch up to the
space station.

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DR. BACON: The shuttle only has
a limited amount of resources

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on board. It can stay in orbit
for about 17 days and we like

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to spend as many of those days
as possible docked

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with the space station.

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JOSH: Once in orbit, the
shuttle travels at the same

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speed as the international
space station,

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28,800 kilometers per hour.

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But if it’s traveling at the
same speed,

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how does it catch up?

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By traveling on in inside
track!

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Here to demonstrate this
concept is Madi and Alice.

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Madi will represent the
International Space Station

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and Alice, the Space Shuttle.

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The shuttle, like a runner on
the inside track, will

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eventually catch up with the
station, represented by the

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runner on the outside of the
track, provided both are

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traveling at the same speed.
It’s because the inside runner

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is covering a shorter distance,
over each lap.

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Checking the numbers, it’s 400
meters around on the inside

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lane of the track. But in lane
8, 456 meters.

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So a runner on the inside track
will circle the track in a

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shorter time than a runner on
the outside track,

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when both are traveling at the
same speed.

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The shuttle and the station
work like the runners.

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The shuttle, on the inside of
the on the orbital plane will

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eventually catch up with the
space station. That’s when

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docking occurs. To get the most
of their resources,

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NASA launches so the shuttle
will rendezvous with the

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Station within 2 days.

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And as for the different launch
times?

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Well that’s got to do with the
Earth, and physics.

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DR. BACON: Because it has a
soft molten core and it is

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spinning, it tends to flatten
out.

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It gets the poles a little bit
closer to the

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center than the equator. It
bulges out.

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JOSH: That bulge creates
gyroscopic effect

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on the orbital plane.

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DR. BACON: Just like a
gyroscope it starts to precess.

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JOSH: Precess is the change of
direction of the axis of

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rotation of a body caused by
external forces.

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DR. BACON: Now what does that
mean to us at the launch

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center? It means that if we are
ready to launch 7:30 in the

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morning on any given day, the
next day that orbit plain will

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have twisted enough that we
have to launch 23

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and a half minutes earlier.

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JOSH: By understanding orbital
mechanics, scientists and

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engineers like Dr. Bacon can
use known variables, such as

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the distance of the ISS from
the center of the Earth at any

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given time, the satellite’s
velocity, and the diameter of

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the satellite’s orbit to launch
the shuttle and make orbital

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changes so it will rendezvous
with ISS at a specified time.

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So as you can see, a lot of
thought, and a lot of

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calculations go into every
shuttle launch.

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Learn more about launches and
all of NASA at www.nasa.gov.

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? [music] ?

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.