? [music] ?
KIDS: Our World!
EVAN: Hey Globey, have you ever wondered what keeps satellites,
like the International Space Station and the space shuttle
in orbit? Once they get to a certain place in space, they
just keep going round and round Earth. But I also know about
inertia - moving objects have a tendency to keep
moving in a straight line. So what’s to stop them from
just flying off into outer space? [crash]
Whoops...
Well, it turns out that there is a balance between inertia
and gravity acting on these satellites. Now we all know
that gravity is the attraction between objects. How big that
attraction is depends on the mass of the objects (or how big
they are) and how far away the objects are from each other.
Objects on or near Earth are caught in this attraction and
are pulled down toward Earth’s surface.
So, even if you throw a ball up in the air,
gravity pulls the ball back down to the ground.
But in space, all that seems to change.
I’m sure you’ve seen pictures of astronauts floating around
inside the space shuttle and we know the astronauts don’t float
around here on Earth. So does that mean there’s no gravity
once the astronauts get in orbit? And what about way out
there in outer space? Is gravity still
important out there?
Actually, gravity is still at work, even as far away as outer
space. So what do you think keeps the space station or
space shuttle from floating away into deep space?
You got it, gravity! But if there’s gravity in space,
why do the astronauts experience weightlessness?
And why do they appear to be floating around? I know.
It’s a bit complicated. So let’s ask a friend to help
explain it.
Dr. John Bacon is an engineer at the
NASA Johnson Space Center in Houston, Texas.
JOHN BACON: Hi Evan. A lot of people ask me about why gravity
seems to go away when you get to orbit.
I can explain that with my keys here. I have a little elastic
on the keys, and I’m going to show what happens when I drop
this whole system. The elastic is all stretched out and
gravity is the most important in setting up how my keys
behave at the moment. But the instant that I let it fall,
gravity is not going to be important anymore.
It’s not going to fit against my fingers. You’ll see this
whole thing change in 3-2-1.
You’ll see that even before the keys hit my hand this little
elastic has immediately relaxed. I’ve changed the ways
the keys behave when I let it fall.
The same thing is true with water on the surface of the
table. It has a nice flat surface for days and days and
days until it finds the edge of the table and is allowed to
trickle off. Instantly when it falls we get surface tension
taking the water into a nice little cylinder and then air
forces start sheering it into drops and you see it do
anything but stay flat. When we take gravity away by letting
things fall we get a whole new behavior.
The problem is if we get high above the planet Earth,
things start falling towards it. No matter where I go around
the equator, I will fall to the middle of the Earth.
Then I’m going to hit the Earth rather hard. So if I want to
get this wonderful experience of falling forever, what I need
to do is go sideways very fast.
If I can go sideways fast enough, I’ll actually fall
right over the horizon.
I’ll just keep going sideways over the center of the Earth
pulling me to the middle. If I’m on board the shuttle or a
space station up here in the vacuum, I’m going to feel like
I’m falling forever. Everybody on the station and all the
experiments with them appear to float in the middle of the
cabin forever. Notice there’s actually lots of gravity there.
It’s what’s holding the space station right next to the
Earth. Gravity is all the way out as far as the Moon.
It’s what holds the Moon in its orbit around us. But when we
get going sideways fast enough, we just keep falling.
EVAN: Just like skydivers, astronauts experience
weightlessness as they fall towards Earth. So even though
gravity is constantly pulling them down, they can’t feel
their own weight as they are falling. And if there are two
skydivers falling together, it seems as if they’re floating
toward each other. Hey, don’t I know you?
But what keeps the space shuttle from falling to the
ground like the skydivers? Well, like Dr. Bacon said, the
space shuttle would indeed fall if it wasn’t traveling
so fast around Earth.
How fast does the shuttle actually have to travel to stay
in orbit? The shuttle is flying approximately 28,000 kilometers
per hour. That’s 17,500 miles per hour!
That’s so fast the shuttle makes a complete orbit around
Earth in just 90 minutes. Can you imagine that?
So, the astronauts are being carried with the shuttle as it
orbits Earth. At the same time, Earth is pulling on the
shuttle and everything inside, but because the shuttle is
moving sideways so fast, the astronauts, and anything else
that isn’t fastened down, are getting pulled to the middle of
the shuttle, making it look as if they are floating.
And that same sideways speed keeps the shuttle
itself from falling back to Earth.
I bet you’re wondering how the shuttle ever comes back to
Earth if it’s traveling too fast to fall to the surface.
The answer is simple. They have to slow the shuttle down.
The shuttle commander actually fires the shuttle’s thrusters
in reverse. This slows the space shuttle down, and because
it’s no longer traveling fast enough to stay in orbit, the
shuttle falls into the atmosphere and begins its
journey back to the surface of the Earth.
Gravity sure is complex. And every time astronauts travel
into space, NASA learns more about this special attraction
that keeps all kinds of things in their places, including us!
Do you think you’d like the feeling of floating forever?
I don’t know. I think it might make me a little sick to my
stomach. I think I ought to rethink that whole
skydiving thing!
? [music] ?
KIDS: Our World!