FOSSUM: Hi. My name's Mike Fossum.
I'm the commander of Expedition 29.
Welcome to the International Space Station.
You may wonder why the International Space Station
doesn't tumble in its orbit around the Earth.
Well, we have a system that maintains our attitude --
our attitude control system, if you will.
There's really two ways to do this, to approach it.
The first way would be to use rockets or small jets
that would keep thrusting and pushing
to nudge the Space Station into its correct attitude.
Problem with using thrusters like that
is it requires a lot of fuel
that we have to continually ship up from the Earth.
So, instead, we use angular momentum and gyroscopes.
Let me demonstrate a little bit.
Before we get into this,
we really have to talk about the definitions of the concepts.
It's really the conservation of angular momentum
that's really important here.
I have this flashlight rotating.
In physics, this angular momentum of an object
rotating about some reference point
right here in the middle --
that's a measure of the extent
to which the object will continue to rotate
around that point,
unless it's acted on by an external torque.
Now, torque has to do with the ability of a force
to rotate an object
and how far away from the center that force is applied.
The angular momentum of a single, rotating body
is equal to the product of its rotational inertia --
that's just physical properties of the mass distribution,
the rotational inertia --
and angular speed, how fast is it moving.
The angular momentum and rotational inertia --
that includes the mass -- are constants in the system.
Angular momentum of the system remains constant
if no external force is acting on the system.
This is the law of conservation of angular momentum.
I can demonstrate this to you
with me actually doing a little bit of a demonstration here,
so I want you to watch this.
I'm gonna have to unclip the microphone,
or I'll tie myself up in knots.
Again, what's going on as my body spins doing this
and moving my arms in and out from being close to the body
is when I bring the arms in closer to the body,
the body's rotational inertia is decreased.
The angular momentum must be conserved.
As the rotational inertia is decreased,
the angular speed increases and vice versa.
This is called an inverse relationship.
Now, no external force acts on the system,
and thus the angular momentum about the axis
must remain the same.