How Do Spacecraft Orbit Earth? Angular Momentum Explained By NASA

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How is it possible for the ISS to stay in orbit? Learn more about the science behind orbiting Earth and more in this NASA "STEMonstrations" video.  Credit: NASA Johnson Space Center

Transcript

00:00Hello, my name is Sultan Alniadi and I'm an astronaut living and working aboard the International

00:20Space Station.

00:22Any idea how it's possible for the space station to continuously orbit Earth 250 miles above

00:27the surface?

00:29And why at 17,500 miles per hour?

00:33What would happen if the station sped up or slowed down?

00:35We are going to explore those questions and more by investigating the connection between

00:40the angular momentum and the orbits in our microgravity environment.

00:45But first, you need to know a couple of other terms.

00:49Let's get started.

00:52Before we dive into centripetal force, it's important to look at Newton's first law of

00:56motion, which states that an object will continue moving with a constant velocity along

01:00a straight path unless acted upon by a net external force.

01:05This means that the space station will move along a straight path if it weren't for one

01:09key external force acting on it, Earth's gravitational pull.

01:14Another name for this external force is centripetal force.

01:18A centripetal force is any net force that keeps an object moving along a circular path.

01:24Gravity, in this case, is a centripetal force because it is the force that is keeping our

01:28space station moving in its circular path around Earth.

01:32Okay, now you know that gravity constantly pulls a moving object with linear momentum

01:41inward just enough to cause it to travel in a curved path, making its momentum angular.

01:49The International Space Station maintains this balance between gravity and linear momentum

01:53by traveling at the required 17,500 miles per hour to maintain an altitude of 250 miles.

02:01This is considered low Earth orbit.

02:03It is high enough to encounter very little interference from the atmosphere but low enough

02:07to be relatively easy to travel to.

02:10Let me show you some examples of angular momentum being conserved in the microgravity environment

02:15aboard the station.

02:16I will apply a force to set this yoyo in motion.

02:19The force of tension is transferred through the string, which is a centripetal force keeping

02:24this yoyo revolving around my hand.

02:26But what happens when I let go of the string?

02:28Once the tension from the string is removed, the object continues to follow Newton's first

02:33law of motion.

02:34It keeps moving at a constant velocity along a straight path relative to the space station.

02:40Now what happens to the motion of the yoyo if we increase the centripetal force by increasing

02:44the tension in the string?

02:46As I'm holding the string between two fingers on one hand to keep the axis of the rotation

02:50stable, I'm going to pull the string with my other hand, increasing the tension and

02:55centripetal force and decreasing the radius of the yoyo's orbit.

02:59As the radius of the yoyo's orbit decreases, its velocity increases.

03:04Angular momentum is the product of an object's velocity, mass, and the radius of its orbit

03:09from an object's center.

03:11If you only have centripetal force, angular momentum must also be conserved.

03:15So if the radius of its orbit decreases, its velocity must increase in order to maintain

03:20its angular momentum.

03:23Let's try this again, but this time I'll decrease the tension on the string, lowering the centripetal

03:29force and increasing the radius of the yoyo's orbit.

03:34If you thought the velocity of the yoyo would decrease, you were right.

03:38Since angular momentum must be conserved, if the radius of an orbit is increased, the

03:43velocity of the yoyo must decrease.

03:48As you can see, there is an inverse relationship between the radius of the orbit and the yoyo's

03:53velocity.

03:54I was able to change the velocity of the yoyo by increasing and decreasing the centripetal

03:58force in the system.

03:59We can't do this with the orbit of the station or other satellites because we can't change

04:04the pull of gravity exerted by Earth.

04:06Instead, to keep the station in a stable circular orbit, we used thrusters that can help maintain

04:12the constant speed of 17,500 mph.

04:18To learn more about these topics, check out the corresponding Classroom Connection to

04:21conduct your own experiment and discover other ways angular momentum plays a part in your

04:26daily life.

04:27Thank you for exploring some physics with me today, and see you soon.

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