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 al-Niyadi and I'm an astronaut living and working on board the

00:20International Space Station. Any idea how it's possible for the Space Station to continuously

00:25orbit Earth 250 miles above the surface? And why at 17,500 miles per hour? What would happen if the

00:33station speed up or slowed down? We are going to explore those questions and more by investigating

00:39the connection between the angular momentum and the orbits in our microgravity environment.

00:45But first, you need to know a couple of other terms. Let's get started.

00:50Before we dive into centripetal force, it's important to look at Newton's first law of motion,

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

01:00along a straight path unless acted upon by a net external force. This means that the space

01:06station will move along a straight path if it weren't for one key external force acting on it,

01:11Earth's gravitational pull. Another 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. Gravity in

01:24this case is a centripetal force because it is the force that is keeping our space station moving in

01:29its circular path around Earth. Okay, now you know that gravity constantly pulls the moving object with

01:40linear momentum inward just enough to cause it to travel in a curved path, making its momentum angular.

01:48The International Space Station maintains this balance between gravity and linear momentum by

01:54traveling at the required 17,500 miles per hour to maintain an altitude of 250 miles. This is

02:01considered low Earth orbit. It is high enough to encounter very little interference from the atmosphere,

02:06but low enough to be relatively easy to travel to. Let me show you some examples of angular momentum being

02:12conserved in the microgravity environment aboard the station. I will apply a force to set this yoyo in

02:18motion. The force of tension is transferred through the string, which is a centripetal force keeping this

02:24yoyo revolving around my hand. But what happens when I let go of the string? Once the tension from the

02:29string is removed, the object continues to follow Newton's first law of motion. It keeps moving at a

02:35constant velocity along a straight path relative to the space station. Now what happens to the motion of the

02:41yoyo if we increase the centripetal force by increasing the tension and the string? As I'm holding the

02:46string between two fingers on one hand to keep the axis of the rotation stable, I'm going to pull the

02:52string with my other hand, increasing the tension and centripetal force and decreasing the radius of

02:58the yoyo's orbit. As the radius of the yoyo's orbit decreased, its velocity increased. Angular momentum is

03:04the product of an object's velocity, mass, and the radius of its orbit from an object's center. If you only

03:11have centripetal force, angular momentum must also be conserved. So if the radius of its orbit decreases,

03:18its velocity must increase in order to maintain its angular momentum. Let's try this again, but this time

03:25are decreased the tension on the string, lowering the centripetal force and increasing the radius

03:31of the yoyo's orbit. If you thought the velocity of the yoyo would decrease, you were right. Since angular

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

03:53I was able to change the velocity of the yoyo by increasing and decreasing the centripetal force

03:58in the system. We can't do this with the orbit of the station or other satellites because we can't

04:03change the pull of gravity exerted by Earth. Instead, to keep the station in a stable circular orbit,

04:10we use thrusters that can help maintain the constant speed of 17,500 miles per hour.

04:17To learn more about these topics, check out the corresponding classroom connection to conduct your own

04:22experiment and discover other ways angular momentum plays a part in your daily life.

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

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