How Do Spacecraft Orbit Earth? Angular Momentum Explained By NASA

Space.com

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. Before we dive into

00:52centripetal force, it's important to look at Newton's first law of motion, which states that

00:57an object will continue moving with a constant velocity along a straight path unless acted upon

01:03by a net external force. This means that the Space Station will move along a straight path if it

01:08weren't for one key external force acting on it, Earth's gravitational pull. Another name for this

01:15external force is centripetal force. A centripetal force is any net force that keeps an object moving

01:21along a circular path. Gravity in this case is a centripetal force because it is the force that is

01:27keeping our Space Station moving in its circular path around Earth.

01:36Okay, now you know that gravity constantly pulled the moving object with linear momentum inward just

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

01:46The 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

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

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

02:23force keeping this yoyo revolving around my hand. But what happens when I let go of the string? Once the

02:28tension from the string is removed, the object continues to follow Newton's first law of motion.

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

02:39Now, what happens to the motion of the yoyo if we increase the centripetal force by increasing the

02:44tension and the string? As I'm holding the string between two fingers on one hand to keep the axis of

02:50the rotation stable, 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. As the radius of the yoyo's orbit decreased,

03:01its velocity increased. Angular momentum is the product of an object's velocity, mass, and the radius of a

03:08its orbit from an object's center. If you only have centripetal force, angular momentum must also be

03:14conserved. So if the radius of its orbit decreases, its velocity must increase in order to maintain its

03:20angular momentum. Let's try this again, but this time I'll decrease the tension on the string, lowering

03:28the centripetal force and increasing the radius of the yoyo's orbit. If you thought the velocity of the

03:35yoyo would decrease, you were right. Since angular momentum must be conserved, if the radius of an

03:41orbit is increased, the velocity of the yoyo must decrease. As you can see, there is an inverse

03:49relationship between the radius of the orbit and the yoyo's velocity. I was able to change the velocity

03:55of the yoyo by increasing and decreasing the centripetal force in the system. We can't do this with the

04:00orbit of the station or other satellites because we can't change the pull of gravity exerted by Earth.

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

04:12constant speed of 17,500 miles per hour. To learn more about these topics, check out the corresponding

04:20Classroom Connection to conduct your own experiment and discover other ways angular momentum plays a

04:25part in your daily life. Thank you for exploring some physics with me today and see you soon.

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