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

00:00 [ Music ]

00:16 >> Hello, my name is Sultan Al-Niyadi

00:17 and I'm an astronaut living and working

00:19 on board the International Space Station.

00:22 Any idea how it's possible for the space station

00:24 to continuously orbit Earth 250 miles above the surface?

00:28 And why at 17,500 miles per hour?

00:32 What would happen if the station sped up or slowed down?

00:36 We are going to explore those questions and more

00:38 by investigating the connection between the angular momentum

00:42 and the orbits in our microgravity environment.

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

00:49 Let's get started.

00:51 Before we dive into centripetal force, it's important to look

00:54 at Newton's first law of motion,

00:56 which states that an object will continue moving

00:59 with a constant velocity along a straight path unless acted upon

01:03 by a net external force.

01:05 This means that the space station will move along a

01:07 straight path if it weren't

01:09 for one key external force acting on it,

01:11 Earth's gravitational pull.

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

01:18 A centripetal force is any net force

01:20 that keeps an object moving along a circular path.

01:23 Gravity, in this case, is a centripetal force

01:26 because it is the force that is keeping our space station moving

01:29 in its circular path around Earth.

01:32 [ Music ]

01:36 Okay, now you know that gravity constantly pulls a moving

01:39 object with linear momentum inward just enough to cause it

01:42 to travel in a curved path, making its momentum angular.

01:46 The International Space Station maintains this balance

01:51 between gravity and linear momentum by traveling

01:54 at the required 17,500 miles per hour

01:57 to maintain an altitude of 250 miles.

02:01 This is considered low Earth orbit.

02:03 It is high enough to encounter very little interference

02:05 from the atmosphere, but low enough

02:07 to be relatively easy to travel to.

02:10 Let me show you some examples

02:11 of angular momentum being conserved

02:13 in the microgravity environment aboard the station.

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

02:19 The force of tension is transferred through the string,

02:22 which is a centripetal force keeping this yo-yo revolving

02:25 around my hand.

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

02:28 Once the tension from the string is removed, the object continues

02:31 to follow Newton's first law of motion.

02:34 It keeps moving at a constant velocity along a straight path

02:37 relative to the space station.

02:39 Now, what happens to the motion of the yo-yo

02:41 if we increase the centripetal force

02:43 by increasing the tension in the string?

02:45 As I'm holding the string between two fingers on one hand

02:49 to keep the axis of the rotation stable,

02:51 I'm going to pull the string with my other hand,

02:54 increasing the tension and centripetal force

02:56 and decreasing the radius of the yo-yo's orbit.

02:59 As the radius of the yo-yo's orbit decreases,

03:01 its velocity increases.

03:03 Angular momentum is a product of an object's velocity, mass,

03:07 and the radius of its orbit from an object's center.

03:11 If you only have centripetal force,

03:13 angular momentum must also be conserved.

03:15 So if the radius of its orbit decreases,

03:18 its velocity must increase in order

03:20 to maintain its angular momentum.

03:23 Let's try this again, but this time,

03:25 I'll decrease the tension on the string,

03:28 lowering the centripetal force and increasing

03:30 the radius of the yo-yo's orbit.

03:34 If you thought the velocity of the yo-yo would decrease,

03:37 you were right.

03:38 Since angular momentum must be conserved,

03:40 if the radius of an orbit is increased,

03:43 the velocity of the yo-yo must decrease.

03:48 As you can see, there is an inverse relationship

03:50 between the radius of the orbit and the yo-yo's velocity.

03:53 I was able to change the velocity of the yo-yo

03:56 by increasing and decreasing the centripetal force

03:58 in the system.

03:59 We can't do this with the orbit of the station

04:02 or other satellites, because we can't change the pull

04:04 of gravity exerted by Earth.

04:06 Instead, to keep the station in a stable, circular orbit,

04:10 we used thrusters that can help maintain the constant speed

04:13 of 17,500 miles per hour.

04:18 To learn more about these topics,

04:19 check out the corresponding classroom connection

04:21 to conduct your own experiment and discover

04:23 other ways angular momentum plays a part in your daily life.

04:27 Thank you for exploring some physics with me today,

04:29 and see you soon.

04:31 [MUSIC PLAYING]

04:35 (dramatic music)

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