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the show you're about to watch concerns

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the space shuttle mission unlike any

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other you've ever seen the tethered

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satellite mission while the mission was

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successful in proving an idea that had

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only been a theory we had some hardware

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problems and the tether did not extend

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as far as we wanted it to go as often

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happens in research we learn from things

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when they go wrong here we learned

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enough to prove that this truly unique

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concept really does work

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in this program we do calculations based

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on the length we expected the tether to

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extend not how far it actually extended

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-

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an ordinary summer's day at the lake the

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Sun the water good fun and physics

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physics you might not think so but yes

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take this water skier for example

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there's a lot of basic physics involved

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in skiing there's the force of gravity

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the pull of the rope the resistance of

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the water the skier balances these

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forces and a few more to keep a stable

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center of mass gliding across the water

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but the physics is so basic he doesn't

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even have to think about it

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he just skis and if he doesn't get it

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right the first time he tries again

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until he does but in space

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we usually get only one shot at a task

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so before we do anything we analyze it

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to get the physics right the first time

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that's important because what you think

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should happen in space isn't necessarily

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what will happen let's say we want to

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speed up the orbiter to rendezvous with

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a spacecraft just ahead do we fire the

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orbiters jets to go faster no if we

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follow intuition and fire the jets to

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speed up will actually be propelled into

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a higher orbit that will end up slowing

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us down so up here in space we can't

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always depend on what we think will

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happen we have to know what will happen

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and that takes a good understanding of

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physics whether we use it to rendezvous

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with satellites in space or describe

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waterskiing on the earth has unlimited

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applications and today we have a really

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unique one to show you welcome aboard

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the space shuttle Atlantis where we're

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about to launch a new type of spacecraft

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it's called the tethered satellite

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that's because the entire time it's

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deployed it remains tethered to the

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orbiter by a two millimeter diameter

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strand of insulated copper this is one

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of the most complex missions ever

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attempted by a space shuttle crew what's

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amazing is that most of the physics that

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we use to fly the tethered satellite is

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the same physics that you study in high

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school it includes concepts like gravity

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angular momentum and center of mass in

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the next few minutes we're going to see

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how these concepts relate to the orbiter

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the satellite and the tether we begin

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the experiment by deploying the tethered

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satellite we do this from the top of a

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12 metre boom to protect sensitive parts

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of the orbiter small invisible nitrogen

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jets are used to get the satellite and

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tether in motion once they are far

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enough away from the orbiter we can turn

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the Jets off and the satellite and

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tether will continue to deploy

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what makes the satellites continue to go

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up after all you can't push on the road

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well that's where the physics comes in

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we take advantage of a force that causes

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the tether to continue extending on its

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own away from the earth it's a force

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that gets stronger as the tether gets

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longer when what is this force well its

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gravity gravity but how can gravity make

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things go up well which one of those

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things that doesn't make sense that's in

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you understand the physics so let's look

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at the physics of gravity all objects

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are cooled by Earth's gravity even

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objects in space but the further away

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from the earthly objects are the less

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the earth gravity pulls on them Sir

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Isaac Newton described the force due to

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gravity as an inverse square law that

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means that an object two times as far

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away from the center of the earth at

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another object with the same mass fields

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only a quarter of the gravitational pull

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it's a mouthful but next to the tether

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gravity is the most important influence

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on our tethered satellite let's find out

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why we'll calculate using Newton's to

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measure the

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when the 500 kilogram satellite is in

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the cargo bay six thousand seven hundred

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kilometers from the center of the earth

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it has a certain amount of force on it

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if we extend its 10 kilometers upwards

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gravity pulls on it about 30 Newtons

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less extending it's another ten

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kilometers reduces gravity is gripped by

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another 13 Newtons

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for a total of 26 we said that gravity

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is an inverse-square force and for our

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purposes the distance involved is from

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the center of the Earth's orbit and to

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the satellite as it moved away from the

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orbital but the distances the satellite

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is moving away from the orbiter are much

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smaller than the distance to the center

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of the earth if you put a force on these

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smaller distances we see that the change

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in force is linearly proportional to the

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change in distance

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we call this difference in the force of

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gravity with distance the gravity

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gradient force Britons being the

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mathematical term for difference

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when the tether length doubles the

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now we have enough information to

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understand why the tether reels out and

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stays out so let's put it all together

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the acceleration caused by the Earth's

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gravity is greater for objects closer to

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the Earth's center

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this means the orbiter is trying to

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accelerate towards the Earth faster than

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the satellite this extra acceleration

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causes the orbiter to want to fall away

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from the satellite creating a stretching

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force along the length of the tether and

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that's the force that keeps the tether

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taut and makes the tethered satellite go

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up but how can gravity make something go

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up doesn't that violate the law of

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conservation of energy

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could we be getting something for

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nothing to understand how gravity can

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make something go up we need to go a

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little deeper into dynamics and look at

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the concept of center of mass the center

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of mass is the point in a system where

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the system is evenly distributed and in

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balance balance or equilibrium is

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important because it means the system is

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stable we're all in the same orbit

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around the Earth Jeff the orbiter and

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these two apples the center of mass

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between two objects such as these apples

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doesn't change just because they're in

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orbit their center of mass is between

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them here when I start them spinning

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their center of mass continues to be in

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the same place if I cut the string so

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the apples are attached by the longer

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string the center of mass is still in

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the same place inside the orbiter which

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means in the same orbit what does this

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have to do with the orbiter and tethered

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satellite when the tethered satellite is

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in the orbiters cargo bay the center of

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mass is inside the orbiter when we

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deploy the satellite do you think the

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orbit of the center of mass changes if

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it did then we would be violating the

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law of the conservation of energy

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however like the apples the orbit of the

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shuttle satellite system doesn't change

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because no external force has been

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applied to the system but the positions

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of the orbiter and satellite do change

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to keep this

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systems center of mass in the same orbit

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and continue its stable equilibrium

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around the earth let's look at that

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concept in action on the ground to make

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a seesaw balance one person sits in the

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middle if we add another person the one

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in the middle has to move to keep the

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it's the same for our tethered system

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when the satellite is extended like the

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heavier person on the seesaw the orbiter

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stays closer to the center of mass let's

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see how much the one hundred thousand

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kilogram orbiter moves away from the

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center of mass when the five hundred

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kilogram satellite is deployed 20

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kilometers you could work this problem

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out for yourselves because the orbiter

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is more massive it moves only about a

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hundred meters away from the center of

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mass while the satellite moves away

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almost 20 kilometers and the center of

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mass of the system stays in its original

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orbit because the orbiter moves so

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little it almost seems like the

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satellite alone is moving up but now we

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know both objects are moving one up and

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one down once you understand this

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principle you can also see this

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situation is symmetrical we could just

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as easily have made the satellite go

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down 20 kilometers in the orbiter go up

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here's another way of looking at it

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think of a pulley system a heavyweight

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moves a little to balance a light weight

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that moves a lot the weights can move in

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either direction the center of mass

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stays constant but in space we don't

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need a pulley a tether will do now let's

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look at the forces that keep the tether

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system stable along a vertical axis when

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this toy is punched gravity makes it

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return to a vertical position the

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tethered system could also be perturbed

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off the vertical axis without any

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restoring force the system would rotate

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like these tethered apples but the

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tethered system stable equilibrium keeps

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it aligned along a vertical axis what

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causes the tethered system stable

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equilibrium and space well once again

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gravity is at work you see gravity pulls

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the pull on the satellite at the upper

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end of the tether makes the system want

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to depart from the vertical while a pole

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on the orbiter at the lower end makes

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the system return do you remember the

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gravity gradient the end closest to the

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earth feels the greatest acceleration so

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the system feels a net torque and the

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tether returns to the vertical it

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doesn't matter if the mass is at the end

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of the system are equal or not the

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gravity gradient is stabilizing the

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system but gravity gradient is only part

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of the story there's another force at

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work remember we saw that there is an

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approximate force of 26 Newtons

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for the gravity gradient over a 20

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kilometer tether

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well when the tether is extended 20

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kilometers our force measuring device

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measures 40 Newtons not 26 Newtons

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now where did this extra force come from

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here is a clue it's the same force we

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feel when we will an object around on a

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string and the strain stretches the

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faster the speed the harder the force

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tries to fling the object away from the

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as you have seen gravity helps keep the

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Tetra vertical

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and you can actually calculate this

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expert force and it turns out it's about

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half the gravity gradient force a

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significant effect

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therefore as it orbits the earth it is

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actually rotating once per orbit about

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its center of mass creating this extra

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force that stretches the tether we can

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see it in action another way if we have

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one of our crews spin a bucket of water

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around in a circle the water will stay

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inside a bucket as long as the motion

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continues stop the motion the force

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disappears and the water obeys the law

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of gravitational attraction like the

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gravity gradient force it is also

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proportional to the length of the tether

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as long as the rate of rotation stays

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constant

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assumed that the tether remains at a

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constant length and this is not always

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the case when we lengthened or shortened

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the tether another physics principle

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that you've heard of comes into play and

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that angular momentum you see it here in

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the motion of the skater angular

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momentum is the product of an object's

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rotational inertia and angular velocity

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about a particular axis when the object

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changes its mass distribution the

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angular momentum stays the same but the

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rotation rate changes we call this the

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conservation of angular momentum and you

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can see for yourself how this principle

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works stand on a turntable with your

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arms outstretched and weights in your

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hands

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have someone start you spinning now

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bring your arms in you speed up that's

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because the more the mass is

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concentrated towards the center of mass

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the faster the mass spins now let's see

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how this part of physics relates to our

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spaceship and the data satellite when

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the tether is pulled in the rotating

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mass of the system is concentrated into

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a smaller area and the system tries to

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speed up just like the skater when the

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tether is let out the mass is

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distributed in a bigger area and the

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orbiter and tether system slow down just

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like the students slow down

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when she extended her arms in the

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orbiter we see this from a slightly

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different perspective because we are

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part of the rotating system remember

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during most of the mission for

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scientific reasons the orbiter is moving

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with the tail forward do you notice how

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from our point of view the satellite

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moves forward with respect to our

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direction of motion and moves backward

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with respect to our direction of motion

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when it's let out and this is called the

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Coriolis effect and it occurs in all

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rotating systems as a consequence of the

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conservation of angular momentum

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it is the tendency of objects in

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rotating systems to move in a curve as

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we'll try to throw a ball from someone

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on one side of a merry-go-round

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to someone on the other side of the

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merry-go-round as we can see the ball is

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thrown in a straight line but not from

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this point of view here it appears the

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balls trajectory is curving and this is

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what makes the satellite move backward

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or forward as we reel it out or in the

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Coriolis effect is what makes hurricanes

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spiral and other weather systems curved

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we won't go into the mathematics here

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except to say that the larger the tether

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reel out or reeling rates the greater

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the backward or forward motion of the

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satellite from the vertical so let's

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review the aspects of tether dynamics

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that we've looked at so far gravity

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center of mass and angular momentum all

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of these concepts are studied in your

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first-year physics course yet all of it

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is used in a way like nothing you've

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ever seen before and there are even more

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uses of tether technology on orbit we

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found we could use the tether to

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generate electricity as the copper cord

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tether flew through the Earth's magnetic

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field

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so the question left is what can we use

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this to other technology for we could

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use tethers to directly study parts of

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the Earth's atmosphere that we can't

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reach by any present-day technology

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tethers could reduce the need for

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chemical fuels in space boosting objects

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to stay in orbit and assisting others to

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deorbit and engineers could use the open

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wind tunnel of the Earth's upper

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atmosphere for testing the hypersonic

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aerodynamics of reentry vehicles and

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aerobraking technology

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it's even possible that one day tether

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driven electric cable cars will ferry

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space travelers from earth to

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geostationary orbit what uses of tether