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  • [ music ]

  • Hey it's me Destin welcome back to

  • Smarter Every Day. So you're probably well aware of the awesome science that comes

  • out of space telescopes, but what you might not be aware of is the awesome science that goes

  • into making these things work. For instance check out this NASA photo. These guys are working on

  • the sun shield for the James Webb Space Telescope, and what I think is so cool about this photo

  • is that this guy in the bottom right corner is my Dad. You see the instrument

  • operates at about 45 degrees Kelvin but you have all this heat from the sun coming from the

  • other side. So what Dad's doing is he's using this 3D laser scanner to make sure

  • that the sun shield is perfect to provide a stable operating environment for the

  • telescope. Thumbs up for you dad, I'm proud of you. So I thought I understood

  • how space telescopes point at what they want to observe, but it turns out it's a little more complicated

  • than I thought. I'll explain two ways that you can apply an external force

  • on the satellite and get it to turn. First up, rockets.

  • [ Rocket sound] OK. Now that our space telescope is on orbit

  • how can we point it at a star. [ pshh pshh ]

  • Rockets are cool, but they use fuel, which clouds up the area around the

  • satellite, and you can run out of fuel, ending your mission. So how do we

  • do this without having rockets. Hang with me on this one. Think about

  • this little compass needle. It's interacting with the Earth's magnetic field, but what

  • if we scale it up and create a big needle like a rod or torque rod, and we

  • put a coil of wire over the top of it. And then we take this wire

  • and we hook it up to this power supply I have on my desk and then turn on the voltage.

  • See that? An electromagnetic field. So is it true

  • that this electromagnetic field in my hand is interacting with the Earth's magnetic field

  • just like this compass needle that we were able to get to rotate.

  • Yes, that's exactly what's happening. So here's the deal. This is called a

  • torque rod. Scientists do this all the time on spacecraft. They take three torque

  • rods and they put them in three different axes and they're able to apply or remove

  • voltage as they need to rotate depending on where they are in the Earth's magnetic field.

  • Now this is a very elegant solution because there's no moving parts, but the

  • problem is it only works in places where you have an external magnetic field,

  • like Earth. This wouldn't work in deep space or even on Mars where

  • you have a really low magnetic field. So in order to rotate a space telescope

  • somewhere like that, we're gonna have to use flipping cat physics. So if you go back to

  • the last episode of Smarter Every Day I used a high speed camera to explain that a cat is

  • able to flip 180 degrees from rest by rotating against himself.

  • Go watch that last video if you need a refresher. But what I didn't explain

  • is that this ability to change the overall state of the system without

  • violating the conservation of angular momentum is a very special problem

  • that's been studied by applied mathemeticians, neuroscientists, robot builders,

  • and even astrophysicists. Just do a quick academic search for flipping cats

  • and you'll find out what I mean by seeing thousands of pages of equations.

  • OK so to explain why cats are so special let's get a block of wood with a hole in it and a ping pong

  • ball with a mark on it. Then we'll mark the block so we can tell how the two line up.

  • If we move the block the ball rolls, and when we return it,

  • the system goes back to the initial state, and this is always the case right?

  • Nope. If we move the block out but return it along a

  • different path, the system doesn't return to the initial state.

  • Aah. This means that the path determines the state of the system right?

  • So just like the ball never slips, angular momentum in both cats

  • and space telescopes never changes. But we are

  • able to change their final rotation. You see special systems like this are called

  • Nonholonomic systems. Look, I know that's a big word, and you

  • might just be watching this video because it's a cat video, but I need you to hear this.

  • The physics of flipping cats is what makes us be able to take photos

  • like this. No, seriously. Flipping cats!

  • One way to keep a space telescope pointed at a particular star is to use a device called

  • a reaction wheel. Multiple reaction wheels are spinning inside the space craft.

  • If a rotation is needed in a particular axis, engineers calculate

  • which wheel should be sped up or slowed down at exactly what times.

  • So I am in a NASA building and we are going to a special lab.

  • We're here at the attitude control components lab.. [ knock knock ]

  • And these guys know about how to control space satellites

  • using cat math. Here's our guide here.

  • (Dean) Come on in. (Destin) OK Dean so thank you for alowing me to come over after I showed you my cat video.

  • - That was pretty cool. (Destin) I appreciate that. So I have a question.

  • So a cat is just free-floating in space essentially and then he rotates.

  • - Correct.

  • - Correct. - How does he control his rotation without any

  • external forces on him? - Well he basically uses his legs

  • and his body to rotate and have momentum transfers.

  • - Oh and that's kind of what you guys do right? You're brokers of momentum.

  • - Yes, we do momentum management. That's what happens on the space station

  • or any system that's flying in space. We have to manage

  • the amount of momentum that's actually in the system so that you can point and

  • focus at a nebula or a star that you want to look at

  • for a telescope. - And so what is this device that you have next to you? - Well this device

  • is a Control Moment Gyro. Now the difference between a

  • reaction wheel and a CMG is in the

  • CMG I can actually rotate it's axis. And you can see this actually

  • has two axes of rotation. And by doing that

  • - You made a lot harder math for yourself didn't you. - Yes we did.

  • But there are a lot smarter people than I am that go off and make sure that

  • they know how to point the satellite or the telescope

  • and they go from one point to another point along a specific trajectory.

  • But if I keep rotating this up you'll see that there's a point where it will stop

  • and if for some reason I were able to get to this point

  • then I can't go any farther, so I have to have some

  • other system to allow me to get out of that

  • blocked position. - Oh so that's where your external energy inputs start to come in.. - That's right

  • that's where your mag torquers or your

  • propulsion jets or thrusters come into play. - And the torquers would be the Earth's magnetic field.

  • - Yes you react against Earth's magnetic field. - OK so

  • here you guys test the components to make sure that these things will work on orbit.

  • - Yes we do, and in this vault over here you'll see we have a life test running

  • - Is that what I'm hearing here? - That's what you hear. - So what am I looking at?

  • - This is the Chandra X-Ray Telescope life test for the

  • reaction wheel, and what we're doing is we're testing the system

  • to see how long it will last, so that if there is a failure on

  • say the wheels, then we can go in there and say

  • we can take the wheel apart. - I notice you have a really old computer running this thing.

  • - Yes well Chandra was built in the early nineties and

  • so the hardware that is used to test Chandra is

  • the same hardware that is flying on Chandra, so you gotta remember

  • that Chandra is now about 13 years old I think? - Oh wow.

  • So it's like you're locked in time. - Yes. - When you launch the thing you're

  • locked at a certain time and you have to continue to use that same hardware. - That's the same thing you had the

  • problem with the space station or the shuttle. They were locked into time

  • with the components that were built way back in the seventies.

  • - Thank you very much. Appreciate it. - You're welcome. - So thank you very much for watching. I hope

  • you learned something about cats that you didn't know, and please consider subscribing,

  • and sending this to your friends. Gigi, get it!

  • Get it.

  • You hear a bird? Hear a bird?

  • We had a ball.

  • The only reason you can do

  • this is because it's a Nonholonomic system and you know flipping cat math. OK

  • I'd like to thank Jimmy Neutron for the use of his farm,

  • and his cat.

  • [ Captions by Andrew Jackson ] captionsbyandrew.wordpress.com

[ music ]

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空間望遠鏡像CATS一樣操縱--每天更聰明 59 (Space Telescopes Maneuver like CATS - Smarter Every Day 59)

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    林宜悉 發佈於 2021 年 01 月 14 日
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