How do space telescopes point themselves?

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Speaker 1: Hey, Jorgey, are you good at navigating? Depends on what do you mean by navigating? Do you mean navigating the complex issues of how to lead a good life? Then no, I haven't figured that one out. But if you mean like getting somewhere, I have a phone with GPS, so I guess i'm pretty good. Well, what have you lost your phone? Like? Or civilization crumbled? Do you know how to oriente yourself in the woods? Well? I imagine I could use a map and a compass, right, do you mean like a basic old school compass or the compass app on your phone? Okay, yeah, that's a good point. I only have the compass on my phone. But I guess you could probably find a low tech original you know, og compass. Yeah, that would let you get low tech original lost. I guess there's civilization crumbles were all lost. I am morehammy cartoonist and the creator of PhD comics. Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I've never honestly been lost in the woods. Well, well, yeah, I think that's self evident, because you're talking to us right now. If you were lost in the woods, I'm not sure we would have from you again. Maybe I'm calling you from my secret woods hideout, or even I don't know where it is, although if you have Wi Fi there and are able to record, I'm not sure you're that lost. Yeah, that's true. But I've often gone on long backpacking trips and wonder if I really could get myself out of the woods if I had to. Yeah, it's pretty tricky because I guess it's hard to see above the trees and know where you are right. I can't see the forest for the tree. It's definitely a particular skill of figuring out how the map represents the world you're seeing around you, and how to figure out where on the map you are. Well, I'm glad you're not lost in the woods, Daniel, it's what of my recurring nightmares. Welcome to our podcast Daniel and Jorge Explain the Universe, a production of our Heart Radio in which we try to avoid being lost in the woods of physics. We try to navigate our way through all of the confusing issues about this incredible universe, figure out how we can actually understand it, what we can make sense of, how big our map of the intellectual cosmos. We really can illuminate it. So I think of this podcast as your GPS for the entire universe, helping you know where things are and how to get there. Because slowly, over hundreds or thousands of years, we have started to build a map of how the universe works. We have a literal map of like what's physically out there in the universe, but we also have a conceptual map one that tells us how things work, how they explain the experience we see, and what they predict about what is to come. Yeah, because it is a pretty big universe and there's a lot out there for us to explore and to check out, and so having a map is a really good thing. Do you know where we sit in the universe? And it turns out that, we say it in a very small corner of one tiny galaxy that's part of a giant supercluster. And it's amazing we've been able to figure that out just looking at the night sky from this little piece of rock. Why do you call our galaxy tiny? I think it's pretty impressive. Well, you know, it could be bigger. You know, always use a bigger house, right, I don't know. I have friends that moved into a bigger house, and they found themselves just screaming at each other from opposite ends of the house all the time. I think they were happier in their tiny, little cramp department. Sounds like they needed an intercom, which is like technology from the eighties seventies. Yeah, exactly. And so if we lived in Andromeda, we'd have an even bigger galaxy to explore to find those aliens unless we had some sort of like alien galactic intercom where we could just talk to everybody. Yeah, you could have like a quantum warp tunnel the intercom. But the universe is quite vast, even beyond our tiny or large galaxy, depending on how you see it. And it's incredible that we have been able to figure out what's out there. Remember when you look at a map of the superclusters or our galaxy, that those are constructed from painstaking work to figure out where everything is. We don't have cameras above the Milky Way or outside of the galaxy. We've basically only ever observed things from Earth or from very very close to Earth, and those technological eyeballs we have built have allowed us to piece together this concept of where we are in the cosmos. Yeah, it's amazing what we've been able to piece together just from our little viewpoint using basically like two pieces of glass. Right, the original telescopes were che tube and two pieces of glass. I mean they're a little fastier now, but essentially the same thing. Yeah. I think you're glossing over a couple of crucial details, like the shape of that glass, but yeah, those are the basic ingredients. Yeah, and so we've been able to look at the stars and other galaxies from our point or an Earth, but we've also been able to look at the sky from the sky. We now have more than a few space telescopes out there in orbit and beyond orbit looking at the rest of the universe. Yeah, we have two really awesome sets of technology ground based telescopes that can get really really big tens of meters across for the primary mirrors, but those can be obscured by all the air that's between us and space. That air wiggles and shimmys and makes a little bit unclear to see what's out there. So we have this other awesome set of eyeballs we built that are actually out there in space above the atmosphere and can see much more clearly, although they can't yet be quite as large, So it's a complementary set of eyeballs. Now, these are not literal eyeballs, Like, we didn't send eyeballs into space, did we. Well, it depends on your definition of eyeballs. Right, they're not human biological eyeballs, but they're more like cameras. Right. They take pictures which are then transmitted to your eyes. Are they in the shape of a ball at least there are definitely some balls on them, right, we'll talk about it in the podcast. But they have spinning wheels and spinning balls, which are crucial elements of their operation. Oh all right, well, so technically there are they are eye and balls. But it is amazing that we have space telescodes. It's pretty cool. It's like, literally we build spaceships that are nothing but or spacecraft that are nothing but a telescope. Right, that's their only function, and they're out there in space doing their job. They're sort of like robotic space telescope spacecraft. Yeah, they're sort of like distant robot eyeballs that we connect to our own minds. It is really incredible. And you know, the telescopes here on Earth. That makes sense how they work. You want to look at something, you can turn the telescope, you point it at that thing that you want to watch. But the telescopes that are out there in space, it's a little harder to understand, like how those work, how they keep track of where they are, how you can turn and telescope in space? And a bunch of listeners wrote in and ask me how does that work? So do they end the program? We'll be tackling the question how do space telescopes point themselves? Now? I guess, Daniel, The question I guess is like, if you telescope here on Earth, you're grounded to the Earth, so you sort of know where you are and which way you're pointing. But maybe the question that the listeners were wondering is like, if you have a telescope out there in space, like, how do you know where you are? And how do you know which way you're pointing? Yeah, I think there's two different parts to it, right, is how do you know which direction you are pointing? And then also how do you change your direction? Right? How do you actually turn something that's up in space? Because here on the ground you can push against the ground it's like connected to something that you can push against. But up in space, right, it's harder to move things around, especially if you wanted to last four decades. M I see, because I guess anything that you do, like if you have jets or anything, then that means that you're expending energy. Yeah, and more specifically mass, right, jets have to push out something. You have to throw something out the back of the jet in order to get the momentum. You mean, we count to throw something at them from here, like to you know, knock them into the linement. That was definitely one of the plans, So I think it was pretty far down on the list. Maybe zap them from Earth with lasers also was pretty far down on the list. Oh, but that would be pretty good, wouldn't it. That's what our strategies for turning asteroids that are coming towards Earth. So maybe we would also work for spacecraft. Yeah, you know, it would indowd used lasers. Actually, I think that would work if you had like sales on the telescope and you could just push it from Earth with lasers. That would be really cool. I can't imagine what could go wrong or why there might be an issue with building an enormous space laser. They should high NASA obviously because we have good ideas. I'll be expecting an email as soon as we're done with this podcast. Well, as usually, we were wondering how many people had there had thought about the space telescopes out there in space and how they turned themselves to point at different stars. So thanks to everybody who answered these questions for the podcast. If you would like to participate for our future episodes, please please please do right to me two questions at Daniel and Jorhey dot com. We'd love to hear a huge variety of voices from all over the world. So think about it for a second. If you earned space pointing a telescope, how would you turn yourself. Here's what people had to say. I haven't thought about it. Maybe by using some geroscopes, either this cameramount that you pointed at the North Star and then it's basically calibrated to turn to compensate for the rotation of the Earth, which is like very consistent. So I'm assuming that space telescopes would do the same. I would guess that the space telescopes point themselves the same way that Alon Musk's SpaceX rockets do with the air pressure thing, I don't know, maybe either that or like a ion engine, I don't know. I learned that the James Webb telescope has a set of wheels that spin and apply some torque, so the whole thing making the twist a little okay, I think I actually remember this one from a previous episode in which we said that it was actually very hard to orient yourself in space, with the exception of being able to use pulsars, which you described as sort of like celestial guiding points that flash very consistently and can therefore somehow be used to triangulate your location, assuming that you already have the known location of two or more pulsars. I believe they use gyroscopes in order to orient themselves, or perhaps they off gas, you know, shooting little jets in particular directions in order to orient themselves in order to point themselves in a particular direction, and they use the background stars to orient themselves correctly. All right, pretty technical answer here, but pretty imaginative. Yeah, our listeners have thought about flying through space, how to get around, how to turn, how to know where you're pointing. We got some pretty smart folks listening to the podcast. Yeah, let's flatter our audience. You guys are awesome, beautiful and brilliant. But I feel like the answer is here, and we're also a little confused about what we're asking in the question, like are we asking like how does a space telescope orient itself? Like how do we how does it know which way it's pointing? And also how does it turn to point at something it wants to look at. Yeah, I think we're asking both questions and have different answers, both of which are really fascinating. So I think all of that is involved. I mean, you have your eyeball out in space, you wanted to look at something in particular, you got to solve both problems. You gotta know where it is now and how to change its position. M do you think there's a there's like a joystick somewhere in NASA or Houston Control Center that points these telescopes? Like who gets to move that joysting? And I wonder if there's a red button on the top of that joystick and if it actually fires something or if it just has a little like sound effect peo pew, or maybe if you press the button like a flag pops out at the end of Hubble boom, or I wonder if anyone that NASA has never been tempted to turn the telescope around and point it at Earth, like, what could it look at? What could it see? You could take a selfie with Hubble? Right? Yeah, oh man, do you probably find all of NASA selling those selfie opportunities. Hubble is quite delicate, and if too much light enters its aperture it could damage it. They have to be very careful about not pointing it, for example, towards the Sun. And I wonder if even the Earth might be too bright a source for Hubble. Well, I guess it would have to be night selfies. Then all right, well, let's dig into this question of how based telescopes orient themselves, how they know which way they're pointing at, and then if they want to point somewhere in particular, how do they move themselves to point in that direction. So, first of all, Daniel stepped us through this. Why this is important and heart wells important because we want to choose what we are seeing. Remember that the telescopes don't see all of space, right, It's not like when you look out of the night sky and you stare up and you basically see the whole sky, or at least the part that's not blocked by the earth. A telescope is very very narrow aperture in comparison, and so you're only really looking at a small portion of the sky, and you want to get to pick which portion of the sky you are looking at. Are we studying this galaxy, are we studying that star over there? Are we tracking something that's moving? So you definitely want to have control over where your telescope is pointed. Yeah, it's sort of like you say, it has a very narrow field of view. I imagine it's sort of like walking around your neighborhood looking through a straw or something like that. Right, that's what it means to have a narrow field of view. Like if you close one eye and the other eye could only look through a drinking straw, your field of view moop be super narrow, and it'd be pretty hard to know where you are. And anybody who looked through a telescope has that experience. Your telescope sort of towards the thing you're looking for, and then you look through the telescope and you don't see it and show wiggle the telescope around and try to find the object. It's not easy when you're looking through a telescope to find that particular object has to be pointed very very close for you to even see it. And a straw is a great example, but it's actually not even dramatic enough. Some of these telescopes, their field of view is so small. It's more like looking at a grain of saying you hold at arm's length, right, that's the fraction of the sky these telescopes can look at at one time. It's like looking at a straw. That's the width of a grain of salt, and a meter lungs what you're saying, Yeah, that's exactly right. So some recent images, for example from James Webb, where they focus on the deep, deep sky, they point at one particular place in the sky and they take a bunch of pictures of that one spot. And the reason you want to hold it there for a long time is that the things that they're looking at are quite dim. You know, these distant galaxies don't send a whole lot of photons per second, so you want to build up a crisp image of them. You've got to wait as many seconds as possible to get as many photons as possible. So you have to keep pointing in the same direction for as long as possible. Yeah, and I imagine that's extra hard because first of all, like that thing that you're looking at might be moving, but also like the space telescope is moving, right, and like these space telescopes are usually an orbit around something, either the Earth or I guess mostly the Earth, but either a near orbit or far orbit. Yeah, we're always moving relative to the Sun. And even if these distant objects aren't effectively moving relative to our galaxy, You're right, our position is moving, and so you have to do something to stay on target. You can't just turn it and point and take pictures. The things you're looking at will change as you orbit the Sun, and so you have to do work. I have to do something to keep pointing in the same direction. M okay. So then, and that's hard to do to move your space telescope because basically there's nothing to push against in space. Exactly, if you're swimming in a swimming pool and you want to turn, what do you do. You hold your arms out and you push against the water. Right, You're pushing against something, and so you turn. But in space, what is there, right, There's no air, there's no water, there's nothing to push against, and so turning yourself is much harder because there's nothing immediately there for you to push against, for you to like boost off of right. And but usually satellites and spacecraft the way they navigate and turn and move around, they have rockets, right, or at least some sort of like a listener suggested, like an ion engine. And the crucial thing here is conservation of momentum. If you're stationary and you want to get moving, then to conserve momentum, you have to throw something going the other direction. That requires mass right the same way that like if you fire a bullet, you feel a recoil. If you're out in space you turn on a rocket, then basically the motion of your ship is the recoil from firing the rocket, because it's basically shooting a bunch of tiny bullets out the back of the rocket. The rocket is not just flames, it's throwing mass out the back of it. So you don't just need fuel to run the rocket. You need some sort of propellant something to throw out of the rocket to move your ship. And that's true both for motion and for rotation. And so if you need a mass to do it, then eventually you're going to run out because you can only bring a limited amount of mass. So the goal is to figure out a way to turn your telescope without using some kind of propellant, right, because I guess if you're using a propellant, even if there are like ion atoms or molecules, you're going to run out eventually, right, You are going to run out eventually. And if you spend billions of dollars and decades to develop this thing, then you want it to last as long as possible. So you're going to try to avoid at all costs having things that run out. Can you make it electric like an electric car? Yeah, you can make it electric, and an ion engine essentially is electric, but it still has to throw something out of the back, right, it's throwing ions which have been accelerated by electric fields. And just bring like a really big gas tank, like one that will last one hundred years, right, Because these missions usually don't have like an unlimited lifespan, right, They usually come with like an expiration date. You can't do that. But then the gas tank is big, which means it's heavy, which means you need more gas to launch it. And usually you want to use all of your available space and mass to design it for science rather than having a huge fuel tank on the back of it. I see. So I guess if you can figure out a smarter way to turn out there in space, then you could have more science than your rugget bigger telescope exactly, more science and more years of science because you wouldn't run out of something. Then you need to turn the thing also be greener, I imagine, right for the space ecosystem, you'd be less pollution. That's true exactly, and so for all of our neighbors out there, we should be consider it. All right, Well, that's why it's important and hard to turn a space telescope and orient it out there in space. And so let's get into how you would actually do this and how you would find yourself if you were lost in space. So let's dig into that. But first let's take a quick right, all right, we're talking about space telescopes, which are telescopes in space basically finally a well named physics object. Well, I know, right, and we're talking about how they point themselves out there in space. So let's tackle maybe the first question is, if you're on telescope out there in space, how do you know where you are? And how do you know where you're pointing? So these telescopes typically have multiple ways to figure out where they are pointing. First of all, they just have a bunch of sensors. Like the Hubble, for example, has several different kinds of sensors. It has a sensor that tells it where the Sun is, which helps it know where it's pointing, but also helps it avoid pointing into the Sun accidentally. It also has sensors for magnetic field, so that you can use the Earth's magnetic field to help figure out where it is. And then there are sensors that look at stars, and there's like a known star map and helps it get an orientation roughly for where it is, So to get a rough idea for where it is and orient itself, it has essentially maps the Sun, the magnetic field, and the stars that give it a sense for where it is. Yeah, that's usually how they do it in science fiction, Like if you're in a spaceship and you land in a place you're not quite sure where you are. Usually the way you orient yourself is by looking at the stars around you, and if you sort of know where they're supposed to be, you can figure out where you are relative to them. That's the idea, right Basically, they're looking at the constellations. They're looking at the constellations, and Hubble is not a traveling spacecraft, so it will never appear in Andromeda and have to figure out where it is. It's always going to be orbiting the Earth, and so we know what the stars look like when you're orbiting the Earth, and so you just need a few examples of particular known stars and you can roughly figure out where you are. So those are the sort of lower precision instruments, sort of baseline that Hubble uses to figure out where it's pointing. But it also has much more precise way to measure how it's so not just like look at the map and figure out where you are, but also understand how far you have turned right. And so in internal to Hubble and almost all of these spacecraft, they have gyroscopes. Gyroscopes are these balls that's been really really fast, and so they're insensitive to the motion of Hubble, and they can measure sort of how far it's turned. M Yeah, that's pretty cool. We use gyroscopes here on Earth all the time also to measure how things turn. But I guess you know as an engineer, the trickling thing with gyroscopes is that they tell you how much, if whether you've turned and how much, But over time they're sort of not calibrated to something fixed like the sun for example, exactly. And so if you're holding a gyroscope and you turn, the gyroscope stays pointing in its original direction, and so you can measure I turned thirty six point two degrees. So it's a relative measurement, as you say, it tells you how far you have turned, doesn't tell you where you're actually pointing. That's why Hubble has this combination of having the rough sensors to tell it the absolute measurements like on pointing and is part of the sky or that part of the sky or this part relative to the sun, plus these gyroscopes to measure very precisely how far it has turned. So it needs a combination of these sensors to get an absolute sense for where it is pointing in the sky. Because I guess if you're using a sensor to track where the sun is. You're basically talking about a camera, right, and so maybe a camera is not that accurate. Yeah, it's basically a low tech camera, and the precision of that is limited by like the pixels of the camera and also basically the width of the object you're looking at. And so the gyroscopes give you the most precise measurement of how far you have turned. And these things need to be again, super duper precise. Like when Hubble is focusing on something and trying to keep it in its field of view, it's like holding a laser beam focused on a dime two hundred miles away. That's how precise we're trying to be. You mean, like how stead of your hand needs to be basically right, Yeah, exactly, And so you're focusing on a dime that's two hundred miles away, plus you're moving relative to that dime, and so it's not just about being steady, it's about slowly tracking, it's about turning your telescope so you can keep on it. So these gyroscopes are super duper important to the operation of these based telescopes, and Hubble has been going for decades, and because these things are so important, they actually went up in two thousand and nine and replaced all six of them. Hobble has six of these things, six gyroscopes. I met six gyroscopes. Yeah, and each one spins at like twenty thousand rpm. Why do they need to be replaced, Well, eventually they degrade. You know, there's always some amount of friction in those things, so they'll rub against each other, they'll slow down, they'll heat up, and nothing is a perpetual motion machine, right, and so eventually these things do need to be replaced. No, when you say it needs to be accurate to the point where you can spot a dime two hundred miles away, is that when you're tracking something, you know, when you're trying to stay focus on a star? Or is that more for finding stars and things like that? I imagine in the gyroscopes maybe don't really help you to find a star. Yeah, the gyroscopes don't tell you what's out there at all. They just tell you how far you have turned. And the scientists need to decide where they want to look. So maybe they've seen something already in the sky near another object, then they want to appear more closely, or they've seen it maybe in the infrared using Spitzer, and now they want to get optical images of it. So they have to already know where in the sky to look. So they have like galactic coordinate systems they used to orient to say where something is in the sky relative to the plane of the galaxy, for example, And so you have to know basically where something is and then go look at it. Is there like a galactic coordinate system. Oh. Absolutely. When you look at the maps, for example, of the cosmic microwave background, those are relative to the plane of the galaxy. So the galaxy runs through the middle of those like a line through the middle of that oval. And then you go above and below the galactic plane. It's arbitrary, right, You could pick an access anywhere in space, and so we pick it relative to the Milky Way center to the like basically the main axis of the Milky Way. Yeah. And if you are out camping and lost in the woods and you look at the sky, you see the sort of Milky Way of stars across the night sky, and that is the plane of the galaxy, right, if you're looking above it or below, you're looking out from the galaxy, because remember, our galaxy is kind of like a disc, and if you're looking at that line and you're looking through the galaxy, which is why it looks so milky, because there's so many more stars and gas and dust and all that kind of stuff. So that's the galactic coordinate system. We used to talk about where things are in space. Well, that gives you the direction, but like, where's the origin of this coordinate system. It's at the center of the Milky Way. If you look at that oval, for example, and you put a dot in the very very center of it, that's where the black hole is. But then when we look at our night sky, it's going to be a little different than that. Right, that's right, we don't see that entire thing. But you can map the sphere of things that we can see onto that coordinate system. But you have to like a little bit of an angle change because we're not at the center of the galaxy, right exactly, we're not at the center of the galaxy. And also our solar system is filted a little bit, so you have to know where the Sun is relative to the center of the galaxy in order to map that on cool But then you said it uses sort of a cameras to see the constellations in a way or a map of the stars. Does it actually do that, like does it actually like track certain stars or constellations? And is that one of those maps you can buy in Hollywood Boulevard to the Stars. Yeah. So Hubble has a bunch of these different systems, right, has the course sun sensors, has a magnetic sensing system. Then it has star trackers, right, and the star trackers determines Hubble's altitude by looking at the location and brightness of stars that it sees, So it has a broader field of view than Hubble sort of main camera. And this lets it like identify unique patterns throughout the sky, which a computer then maps to star maps internal to Hubble and lets it figure out like if there's a correction or if it's slightly pointed in the wrong direction. And then the fine guidance system uses the gyroscopes and everything else to sort of fine tune everything. Now that's interesting. They had to go and replace those gyroscopes. Is that something we can do pretty easily? Like how do we do that? We need to send a rocket with people or do we send robots it's not something we can do very easily. We have to send astronauts up there because it's a complicated job, and so it was done in two thousand and nine, but that was the last time, and it's not something that we can do for James Webb for example. James Webb. Remember it's not in Earth orbit, it's out at a Lagarrange point. It's much much further away, and it's not a place where we can send humans. So either we have to develop robotic repair people or we just can't replace it. So James Webb actually has a slightly different technology than Hubble does. M what does the James Webb telescope do? So Hubble has these spinning balls. They're like mechanical, right, But James Webb tried to look for something that was less mechanical, that didn't require something spinning a really high speed, because that seems like sort of easy to mess up, like a little grain in there can really mess it up. So James Webb actually uses this weird technology. It's a quartzemisp fear that resonates in a particular way, sort of like if you have a wine glass and you rub your finger around it. It resonates and it makes like a ringing sound. That's that wine glass like flexing a little bit. You can't see flexing, but it's actually shaking a little bit. And if you like rotated the wine glass, then the sound would rotate with it. So what happens in the gyroscope inside James Webb is that the quartz hemisphere resonates in this very particular way. It's surrounded by electrodes that are like driving the resonance. They can also detect any slight change in its orientation, Like if James Webb rotates around this quartz hemisphere, they will hear the resonance impacting the telescope at a different location. Well, it's pretty fascinating and so I guess those don't wear out. The hope is that they don't wear out as fast. Right, everything will wear out eventually. This is still moving. Every time James Webb moves, it moves relative to these gyroscopes, and so there's a potential for friction there. But you don't have a spinning mass, right, and so it's less kinetic energy, it's less mechanical, and so the hope is that it will last longer. And so that's how it orients itself. And so if you wanted to point to like a particular galaxy out there that you know about, um, do you still have to kind of like tan around you think, like, do you think there's someone and NASA with the joystick going back and forth, back and forth, opened out, Oh there it is. Or do you think they can just go like point to here, boom, it's pointing there. I don't know the details, but I'm pretty sure it's not a joystick. I think they type in the coordinates and Hubble like pans over. This thing happens very slowly, like when Hubble turns, it turns about as fast as a clock does. It's a hubble, for example, can turn ninety degrees and about fifteen minutes. This is not something you want to spin around very quickly. I see, So it just takes a while with the joystick. I hold the joystick for a while. Yes, it takes patience with a joystick. Probably they do have a joystick that's not actually doing anything. It's just connected like the large change on collider and the the center. They have a big red button you can press that sets off lots of alarm bells and flashing lights, but doesn't actually shut anything down. Wow, that sounds like something the fire department did not approve. All right, well, that's how space telescopes orient themselves. How they know where they're looking at in the night sky or I guess. And if you're if you're in space, every every night is the night sky. It's always night in space. Yeah, unless you're looking at the Sun, I guess. But now let's talk about how space telescopes move, how they actually turn to look at a particular star or galaxy or nebula. So let's get into that, but first let's take another quick break. Or we're talking about how space telescopes point themselves. That's that seems very like self accusatory. I mean, like, what's the point of space telescopes? No, like they have to point at them they're pointing themselves at themselves. I mean, some you've got to do it, right. How introspective are space telescopes? I guess they're not really pointing themselves. We are pointing them, right, somebody is doing it. Yeah yeah, right, the joystick. It's not like they're up there just deciding on their own. Hey I'm going to look at Andromeda today. Yeah yeah, I'm sure there's the NASA joystick person listening to this brain now going, hey, I point the space tells cooes. Do you think space telescodes point themselves? That's right? What do you think the garbage takes itself out just because you're not doing that well? We talked about how space telescopes can know which way they're pointing out out there in space, because I guess it's pretty disorient can be disorient thing. If you're out there in space, you're sort of it's hard to know which way is up and down exactly. And so the second question now is how do they actually turn? How do they like if you're looking in one way looking at a star and you want to look at the star over there, how do you make that term? Because, as we talked about, you don't want to rely on propellants or rockets or ion engines because those are kind of costly. They maybe you might run out at some point in the future. Yeah, and those would be nice, right, you'd like to do that. It's sort of an easy solution because it lets you have a net force. Right, you have your space telescope, you throw something off the side, you're applying a force to that object. That object applies to force back to you. You turn or you move and make some sort of sense. But as we said, that requires some mass, and so now we need a solution that doesn't have any net force or no net torque on the object. Right, you have to figure out how to turn the telescope without applying an overall force to it. Oh, I see what you're saying. Because if you are applying an overall net force or torque, that means you're expending energy in the universe, right, yeah, and not just energy momentum. Right. So if you're going to turn this thing from the outside, you're like, put your hand on it and turn it, then you're applying a force to it, right, Or if you're on the telescope and you're throwing a rock off the side of it, you're using some mass. You're expending momentum. So what we want is a way to turn the telescope without changing its total momentum, because changing its total momentum, by Newton's laws, requires something else to balance that momentum, which means something else with mass, and there's nothing else out there. It's just floating out in space. How do you turn the telescope without applying some overall force to it. That's the physics puzzle, Like how do you change your absolute orientation without changing your overall angular momentum? Kind of Yeah, imagine, for example, you're on ice skates and you're on a super duper slippery surface. How do you turn? Or you can't push against the ice because you're on ice skates in a super slippery So how do you turn your direction? How do you change which way you are pointing? That's basically the puzzle, right, So if you could push against the side, that'd be great, but there is no side. If you could like throw a rock, then that'd be great, but you can't do that. So the question is how do you turn on this slippery surface? Right? Or I was thinking it's more like, you know, if you were stuck out there in space, like if you're an astronaut. So imagine you're an astronaut and your space suit and you're out there and base whether you're looking away from your spaceshipe or away from the Earth, and you want to turn around to look at your spaceship or Earth, but you've run out of fuel and maybe in your jetpack, how do you turn yourself around, Like, you can't just like grab something and pull yourself to look the other way. And you can't just like flail your arms because it would be hard to sort of change your orientation. Yeah, even just flailing your arms won't do it, right. You can't by flailing your arms apply any overall force to yourself. So this seems like an unsolvable problem, and the way to solve it is to find a loophole is to say, well, what if I don't want to turn the whole telescope. What if I only want to turn part of the telescope. So imagine like an invisible dividing line. You say, this part of the telescope I want to turn because it's got the cameras on it, and this other part of the telescope has the electronics and all the other stuff. They can't see anything, so I don't really care about that one. So instead of turning the whole telescope, what if you just want to turn part of the telescope one way? You can do that by turning the other part the other way. I imagine, for example, having two ice skaters that are ski getting together. One of them can start spinning if they push against the other one. Right, So instead of turning the whole telescope, just turn the part of the telescope you want to actually use to look at the universe by pushing it against another part of the telescope, or maybe instead of iceicators. You can imagine our stranded astronaut out there in space. You know, they can't look in a particular way by themselves, but if they had a buddy or a friend, like, one of them could push against the other one and at least one of them can look back at Earth or at their spaceship. Exactly. If you don't care what your buddy gets to see, then you can turn in one direction by pushing against him or her. And that's exactly what they do. On the space telescopes. They have a little part of it called at a buddy. It's got a little space telescope. Buddy, it's got the important part and the not important part, and the non important part is just there to help the other part turn. It's at the buddy, the sidekick, right, and so on a space telescope, this is called a reaction wheel. Essentially, it's a little piece which turns the opposite direction that the spacecraft does, so spacecraft says, I want to go that way. Then the reaction wheel turns the other way in order to balance it. So you're not changing the overall angular momentum of this thing at all. You're only changing the angle momentum of the part that you care about and the part you don't care about. The sidekick gets the opposite angular momentum, so physics is happy and you get to point the part that you want in the right direction. M So I'm imagining like inside of the space telescope, there's basically like a just a big disc maybe right, or like a big donut or cylinder that's that you can spin. Is that the idea? That's exactly the idea. So if you want to turn like clockwise, you would turn the donut or the disc counterclockwise. M Imagine you two astronauts. One of them ones to turn clockwise, so he pushes against the other one and one of them turns one way, the other one turns the other way. Now on the space telescope, you don't want like a second telescope to push against, so you shrink the other part down as much as you can make it massive and make it spin really, really fast, so it can store a lot of angular momentum. And so the space telescope has one of these for each direction it might need to turn interesting like up and down, the side to side, in front, the back exactly. So you need three of these to control your direction complete the in space. Usually they have extras just in case one of them breaks. But they're called reaction wheels or momentum wheels, and they are fixed in place on the sort of on the side of the telescope. They spin many many times, like a thousand or four thousand times a minute. M Now, I guess maybe I have two questions. One is okay, so I'm out there and floating a space, and I want to turn clockwise, So I spin my little wheel counterclockwise, and that gets me to turn clockwise while the spinning wheel is spinning inside of me. Now, let's say I want to stop because I certainly I got some angular momentum turning. How do I stop turning? Do I just spin the wheel the other way? Just spin the wheel the other way exactly, And so you can apply whatever torque you want to yourself as long as you're applying the opposite torque to the wheel and that works in both directions, and so the wheel isn't like ever stationary. What you're doing is you're speeding the wheel up or slowing the wheel down, and I do that with a little electric motor which is solar powered. So it is sort of like your Tesla as you said earlier, yeah, or like the Prios, right, or any any car with battery. Like when you break, you're putting energy into the battery, and then when you need to accelerate, you take energy from the battery. So basically the same concept, right, Basically the same concept exactly. So you want to change your orientation, you have to change the speed of the wheel to create a torque on the rest of the object. And so this thing spins really really fast, so it can store a lot of ing the momentum, but it's still really small and low mass compared to the actual telescope, which means you can't turn the telescope very quickly. But that's good, right, you don't want this thing jerking around. They're not super dupe or powerful, but you don't ever need to ever change the telescope's direction really really quickly. It sort of feels like you got something for free or something for nothing, you know, do you know what I mean? Like that was pointing one way and then I did something, and now I'm pointing it another way. But I didn't lose really any energy. Yeah, there's two different aspects of this, energy and momentum. So momentum conservation is satisfied. Because part of U spunum one way, the other part supund the other way, so it adds up to zero. Just like your two astronauts, they could also split apart if they push against each other, right, they could float away in space when you could get back to the spaceship, and the other one could be lost to infinity, and that would satisfy conservation momentum. There'd be no net force on the pair of them, even though there is a force relative between them, So momentum is satisfied. But you're right, we are using energy, so this is not for free. You need to speed up that reaction wheel or slow down that reaction wheel. That requires some energy, and so this thing is not for free. It does use some energy, but it doesn't need any propellant. Right. A rocket uses both energy and propellant, has to have some math to throw at the side. This doesn't require any propellant, though it does use some energy. Yeah, I guess what I mean. Like in the two astronaut example, if you and I are in space and I'm like, Daniel, save yourself. I'm going to push you towards the spaceship to save yourself, and I push you. You're moving towards the spaceship, but I'm not. I'm moving away from the spaceship. But then I'm what if? And then but then suddenly it's like I changed my mind. I'm like, wait, wait, wait, no, that was a terrible idea, and I pull on the rope that was attached between us to bring us back together. Technically, we would not, like, our center of mask would not have moved. That's right, right. Our center of masks cannot move without some external force. Right. So even if you don't change your mind and I drift back to the spaceship, you're drifting away from the spaceships. Our center of mass is not changing, right, right. But on the spinning example with the space telescope, I kind of it sort of feels like you did get away with something, right. It's like you spun the mass one way and then you spun it the other way and now you're you're in a different spot. Your total orientation change direction. Well, part of the spaceship changes direction and another part changes direction in the opposite way. So the total angle momentum hasn't changed. Right. But then when you slow down to stop, you spin it the other way, and presumably it's the same amount of momentum that you need to take out or put back in. And so you and the wheel are in the same spot you started with, but both of you are pointing in a different direction. Though you're both pointing in a different direction, but the angular momentum hasn't changed. You've expended some energy, but the angular momentum isn't different. Yeah, right, it sort of feels like you're getting something for free. Well, it's sort of like if the astronauts push against each other and they're further away, it costs some energy to change that configuration, but it didn't change the overall momentum. Yeah, but in the astronaut example, they didn't move if they come back together. But in the wheel case, do you do sort of like move, You're not pointing in a different direction, right, Well, in the astronaut a case, imagine we're connected by ropes and you push against me, so that I drifted back towards the ship, and you drift away from the ship. And then you change your mind, and so you tug on the rope to stop my motion, which also stops you. Now we're further apart than where we were, but we have no change in our center of mass, no change in our overall momentum. We've lost is you spend some energy pushing me away and then pulling me back, So in the same way, when you're orienting the telescope, you've changed its overall configuration, but there's no change in its overall angle momentum. That you have spent some energy to change the directions of both parts, the telescope and the reaction wheel. Interesting, so well, well, I feel also that the other part question I had is it isn't spinning a little wheel basically the same as flailing your arms, Like if I was stuck out there in space, could I also just like gonna spin my arm and that would reorient myself? If you could turn your arm effectively into a reaction wheel, then yes, I don't know if you really could get your arm to spin independently along the same access though I had to think about the biomechanics of it. Actually, you're an expert in that, aren't you. I'm not sure if you really can have it spinned independently, or if when you're moving in or way, if you're moving in a circle, if you're effectively pushing back on your body. But yes, if you, for example, ripped your arm off and attached it via mechanical axle to your body, then by spinning it you could change your direction. That seems a little dramatic, but I think the answer, since you say that I'm the expert, I think the answer is yes, I think you could do that. It's kind of the reason why when you jump off a cliff into the water, for example, or of a diving board, people flail their arms. They sort of like moving like a windmill, and that because they're trying not to fall on their face in the water. Yeah. Well, I'll trust you whether that's possible. I prefer the cleaner physics but more gory example where you actually pull the arm off, but I trust you that it's possible even without filling your arms. All right, we're in space. You can rip your arm out, but they don't have to look back at the spaceship, although I'm not sure what you're going to do once you get the spaceship. Are you going to open the door? And I'll do my way and we'll see how that goes? All right, Well, we'll see if the door was designed to be opened one handed, just space was designed for arm removal. I'm not saying it's more practical. I'm just saying the physics of it is clearer. I see, I see, And that's more important than your arm. I guess in this scenario, if it's just hypothetical and I want to give the accurate physics answer, then yes, I prefer the more gruesome but clear physics scenario, right right. I think as an engineer, I would try my way first if it works, rather than getting to the physics dogma here. All right, but you can be expending valuable oxygen as you do your experiment. All right? So then is this how the James Webb space does coupe orient itself? Do they have? Does it have these spinning wheels? Do the Hubble also do this? Yeah? So basically every spacecraft does this. James Webb has six of these reaction wheels that are spinning that help it turn Hubble has these things. Kepler has these things, and Kepler is a fascinating story because these things failed on Kepler, which made it very, very difficult for Kepler to do its mission. What happened? So Kepler launched two thousand and nine, had four of these reaction wheels you only nearly need three, but it had a spare just for good measure. And remember, Kepler is a telescope that's looking for planets to eclipse their stars. So you got to watch a star for a while, for a long time to see it's one ten thousands drop in brightness as a planet goes across the star, so you really got to be focused on it. A few years into itsmission, in two thousand and twelve, one of these things failed and they didn't understand why. But that's okay. They were had four, so they had one spare. They're okay with three, and then the next year they lost weight. I have a question, like, you need one for every direction, right up, down, left, and right from the back. Which one is your spare? Like? Can you spur point in all three directions? Yeah? Good question. I don't know the answer. I guess the engineers have probably figured that out. Okay, So then Kepler lost one and they activated to spare and then what happened, And then they lost another one in twenty thirteen, so now they only had two, which limits how the spacecraft can turn right. And this thing has to be able to turn in three D to track an arbitrary star. So people were pretty bummed. They spent a lot of time and money on this spacecraft, and it also cost money to operate. It's not like once you have it up there in space it's free, it's saying, costs millions of dollars to operate the deep space network and the people and all electronics and everything. So it's a real question of like you just shut the thing down or do you try to figure out another way to operate this telescope. I wonder I'm guessing the answers no, because otherwise it would have figured that out. But I wonder if you can just use two to orient yourself in any direction in space? You know what I mean, because orientations in space are these kinds of weird transformations where you can, like, if you wanted to point to the right, you could, but you don't have something that turns you to the right. You could maybe point down, turn left, or you know, turn the other way and then switch back and do some weird complicated maneuver to get you to point right. Well, these things are orthogonal from each other, and so having only two basically only lets you map out a plane in a three D space. But like it, used one to turn one way, then that reorients the other one, doesn't it, So you essentially kind of can point in any direction. No, yeah, that's a really good point, and I think that that's essentially what they tried to do. But you still need help in that third direction because you don't want to drift right. You don't want to drift in that third direction. And once you've turned and pointed at the star, now you've used your two reaction wheels along those two planes, which means you're susceptible. You're always susceptible to moving in that third dimension. And so in order to correct, you would then need to turn twice basically in order to correct, which you'd bring you off of the star. So they actually came up with an ingenious way to try to prevent that from happening. Oh, I see what you're saying is that if you could point anywhere you want with maybe two reaction wheels active, but you wouldn't be able to maybe track a star smoothly. Yeah, you might have to take like zig zags, right, and which means you couldn't keep it in your field of view. So then what did they do? So they came up with this really cool scheme to use the sun. Right, the Sun is actually pushing on these things. Remember our conversation earlier about like zapping a solar sail attached to telescope with lasers from Earth. They basically are doing that, except they're using sunlight instead of lasers from Earth. So as it moves around the Sun, the solar wind and the photons push against the solar panels on Kepler, and so now instead of compensating for that, they're using that to help keep it stable. Interest in using the solar wind, Yeah, they're actually using the photon pressure, right, not just the solar wind, but the actual photon pressure. It's like a solar sale. So the solar panels are in sort of like a hexagon around Kepler. And if the pointy part where the solar panels meet, if that thing is oriented right along the direction of the photons, then it sort of stays stable and it's turned a little bit, then it's unstable. So they can use that orientation to help either push on the spacecraft or to keep it stable, but would that help it track a start? It really limits what they can do. They can only look at sort of a couple different places in the sky, but for a couple of spots and its orbit around the Sun, they can use the Sun to compensate for the lack of the third reaction wheel and keep it stable and keep a tract on a planet for a little while. So it's not a complete recovery of its abilities by any means, but it's a partial recovery of the science mission cool. Well, that's a pretty clever technology, I guess, although I feel like they should change the name from reaction wheels to flailing arms. It's a really big bummer that these things went bad. They've been trying to understand what happened, and in twenty seventeen there's a paper that came out that suggests that it's due to geomagnetic storms from the Sun. Basically, the Sun has like some big energetic event, it dumps out a bunch of plasma and a coronal mass ejection and as this passes through the spacecraft, it interferes with the operation of the reaction wheel. Wow. Yeah, that's pretty cool and also a pretty convenient story to make up for the fact that era the thing you design did not last as much as you thought it would. Yeah, and these reaction wheels are very specialized technology. This one manufacturer that has been putting these things out it's called Ithaco, and their reaction wheels have failed not just on Kepler but also on other spacecraft. So James Webb actually went to a different manufacturer to produce these things. So we're hoping that James Webb's reaction wheels lasts a lot longer. And so that is a pretty clever way to turn yourself in space to have these reaction wheels. And so basically the space teal skills use them. Do other spacecraft use them like the voyage you're used at, or do some of these like the Parker Solar Probe does it use that too? Some other spacecraft do use these kind of things, But remember they're very slow, so they're not great for navigation. They're really just great for like very gentle orientation. Another example is light Sale Light Sale is one of these things that's testing out the ability to sail on sunlight. There's a huge solar sale that it's using to gather momentum and navigate around the solar system. But they also want to be able to steer this thing, and so they have a reaction wheel on it to try to turn it sort of towards an away from the sun to change how it's sailing. So then it only needs one wheel. It only needs one wheel. Yeah, though it's also sort of experimental craft, and so I think they're trying to be simpler and cheaper. Everybody would love to have more of these wheels, and a lot of the spacecraft have a combination of reaction wheels and chemical thrusters. Chemical thrusters are for when you've like saturated your reaction wheel you can't turn anymore because it's already spinning in it's max rpm, or when you need to turn faster than you can with your reaction wheels, that you want to use your chemical thrusters very sparingly because you just use up the mass and then eventually you're run out cool Well overall, a pretty clever solution to move yourself, at least in orientation in space. Yeah, it's a very clever idea. And when I think we'll be using for a long time in the future, if we can make these things more reliable and if they don't require tearing your arm off, yes, let's try that solution. Second, So first zapping with lasers, second tearing your arm off. Kind that's right, I'm gonna be up up there is space going yes, season, go ahead and shoot the lasers at Daniel and let me know if that works. And if it does, then you can shoot them in me. But I'm going to be flailing my arms out here and I'll see you back at the space ship. I wonder if that big earth laser for zapping astronauts also has a joystick? And who gets to run that one? Oh man? Yeah? Yeah, And what kind of training the prison needs to do? You know, play a lot of asteroids maybe, or a lot of Halo. Perhaps you want someone who can get a good head shot the first try Fortnite experts. All right, Well, hopefully you did not get lost in this discussion, and we navigated your brain to understanding how space tells coopes move and orient themselves to look at the universe out there and This is crucial to our ability to understand what is out there in the universe and to continue to build that physical and conceptual map of how the universe works. Thanks for joining us, See you next time. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. Or more podcast from my heart Radio visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows.