Hey there, listeners, this is Jonathan. Before we get into the episode, I want to acknowledge something. Uh. Twitter user Charlie Tango Bravo helpfully pointed out that I made a mistake. When we first published this episode, I completely misrepresented the inverse square law, and so later in this episode you will hear me kind of interrupt to give a proper explanation. My apologies for that. Uh and um, yeah, I mean, this one's totally on me. I made a mistake, and I feel real dumb about it. But you know, mistakes happen. It doesn't. It just tells me I need to be even more careful in the future to make sure that I'm not misrepresenting something, which I totally was in the original version of this episode. Anyway, let's get to the episode, and when we get to the part where I'll correct it, I'm pretty sure we'll be apparent. Enjoy. Welcome to Tech Stuff, a production from I Heart Radio. Hey there, and welcome to tech Stuff. I'm your host, Jonathan Strickland. I'm an executive producer with I Heart Radio and I love all things tech and listener Terry A. Carlson, and I apologize for very likely Butchering your name, asked if I might cover how space vehicles navigate. So we're going to talk about navigation, and this is one of those things I find really fascinating and also sometimes frequently actually a bit confusing. Now I blame part of that confusion on my own fascination with stuff like Star Wars, which some folks call science fiction. I think of it as fantasy that happens to be set in space, or even Star Trek, which is closer to science fiction than Star Wars, but can play a bit fast and loose with science and technology. And these kind of properties gave me a really cool but an accurate feel for how space navigation works. Terry, you asked that I cover what references and methods do space programs used to actually do space navigation? And that's a great question, because all of this really does rely on reference or relationships between a vehicle and something else, Like it's all relative. You know, we're going to talk about that a lot in this episode. And I guess on some level this is intuitive, but I had not really thought about it in concrete terms before. So, for example, here on Earth, if you're giving directions to someone, you would tell that person how to get to a place relative to where they are right now, Right, I mean, the same thing holds true for space vehicles. But we've got to remember that everything in space is moved ing all the time. So here on Earth, if you need directions, you can start from a position in which you know you're not moving relative to the Earth. You're standing still. On Earth, You're still moving, but that's because the arts moving. We're going to get to that. And you know you're standing still with reference to the Earth. But in space, everything is moving in reference to everything else. Now you could be moving at a similar velocity relative to your surroundings. So from your perspective, it might seem like you're not all really moving together, but trust me, you totes are. Now what I'm about to go into it really matters when we start thinking about the possibility of traveling beyond our solar system to another. But it's it's, you know, something that we have to take into consideration, even when we're talking about travel within our Solar system. So let's start with the easiest stuff first and then work our way up. The Earth travels in a nearly circular orbit around the Sun, right, I mean, this is not news to you. I imagine. So the Earth is moving, it's moving in an orbital path around the Sun. The orbit moves at a speed that's around sixty thousand miles per hour if you prefer, that's about or almost thirty kilometers per second. Now, if we're talking about going to space to do stuff in low Earth orbit, we're pretty much thinking to Earth, right, We're not too worried about everything else that's going on. The orbital speed of Earth isn't as big a deal in that case, we don't have to take that into as much account. But let's say that we want to travel to Mars. Go from Earth to Mars. Well, Mars is further out from the Sun than Earth, right, I mean it goes my very educated mother. So Mars comes after Earth. Oh, in case you've not actually heard that mnemonic device, this is how I learned it. It was my very educated mother just served us nine pickles. That stands for Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto the planets except well, you know, when I was a kid, Pluto was a planet, and then Pluto kind of got the boot as far as being a planet goes. So I guess now you could say, my very educated mother just served us nothing. Thanks mom. Okay. So, Mars is further out from the Sun than Earth is, and Mars's orbit is obviously a bigger circle around the Sun than Earth's orbit is, because you know, Mars is further out from the center of our solar system. Mars also doesn't have the same orbital velocity as Earth. Earth completes one orbit of the Sun every three sixty five Earth days, with a little bit of change leftover. That's why we have to have leap years, and Mars's orbit is just a hair under six hundred eighty seven Earth days long. But beyond that, Mars's orbital velocity is closer to twenty kilometers per second. Remember Earth's is closer to thirty kilometers per second. So Mars is not just moving in a greater distance because it's orbit is larger, it's moving a little slower compared to Earth, you know, and again relative to the Sun. So that means if we want to plot a course from Earth to Mars, we have to take all that into account, right, because we can't travel instantaneously. It takes time for us to get from point A to point B. We can't just point a rocket at where Mars appears to be to us right at that moment and then launch the rocket in that direction. You know, look at the sky, find the red dot and say aim that away, because you know, the positions of Earth and Mars are shifting in their orbits, and Mars will not be at that same spot by the time the vehicle we launch gets there. Heck, it's not even at that spot as we look at it, because it takes time for light to travel from Mars back to us, So we're really looking at where Mars used to be, and we would be shooting a rocket at where Mars used to be a really long time ago. Uh. The vehicle it would take months to travel there, so by the time it would get to that position in space, Mars wouldn't be there anymore. By the way, all this orbital stuff is why when we talk about sending a crew of human astronauts to Mars, we talk about missions that typically are on the order of a couple of years in length. The trip from Earth to Mars could take around eight months, and that's if we time it so that the launch vehicle UH shoots the spacecraft up and has the spacecraft travel the least amount of distance needed in order to go from Earth to Mars. That means you're timing it right. You're aiming at where Mars is going to be, and you want to time it so that Earth and Mars are at one of the closest points in their respective orbits to one another. You out of time it just right. And because of the difference in orbital velocities and the fact that mars Is year is significantly longer than Earth's year, that means that by the time you would arrive on Mars, Earth and Mars would no longer be super close together anymore. Right Like they're they're constantly in motion, so now they're moving apart from each other. That means for you to get back to Earth, you would need way more fuel for your return trip than you did for your trip out to Mars. Potentially, so the most fuel efficient thing to do is to hunker down on Mars and do all your science e stuff and you wait for the planets to reach a point in their orbits where again you will be traveling the least amount of distance you can get away with. And all told, that means a mission needs to last around two years to get all that done. Plus you know, theoretically you could even spend some of that time on Mars making rocket fuel, so you wouldn't necessarily have to carry all of it with you on the way there. Anyway, that's just a simple example of how motion in space matters. But we're just getting started, right, So I've touched on stuff like low Earth orbit and interplanetary travel within a solar system. But it's not just that the planets are moving around the Sun. Our entire Solar system is hurtling through space. We are part of the Milky Way Galaxy, and at the center of the Milky Way Galaxy there is a super massive black hole that's kind of the you know, it's like the Sun is in our Solar system, except there's a supermassive black hole in the middle of a galaxy billions of solar systems. Scientists estimate that our galaxy has somewhere between a hundred to four hundred billion stars in it. So think about that. I can't. I tried, but that's just a number that's just way too big for me to even get a rudimentary grasp on it. Anyway, our solar system is traveling in its orbit in the galaxy at a speed of around four thousand miles per hour or two kilometers per second. Then we have to consider that our galaxy is also in motion. We've got other neighboring galaxies in our neighborhood is just part of a super cluster of galaxies, and that in itself is part of an even bigger super cluster. And we're all hurtling through space at around a thousand kilometers per second. Where are we going? Well, I mean, I want to say, get in loser, we're going shopping, but we're not. We're headed to the Great Attractor. And to my surprise, that isn't Oscar Isaac. It's a gravitational point in the Lania Kia super cluster. Uh. And um, you know the Great Attractor is also moving towards another mass, the Shapley super Cluster. I guess there's always a bigger fish. And you know, we could keep going down this rabbit hole. But what I really wanted to point out is that we're talking about a lot of body ease in motion here, and that makes navigation more tricky, right, I mean, this is kind of like saying that you stop to ask for directions, but the town you stop in is actually moving on the map, and the place you're going to is also moving on the map. Maybe it's moving away from the town you are in, which is still in motion. So the route you are going to take to get there is changing by the minute. It gets complicated, so you have to take, uh, you know, into account a lot of things. You have to take reference points in order to make it all makes sense. And of course that's like talking about a map that's two dimensional. Obviously in space you're talking about three dimensional. You're not limited by a two dimensional plane. You're moving in three dimensional space. This, by the way, is all before we even consider stuff like relativity, which makes things even more weird. Einstein's theories of relativity really show how our universe behaves in ways that we don't often get to observe of directly. So we don't have a lot of direct perspective on these things. But let's use an example to explain some of relativity, and we'll start with special relativity. That's the I would argue the easier of the two to get a grasp on. Alright, so we've got two people and we're gonna name them Alice and Bob. Alice has superpowers, and Alice can travel through space without a space suit and can move at near the speed of light. So she flies through space close to the speed of light, and she goes and flies off on an adventure at top speed. When she comes back and she meets up with her best friend Bob, the two notice something unusual. So to Alice, it'll seem like Bob aged faster than normal, as if more time had passed for Bob than it did for Alice. To Bob, it will seem like Alice has aged less than normal, like not as much time passed for her, like like like less time pass less than it should have. And the reason for this is that the faster your speed is relative to some other reference point. This is why we talk about relativity. It is relative to some other reference point, the slower time will pass for you relative to that reference point. So again, like Bob is our our reference point for Alice. So it appears like, you know, Alice hasn't aged as much because time appeared to pass slower for Alice than it did to Bob. Now to Alice, time will have seemed to pass as normal for our own frame of reference. So in other words, it wouldn't feel to her as if time had slowed down. She would feel like time was passing, just as it would if she were standing perfectly still on Earth. A second would feel like a second to her. And in fact, if Alice were wearing a watch with a second hand, it would seem to be clicking one second at a time, just perfectly. Now, if Bob could somehow observe that watch while Alice is traveling at near the speed of light, Bob would see that the second hand is moving really slowly. It would be taking way longer than a second for it to take each tick. In fact, the faster you go, the slower it gets. And if you got to the point where you could travel at the speed of light, it would stop like the second hand wouldn't move anymore. For Bob, he wouldn't see the second If somehow Alice could move faster than the speed of light, which is as Einstein would put it impossible, it would look as though the second hand was going backwards. She would be traveling back in time. Now that's physically impossible. So I just thought I would throw head in there as an interesting you know, side note. But again to Alice, it would seem like time was passing as normal. And likewise, let's say Bob's wearing a watch. He has a second hand. To Bob, time is passing just as normal. A second takes a second, he can watch the little second hand click on buy on this on his watch. Now, let's say that Alice is able to see Bob's watch. While Alice is traveling at near light speed, it would look to her as if the second hand was going way too fast, like it was just spinning around the watch face. And again, Bob and Alice would each feel the passage of time as if it were just normal. It's only when they compare it to a point of reference, when they are relative to something else, that they see that there is any type of difference. Now, for most of our experiences on Earth, we don't notice this effect, and that's partly because we're usually traveling at a similar velocity relative to one another. We're all on this planet. Most of us aren't going super duper fast. However, there are cases where we can measure a difference. It wouldn't be observable like to our normal senses, but with very sensitive you know, metrics, we could see that there was a difference there. It's settled because we are not able to go anywhere close to the speed of light, at least not yet. So for example, we have the case of Mark and Scott Kelly. These are twin brothers who are both astronauts. Now, both of the Kelly's have spent time in orbit. Mark Kelly was born first, he's the older of the two twins, and he's logged fifty four days in space. Pretty respectable, right, I mean extremely respectable, Mr Kelly. I don't mean to, you know, dismiss that incredible achievement. But Scott Kelly has spent five hundred twenty days in orbit, almost ten times as much time in orbit, and a lot of that was aboard the International Space Station, which orbits the Earth at the speed of around twenty eight thousand kilometers per hour or seventeen thousand, five hundred miles per hour. So for a significant amount of time, Scott was traveling much faster relative to his brother Mark, who was back here on Earth. And since traveling faster means that time passes more slowly relative to an outside observer, it means that the gap between Mark and Scott actually got bigger. Mark aged faster here on Earth than Scott did out in space because of that speed of travel. Mark once said that he used to be six minutes older than his brother, but now, thanks to all that space travel that Scott did at high speeds, Mark is six minutes five milliseconds older. And that's a funny thing to say. Uh And sure you could argue, well, that's not really significant to our normal frame of reference. I mean, what's five milliseconds, But it does show that we actually have to keep this in mind when it comes to space travel. It does matter. Now when we come back, we're gonna tackle a little bit more of a relativity. But first let's take a relatively quick break. Okay, we're back, and we are not done with relativity yet. The speed and time thing is all part of special relativity. But Einstein was a real you know Einstein, and he also published his theory on general relativity. This includes an explanation that if you were to have two clocks and one of them is closer to a gravitational mass than the other one, the other one is much further out from that gravitational mass. The one that's closer to the gravitational mass will take more slowly than the one that's further out. This is separate from the whole you know, speed thing. So in other words, gravity also affects the rate at which time passes. It passes more slowly when you're closer to a big center of gravity. And this becomes really important for navigation just here on Earth, not even just space navigation, but navigation here. And you might wonder why that is. Well, a lot of us depend upon GPS apps or devices, right like we pull that up whenever we're going someplace new, and these devices they work by relying on signals that are coming from GPS satellites. It gets a few of these different signals and then it's able to pinpoint the location where on the surface of the Earth we happen to be at that given time. Well, those satellites are really far away from us. They are beyond low Earth orbit, so they're beyond where the International Space Station is, for example. They're out in medium Earth orbit somewhere around twenty two KOs out from the Earth, or around twelve fifty miles. So these satellites are much further out from the Earth's gravitational mass than say your watch or come on, we'll be real here your smartphone. So because of that, the clocks on the GPS satellites tick slightly fat stir. Then the clocks here on Earth tick. Remember, the closer you are to a gravitational mass, the slower time will pass for that particular frame of reference compared to a different frame of reference. I always have to throw that part in because again, in the moment it passes, the way time passes, like the way our experience of time is remains the same, not to our own frame of reference. Now, the GPS satellites are also traveling really fast relative to us, so that actually means that we have to take a bit off the top right, because we know that the faster you travel, the slower time uh affects you relative to someone who's not traveling at that speed. So the effects of general relativity mean that a clock on a GPS satellite has on average around forty five micro seconds more than an Earth clock at the end of a day, a full day. But again, because these satellites are link faster relative to us, the clocks also have a negative seven micro seconds two factor, and compared to our clocks, due to special relativity, so we have to combine those two together forty five and negative seven. That gives us thirty eight micro seconds that are extra on the GPS clocks. So if if we were to stop it right at midnight for both clocks, you know here on Earth and at the GPS satellite, we would see midnight on our clock and midnight plus thirty eight million micro seconds, not milliseconds micro seconds on the GPS side. Now, because our navigation depends upon taking signals from satellites and essentially measuring how long it took for that signal to go from the satellite to us, in order for us to calculate where we are on Earth, we actually have to account for that difference between our clocks and the satellite satellites clocks, or else we start to get some drift, and that means that over time, and and we're actually talking about years here, but it does happen, our navigation systems would become less accurate, which would eventually get to a point where our GPS device wouldn't even really show us where we are because it would be miscalculating based upon the the differences in the clocks on the GPS satellites versus the clock on our phone or that our phone is connected to, and instead it would show you where it thinks you are, but there would be a growing gap between where it thinks you are and where you really are. Based upon this gap in time. It would take like seven years for that to get to a point where we might even notice it, and we do correct for it. So essentially what happens is we shave thirty eight micro seconds off the clocks every midnight that the that the clock's hit. So when the clocks hit midnight on the GPS satellites, they kind of hold for thirty eight microseconds, which puts them I can sync with the clocks here on Earth. And then we have to do it every single day because every single day we get the effects of relativity. All right, well, that's something we have to take into account with navigation, Like these are things that we don't again, we don't necessarily have to think about here on Earth. Typically, most of us don't come into situations where special and general relativity have a noticeable impact on our day to day experience. UM. One really important element in space navigation is something called the deep Space Network, which is not a really cool science fiction channel, uh, no, it's it's actually a bunch of antennas, and it's it's more than that. But that's a big part of it. If we go back to the nineteen fifties, we had the space race ramping up. In nineteen fifty seven, the then Soviet Union launched spot Nick into orbit. The US was already working on its own satellite, and of course there were reasons for this beyond the scientific push. Scientific push was a big part of it, but there were other political reasons. For one thing, Demonstrating that you could put a payload into space also sent the message of hey, comrade, we can build rockets big enough to reach you, even though we're on the other side of the world, and you know, nuclear weapons are a thing, so you know, this wasn't just you know, a science thing. But that's another tangent that I won't go down any further. But on the U s side, one of the things that the Jet Propulsion Laboratory or JPL undertook was a job from the United States Army, which effectively you know, ran the jp L to establish radio tracking stations in certain parts of the world, including places like Singapore, Nigeria, and California. So upon, launching the first successful US satellite, which was called Explorer one. These ground stations would receive data from the satellite to be able to track it as it passed over overhead. This was essentially telemetry data, and you might wonder what does that mean. Well, telemetry is essentially the process of using some sort of device to measure something. It could be temperature, it could be pressure, it could be speed or velocity, and then it transmits that information to a distant receiver. Now, in this case, the telemetry was mostly about the Explorer one's orientation and velocity as it went through its orbit. Engineers at mission control could take that data and plot out the Explorer one's orbital path. In October, the US government established NASA, and this was really to consolidate the space efforts from various independent groups, mostly in the military, like the Army, Navy, and Air Force all had independent space exploration UH initiatives, so this was to kind of bring them all under one civilian umbrella and combine all those resources to be more effective. So this included the Jet Propulsion Laboratory that had previously been run by the U. S Army that became part of NASA. In December of nineteen and NASA would assign to the jp L the responsibility of planning out planetary and lunar exploration missions that would use unscrewed spacecraft, that is, spacecraft that did not have human beings abort robotic spacecraft if you prefer. Now. That would necessitate a network system of receivers here on Earth to be able to receive communications with and then to send communications too, as well as to just keep track of these robotic spacecraft as they traveled away from the Earth. You can't just have one really big antenna in Houston, because you know, the Earth rotates and sometimes Houston would be pointing the wrong way. Get together, Houston. So yeah, to establish that these antenna around the Earth need to be pointing outward in such a way that you have maintained contact with distant spacecraft. Moreover, as the spacecraft move further from the Earth, the signal strength will decrease. In fact, you know, here on Earth we describe radio frequencies as obeying the inverse square law, which means that the power of a signal is inversely proportional to the distance from a source. Hey, it's Jonathan from one day further out from when this episode originally published. Okay, so the first time I tried to explain this, I just playing god it all wrong. Uh, we published the episode, they had an error in it, and this was because of a fundamental misunderstanding on my part. But fortunately Charlie Tango Bravo on Twitter set me straight and I'm gonna try harder to get it right this time. So the inverse square law will first imagine that you have a source of electro magnetic radiation. And this can be anything from you know, a radio antenna to a microwave source to light. In fact, let's talk about light because that's pretty easy for us to wrap our heads around it because we can directly observe it. Right, We've all seen that as you move away from a source of light, then you get less light to work with, Right, that's intuitive. So if you're walking around I don't know, a spooky attic and it's lit by a single light bulb hanging down from a chain, well you know that as you get towards the corners, a ghost is gonna get you. I'm kidding. Ghosts don't exist, but you do know that as you move further away from the light, bulb, it gets darker, Right, That just something we've experienced. Well, we can actually describe this phenomena with the inverse square law of propagation. We can think of radiation moving out from a source as similar to that of an expanding sphere, like it's going out in all directions and growing as it moves outward. Right, the sphere gets bigger and bigger. The center is still stationary in this frame of reference. So the further we get out from the center of the sphere, the more surface area that electromagnetic radiation is covering. Right, the the outside of the sphere is bigger. That means that the signal strength is growing weaker. You have the same amount of signal to go around, but you're covering a larger area, so you can think of it as spreading across more space. And we can describe the relationship between signal intensity and distance as intensity equals one divided by r, that being the distance squared. So if you double the distance between you and a source, if you make are twice as big, the intensity you observe will drop by a factor of four. So if you went from one to two, then the intensity would go down to a quarter of what it used to be, you've reduced it by a factor of four. If you were to triple the distance between you and the source, then the intensity would reduce by a factor of nine. Three squared is nine, and so on and so forth. So I described in the previous version that signal does decrease as distance increases. That part was right, but the relationship I got totally wrong. The important thing to remember is that the signal strength drops off as we get further away from the source. So for spacecraft that are traveling further from Earth, that's a big factor we have to take into consideration. All right, let's get back to the original episode, and again, thank you to Charlie Tango Bravo for setting me on the right path. I appreciate it. And we also have to remember that there's a lot of stuff that generates radio signals. I mean, there's a ton of stuff here on Earth that we create, like TV and radio and cell phones and that kind of stuff. Those generate radio signals. But they're also a lot of things in space that generate radio signals, like pulsars and nebula and quasars. So in other words, there's a lot of potential noise to deal with when we're looking for a radio signal. So again, finding that that signal amid all the noise is a big challenge. It doesn't it's not just important that our antenna is sensitive. We have to have it really directional so that we can make certain that we're pointing at the thing we want to to listen to. Otherwise we might mistake some errant Earth generated signal as being our spacecraft and then we're on the wrong track. So to meet these challenges, NASA, through the JPL, established the Deep Space Network or d s N. The DSN has three facilities that are approximately a hundred twenty degrees apart in longitude, so that means they're roughly equidistant from each other Longitudinally, you multiply one twenty by three, you get three sixty. That's a circle, right. So one of the three is in Goldstone, California. That's actually in the desert. It's northeast of Los Angeles, it's south of Death Valley, and it's pretty far away from stuff that generates radio waves. The second is in Madrid, Spain, so that's almost smack dab in the middle of Spain. The third is in Canberra, Australia, which is on the coast in the southeast of Australia, and it's probably covered with venomous animals. I mean it's Australia, so it's a safe bet. So again, because these sites are a hundred twenty degrees apart from each other, we get that three hundred sixty degree view if you will, of the space around Earth. And what this means is that at any given time, at least one of the three DS insights has the ability to establish a line of sight communication channel with a distant spacecraft. And as the world rotates and one of these sites begins to lose contact, the next one will pick it right up, so communications can remain online. You don't have an interruption. This is a big thing. Like you might remember when I was talking about space stations. When the most recent as of this recording, anyway, when the most recent module joined the International Space Station, it was the Naka from the from Russia. When it docked, its thrusters misfired. They weren't supposed to fire, but they did, and it caused the space station to rotate and move into the wrong orientation relative to the Earth. Well, when that happened. The space station was not in range of Russia's mission control because that was on the wrong side of the Earth at the time, and that was a problem because Russia was the only entity that had the ability to control the thrusters on the Knocka. So that's an example of why it's important to have established these points where we can have uninterrupted contact. All right, we've got more to say about navigation, but before I get totally lost, let's take another quick break. So at these different sites at Goldstone, California, and Madrid, Spain, Canberra, Australia, they have a series of radio telescopes, including one really big one at each of these sites, and the deep space radio antenna can be extremely impressive. So the Goldstone Mars dish, that's the biggest one at gold Stone, California. That one was built in nineteen but it was later upgraded in night so today it measures seventy eaters across, that's about two thirty feet. Now, this dish has a surface area of around an acre or square feet or three thousand, eight hundred fifty square meters. It weighs nearly three thousand tons. It's mounted on massive machinery that can tilt and turn the dish so that it can be aimed precisely where a spacecraft is overhead and get you know, that laser like focus with a communications channel with a spacecraft that could be millions or billions of miles from the Earth. And the purpose of those big big dishes, I mean, it's all about collecting the very weak radio energy that's being sent back from the spacecraft. You know, because again that distances is intense, like the signals are very very weak by the time they get back to Earth, especially for super distant spacecraft. So this big dish is able to like that energy and then direct it toward the antenna itself, so you can think of it this parabola. It's all focusing that radio energy to a specific point, that point being the end of the actual radio antenna. Otherwise the signal would be so weak that it would be difficult, if not impossible, to detect that signal through all the noise, and engineers have to position and antenna precisely to beam radio instructions back to the distant spacecraft. If you're off even by a little bit, the message is not going to end up going to where you need it to. Go, and the further out the spacecraft is from Earth, the more critical it becomes to get that just right. So all of that is to set up the actual talk about navigation itself. We had to set all those parameters to talk about the process. Without the system like the Deep Space Network in place to communicate with spacecraft navigation would be impossible. We would not be able to build a spacecraft capable of detecting its own orientation and velocity and to make changes on the fly. It's not like Star Trek or Star Wars, where you just tell an on ship computer to plot a course for Javin or whatever. NASA describes space navigation as being the domain of three large departments within NASA. There's mission design, there's orbit determination, and there's flight path control. These three things all inter relate to one another. So first we have mission design. Now, this is the part of navigation in which mission control has to determine what the intended trajectory is for the spacecraft, where is it supposed to go. Now, this alone is tricky for all the reasons that I've talked about earlier in this episode. You've got to take all those different elements into consideration, like relativity. You know, what is it that you're hoping to do? What what is the purpose of the spacecraft? What are the things that are going to affect the spacecraft as it travels from Earth to get to where it's supposed to go. How do you account for those things? And how do you either incorporate stuff so that it, you know, it becomes part of your mission, or how do you find a way around an obstacle or challenge. Now, keep in mind, we learn new stuff the more we send spacecraft up, Like we learn more every single time, and some of that stuff is important and it affects calculations, like important for the sense of navigation, that is, it's always important. Someone has to take the things we've learned and then build that into software that we use to calculate complex equations in order to plot out navigation. So space navigation is something that has evolved over time, and as we learn more and incorporate what we've learned into the next generation of software, it's constantly in a state of change. So one thing that engineers have to factor in is the fact that a spacecraft is always orbiting. Something I kind of indicated this at the top of the show. You know, it could orbit the Earth. It could be in low Earth orbit and just stay there, but it's in orbit. Or you might want it to leave Earth orbit and enter into a solar orbit, so now it's orbiting the Sun just like Earth or Mars, or you know, any of the other planets are. Maybe you want to enter into a different planets orbit. Maybe you want it to leave the Solar System entirely, in which case it's in a galactic orbit. It's orbiting the center of the Milky Way, just like our Solar system is. It does require an awful lot of velocity in order to escape the Solar System. By the way, but we have done it, not with you know, humans obviously, but with spacecraft we've sent out anyway. Orbits are a huge part of navigation calculations. And again you're talking about an origin and a destination, and that are from an outside reference both in motion, and we have to take into account the effects of stuff like relativity with spacecraft. That's going to affect things like the clocks on board the spacecraft. That means we have to account for those changes due to relativity or else we risk losing track of the spacecraft. We might point an antenna at where a spacecraft either used to be or might be in the future, but isn't right now, just because we have these deviations from between our earthbound clocks and the ones that are aboard the spacecraft. So the software has to plot a trajectory that has to take all these different things into consideration, and as you can imagine, this means those calculations get pretty darn complicated, particularly when you're looking at something you know, really ambitious, which sounds a little weird to say, because I still think just getting something into low Earth orbit is being really ambitious. But you know, there's there is a scale, I guess, and as I'm sure we're all aware, software does not always come out perfect, right. You've probably used software where you've encountered a bug or a glitch, Like maybe you're playing a video game and the textures failed to load and everything looks weird. Well, that's irritating when it's a video game, but when you're talking about like interplanetary navigation, a bug or glitch can become an enormous challenge. Now, it might not be a show stopper. You might be able to work around it, but it likely will require a lot of people to work out a solution on the fly in order to make a sure that a spacecraft's route is in fact the right one to do whatever it is you want that spacecraft to do, all right. So, once all of that has been taken into account, engineers calculate the navigational route for a spacecraft. This planned route is the reference trajectory, so this is the route the spacecraft should be on. Also, there's a cesall relationship between the navigator for a mission and the software developers who are making the navigation software. So as navigators find bugs or they encounter new situations that necessitate new features in the software, they can relay that to the developers, and then they take the feedback and they produce new versions of the software. I imagine that gets increasingly challenging to do, especially to incorporate new features. Any developer can tell you that it can be a nightmare to put something new into code that you've just gotten to work, because the chances are you're going to break something that previously had been working. Okay, just imagine that for things that are traveling through space and you have to deal with relativity and stuff. Now, even when you do that correctly, which you know obviously requires a lot of work. It does not mean that a spacecraft is just going to magically stick to that reference trajectory. All sorts of things can cause the spacecraft to deviate from the planned route. In some cases, it might be on purpose us such as you know, you might have to do a maneuver to avoid a potential collision. Or it might be that your pathway is taking you close to a planet and you're planning on using a gravity assist two make the spacecraft continuance journey. But in other cases, something might pop up that wasn't anticipated, Like it happens very quickly. Maybe the spacecraft passes some large asteroids and the gravitational attraction between the spacecraft and the asteroids pulls the craft out of its trajectory a bit. Or maybe it turns out that the software had a bug in it or a blind spot that failed to account for something, and the spacecraft is veering off course a bit as a result. I mean, even solar pressure, that is pressure from light itself hitting the spacecraft can be enough to push the spacecraft off its reference trajectory. This is where orbit determination comes in. Now, as the name implies, this part of space navigation is just keeping track of a spacecraft's actual position. We know where we want the spacecraft to go, but this is about us figuring out where the spacecraft actually is. And that is another kittle of fish. NASA breaks down orbit determination into three sub processes or subgroups, So there's orbit reconstruction. This is asking the question where has this spacecraft been? This is all about determining the past route, the past locations for the spacecraft to understand its actual trajectory versus the reference trajectory. Then you've got orbit determination. This is like asking where the heck is the ding dang thing right now? Then you've got orbit prediction. This is like asking where the heck is this thing going to next? And and at this point I kind of wish I hadn't burned the mean Girls reference about gettin loser we're going shopping, But I did that one already. Anyway, A bit of consideration reveals that all three of these things are important. See, we're not controlling these spacecraft in real time. You know, you don't have someone sitting looking at a monitor. They get a first person view of a spacecraft's viewscreen. They've got a joystick and they're just making it fly all over the place. That's not how this works. And it does take time for a signal to pass from one point in space to another. The fastest that this can happen at is the speed of light, and you know, light is wicked fast. It's in fact the fastest stuff there is. But light can't traverse great distances in an instant. I mean it takes about eight minutes for light to go from the Sun to hit us here on Earth. So as as spacecraft gets further from the Earth, the information that we get back from those spacecraft becomes more dated. Right, it's more about where the space was at the time it transmitted, but minutes have passed between then and when we're able to actually look at the data. So as engineers get the latest telemetric data, they're actually looking at stuff that's several minutes old. So we're not observing the spacecraft directly. We're getting information back from the spacecraft, and then we have to draw conclusions about what's going on based upon the information we have. So if the data indicates that perhaps a ship is drifting away from its reference trajectory, we need to be able to look back and see what the data has said about that. When did that deviation begin? How long has it been going on? Where is the spacecraft now? Based on the information we have, keeping in mind we're projecting forward a few minutes because the information we have is older. And then, knowing that our ability to pinpoint a spacecraft position with precision decreases the further out the spacecraft gets from us, we have to start building in margins of error, and then, based upon all we know, where is it heading to now and how can we get it back on track? If we were not able to determine this, we wouldn't know where to point the antenna with the d s N in order to track the spacecraft. We would lose the spacecraft, let alone being able to figure out how to correct its its course. And you know space is big, so if you lose something like a spacecraft, good luck finding it again, because you'd you'd just be scanning regions of space looking for the faintest of radio signals to try and get back on track. It's not something you want to have happen. Well, we've got some more to talk about with navigation before we wrap all this up, So let's take one last break. Okay, I was talking about orbit re construction before the break, and we also need to reconstruct the path of the spacecraft in order to make sure that the scientific data that we're collecting with this satellite. I mean, presumably we've sent it up there to do something, well, some of it is determined by the trajectory path, Like we have to know the trajectory path in order for the data to make sense. There's imaging data. There's like a type of imaging called synthetic aperture radar imaging, and that requires that we have a precise knowledge of the spacecraft's trajectory so that the software we use to process that information can create a meaningful image from that data. Like it's not like it's sending a JPEG to us. It's sending us data that we then use to create an image with sophisticated software we have running on computers here on Earth. If we don't know the trajectory precisely, then it's almost like we're building a picture, but we're doing it from the wrong perspective, and it would leave us with an image that's nothing like what we were actually trying to capture with the satellite. So it is imperative that we know the actual trajectory of a spacecraft. Then we have flight path control, the third of the three departments I was talking about. Again, you could probably guess what this is about based on the name. We can summarize this by saying that this is the part of space navigation where we figure out how to get the spacecraft from where it actually is to where it is supposed to be. How can you get the spacecraft to return to the reference trajectory that you had set in the beginning. And here's another good reason to put so much emphasis on orbit determination, because spacecraft have a very limited set of options that they can use in order to return to the correct course. An interplanetary spacecraft, like a satellite that's designed to fly by, say, one of Saturn's moons, it needs to on course once it separates from the launch vehicle. You know, once it separates from the rocket that pushes it up into space. So this is kind of like a bowler releasing a bowling ball. You know, once that ball leaves your hand, there ain't no amount of you waving or leaning that's really going to affect the ball's trajectory. You've set it on a path and you can't really influence it anymore. Well, we can still influence spacecraft a little bit, but that initial release from the launch vehicle is the primary thing of putting it on its proper trajectory. So spacecraft can have like thrusters or rocket engines that can fire to adjust their course a little bit. But obviously there's actually a limit to how much fuel any spacecraft is able to carry. And there ain't no gas stations in space, at least not in our neighborhood. So fuel is a limited resource. And that's you know, an understatement, but it's important to remember. So let's say a spacecraft has gone a little bit off course. The flight path control group then has to figure out how far off course is it. Then they have to figure out the commands needed to have the spacecraft returned to its reference trajectory. So they take the data from the orbit determination group that tells them, okay, how far off course are we. Then they start running the calculations how much is it going to take for us to get back to where we need to be? Uh? This means figuring out the proper change in velocity for the spacecraft and if it's been a while since you've had physics, I want to remind you that velocity isn't just speed. A lot of folks use the word velocity to stand in for speed, but that's just part of it. Velocity is a vector. So in addition to speed, we have to have a direction. So a change in velocity is an acceleration. It can be a change in speed or direction or both. The flight path control team figures out what change in velocity is necessary, so they indicate the magnitude and the direction that is required in order for the spacecraft to return to its reference trajectory. Now, the flight path control team doesn't initiate the actual maneuver. They just designed the maneuver, or at least they design the parameters of the maneuver. It needs to have this change in direction and this magnitude. They send that information to another team, a spacecraft engineering team, and it's this group that then use stuff like attitude control systems and thrusters or rocket engines to produce the change in velocity that was indicated by the flat flight path control teams. So, in other words, they're the ones to take this data that says, here's what we need to have happen, and they're the ones to actually activate the systems to make it happen. NASA refers to small flight path control maneuvers as trajectory correction maneuvers, which makes sense. You're trying to correct it move it back to its reference trajectory. Now, as we conduct more space exploration, we learned things that are really able for future missions. So, for example, if we were to plot out a fly by satellite to go to Saturn, we would incorporate some gravity assist fly by. It's most likely this is where you leverage the gravitational pull of celestial body like another planet, to assist the spacecraft on its way by giving it kind of a slight pull slash push towards its destination and a change in velocity. It's a boost, kind of like someone giving you a little push when you're swinging on a swing set, although it can also you know, change your direction somewhat. And as we understand these things and we're able to build it in, we can have that planned from the beginning, so we can build them into an actual mission. When we've got a good handle on those things, NASA can call any sort of velocity maneuvers that we know we're going to have to do deterministic. That is, we have already determined the velocity maneuvers that we will need to conduct in order to maintain our reference true jectory given the route that we're following. We know that they are going to be maneuvers necessary to stay on course, and we have a good idea of what they are and when we will need to execute them. But there are other types of maneuvers that will call stochastic. These are maneuvers that we know we're gonna need to make, but we don't necessarily know. More like, we don't know what the magnitude of those changes might have to be. We might not fully understand the effect of those maneuvers ahead of time, because we're going through uncharted ground, if you if you will. We're not following something that we've already done, where we already kind of have a grasp on what we need to do. And then, of course occasionally sometimes we have to do these maneuvers when something we didn't anticipate at all happens and we have to design and conduct a maneuver kind of on demand. Also, I should mention that attitude control I mentioned it earlier Attitude control is not about whether or not you're spacecraft is sassin you. It has nothing to do with sass. Attitude refers to the angular orientation of a spacecraft given some other point of reference. Again, you have to have a point of reference, because I mean, if you think about it, outer space doesn't really have an up or down. You have to have a point of reference and compare your position to that point of reference, or you have to, you know, use your point of reference to in order to make a determination about something's positions. So you've gotta have a point of reference to start from in order for you to say something like that durned things upside down or backwards or you know, or whatever. Also, there are interesting ways to change the angular orientation of a spacecraft, and some of them do not involve thrusters or rocket engines. Instead, they might involve something like momentum wheels, as in, you know, physical wheels a rotor spun by a motor. Now I would get into this further, but that requires a pretty long discussion about things like the conservation of momentum and equal and opposite reactions and stuff like that, and we don't really have time for that this episode. Is already going super long, and I don't want Tardy to hate me more than she does already. So she doesn't hate me, folks, She's super nice to me. I just want to make that clear. That was more of a jest. But the short version of all this is that using stuff like momentum wheels can make it possible to change the attitude of a spacecraft without having to use thrusters. And as we've previously established, fuel is a precious resource. So that's a good thing to know, right to build in systems that allow us to make changes to a spacecraft's orientation, for example, without having to burn fuel to do it. Still, it's not uncommon that the spacecraft engineering team will actually have to initiate a rocket engine or a thruster ignition to provide the thrust needed to get back onto the reference trajectory. Uh, the timing on these maneuvers has to be incredibly precise, both because you don't want to waste even a drop of fuel if you can help it, and also that if you fire a thruster for too long or not long enough, then you're not going to return to your reference trajectory. So imagine, for a moment that you're sitting in mission control and you are relying on complex calculations from data sent back to you from space just to tell you where that something is right, and that's something the spacecraft is so far away that there is no means for us to observe it directly. All you have is the data coming back to go by, and then you have to come up with the command to send via radio back up to this object to get it back onto the course it's supposed to be following the data you have indicates that the thing you know was in this one particular position several minutes ago, So you have to figure out where it is now based on calculations with the data that you have, knowing that that is at best maybe a precise approximation, but still an approximation. Then you figure out the command you need to send to where the spacecraft is going to be in order to get it to take the action to get it to where it should be. This whole thing is mind boggling to me. It kind of makes me think of like submarine navigators who use precise charts and timing like a stop watch in order to plot underwater courses. Because they're not able to just look outside. I mean, at the depths that submarines can travel. You can't have windows because they would collapse in from the pressure. So you're in a tin can underwater, and you're using very precise maps and a stopwatch and knowledge of how fast you're moving in order to make calculations to determine whether or not you're going to bump into something. It's hard for me to even contemplate. Well, once the spacecraft engineering team has done their thing, that whole process has to repeat itself. The orbit determination team has to figure out if the spacecraft is in fact back on its reference trajectory, or if they'll need to conduct another maneuver, and so on and so forth. And because there's so many little things that can pull a spacecraft off track, this becomes a continuous process. It's incredible to me that people figured this stuff out, that they figured out how to not just not just how the universe works, Like it's already incredible to me that Einstein was able to determine these in incredible theories of special and general relativity, but then for people to build on that and to make technologies that take that into account, it's it's phenomenal. It also tells you by the way science works, right, because if science didn't work, the technology we build that leverages that science, it wouldn't work. So science works or else are tech wouldn't work, especially when it comes to space navigation. Now, there is a lot more that we could say about spacecraft navigation. I haven't really gone into deep details here, uh like how do you determine what a spacecraft's velocity is? For example? I haven't talked about that process, but I felt this was a good if you will high level overview of the topic. If you would like to know more details, let me know. I'll research and write and do a follow up episode and it will reference this one, but it will go into much more detail to talk about the actual processes and technologies we use to do the things I've talked about in this episode. If you want that episode or any other topic on tech Stuff, reach out to me on Twitter. That's the best way to get in touch with me. The handle we use for the show is tech Stuff H s W and I'll talk to you again really soon. Yeah. Text Stuff is an I Heart Radio production. For more podcasts from I Heart Radio, visit the I heart radio, app, Apple podcasts, or wherever you listen to your favorite shows.