Ryan McLinko explains why different satellites orbit at different altitudes: imaging satellites need low Earth orbit for resolution, GPS uses medium Earth orbit to reduce satellite count, and geostationary orbit stays fixed 35,000 km up. Getting to orbit requires reaching 8 km/s horizontal velocity, but the Tsiolkovsky equation shows only a few percent of a rocket's mass can be payload, making launch expensive.
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Just to give you guys, again, a feel about orbits. So these orbits are to scale. So we've got the size of the Earth has a radius of about 6,000 kilometers. And then we have the different places where satellites can be. Now, all of the different orbits that you're used to, whether it's the International Space Station, whether it's these orbits that all these remote sensing satellites are going to, they're actually all the way down here, just barely outside of outside of Earth. In fact, no human has since— ever since the Apollo missions when humans went to the moon, no human has been outside of this for whatever that was, 40 years or so, and I'm sure that'll change soon for a number of different reasons, but just to kind of, again, put things in perspective. Things that— and the reason that the Earth imaging satellites are so close is because of resolution. So imagine that you want to be looking at something. You want to be close to what you're looking at if you want to get any sort of good view. If you want to get a general kind of wide-angle view, you might go up higher. But if you want to be able to see things that are useful, then you want to be actually pretty close to the ground. As compared to things like GPS, and Galileo, GLONASS, and Compass are all similar type— actually, just different companies' versions of GPS, which is an American constellation. Those are a little bit higher, because you don't want to have to have— basically, if you put GPS at the very low orbit, you'd need to have 1,000 satellites in order to put it up. As a result, you push it out farther, so that way you need less satellites. Makes it a little bit harder, but GPS is such a low data rate application that you basically send these tiny little bits of data, and even though it's farther out, it still works out okay. Compared to geostationary orbit, which is at 35,000 kilometers out, at the kind of upper end of what's called high Earth orbit. These guys were called low Earth orbit. These called medium Earth orbit. Very original, I know. And then geostationary. Now the nice thing about geostationary, so it kind of, it's kind of annoying because it's really far away, which means that it's hard to close your link budgets. It's a very long distance and anything spreads. So if you're doing communications, you have something called the inverse square law, which you guys are probably all familiar with. Which means that any sort of energy, including radio energy, will spread out as it goes through any sort of distance. So it's really far away, which means that you have to have a much better receiver on the other side to be able to hear it. The cool thing about geostationary, though, is that what happens is that you orbit at the same rotation rate as the Earth rotates. As a result, you stay over the same part of the Earth at any point in time. So if we have a dish that's on our roof that wants to give us internet, then there's a satellite that's going to be right there. And at all points during the day, the satellite is always right there, because it maintains the equivalent position within the reference frame that is us standing on the Earth, which is very handy, because you don't want to have to have tracking dishes, which are much more expensive. Again, talking about scale, because I like putting things into reference to give everybody a feel for kind of what things actually look like in reality. And it's hard to get— it's hard to remain in perspective when you're looking at numbers, just because of how our brains work. So here we have the Earth, low Earth orbit, where most of these satellites, most things that we do, are located only within here. And you go all the way over here, and that's where geostationary is. So it's 6x. Farther away from the Earth than the Earth's radius, or low Earth orbit, then the Moon, just for reference, is then 3x further away than that. So— or actually, 6x— another 6x farther away from where geostationary is. So when you think about, again, referencing back to, say, Apollo, why it took days of actually putting a significant burn to push you into a trans— LEO. lunar trajectory, and then it took multiple days in order to get there. And then again, if we think back to what I was showing before about how far the planets are apart, and those are actually the planets in their orbit rings, so the planets could actually be even farther apart if they're on opposite sides of the Sun from each other, why the notional transit time to go to, say, Mars, our closest planet, is 8 months. So, yeah. All right. How do we get to space? We talked about all these cool things that we can do with space and what those might look like, what satellites are, but how do we get satellites into orbit if they're not already there? So we use things called launch vehicles, basically rockets, which are basically controlled explosions. So you take a big bunch of very explosive compounds that will, when you make them touch each other, they will make a big explosion, basically a bomb. And then you put a nozzle on the end of it, and then that results in the bomb sending you in the other direction basically by Newton's second law. So you push something one way, you go the other way. So the way that looks, you launch, you then go up for a little bit. You actually go up for a little bit just to get yourself out of the atmosphere, and then you kick over and you go straight. You go horizontal. Now the reason for that is that getting to space isn't about going up. As we looked before, we saw that— whoops. We saw that we don't need to go up very much to go to space. We mostly need to go horizontal. So what orbit means, orbit is just going so fast that as you fall, you keep missing the Earth. So the Earth is round. So you keep going forward, and you keep falling, but you happen to be falling at the same rate that you're just missing the Earth as you're going around. Now, as it turns out, you need to be going quite fast for that. You need to be going about 8 kilometers per second for a low Earth orbit. So, and to put that in perspective, for that suborbital trajectory that I showed you guys before, the amount of energy necessary to achieve that trajectory, that ballistic trajectory, versus the amount of energy that you need in order to go into orbit is about a 10x, a factor of 10, different amount of energy. Which is why it's so much harder to go actually and put something even in orbit, even in a low Earth orbit, even something relatively small. So not an easy problem. Just to show you guys some more figures, this would be an example launch trajectory. You go up a little bit, and then you kick over and start going horizontal. And then similarly, an actual launch— this is a long exposure picture showing launch and then going into the curved trajectory. That's called a gravity turn. Gravity churn, by the way. If you are listening to any sort of launch broadcast or anything like that, you'll hear that. Oops. Just showing a couple of different rockets here. That's an Atlas V, kind of one of the more traditional type rockets, something that was made by a company or is made by a company called United Launch Alliance. They've been working for a very long time, have had many, many different rockets with no failures, and they're kind of traditional go-to rocket company. On the right, we've got SpaceX. Who all— has anybody— who has heard of SpaceX? OK. Just wanted to check. So SpaceX on the right, a new upstart with building rockets. Actually been around for quite a while now. But at this point, they're starting to get the cadence to be going up. And so— Have reusability, things like that. So pretty important there. This is just showing a bunch of different other types of rockets. There's actually lots of different rockets that are available from different countries. And there's probably not much reason for us to go into that at the moment right now. Some current upcoming and existing innovations in launch. So launch sucks. You're basically expending this chemical energy in order to put us into orbit. But back on these rockets, so the payload is actually like this, and then your rocket is this. And that's because of something called the rocket equation. The rocket equation— and it's exponential. So you have an exponential, and you have a tiny little mass divided by your total mass, and that determines the amount of delta-v that you need, change in velocity, which, as I said, was 8,000 meters per second, which is a relatively big number to be putting into that exponential mass fraction. As a result, even with a staged rocket design, you're looking at getting maybe 10%, probably more like 5% of the rocket's total mass is your payload. So this—
