It’s not a bad example. Because I would follow that response with another question:
What direction are those “little nudges” pushing the station? Why?
It’s important to understand that these nudges are not pushing the station away from the Earth, the way someone might imagine nudging a falling object; like the way people nudge a balloon to keep it aloft, or nudge a concert beach ball to keep it from falling to the ground. The nudges are to increase the speed of the station. The drag from the atmosphere is slowing the station down, and if the station is not going fast enough, it will not stay in orbit.
This must be understood to move on to the sun launch scenario.
Maybe this is better. But I don’t mind the conversation that follows from the ISS scenario.
Dinsdale, keeping on the subject of geosynchronous satellites, I have a question for you. If you have a satellite at the end of your rocket and you launch it straight up to an altitude of 36km above the Earth, what happens to the satellite? Does it stay up there in orbit? After all, you’ve reached the required altitude, right? What happens to it?
I’m seriously considering making a video for this thread and posting it online.
So, with all this great explanations everyone is giving, the scene in the Star Trek Generations movie where the guy launches the sun-killing rocket into the sun from the planet is not possible?
Two possibilities. If you have a Scifi level ship with enough energy to reach escape velocity going straight up, then you will leave the orbit of the Earth.
Otherwise, you’re going to “move your orbit” without changing it’s shape. If you are in a circular orbit with a radius of 100,000km and you burn “up”, you’re still going to have a cicular orbit with the same radius. But now, one side of the circle will be further from the Earth, and the other side will be closer. the Earth is no longer in the center of your Orbit. In Gorsnak’s examples, speeding up and slowing down changed the circle orbit to an oval orbit. One side stayed the same height, and the other side got higher (when speeding up). But burning straight up will not change your circle. So, it moves one side further away, while moving the other side of your orbit an equal distance closer. So, if burn “away” from the Earth hard enough (but not hard enough to reach escape velocity, of course), then you will crash into the Earth on the other side of your orbit. If you burn “up” at the 3 o’clock position, you will crash into the Earth at the 9 o’clock.
If you’re currently in a circular orbit, you’ll end up in an elliptical orbit where the high point of the orbit is higher than your current orbit, and the low point is lower. If you’re already in an elliptical orbit, then you’ll either end up more elliptical or less elliptical, depending on where you are in the orbit when you make the burn. In any case, if you burn at 90 degrees to your direction of travel you are neither adding nor subtracting energy from your orbit, so your average altitude (technically the semi-major access of your ellipse) won’t change.
Thanks for the effort, guys. Gotta tell you that right now my brain is fried at the end of a rough day at work, so I’m gonna have to get back to this later.
Gorsnak, your 1st 3 paragraphs make perfect sense, but my brain locked up on the final par. Same reason I can’t even begin to answer your question B N. My ignorant assumption is that once something gets to the right altitude at the right speed, it keeps falling and missing, so it appears to stay there. But I know that is so simplistic as to distort. My kid used to work for ULA and now works for Ball, so believe me, he has tried to explain all this to me before - along w/ space elevators, Lagrange points, etc. But my brain tends to glaze over pretty quickly. Like I said, he’s coming into town in a couple of weeks, so I’ll try to bone up on some of this sort of thing to be better able to discuss it with him.
My kid who is a microbiologist used to work as a library shelver. One time she brought home a book which - in her words - did a pretty good job of explaining what she is interested in. And even tho it is written for the general public, it does not distort things overly. When I told her I made it thru 16 pages before I thought my head would explode. To which my loving spawn said, “Next time I’m working in juvie I’ll see if they have anything for you w/ pop-ups!” It is frustrating to consider oneself reasonably competent and intelligent, yet feel SO STUPID when trying to figure out things like this. Don’t even TRY to tell me about the pixies inside my computer
They’re pretty much the same, except Bear Nenno describes the ensuing orbit as “still circular but with one side of the orbit lower than the other” which isn’t actually a thing. Orbits are only circular when they’re exactly the same altitude all the way round. The ensuing orbit is higher on one side and lower on the other, but it’s also squashed into an ellipse. This squashing might well be very minor if the difference between high and low points isn’t very much, but it is always there.
I actually took an undergrad Astrophysics course in college, taught by one of the Professors who was putting together instruments and experiments for the Hubble Space Telescope (whose operational center is conveniently right on campus). The course included orbital mechanics and the like, and I remember my mind being completely blown over and over again by the seeming contradictions of it. To go up, you must go down. To slow down, you must speed up.
But every once in a while, I would be able to conceptualize why it had to be that way; those were Golden Moments when my brain briefly achieved a slightly higher level of functioning. Sadly they never lasted for more than 8 seconds before I lost the insight. But I remember I had it once, briefly. A tiny tiny bit of it, anyway.
Yes absolutely. Your orbit describes an ellipse with the centre of mass at one focus of the ellipse. If that ellipse intersects the surface of the earth then (assuming you don’t burn up in the atmosphere) you engage in what people like to call lithobraking. When you thrust “up” in this way you don’t change the semi-major axis of the ellipse, but you can certainly make the minor axis a lot smaller.
No. I’m trying to keep it as simple as possible. The simplest way to view orbits when learning, I think, is to picture them as rigid circles. If you’re sitting on a ship that has rocket thrusters pointing forward, backwards, up, down, left and right, the only ones that will change the size and shape of your orbit are the ones that face forward or backwards. These increase or decrease your forward speed. If you are at the 3 oclock position and you fire your rear thruster, then your altitude at 9 oclock increases. You’ve made an oval. If you fire your forward thruster (decreasing your velocity), then you decrease the altitude at the 9. In both cases, your altitude at the 3 doesn’t change. If you fire your left thruster, pushing yourself to the right, you will move your orbit without changing the shape. Picture a bike wheel suspended at the hub by a string. What happens when you press down on one side? The other side goes up. That’s similar to what happens if you fire your left and right thrusters. It moves your current position at the 3, and moves your orbit at the 9 oclock and equal distance in the other direction. Just like the bike tire. Similarly, if you pull one side of the tire “away” from the center, the other side moves “toward” the center. If you fire your thrusters toward the Earth, then you move your 3 o’clock position closer to the Earth, and move your 9 o’clock position further away. It’s very simplified, of course, but I think it helps people visualize what’s happening.
Down lowers the altitude at the current position of your orbit, while raising it on the opposite side. So trying to go “toward” the Earth will make you go further away. Of course, as you come back around in your orbit, you’ll be closer again, but then you will get further away as you approach the other side of the orbit. You could possibly thrust enough to lower your current position enough to crash into the Earth (or at least enter the atmosphere which would slow you down enough to crash). But that takes much, much, much more energy than simple slowing down a little bit and lowering your altitude that way.
Assuming you launch from the equator, it’ll stay in geosynchonous orbit. This is one case where you don’t have to give it sideways thrust to stay in orbit. The satellite has sideways velocity inherited from being on Earth, which is rotating once per day. That’s exactly how much speed it needs to stay in orbit at geosynchronous altitude.
Now if you launch from somewhere else, it’ll have less velocity inherited from Earth, so the satellite won’t stay in a geosynchronous orbit. Depending on the launch site latitude, it could either go into a different orbit or fall back to Earth.
One of my favorite moments from putting 3 kids thru college was one time I saw my son’s upcoming semester courses. Just about every class was some kind of advanced math. Social science major that I was, I asked him, “How much math do you have to take?”
His deadpan response: “All of it, dad - ALL of it!”
Sorry for the confusion, I meant the thruster is thrusting “up” as in the flames are 90 degrees away from the earth.
Sounds like it’s more or less the same answer either way. Thanks! Was having trouble wrapping my head around it even though I understand the slow down to speed up counter-intuitiveness of orbital mechanics.
It is frustrating to consider oneself reasonably competent and intelligent, yet feel SO STUPID when trying to figure out things like this. Don’t even TRY to tell me about the pixies inside my computer 
