If I’m not mistaken, a body in orbit needs to accelerate to go into a higher orbit. But object in a higher orbit have a slower orbital speed e.g. the ISS has an orbital speed of just under 285 mi/min while a satellite in geosynch orbit has an orbital speed of just under 115 mi/min.
I know I’m missing something obvious. Why do you go faster to go slower?
Paging Buzz Aldrin.
Object in higher orbit have greater potential energy with respect to the Earth - that’s where the work of pushing them goes to. When you boost an object in orbit, you’re not really making it go faster *around *the Earth, you’re just making it go in a slightly straighter line for a while, which resolves to a higher orbit.
Ah!
Thanks.
I don’t think the question’s been answered. My understanding is that there’s only one speed for a stable orbit at any altitude - e.g., ISS can’t decide to speed up to 300mi/min or slow down to 250 without changing its orbit height. Is there a smooth line or curve for speed/altitude?
It seems paradoxical to me that orbital speeds slow as the distance grows - my “common sense” tells me that a geosync satellite must travel faster to achieve its 24-hour orbit than a LEO object must to travel a much smaller distance. (I know that’s wrong, just counterintuitive and teh basis of this question.)
But the ISS’s orbit of 90 minutes is much quicker than a geosynchronous one. If you consider that the force of gravity is felt more the closer you are to a body, you might be able to intuitively see that as you get closer you must travel faster horizontally to overcome that force.
There’s only one speed for any circular orbit - but a range of speeds possible for a non-circular orbit: the equation is v[sup]2[/sup]= G(M+m))(2/r-1/a) where M is the mass of the object being orbited, m is the mass of the orbiting object, r is the distance to the center of the object being orbited and a is the semimajor axis of the orbit, which amounts to being the average between the furthest the orbiting object gets to the center of the orbited object and the nearest it gets.
An intuitive way to think of it is how you’d go from a low orbit to a higher one, using only a couple of speed changes. First, you’d speed up with a burst of rockets (then shut the rocket off). This would put you in an elliptical orbit, with your current altitude as the low point, and the high point determined by how much you speed up. As you follow that elliptical orbit, further away from the planet, your speed will be dropping, right? Then you’ll get to the point of maximum altitude, but by then your speed will be too low to maintain that altitude, so you’ll have to speed up again with another burst of rockets, to reach a speed that will let you orbit in a circle at that altitude.
Remember, you’re constantly accelerating in orbit, as your velocity has a different direction at every moment. To reach a higher orbit, your velocity needs to point outwards for a while (rather than tangential to the original orbit). It takes energy to overcome the force of gravity which binds you to the original orbit.
It helps to think of this in terms of energy and forces rather than speed and acceleration. Especially since the speed is constantly changing.
Pretty much the answer.
You are in a lower, ciruclar orbit. You accelerate with a burst of rocket power, to add velocity along that circular orbit path. You are now in an elliptical orbit, your craft slows as it climbs higher - gravity.
If you do nothing, you reach the high point of the orbit, then begin to accelerate as you come back down to the same point you fired the rocket burst. you pass through that point to repeat the climb.
If at the high point, tangent to the orbit “apogee” you fire another burst from the rocket, you are going faster, you will not fall as far, your orbit is elliptical closer to circular; but that is still your apogee.
Fire the right amount, your orbit is now circular at the higher altitude.
Fire too much, and you are back where you began, an elliptical orbit - except the “low point” or perigee is now where you fired the burst from, and the high point of the orbit, apogee, is even higher.
So short answer - you add speed, but gravity slows you down as you climb - just like a baseball in a pop fly… then pick that speed up again as you return closer to earth.
click
Understood. I am not sure why I couldn’t see that before.
It might also perhaps help to think of a car on a road, with an uphill grade in front of you. You can speed up…and then put 'er in neutral and coast up the hill, such that, at the top, your speed has dropped to nearly nothing.
The actual climbing of the hill – moving to a higher orbit – takes away all of that extra speed.
Ask the Gemini pilots about “forward to go up, up to go backward,” etc. If you really want the full pleasure of motion sickness, try a Whifferdill Turn!
(“Geminii pilots?” 
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