Space mysteries: How does the ISS stay in orbit without falling to Earth?

Jan 16, 2025
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Overall good article, but orbital mechanics are not intuitive. The sentence saying "but there's still enough to resist the ISS's motion and create enough drag to slow it down" is wrong. The drag speeds it up. For it to come down to a lower orbit, the speed has to increase.

My credentials are that I have a MS degree in aerospace engineering, and I have a brother who was commander of the ISS.
 
As was already posted, it is counter-intuitive.

If you are in a circular orbit, and fire a thruster to increase your forward speed, you are going to go into an elliptical orbit with the highest point being above the original circular orbit, and the lowest point being the same as the circular orbit. When in the new elliptical orbit, you will be going faster than the circular orbit at the low point, but slower at the high point, because you have gone "up" in altitude with what was the "extra" velocity.

And, when at the high point of the elliptical orbit, you will be going slower than the speed needed for a new circular orbit at that new high point. So, if you fire your thruster to get more forward speed while at the high point, you end up raising the low point. If you do it just enough, you end up with a new circular orbit at the altitude of the high point of the elliptical orbit.

Getting back to whether the article is stating things correctly, yes it does. The "drag" from the thin atmosphere at the ISS altitude does slow it down. The orbital dynamics then cause it to drop its low point, where it will be faster than it was at its high point. The opposite of what I described above. But, because drag is a continuous process, rather than a discrete impulse, the resulting orbit is a spiral, not an ellipse. It still stays close to circular, because the drag is slight compared to its total momentum.
 
Jan 16, 2025
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It's simple. When you swing a conker around on a string, what keeps it from flying off? It's the tension in the string. Replace that with gravity and there's your answer. If the string snaps, the conker flies off in a straight line at a tangent to its circular motion. With satellites, speed up and takes a wider orbit until it reaches terminal velocity, which is about 12 km per sec. Then you are on your way to the moon. This is High school physics.
 
Maybe "nicks" was not paying attention in his high school physics class?

There is little in common between whirling a mass on a string and orbiting a free-falling object in space.

The main difference is that the string's tension does not change negatively as it is stretched, it instead increases with stretching, which produces greater distance of the mass from the center of its circular path. Gravitational attraction gets weaker with distance, as a function of the square of the distance. And that is crucial in determining the shape of an orbit.
 

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