I have an app that shows live feeds, positioning, and other current information about the ISS (International Space Station). I noticed that its speed seems to vary. How can that be? Doesn’t the speed need to be constant for it to stay in orbit?
In: Physics
In orbital dynamics, speed and altitude are inversely related. The lower the altitude of your orbit, the faster your orbital speed, the higher the altitude of your orbit, the slower your orbital speed. No orbit is a perfect circle, but rather an ellipse. Think of it like an egg shape. The orbit of the ISS is nearly circular, but not quite. It varies a bit, but currently, its apogee (the point of its orbit farthest from the surface of the Earth, like the pointy part of the egg) is 423km, and its perigee (the lowest part of its orbit) is 418 km. So as it goes from apogee to perigee, its altitude is decreasing so it’s speeding up a bit, and once it hits its perigee and starts going back towards its apogee, its altitude is increasing so it’s slowing down a bit. Next time you watch the app, keep an eye on the speed *and* the altitude, and you’ll notice this. If the speed is increasing, you’ll notice that its altitude is slowly decreasing, and vice versa.
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