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
No orbit is perfectly circular. It is an elliptical orbit, and as such, with vary in speed and height throughout its many rotations each Earth day.
If the speed is being reported relative to the earth’s spinning surface, then it will vary even if the ISS is traveling at constant speed relative to the earth’s center. The ISS’s orbit is angled about 50 degrees from the equator, so when it is directly over the equator, the earth will be spinning faster than when it is over the higher latitudes. The north-south component of its orbit will also be fastest over the equator.
In addition, objects only maintain a constant speed in a perfectly circular orbit. The ISS’s orbit is nearly circular but does vary by 2 km (408 vs. 410 km) over the course of each orbit.
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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