How accurate does the ISS's velocity and altitude need to be to maintain orbit?

Question

We know the the ISS is not drifting weightless in space, but rather is constantly falling as it circles the Earth. To do this it must be at a specific altitude and moving with a specific velocity. How accurate must these be to maintain orbit? Must it be within a few meters altitude and kilometers per second in velocity? Or is there a wider range allowable? Or do they constantly adjust the velocity to account for changes in altitude?

Answer 1

For a particular orbital period and altitude (or altitude and eccentricity, or eccentricity and period), there is exactly one velocity that produces the correct orbit.

Of course the ISS doesn't have an exact velocity, because it's actually constantly slowing down due to atmospheric drag and solar radiation pressure (we think of "space", 100 km up, as a perfect vacuum normally, but in fact there's still low-pressure air up there - dense enough that for low orbits you usually have to account for it and add orbital maneuvering thrusters to spacecraft to keep them from crashing). In reality, it has to frequently burn small engines to keep it in its approximate place. A deviation from its altitude by more than a kilometer or so would be a big issue, but it also wouldn't be fundamentally difficult to solve. They just have to activate their orbital maneuvering thrusters for a moment in the right direction, and they're back on track.

Really it's less of an exact altitude and more of a "we're this many kilometers up (plus or minus a hundred meters) in a circular 55-degree-inclination orbit", and when atmospheric drag or solar radiation pressure change that measurably, they just fix it.

To answer your question: they don't usually have to change their velocity because, in a circular orbit, velocity is constant throughout the entire orbit. But due to drag in the upper-upper atmosphere and radiation pressure, they occasionally have to fix their orbit to stay roughly where they are; generally their velocity stays mostly-constant and their altitude might vary by a few meters over the course of a few years.

They won't be doing this for much longer. The ISS is planned to be deorbited in the early 2030s; instead of keeping themselves in a roughly-circular orbit, a deorbit vehicle is going to (somehow) drop the ISS out of orbit. This will involve a very large change in velocity and altitude, which might interest those seeking to determine the accuracy of its altitude.