Why is the speed of the ISS so high?

Answer

For the sake of simplicity, let’s work with circular orbits. Two forces act on a satellite:

– gravity : G.Ma.m/R²

with G the universal gravitational constant, Ma the mass of the earth, m the mass of the satellite and R the radius of the orbit of the satellite (so measured from the center of the earth)

– the centrifugal force (let’s work with that for the sake of convenience) : pl² / R
it tries to move the satellite away from Earth. v is the speed of the satellite

Friction with the outer layers of the atmosphere is not considered here.

If the satellite wants to move in a circular orbit, these two forces must balance each other:
G.Ma.m/R² = pl² / R

and if you separate v from this : v = square root (G.Ma/R)

Now G and Ma are fixed known numbers. So if you fill in R, v is irrevocably fixed. If you want to fly slower with the ISS (in km/s), you will have to raise the orbit. But once the track height has been chosen, the speed is fixed. You can no longer choose it.

ISS orbits at an altitude of 350 to 400 km. Let’s say at 380 to take something, so at 380 + 6371 km from the center of the earth, so that R = 6751000 meters.
Mon = 6 10²⁴ kg, and G = 6.67 10^-11 m³ kg^-1s^-2

With these numbers this gives as speed 7699 meters/second = 27 718 km/h

So, this speed follows irrevocably from the chosen track height. A shuttle that does not have to go to the ISS usually stays on a slightly lower orbit and will therefore also have a slightly higher speed. But there too, that speed is fixed once the runway height has been chosen.