The shock arises because solar wind particles are emitted from the Sun at about 400 km/s, while the speed of sound (in the interstellar medium) is about 100 km/s. (The exact speed depends on the density, which fluctuates considerably. For comparison, the Earth orbits the Sun at about 30 km/s, the ISS orbits the Earth at about 7.7 km/s, airliners fly over the ground at about 0.2-0.3 km/s, a car on a typical limited-access highway achieves about 0.03 km/s, and humans walk around 0.001 km/s.) The interstellar medium, although very low in density, nonetheless has a relatively constant pressure associated with it; the pressure from the solar wind decreases with the square of the distance from the Sun. As one moves far enough away from the Sun, the pressure of the solar wind drops to where it can no longer maintain supersonic flow against the pressure of the interstellar medium, at which point the solar wind slows to below its speed of sound, causing a shock wave. Further from the Sun, the termination shock is followed by the heliopause, where the two pressures become equal and solar wind particles are stopped by the interstellar medium.