a standing shock wave, the solar wind falls below the speed of sound and becomes subsonic. It was previously thought that once subsonic, the solar wind would be shaped by the ambient flow of the interstellar medium, forming a blunt nose on one side and comet-like heliotail behind, a region called the heliosheath. However, observations in 2009 showed that this model is incorrect. As of 2011, it is thought to be filled with a magnetic bubble "foam". The outer surface of the heliosheath, where the heliosphere meets the interstellar medium, is called heliopause. This is the edge of the entire heliosphere. Observations in 2009 led to changes to this model. In theory, heliopause causes turbulence in the interstellar medium as the Sun orbits the Galactic Center. Outside the heliopause, would be a turbulent region caused by the pressure of the advancing heliopause against the interstellar medium. However, the velocity of the solar wind relative to the interstellar medium is probably too low for a bow shock. Termination shock The termination shock is the point in the heliosphere where the solar wind slows down to subsonic speed (relative to the Sun) because of interactions with the local interstellar medium. This causes compression, heating, and a change in the magnetic field. In the Solar System, the termination shock is believed to be 75 to 90 astronomical units from the Sun. In 2004, Voyager 1 crossed the Sun's termination shock, followed by Voyager 2 in 2007. 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