Predicted to be a region of hot hydrogen, a structure called the "hydrogen wall" may be between the bow shock and the heliopause. The wall is composed of interstellar material interacting with the edge of the heliosphere. One paper released in 2013 studied the concept of a bow wave and hydrogen wall. Another hypothesis suggests that the heliopause could be smaller on the side of the Solar System facing the Sun's orbital motion through the galaxy. It may also vary depending on the current velocity of the solar wind and the local density of the interstellar medium. It is known to lie far outside the orbit of Neptune. The mission of the Voyager 1 and 2 spacecraft is to find and study the termination shock, heliosheath, and heliopause. Meanwhile, the IBEX mission is attempting to image the heliopause from Earth orbit within two years of its 2008 launch. Initial results (October 2009) from IBEX suggest that previous assumptions are insufficiently cognisant of the true complexities of the heliopause. In August 2018, long-term studies about the hydrogen wall by the New Horizons spacecraft confirmed results first detected in 1992 by the two Voyager spacecraft. Although the hydrogen is detected by extra ultraviolet light (which may come from another source), the detection by New Horizons corroborates the earlier detections by Voyager at a much higher level of sensitivity. Bow shock It was long hypothesized that the Sun produces a "shock wave" in its travels within the interstellar medium. It would occur if the interstellar medium is moving supersonically "toward" the Sun, since its solar wind moves "away" from the Sun supersonically. When the interstellar wind hits the heliosphere it slows and creates a region of turbulence. A bow shock was thought to possibly occur at about 230 AU, but in 2012 it was determined it probably does not exist. This conclusion resulted from new measurements: The velocity of the LISM (local interstellar medium) relative to the Sun's was previously measured to be 26.3 km/s by Ulysses, whereas IBEX measured it at 23.2 km/s. This phenomenon has been observed outside the Solar System, around stars other than the Sun, by NASA's now retired orbital GALEX telescope. The red giant star Mira in the constellation Cetus has been shown to have both a debris tail of ejecta from the star and a distinct shock in the direction of its movement through space (at over