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The heliopause is the theoretical boundary where the solar wind and interstellar wind counter balance each other. The area will remain a virtual mystery until Voyager 1 and Voyager 2 enter it on their way out of our solar system. Voyager 1 should be the first to arrive, entering in 2014.
There are multiple theories concerning the heliopause. According to one, there is an area of hot hydrogen(the hydrogen wall) that exists between the bow shock of the solar wind and the heliopause. In this area, interstellar material freely interacts with the edge of the heliosphere. A second theory posits that the heliopause could be smaller on the side of the solar system facing the Sun’s orbital motion and may vary based on the velocity of the solar wind and the density of the interstellar medium.
The exact distance from the Sun to where the heliopause is located is unknown, but it is known to lie far outside the orbit of Neptune. After Voyager 1 and 2 completed their original missions, they were repurposed to find and study this region, the termination shock, and the heliosheath. Voyager 1 reached the termination shock on May 24, 2005 and Voyager 2 reached it on August30, 2007 according to NASA released data. Their observations are being supplemented by the Interstellar Boundary Explorer(IBEX) mission. IBX is studying the heliopause from Earth orbit.
Early data from IBEX suggests that scientific assumptions about the heliopause were grossly inadequate. The true complexities of the region’s environment are well beyond current theory. It appears that when particles from the Sun interact with interstellar particles, they slow down while releasing energy. The many particles that accumulate in and around the heliopause are highly energized by negative acceleration and creating a shock wave. Based on this, it ca be said that the heliopause is a magnetopause between the solar system’s magnetosphere and plasma currents from the greater galaxy region. Scientists are digesting this new information with the full expectation that everything could drastically change when the Voyager spacecrafts reach the heliopause.
We’ve also recorded an entire episode of Astronomy Cast all about the Voyager Program. Listen here, Episode 199: The Voyager Program.