 The sun is moving through the galactic medium like a ship in the ocean, and we're just tagging along. As we discussed earlier, the sun is a thermonuclear fireball that continuously ejects large quantities of high energetic particles into space. This is the solar wind and it goes out in all directions. The extent of the solar wind defines the final frontier of the influence of the sun. This is called the heliosphere. The solar wind's strength dies down as it spreads over ever increasing volumes of space. As it approaches the strength of the interstellar wind, coming from the rest of the Milky Way, its motion slows down abruptly. This is called the termination shock. Beyond this is a transitional region called a heliosheath that terminates at the outermost edge of the heliosphere called the heliopause. And like ocean water being pushed aside by the bow of a great ship, the sun, with its solar wind, does the same in the galactic medium. That's why they call the final boundary the bow shock. This marks the final extent of the solar wind and defines the outer limits of the heliosphere. And like the other objects in the solar system that can be seen and triangulated to determine how far away they are, the only way to find out how far away the termination and bow shocks are, and therefore calculate the full size of the solar system, is to go there, measure which way the wind is blowing, and report back how far you've gone. This is exactly what Voyager 1 and Voyager 2 have done. Crossed in 1977, both spacecraft have passed through the termination shock. Voyager 1 crossed at 94 astronomical units in December 2004. Voyager 2 crossed at 84 astronomical units in August 2007. So we see that the boundary is not exactly a sphere. As of October 2017, both voyagers are deep into the heliosheath. After 40 years, their instruments still report on cosmic rays, charged particles, magnetic fields, and plasma waves. When they detect a change in the direction of the wind, we'll know that they have entered interstellar space. They are expected to have enough energy to continue reporting through 2020, so there is still a chance we'll see it happen. Recent magnetic field data from the Voyager probes, Cassini and Ibex, the interstellar boundary explorer mission, indicate that the heliosphere may be more rounded than previously thought. If correct, the tail we see here will be replaced by the interstellar medium. Research is ongoing in this area.