The global heliosphere is created by the supersonic solar wind diverting the interstellar plasma flow around the Sun. Interstellar ions and neutral atoms flow at 26 km/s relative to the Sun. The solar wind, flowing outward at 400-800 km/s, makes a transition to subsonic flow at the termination shock. Beyond this, the solar wind is turned toward the heliotail, carrying with it the spiraling interplanetary magnetic field. The heliopause separates solar material and magnetic fields from interstellar material and magnetic fields. Interstellar neutral atoms can penetrate the heliosphere, but interstellar ions are diverted around it. Beyond the heliopause there may also be a bow shock formed in the interstellar medium

Although there are presently no direct measurements of the size of the heliosphere, a combination of theory, modeling, and a few key measurements leads to the conclusion that the termination shock is presently located between ~80 and ~100 AU from the Sun. In this case the heliopause (the boundary between solar wind and interstellar plasmas) is expected to be somewhere between ~120 and ~150 AU. It is anticipated that Voyager 1, presently at ~76 AU, will reach the termination shock within the next few years and establish the scale size of the heliosphere. Although the Voyager spacecraft will undoubtedly make fundamental discoveries about the nature of the boundaries of the heliosphere, the Voyager instruments were designed to investigate the outer planets and their satellites and magnetospheres, and there are many key properties that they are simply unable to measure There is a need for a new mission carrying an instrument package specifically designed to make comprehensive measurements of the boundaries of the heliosphere, and to begin the direct exploration of nearby interstellar space.