The Sun's million degree corona is continuously evaporating into space, producing an expanding ionized gas or plasma called the solar wind. This supersonically expanding, magnetized wind inflates a bubble, called the heliosphere, in the Local Interstellar Medium (LISM), a medium that contains the remnants of exploded stars and other stars' stellar winds. At over three times the distance to Pluto, the innermost boundary of the heliosphere is thought to be a termination shock, where the solar wind abruptly slows prior to meeting the LISM. Because the Sun is moving rapidly through the galaxy, the heliosphere is compressed and a bow wave or shock forms in the LISM ahead of it. This complicated and fascinating region where the solar wind and LISM interact around the edges of the heliosphere is truly the last unexplored portion of our Sun and solar system's place, and thereby our home, in the galaxy.
All that we know about the interstellar interaction has been gleaned so far from a combination of indirect observations, modeling, and the single-point observations from Voyager 1, our humankind's most distant robotic probe. Voyager 1 crossed the termination shock toward the end of 2004, becoming the first spacecraft to examine the termination shock and the heliosheath beyond it. Voyager 1's observations are stirring important scientific controversies concerning the global structure of the shock, its time history, and how (and whether) it accelerates anomalous cosmic rays. One thing is clear: the termination shock is far more complex than previously thought. Voyager's critical single-point observations highlight the need for global, all-sky observations of the distant interstellar interactions.