The Voyager spacecraft was launched by NASA twenty-six years ago. For years this probe sent back amazing pictures of the outer planets in our solar system before sailing off into space. Now, Voyager is ninety times as far from the sun as we are, and is about to reach the Termination Shock.

It may sound sci-fi, but the Termination Shock is one of two proposed boundaries to our solar system. You can say that the solar system ends with Pluto. Astrophysicists, however, think of the solar system as extending farther out to include two less obvious, but just as real, outer edges.

One of the outer edges is the Termination Shock, where the charged particles blown out by the sun--and known as the "solar wind"--slow down abruptly, thanks to the pressure from other charged particles coming from deep space. Beyond that is the edge of the Heliosphere, which is where the solar wind finally ends.

The Voyager probe is just now crossing the Termination Shock. In another fifteen years or so it will pass the outer edge of the Heliosphere--a human-made object moving for the first time in history into true interstellar space. It's an awesome thought.