It's obviously a major milestone for space exploration which is probably why scientists have been arguing for months over whether or not Voyager 1 had crossed the threshold into interstellar space. In the end, it all came down to the plasma surrounding the spacecraft. After a burst of solar wind and magnetic fields caused the plasma around the spacecraft to oscillate in April, researchers realized plasma was also 40 times denser at that point than it was in the heliosphere. This was a sign that the Voyager 1 had entered interstellar space, and the team ultimately determined that the spacecraft crossed the line in August 12 of last year. (Listen to the sound of interstellar space below.)
"Voyager has boldly gone where no probe has gone before, marking one of the most significant technological achievements in the annals of the history of science, and adding a new chapter in human scientific dreams and endeavors," said NASA’s associate administrator for science John Grunsfeld. "Perhaps some future deep space explorers will catch up with Voyager, our first interstellar envoy, and reflect on how this intrepid spacecraft helped enable their journey."
In the meantime, all eyes are on Voyager 2, which is nipping at its sibling's heels, speeding fast into interstellar space. (That is, if 2 billion miles can be considered nipping at its heels.) Either way, Voyager is now on its way to another star. At it's current speed of 100,000 miles per hour, it'll only take her 40,000 years. More here.