Last week, NASA announced that the Voyager 1 spacecraft had become the first human-made object to enter into interstellar space. As a writer, I liked how they said "human-made" in the press release, as it both directly avoids the chauvanistic "man-made" term and implicitly allows for non-human species to have beaten us to this new frontier. As an astronomer, I didn't like some of the resulting press that made the natural, yet wrong, conclusion that Voyager has left the solar system. The blame is partly on NASA, who did not make the distinction in the release, but rather in an accompanying article.
First, let's deal with interstellar space. Interstellar means "between the stars," and hence refers to anyplace that is inside our galaxy and outside of the atmosphere of a star. The important question for today is defining where the atmosphere of a star ends. (I'll leave the question of "inside our galaxy" for a later disussion.)
As seen during total solar eclipses and by several space missions, our Sun has a large extension to its visible atmosphere called the solar corona. The corona, in turn, blends into the solar wind that streams past Earth and the other planets. The boundary where the solar wind dies out is the place where interstellar space begins.
As the solar wind extends to larger and larger volumes about the Sun, the pressure it exerts drops. At some point that pressure is counter-balanced by the pressure of the gas in interstellar space. For Voyager 1, the boundary crossing was marked by measurements that indicated changes in density starting on August 25, 2012. That date is now accepted as the beginning of humanity's interstellar adventure.
Second, let's consider the extent of the solar system. Earth is 93 million miles from the Sun; a distance we define as one astronomical unit (AU). The farthest planet, Neptune, orbits at 30 AU. The thousands of small, icy objects (including Pluto) discovered in the Kuiper Belt are generally out to 50 AU. The most distant object yet discovered orbiting our Sun is Eris, at about 97 AU. With Voyager 1 at 125 AU and Voyager 2 at 102 AU, both are beyond these distances.
However, the orbits of long-period comets extend out to around 500 times farther still. The comets are too small and faint to be seen beyond the outer planets, but the parameters of their orbits are well measured. At an outer distance of roughly 50,000 AU, the Oort Cloud of comets represents the boundary of objects orbiting the Sun. Considering a scale model where Earth is one inch from the Sun, Voyager 1 is about 10 feet away and the Oort Cloud extends for over three-quarters of a mile. It will take tens of thousands of years for the Voyager spacecraft to reach such distances and truly exit the solar system.
The apparent contradiction is that Voyager 1 has crossed into interstellar space, yet remains inside our solar system. It is not really a conflict, as the two measures use different criteria. The edge of interstellar space is a pressure boundary, while the extent of solar system orbits is a gravitational boundary. There is no reason the two boundaries should coincide, and, in fact, they differ greatly.
Voyager 1 is not the only long-distance space journey in the news these days. Comet ISON, when its orbit is extrapolated backward, has spent most of its time well outside the boundary of the solar wind. Its passage into the inner reaches of the solar system is vanishingly brief compared to its entire orbit. If and when it flares up this fall / winter, consider it a beautiful and dynamic greeting from a passing interstellar traveller.