Do you remember the Great Barrier at the edge of the galaxy in Star Trek? Well, we’re still a long way from getting there, but human-built spacecraft have reached a different boundary at the edge of our solar system. It’s called the “termination shock”, and it’s where the solar wind dies down under pressure from gas in the interstellar medium (give yourself some geek points if you already knew that space outside the solar system isn’t completely empty).
And what spacecraft made this journey? None other than Voyagers 1 and 2, coincidentally (huh, small galaxy). Well, Voyager 1 actually crossed the boundary back in 2004, but we missed it because nobody was listening to the transmissions (WTF?). But we had our dishes turned in the right direction when Voyager 2 crossed on August 30, 2007.
What scientists learned was that the termination shock isn’t all that shocking. The solar wind slows as you approach the termination, with a noticeable but smaller than expected drop at the boundary itself.
At the point where Voyager 2 crossed, the termination shock is 84 AU from the Sun — that means that it’s 84 times as far from the Sun as the earth is. To put that in perspective: if the Sun is a ball 8 inches (20.32 cm) in diameter, then the Earth is a peppercorn 26 yards (23.77 m) away. In this model, the termination shock lies 2184 yards away from the Sun, or about 1.24 miles (2 km). The little peppercorn launched two infinitesimally small objects that managed to get there, after about 30 years.
It should take about another 10 years for both Voyagers to reach their next milestone: the heliopause, which marks the farthest reach of any solar wind — and the beginning of interstellar space, the final frontier.