Voyager 1's passage into interstellar space
It's official: Voyager 1 is finally out of the Solar System's bubble. Measurements by NASA published this week1 show that in August 2012 the US probe detected a sharp increase in the number of electrons hitting its instruments. It was the sign the mission team expected to see as the spacecraft crossed the heliopause, the region where solar winds end their outward run as they collide with interstellar particles.
Voyager 1 has been in space for 36 years now; at the time when it detected its transition through the heliopause it was about 18 billion kilometres from Earth. Sister craft Voyager 2 is about 3 billion kilometres behind and is expected to cross the heliopause in a few years.