Almost certainly – after a 35-year journey out of the Solar System, Voyager 1 seems to have finally left the heliosphere (the region dominated by particles flowing out from the Sun in the solar wind). Scientists think it achieved this landmark around August 2012, when its detectors began to pick up increasing numbers of cosmic rays – actually high-speed particles from distant stars and galaxies.
This indicates that it has almost certainly crossed the heliopause – the boundary between the heliosphere and the interstellar medium – and is now drifting through a turbulent region just beyond the Sun’s protective bubble. However, some scientists want to wait and see how conditions settle down beyond this bubble before admitting that the boundary has been crossed.
Answered by Giles Sparrow