Six years ago, the Voyager 1 spacecraft informed scientists that it had become the first man-made object to enter interstellar space. Now, Voyager 2 has begun to return signs that its own exit from the Solar System could be coming soon.

Two of Voyager 2’s instruments have measured an increase in the number of high-energy particles called cosmic rays hitting the spacecraft, according to a NASA release. Scientists think that the heliosphere, the region of particles and magnetic fields under the Sun’s influence, blocks some cosmic rays. An increase in their rate means that the probe could be nearing the heliopause, the heliosphere’s outer boundary.