In November 2018, after an epic, 41-year voyage, Voyager 2 finally crossed the boundary that marked the limit of the Sun’s influence and entered interstellar space. But the little probe’s mission isn’t done yet — it’s now sending home information about the space beyond the Solar System.

And it’s revealing something surprising. As Voyager 2 moves farther and farther from the Sun, the density of space is increasing.

It’s not the first time this density increase has been detected. Voyager 1, which entered interstellar space in 2012, detected a similar density gradient at a separate location.