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upstart writes in with an IRC submission for nutherguy:

Voyager Spacecraft Detect an Increase in The Density of Space Outside The Solar System:

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.

Voyager 2's new data show that not only was Voyager 1's detection legit, but that the increase in density may be a large-scale feature of the very local interstellar medium (VLIM).

[...] One theory is that the interstellar magnetic field lines become stronger as they drape over the heliopause. This could generate an electromagnetic ion cyclotron instability that depletes the plasma from the draping region. Voyager 2 did detect a stronger magnetic field than expected when it crossed the heliopause.

Another theory is that material blown by the interstellar wind should slow as it reaches the heliopause, causing a sort of traffic jam. This has possibly been detected by outer Solar System probe New Horizons, which in 2018 picked up the faint ultraviolet glow resulting from a buildup of neutral hydrogen at the heliopause.

It's also possible that both explanations play a role. Future measurements taken by both Voyager probes as they continue their journey out into interstellar space could help figure it out. But that might be a long bet to take.

"It is not certain," the researchers wrote in their paper, "whether the Voyagers will be able to operate far enough to distinguish between these two classes of models."

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