Voyager 1 discovers faint plasma 'hum' in interstellar space

Four and a half decades after launch and over 14 billion miles from Earth, Voyager 1 still makes new discoveries. The spacecraft has picked up the signature of interstellar space itself, a faint plasma "hum" scientists compared to gentle rain. 

Plasma has been part of Voyager 1's mission from its launch — the spacecraft discovered lightning strikes in Jupiter's atmosphere and studied how the solar wind tapered off in the outer solar system.

And since 2012, scientists have turned the spacecraft's instruments upon a completely unexplored part of distant space. That's when Voyager 1 crossed the heliopause, where the solar wind — the constant stream of charged particles that flows off the sun — is no longer strong enough to hold back the interstellar medium that surrounds our little neighborhood. Since 2012, as Voyager 1 has drifted ever farther from the sun, the spacecraft has measured the plasma around it.

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This part of the interstellar medium is, mostly, quiet. "It's very faint and monotone, because it is in a narrow frequency bandwidth," Stella Koch Ocker, a doctoral student at Cornell University who led the new research, said in a statement. "We're detecting the faint, persistent hum of interstellar gas."

But every few years, the solar wind pushes back. Voyager 1 picks up those events as shockwaves. "In the case of a solar outburst, it's like detecting a lightning burst in a thunderstorm," senior author James Cordes, an astronomer at Cornell, said in the same statement. "Then it's back to a gentle rain." 

For a time, scientists thought those shocks were the only way that Voyager 1 could measure the density of plasma out there.

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But now that scientists have heard this unexpected hum, they can track the interstellar medium between shocks, which can help them understand much more about a largely undiscovered expanse of space. Ocker believes there's much more low-level activity in the interstellar medium than scientists previously thought.

"Now we know we don't need a fortuitous event related to the sun to measure interstellar plasma," Shami Chatterjee, a Cornell astronomer, said in the same statement. "Regardless of what the sun is doing, Voyager is sending back detail. The craft is saying, 'Here's the density I'm swimming through right now. And here it is now. And here it is now. And here it is now.' Voyager is quite distant and will be doing this continuously."

Voyager 1 and its twin, Voyager 2, will sail away to the stars for time immemorial. But for scientists here on Earth, the spacecraft's days are numbered. Sometime within this decade, the spacecraft's plutonium power sources will finally run dry. 

In the meantime, scientists are savoring every last bit of data that trickles back. "It's the engineering gift to science that keeps on giving," Ocker said.

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