Humans can’t hear sounds in space, but there are sounds waves moving through the rarefied plasma that fills interplanetary and even interstellar space, the gaps between the stars. Now, thanks to Voyager 1, we can say what interstellar gas sounds like: a constant hum.

Voyager 1 is the furthest human-made object from Earth. Since 2012 when it crossed the heliopause and left the Solar System, it has been traveling in interstellar space, giving humanity our first direct observations of what the space between stars is really like. Previous studies of this interstellar plasma focused on shockwaves, some of which were triggered by the Sun. This new study, published in Nature Astronomy, looks at the constant background noise of the plasma waves.

"It's very faint and monotone, because it is in a narrow frequency bandwidth," lead author Stella Koch Ocker from Cornell University said in a statement. "We're detecting the faint, persistent hum of interstellar gas."

The authors believe that there is a lot of low-level activity in the interstellar plasma and that this data is just the beginning. The researchers looked at four years’ worth of data establishing the density of plasma over 1.5 billion kilometers (932 million miles). This allowed them to work out the intensity of waves moving through this plasma.

"We've never had a chance to evaluate it. Now we know we don't need a fortuitous event related to the Sun to measure interstellar plasma," Shami Chatterjee, a research scientist at Cornell, said. "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."

The years analyzed were towards the Solar minimum when the Sun is at its least active in its 11-year cycle. This allowed them to study the interstellar space plasma without too much disturbance from our star.

"The interstellar medium is like a quiet or gentle rain," said senior author James Cordes, the George Feldstein Professor of Astronomy. "In the case of a solar outburst, it's like detecting a lightning burst in a thunderstorm and then it's back to a gentle rain."

Voyager 1 left Earth in September 1977, traveling past the edge of the Solar System, through the heliopause, in 2012 after its primary mission of studying the gas giants ended. It takes 21 hours for a signal from the spacecraft to reach Earth and data is downloaded at 160-bits-per-second, roughly the speed of your internet connection when you urgently need to be online.

Voyager is powered by radioisotope thermoelectric generators, which are expected to produce enough power to keep the spacecraft running until 2025.

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