NASA Recorded the Hum of Interstellar Space in 2021

Voyager 1 crossed the heliopause — the boundary where the sun's solar wind yields to interstellar space — in August 2012. By 2021, it was more than 23 billion kilometers from Earth and still transmitting, which is itself remarkable. What it sent back that year was stranger: a persistent, narrow-band hum in the plasma wave data.

Cornell astronomer Stella Koch Ocker and colleagues identified it as the signature of low-level plasma oscillations in the interstellar medium — the thin, ionized gas that drifts between star systems. The interstellar medium is nearly empty: roughly 0.055 electrons per cubic centimeter, or about one electron per coffee cup of space. But there's enough of it that Voyager's instrument could pick up its faint vibration continuously, even when no solar event was driving a louder plasma burst.

Before this detection, researchers could only measure the density of the interstellar medium when a solar shockwave lit it up from behind, like catching dust in a beam of light. The persistent hum gives them a continuous baseline — a way to map the medium the spacecraft is drifting through, not just momentary spikes.

The signal frequency sits near 3 kilohertz, well below human hearing range in any direct sense, though NASA released a converted audio file in 2021 that maps it into something audible. The spacecraft that recorded it launched in 1977, powered by a plutonium thermoelectric generator that loses about four watts of output per year.