More than 14 billion miles from Earth, NASA's Voyager 1 spacecraft is listening to the background music of interstellar space.
The probe, which launched more than 40 years ago, exited our solar system in 2012. Five years after its interstellar foray began, Voyager 1 began picking up a monotonous droning.
According to a study published Monday in the journal Nature Astronomy, the spacecraft is detecting the constant burble of charged gas, known as plasma. The sound is created as this plasma ripples through the interstellar medium — a soup of radiation, gaseous particles, and dust that fills the space between stars.
"We're detecting the faint, persistent hum of interstellar gas," Stella Koch Ocker, a Cornell University astronomer and co-author of the new study, said in a release.
The humming would not be audible to the human ear, Ocker said, but Voyager 1's onboard instruments can pick up the vibrations.
"If we could hear it, it would sound like a single steady note, playing constantly but changing very slightly over time," she told Reuters.
Keeping tabs on those vibrations as Voyager 1 travels further could help astronomers learn more about what interstellar space is like, and whether the mixture of gas and radiation differs from place to place in the universe.
'A quiet or gentle rain'
Voyager 1 has traveled farther from Earth than any other spacecraft in history. It's one of only two human-made objects that have ever crossed into interstellar space — the other i sits counterpart, Voyager 2, which left the solar system in 2018.
NASA launched the twin Voyagers in 1977 to swing around Jupiter and Saturn over the course of a 5-year mission. The spacecraft taught astronomers about the composition of Saturn's rings and the existence of volcanoes on Jupiter's moon, Io. Then Voyager 2 went on to visit Uranus and Neptune, while Voyager 1 continued on toward the edge of our solar system.
To truly reach interstellar space, both Voyager probes had to cross the heliopause — the outer boundary of the stream of charged particles shot out by the sun. This solar wind, as the stream is known, extends beyond our solar system, but at the heliopause, it encounters pressure from the wind in interstellar space and gets turned back toward the sun.
Research suggests the sun's influence can sometimes extend beyond the heliopause, though. Occasionally, the sun spits out billions of tons of charged particles from its atmosphere in the form of solar flares. These eruptions can cause violent disturbances in interstellar gas. Voyager 1 has detected such shockwaves before.
But the farther into the heliopause Voyager 1 goes, the less the sun's energy should ooze into the interstellar medium and push or pull on the gas therein.
According to NASA, there will come a point in Voyager 1's travels when solar particles no longer influence its measurements. Once the probe gets there, "it will sense the stirrings from sources deeper in the cosmos," the agency said.
Indeed, the humming outlined in the new study seems unrelated to what the sun is doing — unlike those erratic shockwaves, this is a constant vibration in the interstellar medium, which suggests a different source of energy that astronomers might not know about yet.
"The interstellar medium is like a quiet or gentle rain," James Cordes, a Cornell astronomer and another study co-author, said in the release. "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."
Listening for the hum of that gentle rain could help researchers better understand how dense the space between stars is — and whether that density changes — as Voyager 1 gets farther and farther from Earth. It could also reveal clues about how and where the most stellar nurseries are, since stars form in high-density interstellar clouds.
