Beyond the edge of the solar system, more than 14 billion miles from Earth, a NASA spacecraft has detected a curious and persistent "hum" in interstellar space.
The faint but constant vibrations were picked up by the Voyager 1 spacecraft, which, after more than four decades journeying deep into the cosmos, is the most distant human-made object in space. Scientists say the new discovery, published Monday in the journal Nature Astronomy, is providing a unique and never-before-seen glimpse of the interstellar environment — the frontier beyond the reaches of the sun and planets in our cosmic neighborhood.
"Voyager 1 is in an interesting region of space that is outside this thing called the heliosphere, which is the protective bubble that encases all the planets in the solar system," said Stella Ocker, a doctoral student at Cornell University in Ithaca, New York, and one of the authors of the new study. "So, it's really our only tool for directly sampling the nature of interstellar space."
Ocker and her colleagues don't yet know what's causing the "hum," but it was measured through ripples of plasma in what's known as the interstellar medium, the hodgepodge of gas, radiation and particles that make up the space between stars. While it's not an actual audio signal, the faint drone showed up as vibrations in a narrow frequency bandwidth, Ocker said.
Previously, scientists could only take fleeting measurements of the interstellar medium after periodic but isolated eruptions from the sun, which would unleash shockwaves that coursed through the solar system and beyond.
The new findings suggest that by tracking these persistent vibrations in the interstellar medium, it may be possible to tease out specific properties of this environment, such as its density. This, in turn, will help astronomers better understand the mysterious environment beyond the solar system.
"Rather than trying to map the geography around my house based on one or two trees in my yard, this will let me map all the way from my house to the next neighborhood," said Merav Opher, a professor of astronomy at Boston University who was not involved with the new research.
Voyager 1 crossed into interstellar space in 2012, but the plasma "hum" was first detected in 2017, Ocker said. It's not yet understood why the signal didn't appear sooner or what that lag could mean, she added.
The scientists are also keen to see whether the persistent drone continues as the probe travels deeper through interstellar space.
Opher called the new findings "phenomenal," because they could reveal much about how the cocoon-like magnetic bubble around the solar system interacts with what lies beyond it.
There are questions, for instance, about how much the sun's activity shapes the solar system's protective cocoon and interacts with the interstellar medium.
"It's almost 10 years now that Voyager 1 has been in interstellar space, and we see that the influence of the sun is still pretty strong," Opher said.
Voyager 1, and its twin Voyager 2 probe, were launched in 1977. Both spacecraft flew past all of the giant planets in the outer solar system before crossing the heliosphere's boundary. Voyager 1 entered interstellar space in 2012, and Voyager 2 followed in 2018.
Ocker said the new research is a testament to the pioneering Voyager 1 mission, which continues to beam back data 44 years after its launch.
"It’s the engineering gift to science that keeps on giving," she said.