Voyager 1, launched from Earth in 1977, is currently 14 billion miles away, making it the most distant human-made object. The probe is so farflung that it has spent about a decade in interstellar space, where it has made mind-boggling discoveries about the great expanse beyond the “heliopause,” a boundary where the Sun’s solar wind gives way to the winds of distant stars.
For instance, Voyager 1’s latest dispatches include the detection of a tantalizing hum of plasma waves—emission generated by hot gas in interstellar space—with origins that have yet to be fully explained. The probe has been picking up this “extremely weak, narrowband plasma wave emission” since 2017, a period in which the spacecraft has traveled about 10 times farther than the distance between Earth and the Sun, according to a study published Monday in Nature Astronomy.
The faint hum is likely the first continuous measurement of the density of matter in interstellar space, and sheds light on the interactions between star systems and the vast interstellar medium between them.
“These discoveries really tell us a lot about how the interstellar medium is structured right outside of the solar system,” said Stella Koch Ocker, a Cornell doctoral student in astronomy who led the study, in a call.
The new signal was recorded by an instrument on the Voyagers called the Plasma Wave System (PWS), which was primarily designed to detect particle interactions within the magnetospheres of the solar system’s giant planets: Jupiter, Saturn, Uranus, and Neptune. Once Voyager 1 crossed the heliopause in 2012, the instrument began to pick up plasma waves in interstellar space with distinct signatures, lasting in duration from days to a year, that could be traced back to specific solar events, such as flares.
But as Ocker and her colleagues pored over more recent data from the PWS, they noticed a faint emission pattern that was barely over the threshold of what the instrument could record.
“We had wondered whether there were any signals in the Voyager data that were still waiting to be discovered, so we were really interested in trying to dig deep into the data, and peel back the layers and look for anything different from what had been found before,” Ocker explained. “We weren't necessarily expecting to find something. But we did.”
The hum of plasma waves was clearly distinct from the emission that had been previously captured by Voyager. In addition to being relatively weak, the signal occupied an unusually narrow band of frequencies around 3 kHz. It was also a continuous buzz, in contrast to the plasma emission events linked to solar activity, which had clear beginnings and ends.
Ocker and her colleagues concluded that Voyager 1 had managed to pick up the ambient waves of interstellar plasma, which is a roiling stew of protons and electrons. Just as the PWS was able to observe the behavior of particles around gas giants, it is now likely eavesdropping on similar processes beyond the heliosphere.
“The most likely explanation is that the faint plasma waves are caused by thermal motions of electrons in the plasma,” Ocker said. “In a plasma, those components are not just sitting in place. They have some underlying level of motion at any given time, and so the motions of those electrons could be inducing very weak plasma waves.”
Still, the exact mechanisms behind the hum are not yet known, leaving exciting new questions open to future studies. The team is optimistic that Voyager will continue recording the intriguing signal, which will help them to constrain its properties and origin. Those results will reveal new details about the density of matter at the border of our own solar system, which is useful for understanding this otherworldly realm. The data can also help explain more fundamental processes, such as the formation of stars.
The study is just the latest testament to the productivity and longevity of the Voyager mission, which has been reporting back from increasingly distant vistas for nearly a half-century. Decades after the twin probes were launched and years after they entered the interstellar realm, they continue to send snapshots of the cosmos back to new generations on Earth.
“Voyager is truly an incredible mission,” Ocker said. “I think it's a real accomplishment and tribute to the resilience of NASA missions and the extraordinary wealth of science that they can produce.”
“It's been just a very exciting experience for me to have the opportunity to learn from people who have been with this mission since its inception and have seen it go through all kinds of discoveries and setbacks—the full rollercoaster of a 40-year mission,” she concluded.