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Powerful telescopes like Hubble and TESS have given us an idea of what interstellar space looks like, but have you ever wondered what it sounds like out there?
Voyager 1 is now getting to hear the music of the cosmos outside the solar system. The probe, which has been boldly going where no spacecraft has gone before since 1977, may be primitive by today’s standards but is picking up some chill vibes from the interstellar medium now that it has long since left the heliosphere. Turns out even the parts of space that seem empty have something going on even when things aren’t flaring or exploding. As it cruises through the void, the probe is tuning in to a faint humming sound that is the music of the stars.
Even if the stars themselves aren’t creating some sort of symphony, so long as there is no catastrophic phenomenon going on, the interstellar medium hums infinitely. It’s a faint humming sound that means much more to a scientist than it might to someone randomly downloading it. The sound of the interstellar medium and what it could tell us about the vast expanse beyond the heliosphere has never been analyzed before. Voyager’s instruments are transmitting messages to Earth that have been incrementally telling a research team back on Earth variations in the density of interstellar gas as it flies through it.
Researcher Stella Koch Ocker of Cornell University, who led a study recently published in Nature Astronomy, and her team have been following the prove as it treks further and further into the unknown.
"The Voyager 1 Plasma Wave System (PWS) measures the voltage difference across the spacecraft antenna," Ocker said in an interview with SYFY WIRE. "This voltage difference can be used to determine how the electric field in the plasma changes over time and frequency. We detect the plasma waves by finding variations in the electric field. These plasma waves occur at radio frequencies, which is why we often refer to them as the 'sounds' of interstellar space."
Finding out about plasma density used to mean having to wait for the Sun to throw a tantrum (such as a solar flare or coronal mass ejection), but not anymore. The monotone humming might be more like cosmic elevator music than screaming metal — except for when there is a solar event that disturbs the surrounding plasma — but it can tell us how plasma is distributed in space. It can also tell us about the subtle activity that happens in interstellar gas. Most of this gas is hydrogen and helium, the same stuff stars are made of, and some of it is left over from their formation.
"Plasma oscillation events are large vibrations in the plasma that are triggered by solar activity (like coronal mass ejections), and they only last a finite period of time," said Ocker. "The weaker plasma waves we have found persist over a much longer period of time—about 3 years in the data we analyzed for this study—and they last between previously detected plasma oscillation events."
Stars burn hydrogen and turn it into helium until they either explode into supernovae, collapse into black holes or diminish into white dwarves. Voyager 1 would pick up plasma disturbances if something like a supernova were to happen far away enough so as not to destroy the spacecraft. Supernova remnants are scattered all over the interstellar medium, and the dust and gas left over from both star formation and death is often recycled in the formation of new stars. As Voyager 1 continues traversing the skies, it will be interesting to see if it hears any signals that could give away a star blazing into existence or gasping for its last breaths, though that is probably unlikely.
"It is not yet understood what is exciting the persistent waves," They may be simply thermal in nature (the gas is hot and can excite waves at a low level)," Cornell space physicist Jim Cordes, who also contributed to the study, also told SYFY WIRE. "It is conceivable that there are also external excitations from events on other, nearby stars or other activities, though those are probably quite rare.
Another thing that this remarkably persistent spacecraft could let us know is how interstellar gas interacts with the heliosphere and possibly affects its shape. While the bubble that contains our solar system was thought to be an oblong shape, NASA scientists found that it is more likely to look something like a smushed croissant.
"What these particular oscillations indicate, given that they are persistent, is that the waves are continuously excited as opposed to much stronger events that have been previously observed," Cordes said.
Solar particles inside the heliosphere are thought to have warped it into this shape. Now that Voyager 1 is listening to the interstellar plasma wave emission on the outside, it could beam back evidence for the effects of forces no one had suspected in the distant reaches of the final frontier. If Ocker had superpowers to modernize Voyager 1 from this distance so it could reveal even more, she woudl use them to update a few features.
“A longer antenna would probably be able to better detect these weak plasma waves," she said. "I think even the most basic upgrades, such as modern data storage capabilities, would be useful. Another big technology consideration is having a strong array of telescopes on Earth to be able to communicate wtih Voyager 1, or any future interstellar probe, from so far away."
Cordes is still optimistic about what the spacecraft that never gives up could tell us about what may only appear to be a a dark expanse of nothingness.
"The in situ measurements of Voyager are unique and it would be good to have a follow-on mission to since its power supplies will be dead some time this decade. Our science program includes comparison of Voyager results with those that we obtain with radio telescopes by observing pulsars, which tell us about turbulence in the interstellar medium at large," he said. "We want to see what the differences and similarities are."
Too bad we can’t hear it through our airbuds or headphones.