Come Hear the Weird Interstellar Space Sounds Captured by NASA's Voyager

According tomedia reports, until recently, every spacecraft in history took measurements inside our heliosphere, the magnetic bubble inflated by the sun. But on August 25, 2012, NASA's Voyager 1 changed that. When it crossed the boundary of the heliosphere, it became the first man-made object to enter and measure interstellar space.

Voyager 1 has been on an interstellar journey for eight years, and careful listening to its data has shed new light on the frontier.

If our heliosphere is a ship sailing in interstellar waters, Voyager 1 is a life raft just dropped from the deck, determined to survey ocean currents. At present, any turbulent "sea water" it feels is mainly from the wake of the heliosphere. But further afield, it can sense disturbances from the depths of the universe. Eventually, our heliosphere will disappear entirely from its measurements.

"It's fair to say that we have some idea of ​​how far Voyager needs to go to start seeing purer interstellar water," said Stella Ocker, a Ph.D. student at Cornell University in Ithaca, New York, and the newest member of the Voyager team. , "But we're not entirely sure when we'll get to that point."

Ocker's new study, published Monday in Nature Astronomy, reports what may be the first continuous measurement of the density of matter in interstellar space. "This detection gives us a new way to measure the density of interstellar space and opens up a new avenue for us to explore the structure of the very close interstellar medium," Ocker said.

When depicting the matter between stars -- what astronomers call the "interstellar medium," one thinks of a dispersed soup of particles and radiation -- a calm, quiet, serene environment. And that would be an error.

"I've used the term 'stationary interstellar medium' -- but you can find a lot of places that aren't particularly stationary," notes Jim Cordes, a space physicist at Cornell University and co-author of the paper.

Like the ocean, the interstellar medium is filled with turbulent waves. The largest one comes from the rotation of our galaxy, which arises as space churns against itself and creates fluctuations that span tens of light-years. Smaller waves (still large, in fact) emerge from the supernova explosion, stretching from crest to crest for billions of miles. The smallest ripples usually come from our own sun, and are produced when the sun erupts, sending shock waves through space and then penetrating into our heliosphere.

These collision waves reveal clues about the density of the interstellar medium -- a value that affects our understanding of the shape of the heliosphere, how stars form and even our place in the Milky Way. As these waves reverberate through space, they vibrate electrons around them, and those electrons are emitted at specific frequencies, depending on how tightly they are packed together. The higher the pitch, the higher the electron density. Voyager 1's plasma wave subsystem -- which includes two "rabbit-eared" antennas that stick out 30 feet (10 meters) behind the spacecraft -- is designed to hear that sound.

In November 2012, three months after leaving the heliosphere, Voyager 1 heard interstellar sounds for the first time. Six months later, another "whistle" came -- this time louder and higher pitched. The interstellar medium appears to be getting thicker and faster.

In today's Voyager data, these momentary whistles continued at irregular intervals. They are an excellent way to study the density of the interstellar medium, but do require some patience. "They're only found once a year, so relying on these chance events means that our map of the density of interstellar space is a bit sparse," Ocker said.

Now, Ocker and their team set out to find an operational measurement of interstellar mid-density to fill in these gaps -- one that doesn't rely on the occasional shock wave propagating from the Sun. After sifting through Voyager 1 data for weak but consistent signals, she found a promising candidate. In mid-2017, just as another whistle blew, it started to increase.

"It's actually a single tone," Ocker said. "We do hear it change over time, but the way the frequency moves tells us how the density changes."

The new signal, which Ocker calls a plasma wave emission, also appears to track the density of interstellar space. When there is a sudden whistle in the data, the pitch of the transmitted signal also rises or falls. The signal is also similar to that observed in Earth's upper atmosphere, which is known to track electron density there.

"It's really exciting because we're able to regularly sample the density of a very long stretch of space, the longest stretch of space we've ever had. This gives us the most complete density the traveler has ever seen. and the interstellar medium map," Ocker said.

According to the signal, the electron density around Voyager 1 began to rise in 2013 and reached its current level around mid-2015, during which time the density increased by a factor of about 40. In the entire dataset they analyzed through early 2020, the spacecraft appeared to be in a similar density range, albeit with some fluctuations.

Now, Ocker and her colleagues are trying to build a physical model of how plasma waves are created, which will hold the key to explaining them. Meanwhile, Voyager 1's plasma wave subsystem keeps sending data back further and further from Earth, where every new discovery has the potential to make us reimagine our home in the universe.