Forty-four years ago, NASA launched the twin probes Voyager 1 and 2 into space. With no return mission planned, the two spacecraft will continue journeying for as long as their generators will allow, transmitting data back to Earth along the way. Voyager 1 allowed astronomers to study Jupiter and Saturn before crossing the heliopause in 2012. Now, the most distant human-made object – a whopping 14 billion miles from Earth – has detected the faint, constant “hum” of deep space.

Heliopause: From helios, the Greek word for sun; the border between our solar system and interstellar space, found at the edge of the heliosphere (below).

Heliosphere: A bubble-like cavity surrounding the Sun, filled with a gas of ions and free electrons originating from the Sun (also known as solar plasma/solar wind).

Most believe that the interstellar space between star systems is a complete vacuum, thereby preventing any sound from travelling within it.  In reality, deep space isn’t quite so empty. The distance between galaxies contains the interstellar medium, most of which is in an electrically charged state called plasma. Typically, Voyager 1 records oscillations in the interstellar plasma that are caused by solar flares: bursts of activity from our Sun. The study published in Nature Astronomy in May 2021, however, reveals that even in the absence of a solar flare, the small amounts of gas filling this medium are constantly vibrating. Though these oscillations are too weak to be picked up by the human ear, lead study author Stella Koch Ocker has an informed idea of how the audio signal would sound: a single steady note, playing constantly and at a single frequency (like a hum), but changing slightly over time. Ocker’s co-author on the study, Professor James Cordes, compares the persistent sound of interstellar space to a “gentle rain,” sometimes interrupted by “lightning bursts” caused by solar flares.  

Even though Ocker and colleagues still don’t know what’s causing the persistent oscillations, discovering the “sound” of the cosmos has big implications for the information Voyager 1 sends home. Previously, astronomers were reliant on solar flares for the vibrations necessary to gather data on the interstellar medium. Now, Ocker’s findings show that certain properties of the interstellar medium, such as its density, can be measured consistently. 

The recordings of deep space transmitted by Voyager 1 are in very good company. Each Voyager space probe carries a gold-plated audio-visual disc in case the spacecraft is ever intercepted by extra terrestrials (no, we’re not kidding). The audio files on these discs paint a beautiful picture of life on our planet: the sound of waves on a shore, babies crying, and even Chuck Berry’s “Johnny B. Goode.” Voyager 1 will continue data collection until about 2025, when its thermoelectric generators will begin to fail – but it will carry Earth’s songs with it for as long as the cosmos extends.

Ad astra.  

Journal references

Ocker, S.K., Cordes, J.M., Chatterjee, S. et al. Persistent plasma waves in interstellar space detected by Voyager 1. Nature Astronomy (2021). https://doi.org/10.1038/s41550-021-01363-7