evidence: NASA’s Voyager 1 spacecraft captured these sounds of interstellar space. Voyager 1’s plasma wave instrument detected the vibrations of dense interstellar plasma, or ionized gas, from October to November 2012 and April to May 2013. Credit: NASA/JPL-Caltech [...] These crashing waves reveal clues about the density of the interstellar medium – a value that affects our understanding of the shape of our heliosphere, how stars form, and even our own location in the galaxy. As these waves reverberate through space, they vibrate the electrons around them, which ring out at characteristic frequencies depending on how crammed together they are. The higher the pitch of that ringing, the higher the electron density. Voyager 1’s Plasma Wave Subsystem – which includes two "bunny ear" antennas sticking out 30 feet (10 meters) behind the spacecraft – was designed to hear that ringing. In November 2012, three months after exiting the heliosphere, Voyager 1 heard interstellar sounds for the first time (see video above). Six months later, another "whistle" appeared – this time louder and even higher pitched. The interstellar medium appeared to be getting thicker, and quickly.