Voyager 1 has left the solar system, and this time – this time – NASA says it's official. To be honest, after so many months of ambiguity, we found ourselves a little underwhelmed. But that was before we heard Voyager's recordings of interstellar space. Seriously, these will give you chills.
As this video from NASA's Jet Propulsion laboratory explains, the "sounds" you hear reflect the detection of dense, ionized gas (the "interstellar plasma" that fills the space between star systems like ours and, say, Alpha Centauri) by Voyager's plasma wave instrument. Via JPL:
The soundtrack reproduces the amplitude and frequency of the plasma waves as "heard" by Voyager 1. The waves detected by the instrument antennas can be simply amplified and played through a speaker. These frequencies are within the range heard by human ears.
These shrill, wraithlike cries are what helped the Voyager science team determine the density of the interstellar medium, and ultimately deduce that the spacecraft had really, actually, finally left the solar system...
...They're also, as you've surely realized by now, pretty much perfect for a horror movie soundtrack.
Read more about Voyager's crossing into interstellar space over at NASA.