In space no one can hear you scream – or can they?
We think of space as a silent place – a vacuum through which sound waves cannot travel. But that's not quite true.
Sound does exist in the form of electromagnetic vibrations that pulsate in similar wavelengths and NASA has designed a special instrument to record them.
The eerie sounds – worthy of a film soundtrack – are the result of interactions of charged electromagnetic particles from the solar wind, the ionosphere, and planetary magnetic fields.
Strictly speaking, the instrument does not detect sound, but senses waves of electrons in the ionised gas or "plasma" that no human ear could hear. But because they occur at audio frequencies they can be played through a loudspeaker and be heard.
The sounds have a practical use as well.
"The pitch and frequency tell us about the density of gas surrounding the spacecraft," Don Gurnett, the James Van Allen professor of physics at the University of Iowa and the principal investigator for the Plasma Wave Science instrument on space probe Voyager 1.
In 2003, Gurnett used the sounds of the plasma wave data as evidence that Voyager 1 had left the heliosphere – the vast bubble of magnetism that surrounds the sun and the planets in its system.
The video above has recordings from within the solar system, but Voyager gave us a sneak preview of what we might hear in interstellar space.
When Voyager 1 was inside the heliosphere, the tones were low, around 300 Hz, typical of plasma waves travelling through the solar wind. Outside, the frequency jumped to a higher pitch, between 2 and 3 kHz, corresponding to denser gas in the interstellar medium. It was this transition that proved to Gurnett that the probe had moved into deep space.
Apart from Voyager 1, plasma wave antennae are fitted on INJUN1, ISEE1, and Hawkeye space probes. They record the vibrations within the range of human hearing – 20 - 20,000 Hz.