Explainer: Is there sound in space?

If you’ve seen the famous promotional poster for the 1979 cult science fiction horror film Alien, then you might be under the impression that we can’t hear any sounds in space – let alone screams of abject terror.

So how could the 2018 SCINEMA International Science Film Festival Best Experimental Film, Astroturf, use the strange sounds of recordings from satellites in space?

No, there isn’t sound in space.

Sound doesn’t exist in space, at least not the way we experience it on Earth. This is because sound travels through the vibration of particles, and space is a vacuum. On Earth, sound mainly travels to your ears by way of vibrating air molecules, but in near-empty regions of space there are no (or very, very few) particles to vibrate – so no sound.

We’re lucky that’s the case, because otherwise the sound of the Sun would roar at an impressive 100 decibels to us on Earth – like hearing a rock concert all day every day. 

Sound travels in what’s known as a longitudinal wave, which causes back-and-forth vibration of the particles in the medium through which it is moving. It propagates through a medium at the speed of sound which varies from substance to substance – generally more slowly through gases, faster in liquids, and fastest in solids.

This back and forth causes regions of high pressure where particles are compressed together (compressions) and regions of low pressure where they are more spread apart (rarefactions). The distance it takes to complete one wave cycle – for instance, the distance between each repeating compression – is what’s known as its wavelength.

The frequency of the wave is measured in hertz (Hz), which is a measure of the number of waves that pass through a fixed point in a second. So, the longer the wavelength the lower the frequency, and vice versa.

Human beings can usually hear sounds within a narrow range of frequencies, usually between 20Hz and 20,000Hz.

So, what are we hearing in Astroturf?

This short film is just one part of a greater anthology, where independent filmmakers were challenged by the Space Sound Effects (SSFX) project to create short films which incorporated sounds recorded in space by satellites.

Now, although we just reminded ourselves that space is a vacuum, it should be clarified that it isn’t completely empty. For instance, it contains solar wind which streams off the Sun – a constant flow of charged particles (plasma) which Earth’s magnetic field protects us from.

The magnetosphere shields us from this ionising radiation and from erosion of the atmosphere by solar wind, but the interactions occurring here are complex and dynamic, and can result in phenomena which disrupt the technology we rely upon, such as electrical grids, global positioning systems (GPS), and weather forecasts.

It’s these plasma waves, electromagnetic vibrations, which can be measured. But the waves fall within the ultra low frequency (ULF) range – with frequencies from fractions of a millihertz to 1Hz – that are undetectable to human hearing. For scale, that’s wavelengths of around 300,000km, and pressure variations so small you’d need an eardrum comparable to the size of Earth to hear them.

But satellites can still observe them. Scientists took a year’s worth of these recordings, dramatically sped up their playback, and condensed them down to just six minutes of audio at frequencies within the human auditory range. This is a process called sonification – like visualisation but instead with sound – where non-speech audio is used to convey information or data, the most famous application of which is the Geiger counter.

This audio was also used in a citizen science project in which high-school students identified an interesting sound-stamp that, when further explored by the scientists, turned out to be a coronal mass ejection – or solar storm – arriving at Earth. By making the data audible they were able to pinpoint an interesting event which the researchers wouldn’t have otherwise spotted.

Detecting the pitch and frequency of electromagnetic waves has also been used to tell us about the density of gas surrounding the Voyager 1 spacecraft – the space probe launched by NASA in 1977 to study the outer solar system and interstellar space.

From this they were able to determine when Voyager 1 had left the heliosphere – the vast bubble of magnetism surrounding the Sun and planets in the solar system – and moved into the denser gas in the interstellar medium between planetary systems.

Are there other instances of the sonification of space data?

There are a lot of these “sounds of space” collected by instruments on various spacecraft, from Juno spacecraft observing the plasma wave signals emanating from Jupiter’s ionosphere, to Cassini’s detection of radio emissions from Saturn (which are well above the audio frequency range and are shifted downward so we can hear them).

Another example is gravitational waves. These stretch and shrink space and can be detected through the distortion, or vibration, of space between masses – but needs to be amplified a billion times to be audible.

Going another route, X-ray, optical and infrared light can be translated into sounds in an ensemble musical piece to represent the position and brightness of light sources in a region of space in the Milky Way.

So, while we can’t hear sound in space as we can on Earth, it’s still possible for us to convert the emissions of space into something the human ear can perceive – and isn’t that much nicer to listen to than a scream, anyway?