Detecting Sounds In Space

As a science writer, and during my tenure as a cruise lecturer (pre-pandemic), I often get questions that are difficult to answer. I once got into a discussion with a doctor on board a ship who asked about sound in space. He had retired from a career in ear, nose, and throat and asked about what we would “hear” on another planet. From there, we talked about Jupiter’s sounds and the noise of a star exploding.

Detecting Sounds on Mars

Well, it turns out, if he’s still around, there are some good examples of sounds in space sent back from spacecraft recently. The first one comes from the Ingenuity helicopter on Mars. Its flights are wildly popular with viewers here on Earth. Each one takes the rover further and higher and they all provide new challenges for the tiny craft. During the April 30th flight, the microphone on the Perseverance rover picked up the actual sounds of Ingenuity’s rotors flipping around at about 2400 rpm (rotations per minute).

Ingenuity’s flight noises became the first recorded ones on a planet where only the sound of the wind existed heard for billions of years. (Yes, of course, each spacecraft landing made a sound, but there were no ears, or microphones, to pick up their noise.)

Now, I can imagine someone asking if it would sound like this to someone standing outside on the surface of the planet. Probably not. Anybody standing outside on Mars will have to wear a spacesuit (and make their own oxygen so they can safely walk around) and will listen to radio communications. That’s going to change what they hear and nobody will ever get to walk around on Mars without protection.

Sounds from Deep Space

Mars isn’t the only place where scientists record “sound” vibrations. And, that’s what sound is: a disturbance, a vibration through a medium. On Mars, the vibrations from the helicopter move through the thin Martian air. But, “sounds” exist in interstellar space (where there is no air, as such). And, they can be detected.

As it turns out, the venerable Voyager 1 (V1) spacecraft does a little sonic exploration of its own, as it passes through interstellar space. It uses its Plasma Wave System to detect a narrow band of vibrations in the plasma environment. V1 has been in interstellar space since 2012 and completed its planetary studies long before that. Studies of the ISM, as it’s called, comprise an important part of its long-term science mission.

Sounds of Silence? Not!

I once had an astronomy professor describe the environment of deep space as “the sounds of silence.” Today, we know that deep space is not empty and we know it’s not “silent.”

Interstellar space contains atoms and molecules of gases and other compounds, and those can move, vibrate, and spin. Those actions send out vibrations that can be detected by sensitive instruments onboard spacecraft passing through the region.

Sampling Sounds in Space

All the spacecraft headed out to interstellar space can be used to sample the density of this “stuff”. So, Voyager 2 can do this, as can New Horizons. In the case of V1, the Plasma Wave System detects a faint “hum” from the low-level actions of interstellar gas. Think of it like the sound made by the constant, gentle patter of rain on a roof. However, instead of water falling in a storm, low-level activity by interstellar gas molecules sends out a monotonic plasma-wave signal. That “hum” is what gets picked up by V1’s instrument.

From time to time, the Sun interrupts this hum with outbursts of its own. Those disturbances travel out to space and Voyager detects them. However, solar events only disturb the interstellar medium for a short time. The rest of the time, the faint hum of gases is all that Voyager can detect” in the plasma wave frequency range.

Now, this isn’t something that human ears are going to hear, even if we could figure out a way to exist in interstellar space without spacesuits and helmet radios. This signal would need to be boosted and processed quite a bit for human ears to hear it. That’s not the point of the study, however. Instead, what it really represents is our exploration of interstellar space in a very direct and evocative way.

(Want to read more about the V1 finding? Check out the research paper where the team involved in monitoring V1’s signals reports their work.)

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