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.)

The last time I posted, in “Taking Flight on Mars,” it was a few days before Ingenuity’s first flight and we were all anxiously awaiting that achievement. Well, it happened, and now we’ve had two more flights — and more to come! Plus, there’s a firehose of information, including great images, all being made available for public appreciation as fast as the science teams can post them.

Furthermore, the MOXIE instrument on Perseverance has sucked in some carbon dioxide from the Martian atmosphere and breathed out oxygen! In the coming days and weeks, the rover will continue its science experiments with studies of the chemical composition of the rocks in the region around its landing site.

Making Oxygen on Mars

We all get the idea about the flight of the Ingenuity chopper — it’s a big step in extending surface exploration on Mars. But, what’s the deal with the MOXIE achievement? Let’s look at it this way: if you were a Mars explorer making your way across the Red Planet in a spacesuit, you’d need oxygen to breathe. And, there’s not a lot on Mars, nowhere near enough to sustain a person’s breathing needs. That’s where MOXIE comes in handy. It’s a small experimental instrument onboard the Perseverance rover that is designed to convert CO2 into oxygen. During future explorations, it will be much easier to convert that stuff in situ than it will be to haul huge oxygen canisters on board a spaceship from Earth.

Not only will CO2-to-O2 converters help give explorers the air they need to breathe, but large-scale conversion efforts will be needed to create reserves of fuel for future rockets that can be used at Mars to transport people to space (or to other parts of the planet).

Short-term Accomplishments on Mars for Long-term Success

There’s been a lot of talk in the past few years about sending people to Mars — which is not a new idea. The exploration of Mars has, if not always stated explicitly, been about learning more so we could go there. But, lately, the interest levels seem to have risen quite a lot. For example, the Chinese have made it clear they intend to send people to Mars. Elon Musk has ambitious ideas about sending a whole city’s worth of people to the Red Planet, and as soon as he can do it. NASA and ESA have been pursuing long-term robotic exploration goals in support of eventual human missions.

Most of the commentary centers on whether we need to do a “plant the flag and bring home rocks” kind of mission, as NASA did with the Apollo trips, or send long-term exploration teams to do science. In my mind, there’s more merit in the second approach, for a number of reasons. First, a trip to Mars is not a three-day “one and done” kind of deal. The mission hardware has to support a long trip supporting the crew. When they get to Mars, they actually have to live there for months at a time. You don’t just go there, do a few things for a few hours, and then head home. Orbital constraints will teach you otherwise.

Supporting Human Missions to Mars

Now, all the missions that have gone to Mars so far have functioned as science projects as well as scouts. They give us info about conditions on the trail ahead. That gives anybody who sends people to Mars a good start on preparations for living and working on Mars. Like lunar explorers, Mars crews will need to bring their own homes and supplies to tide them over. Their first task will be to set up units like MOXIE so they can start making their own oxygen. And, they’ll need to “mine” water, since that’s also important. They’ll be doing all this, along with achieving some science goals along the way. Essentially, you need to guarantee your survival before you can focus on other projects.

Those first missions probably won’t look like the Martian cities Elon dreams of; those will take a while to achieve. But, they’ll be a start on the eventual (and some would say, inevitable) colonization of Mars. The missions we see today are still precursors, but the future Martians will be more grateful for them than you can possibly imagine.