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The Perseverance rover is well on its way to Mars. It's more than halfway there, in fact, in kilometers traveled. In 91 days — on 18 February, 2021 — it will begin its descent to the Red Planet, ramming the thin air there at hypersonic speeds, slowing to where a parachute can be used to slow it further, then lowered by a rocket-powered sky crane to the surface.
One difference, though, is a specially installed microphone that will record the whole thing. It will hear the wind rushing, the sky crane firing, and perhaps even dust and sand rustling as it touches down.
But, on 19 October 2020, engineers here on Earth ran a checkout of various Perseverance systems, including the EDL mike. They turned it on and let it record what it heard.
And what it heard was the heat rejection fluid pump humming along:
Now I know: The sound itself is a little underwhelming. But you have to understand what you're hearing. And why.
Martian winters are extremely cold. It's not exactly Florida there even in the summer, but in the winter it's cold enough to freeze carbon dioxide right out of the air. A lot of equipment on the rover won't work well in that kind of chill, and needs to be heated.
Happily, Perseverance is carrying a chunk of plutonium along with it.
Oh, you didn't know that this sucker is nuclear? Yup. Solar panels work on Mars, but the rover has a lot of moving parts that need more power than solar can provide, so it has what's called a radioisotope thermoelectric generator (or RTG), which uses the decay of plutonium to make electricity. It's very efficient — it generates 110 Watts of power — and lasts a long time. They've been used successfully on many missions to the outer solar system, and were even used by the Apollo missions to the Moon.
It also generates a substantial amount of heat (2000 Watts worth, about the same amount of heat 20 people give off just being alive). So the rover has pipes running through it filled with coolant fluid (CFC-11 and Galden HT-170, if you must know — the chassis and some architecture of Perseverance is the same as for Curiosity). They take heat from the RTG and distribute it through the rover where it's needed. Moving that fluid takes pumps to move it along, and those pumps have parts that spin rapidly, creating the whirrrrrrrr the mike picked up.
So that somewhat underwhelming low, subdued hum you heard? That's actually rapidly spinning parts inside a pump meant to move antifreeze through a one-ton roving nuclear powered mobile chemistry lab that will look for signs of truly ancient life in a dried up lake bed on another planet. While it was still in space on its way.
Now does that sound make more of an impact?
You may wonder why we can hear anything at all, given that the rover is in the vacuum of space. Sound needs a medium to travel through, some sort of substance, and for us on Earth's surface that's usually air. But sound can travel through water and even solid, compressing and rarefying the material it passes through. The pumps are pushing fluid, and the steady vibration of that is transferred through the body of the rover into the microphone. Voilà. Sound.
Oh, incidentally, there's a second microphone on Perseverance, intended to listen in on the SuperCam, an instrument located in the rover's head, which also has a powerful laser that will zap rocks, vaporizing them and causing them to briefly glow. A spectroscope in the camera will then break that light up into colors that will allow scientists to determine what's in the rocks and at what abundances.
So Perseverance is actually a one-ton roving nuclear powered laser-eyed mobile chemistry lab.
Finally, having said all that, did you know that it is possible to have sound moving through space? You just need the space to be not completely empty. In nebulae — gas clouds — the gas is incredibly thin, basically a laboratory grade vacuum, with maybe only a few thousand atoms per cubic centimeter. But when you're a few light years across that makes a lot of cubic centimeters, which adds up to a substantial amount of gas. It's dense enough that very low-frequency (or long wavelength, if you prefer) sound waves can travel through them. These sometimes have visible effects, like when a black hole has carved huge ripples in surrounding gas, or even ripples made in the primordial plasma filling the Universe shortly after the Big Bang (try dropping the phrase baryon acoustic oscillations into your next conversation).
Of course, those are pretty violent events. I guess if you do it loudly enough in space, everyone can hear you scream.
Or, in the case of Perseverance, quietly hum.