Styx, Nix, and Tango - The Dance of Pluto's Moons


Four years ago I was in Buenos Aires as part of UT Austin's Business Honors Program summer abroad in Argentina, and today's article was inspired by one of my friends on that trip! One of my classes was called "Organizational Management," and I found it excruciatingly boring, so much so that rather than waste my time taking notes, during lectures I preoccupied myself with something far more exciting. It was June 2015, and the New Horizons probe was just a month away from its historic encounter with Pluto, so day in and day out I was reading articles on everything I could find about the wayward dwarf planet. Lo and behold, my classmates sitting behind me were so perplexed by the nerdy esoteric babble on my laptop screen, finally one of them straight up asked, "I'm sorry, but I'm just so curious. Do you have some sort of obsession with the moons of Pluto?" Why yes, yes I do, thank you very much, hehe!

Pluto and its moons to scale! See the next image for what the 4 smaller moons actually look like

For some reason, the only discussion people seem to find worthwhile about Pluto is whether it's a real planet (as it was traditionally known) or a dwarf planet, since it was reclassified in 2006. I personally find the debate a little sophomoric - can't we all agree it's a dwarf planet and instead focus on the way cooler things Pluto has to offer?!

These are the very best images of Pluto's smaller moons that New Horizons was able to capture as it whizzed by

For starters, imagine living on a world where you have an atmosphere for only part of the year. When Pluto's highly elliptical orbit takes it somewhat closer to the sun, its surface thaws and allows nitrogen and methane to billow out and cover the dwarf planet in a thin haze of hydrocarbons. But when Pluto travels back away from the sun, the temperature plunges so much (I'm talking -370 degrees Fahrenheit) that the atmosphere literally freezes solid and rains back down onto the surface!

A super HD image of Pluto's atmospheric haze, taken just 15 minutes after New Horizons' closest approach

But even more bizarre than Pluto itself are the moons that surround it. Charon is by far the most notable; at just over half the diameter of Pluto, it's the largest satellite in the solar system relative to its parent body, and some people even consider the Pluto-Charon system a binary system, rather than a satellite orbiting a central body, since the two orbit around a mutual barycenter. What's more, Pluto and Charon are the only instance of mutual tidal locking in the solar system, where the same hemisphere of Pluto permanently faces the same hemisphere of Charon while the other sides never coincide. Said differently, if you lived on the side of Pluto not facing Charon, you'd spend an eternity never knowing you were being orbited by a cosmic companion! (To learn more about tidal locking, check out my previous article here)

Pluto and Charon's mutual tidal locking is unlike the Earth-Moon system, where only the Moon is tidally locked

The four remaining moons are tiny, and they revolve around the Pluto-Charon center of gravity in an absolutely chaotic dance (almost as out of control as me at the tango class I went to in Argentina!). As this great article explains, the wild motion occurs because the presence of the much larger Pluto and Charon oscillating back and forth continuously disturbs the gravitation field that pulls on them, sending Styx, Nix, Kerberos, and Hydra tumbling through their orbits. If you lived on one of these moons, you'd have no idea when the Sun would rise each morning, or from which direction!

Computer animation of the chaotic spin of Nix from HubbleESA, showing the unpredictable changes in reflected light as it orbits Pluto-Charon

Pretty cool stuff if you ask me! Who knew Argentina was the place to learn about Plutonian astrophysics?! And I know Astronomical Returns isn't a travel blog, but I highly recommend visiting - you won't find better pizza, steak, or wine anywhere else on Planet Earth

There's me! (back row, farthest to the left) - June 2015




1 comment

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