Hey, hey everyone! Happy New Year! It’s been a tough year for lots of us, for lots of reasons, but I hope no matter how your 2016 was, your 2017 will be better.
Tonight is also the 8th night of Hanukkah, the last night, so we’re at number 1 on our list of the best moons of 2016! Let’s run them down again if you’re just tuning in:
- Phobos (at Mars).
- The Moon (Earth)
- Miranda (Uranus)
- Dysnomia (Eris)
- Neso (Neptune)
- Io (Jupiter)
- Titan (Saturn)
It must be hard to beat out Titan. When you think about all the news coming in about various moons around the solar system, there was really only one that I kept turning to over and over again. Only one that made my jaw drop more than any other. Only one, surprisingly enough (assuming you haven’t read the title of this post yet), as great as so many other places are. As much as I’ve learned by researching the list, there’s only one it could be.
In July of 2015 our piano-sized space probe friend New Horizons made its way to within 7,800 miles of Pluto as it sped through the Pluto system at over 30,000 miles per hour. Since then, it’s been returning a huge amount of new information about the formerly smallest planet. Depending on how you count, it’s now the biggest dwarf planet, even though Eris is heavier.
Pluto has five moons: Styx, Nix, Hyrdra, Kerberos, and the biggest of them all, Charon. Charon and Pluto are an interesting pair, unlike any others in the solar system. They’re very close in size as these things go. Charon is about half the size of Pluto. Also, they’re only about 12,000 miles apart. This relationship does a couple of interesting things.
First, they’re tidally locked. You know how when you look up at the Moon in our sky, you always see the same side of it facing us? Over time, the Earth’s gravity has slowed the Moon’s rotation so that one day on the Moon is the same length as its orbital period. While we always see the same side of the Moon from Earth, the Earth rotates in the lunar sky; Alan Shepard saw the face of the Earth change during the time he was walking around the surface, hitting golf balls off into the regolith and stuff. In the case of Pluto and Charon, this has happened, too, but there, they’ve slowed and become tidally locked to each other. They both always see the same face of each other in their skies. They orbit the Sun together like a giant, wobbling dumbbell.
But wait, there’s more. When two things orbit each other, they don’t actually orbit each other. Um. It’s true. Check this out.
The Moon doesn’t actually orbit the Earth; the two both orbit a center of mass called the barycenter. In our case, and it’s similar nearly everywhere else in the solar system, the barycenter is within the Earth. So, the Moon is going around the Earth, which is where the barycenter is, but the Earth is also revolving slightly, pulled on a bit by the Moon’s gravity. It’s in a bit of a wobbly orbit around itself.
Since Charon is so big and has so much gravity relative to Pluto, their barycenter is actually outside the surface of Pluto. So, they’re both orbiting it, orbiting each other, revolving, turning slowly around that shared point off in space once every seven Earth days or so. I like imagining what that must look like, seen from a distance. I wonder what it must be like to watch them loop around each other over and over, like two old lovers spending another New Year’s Even on the dance floor, with the Sun nothing more than a bright light among the rest, shining on them from over three billion miles away. This relationship is so strange that lots of people, though not the International Astronomers’ Union (IAU), have come to think of the two, not as a primary (Pluto) and satellite (Charon), but as the Pluto-Charon binary planet.
Orbital mechanics aside, Charon has a lot going on on its own. Would you be surprised if I told you it was covered with water ice? It seems like the more and more places we explore the solar system, the wetter the solar system becomes. Wouldn’t it be something if… nope… I’m not going to say it, but wouldn’t it? It also has a bright red cap, which, earlier this year, was determined to be there because Charon’s gravity is able to grab onto escaping methane from Pluto’s atmosphere, which then freezes and sticks to Charon.
Then, there’s the long canyon near Charon’s equator, the spectacularly named, Argo Chasma. Argo is hundreds of miles long and deeper than the Earth’s Grand Canyon. Cliffs, and gorges. Remember what I was saying about Miranda the other day? There’s more of that here, too.
This system is so strange and gorgeous that it always makes me think of how sometimes, even though we think we know enough about something, there’s always more. Years ago, we knew there was nothing farther from the Sun than Pluto, and we were wrong. Who knows what we’ll find out next? Charon seems to be one of those places. We’ve only visited the Pluto system once, but we have visited it. With that, we’ve visited all of the objects in the solar system that have at one time been called planets, and have probes in action at three of them plus the Moon. Crewed missions or not, this is a great time for space exploration, and, to me, Charon’s a bright symbol of this golden age.
Here’s to Charon, the best, most interesting and inspiring moon of 2016!
Thanks for reading along this week. I hope you’ve enjoyed the list and have learned a few things. What do you say? Were there any moons I missed? Any better places you like?
Have a happy, healthy and safe new year, and thanks for reading along all this year. Thank you, deeply, for making this website part of your life, and I hope you will next year, too. Here’s to a great 2017! Happy new year, and clear skies, everyone!