Pluto’s Moons Wrestle With Chaos


Simulation of the moon Nix which orbits Pluto every 25 days. Here that’s sped up to one orbit every two seconds, so we can see its chaotic spin more clearly.

Little moons in the solar system have it tough. Some were once free-roaming asteroids until they were captured by the gravity of a planet. Others, because of their small size and irregular shape, get pushed around in unpredictable ways by their parent planet.

Tumbling end-over-end like a dropped baton in a relay race, it’s fascinating to watch Pluto’s moon Nix wobble about in a state of orbital chaos. That’s what scientists call it. Chaos. It means no matter how many times you solve the equation, you will never predict with certainty how a body will spin. Analysis of data from NASA’s Hubble Space Telescope shows that two of Pluto’s moons, Nix and Hydra, wobble unpredictably.

This set of computer modeling illustrations of Pluto’s moon Nix shows how the orientation of the moon changes unpredictably as it orbits the “double planet” Pluto-Charon. Credits: NASA/ESA/M. Showalter (SETI)/G. Bacon (STScI)
An Idaho spud if there ever was. This set of computer modeling illustrations of Pluto’s moon Nix shows how the orientation of the moon changes unpredictably as it orbits the “double planet” Pluto-Charon.
Credits: NASA/ESA/M. Showalter (SETI)/G. Bacon (STScI)

If you lived on either, you’d never know when and in what direction the Sun would rise. I don’t know if that would be fun or frustrating. Definitely a challenge for the old biological clock. Imagine if the Sun rose in the east  on your way to work one day and then rose in the west on the next. Or if it stayed up 5 hours today and 12 tomorrow. I have to agree. Chaos.

Nix and its fellow small moons Kerberos, Styx and Hydra wobble because they’re embedded in the strong-arm gravity field of the Pluto-Charon duo. I say duo because Pluto and Charon are similar enough in size to be considered a “double planet”. Charon’s 750 mile (1,212 km) girth is half as big as Pluto’s.

Charon is large compared to Pluto, so the orbit about their common center of gravity located in the space between the two bodies. Credit: Wikipedia
Charon is large compared to Pluto, so they orbit about their common center of gravity located in the space between the two bodies. Credit: Wikipedia

As the duo dances an orbital duet about their common center of gravity, their variable gravitational field causes the smaller moons to tumble erratically. The effect is enhanced by their irregular and elongated shapes. These diminutive satellites have company. Saturn’s moon, Hyperion, which we visited in a recent blog, also tumbles chaotically.

The discovery was made by Mark Showalter of the SETI Institute and Doug Hamilton of the University of Maryland using the Hubble Space Telescope and published in today’s issue of the journal Nature. 

Pluto (upper right) and its largest moon Charon form a "double planet" as seen in this photo taken by NASA's New Horizons probe which is set to make a close flyby of the Pluto system on July 14. Credit: NASA / NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute
Pluto (upper right) and its largest moon Charon form a “double planet” as seen in this photo taken by NASA’s New Horizons probe which is set to make a close flyby of the Pluto system on July 14. Credit: NASA / NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute

That’s not all. The two gents also determined that Kerberos is as dark as the charcoal briquettes you use to grill your burgers and brats, while the other moons reflect brightly like beach sand. That’s because they’re mostly made of ice. So what makes Kerberos different? Astronomers and you and me want to know.

This illustration shows the scale and comparative brightness of Pluto’s small satellites. The surface craters are for illustration only and do not represent real imaging data. Credits: NASA/ESA/A. Feild (STScI)
This illustration shows the scale and comparative brightness of Pluto’s small satellites. The surface craters are for illustration only and are not real.
Credits: NASA/ESA/A. Feild (STScI)

So how did all these little moons get to orbit an object that’s so small it’s not even considered a planet? It’s thought that a collision between the dwarf planet and a body of similar size happened over 4 billion years during the solar system’s infancy. The moons are leftover shards from the crack-up that eventually took up orbits around the present-day Pluto. Outside of Charon, the biggest piece, the other four moons are only a few tens of miles across. What do you bet there are more there for the finding. Small and reflecting the light of the remote Sun, they remain hidden for now. Perhaps New Horizons will find them when it whizzes by the Pluto system on July 14.

10 Responses

  1. caralex

    Bob, how stable is Nix? What prevents it from tumbling and crashing into another moon, or into Pluto itself? There must be some degree of stability even in such instability. How does it work?

    1. astrobob

      Carol,
      Like Hyperion, it’s orbit is fixed, but the moon itself is tumbling and wobbling as it orbits.

  2. Hi Bob,

    When do you suppose we will see a clear image of Pluto? We are into June now and I thought we would be counting crators by now.

    1. astrobob

      Hi David,
      As of today, we’re still 30 million miles from Pluto. That’s about the same distance Earth is from Venus when our two planets are closest. At that time Venus is about 1 arc minute across as seen from Earth. Since Pluto is more than 5 times smaller, it’s only around 10 arc seconds in diameter right now So we’re still looking at a tiny dot just 0.55% as big as the moon. No way to see crater yet just broad regional features. We won’t really start to see clear features on Pluto until about July 7.

  3. caralex

    Thanks, Bob. But will it always remain stable, or does that tumbling and wobbling gradually degenerate into instability in its orbit?

    1. astrobob

      Carol,
      Great question. From what I understand, Hyperion’s orbit has been primarily shaped and stabilized by its interaction with Titan, but its chaotic spin is subject to change. Here’s a passage from a book that provides more details: http://bit.ly/1GoAegQ

      1. caralex

        Oh, wow! Looks like a fascinating book! Thanks for that link, Bob. I think I’ll buy it. 😀

        (On the subject of the glitches on your blog – I have to fill in my name and e-mail address EACH time I post now – I never had to do that before. Also, when I type a reply, it vanishes – it doesn’t show up as a post ‘awaiting moderation’, as it used to. Just so you know!)

        1. astrobob

          Hi Carol,
          Thanks for telling me about the problem. I’m going to forward your comments to admin.

          1. caralex

            You’re welcome, Bob. The need to fill in my name and e-mail address happened on one other occasion – when you changed your blog format a few weeks ago. Then it was resolved. It’s been happening again, since your glitch a few days ago, when the ability to comment wasn’t available.

            The comment ‘vanishing’ after posting, happened on one other occasion too – again, when you had the ‘comments unavailable’ glitch I mentioned above.

          2. astrobob

            Carol,
            Another person is having the same problem. I sent yours along earlier today. I hope they can fix it. Sounds annoying.

Comments are closed.