June 5 – How High The Moons

Today’s factismal: Pluto’s moons are not shaped like footballs.

Odds are you’ve seen the press release or read some reporter’s gloss of it. This week the folks at the Space Telescope Science Institute (the home of the Hubble Space Telescope) announced that they had exciting and interesting news: Pluto’s moons were like badly behaved children that ran all over the system without rhyme or reason. But, of course, the part of the press release that caught the attention of the reporters was this sentence: “This torque is strengthened by the fact the moons are football shaped rather than spherical”.


The artist's deception of what the moons of Pluto might look like (Image courtesy NASA)

The artist’s deception of what the moons of Pluto might look like
(Image courtesy NASA)

Like all good science reporting, the sentence is both true and false. (Schroedinger started as a news reporter.) It is true in that the moons of Pluto are not nice, spherical orbs like ours. Instead, they are crazy, elongated piles of rock that have been pulled like taffy between the competing gravity fields of Pluto and Charon. But that doesn’t even begin to describe how wonderfully odd these piles of rock are. Instead of being neat pill-shaped objects flipping about Pluto’s system, they look more like a three year old’s art project made up by slapping small bits of clay together.

And that’s because that’s what they are. With the exception of Charon, which is about 745 miles across, all of Pluto’s moons are tiny little things. Styx is the smallest known moon of Pluto (we expect to discover more as New Horizons closes in); it is a mere 17 miles across at most. Next comes Kerberos, which is 19 miles across. Then there are the two oddballs, Nix and Hydra, which are the most elongate and the largest of the small moons at about 35 miles across. Put another way, you could hike Pluto’s small moons in just about two days (assuming you could get your tent pitched in the 0.0001 g). So how did Pluto’s moons get so weird? And why do we call them Pluto’s moons instead of “the moons of Pluto and Charon” (which would be more accurate)?

A slightly more realistic view of what Pluto's moons probably look like (Images courtesy NASA, assembled by me)

A slightly more realistic view of what Pluto’s moons probably look like
(Images courtesy NASA, assembled by me)

Pluto’s moons are weird for the same reason that Charon exists. The current best hypothesis for the moons of Pluto is that at some time in the distant past Pluto has a “Big Whack”. A large body almost as big as Pluto ran into the proto-planet, splitting it into lots of pieces. The largest bits coalesced into Pluto. Another large bunch became Charon. and the left over pieces glommed together into at least four “rubble piles”. These heaps probably resemble what we planetology geeks call “rubble pile” asteroids (which is why I used Ida, Phobos, 25143 Itokawa, and an un-named asteroid for the models) with lots of little rocks loosely piled together.

Of course, we won’t know for sure what the moons look like until New Horizons zooms past Pluto. But what we do know is that things are just going to get more interesting. The pictures will rapidly get much better right up to the point when New Horizons whizzes by Pluto on July 14. And that’s where the citizen science comes in. Because the data will be coming in hot and heavy during the few days when the probe is close enough to get a good view of the planet and its moons, the scientists are lining up the names for those things now – and they want your help! They have set up a web form for us to vote on various names for Pluto’s features and to propose names of our own. Of course, even if a name is popular, that’s no guarantee that it will be used; the IAU (the folks who don’t know what a planet is) overruled the name picked by the discoverer of Pluto’s fifth Moon (Vulcan) for one of their own (Kerberos). But it will still be fun to name the features and learn more about this amazing planet as New Horizons makes a fifty-year old dream come true in less than a month. For more information, zoom on over to:

2 thoughts on “June 5 – How High The Moons

  1. Pingback: July 14 – Calling All Planets | Little facts about science

  2. Pingback: July 19 – Heart of Brightness | Little facts about science

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