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Help Name Two Of Pluto’s Moons

The views expressed are those of the author and are not necessarily those of Scientific American.


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Pluto's currently known moons (Credit: NASA/HST)

Far from the Sun planetary bodies can hold onto many more moons. The latest count for Pluto is five satellites, and the most recent two need names.

Back in 2011 and 2012 it was announced that Hubble Space Telescope observations of the Pluto system had spied first one and then another new candidate moon. For a long time the only known satellite of this distant dwarf planet was the object Charon. With about 12% of Pluto’s mass Charon orbits once every 6 days and 9 hours – close enough to cause the center-of-mass of the system to actually lie outside Pluto’s surface.

But in 2005 astronomers spotted additional satellites, Nix and Hydra. These were inferred to have masses barely 0.03% that of Pluto, mere crumbs about 40 to 60 kilometers across orbiting some 2-3 times further out than Charon. And now we know that there are at least two more moons, currently designated as P4 and P5.

It’s a fascinating example of how the long-term stability of planetary satellites improves with distance from a star. Even though Pluto is a mere 0.00218 times the mass of the Earth, out at this distance from the Sun the gravitational tides that make it hard for inner planets to keep moons are so small that satellites can abound.

It’s not all good news though. P5 was detected because observations were made to help plan for NASA’s New Horizons spacecraft flythrough of the Pluto system in 2015. Going at some 30,000 miles an hour, New Horizons is exceptionally vulnerable to damage from even the tiniest chunks of material. Finding P4 and P5 adds evidence to suggest that Pluto may be surrounded by lots of tiny pieces of stuff – perhaps remnants from a collision between Pluto and a Kuiper belt object that helped form the larger moons.

New Horizons is already so far from Earth that signals take about 4 hours to reach it, so real-time piloting through the Pluto system is not an option. Luckily it may be that just a simple course correction is needed to pass a little further from Pluto than originally planned to minimize the risk of collision.

Regardless of that, tradition has it that the moons P4 and P5 need ‘proper’ names. There’s a mandate from the International Astronomical Union (IAU) that these follow the naming of the rest of the Pluto system – drawing on Greek or Roman mythology about the Underworld. However if good enough alternatives are thought up it’s possible the IAU would be tempted.

And here’s where YOU can play a role. The SETI Institute has just launched a ‘Pluto Rocks’ website where anyone can pitch in to vote for both their favorite classical name choices for P4 and P5, and write-in for something original.

So why not take a little time to help name some of the latest moons in our solar system!

Caleb A. Scharf About the Author: Caleb Scharf is the director of Columbia University's multidisciplinary Astrobiology Center. He has worked in the fields of observational cosmology, X-ray astronomy, and more recently exoplanetary science. His latest book is 'Gravity's Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos', and he is working on 'The Copernicus Complex' (both from Scientific American / Farrar, Straus and Giroux.) Follow on Twitter @caleb_scharf.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. jtdwyer 4:18 pm 02/11/2013

    With Charon providing ~10% of the total system mass, it seems this might should more properly be called the Pluto-Charon dwarf planetary system… BTW, could some of the system’s small moons have only recently been captured? Perhaps some small objects periodically migrate between this system and the Kuiper belt…

    Link to this
  2. 2. George Smith 8:02 pm 02/11/2013

    I love how they tell us not to “vote more than once per day.”

    Link to this
  3. 3. Mark Robinson 10:01 pm 02/11/2013

    “…close enough to cause the center-of-mass of the system to actually lie outside Pluto’s surface.”

    Caleb, that reads as if the closer Charon is to Pluto, the more the c of m will lie outside the surface of Pluto, which is the opposite of what is. I think the main factor contributing to the c of m being above Pluto’s surface is the relatively large mass of Charon when compared with Pluto.

    Link to this
  4. 4. cccampbell38 1:45 pm 02/14/2013

    Name Pluto’s two unnamed moons? That’s easy: Mickey and Minnie.

    Link to this

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