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Pluto: Moons, Debris and New Horizons

When I was a boy, I became fascinated early on with the outer planets. The further out, the better as far as I was concerned, and as you might imagine, I had a special fascination with Pluto. In the summer, I used to haunt the library in the nearby suburb of Kirkwood (in St. Louis, where I grew up), working my way through all the books on astronomy and space I could find. Because I was reading all of them, I would encounter older volumes, some pre-dating the discovery of Pluto, and more recent tomes with details about the planet I didn’t know. It didn’t matter; I just kept reading.

What was fun about all this was that I kept expecting to find something new each time I opened a book, and was sometimes rewarded with a fact that brought this distant realm into perspective. The news that Hubble has now found a fifth moon orbiting Pluto awakens that same sense of satisfaction, for as we keep tuning up our observing skills, we’re learning much about the outer system that surprises us. The fact that this tiny dwarf planet — I would prefer to think of it as a ‘double planet’ more than a ‘dwarf’ — has such an elaborate set of moons is unexpected.

Image: Pluto’s newly discovered moon P5 (circled). Researchers are intrigued that such a small planet can have such a complex collection of satellites. The new discovery provides additional clues for unraveling how the Pluto system formed and evolved. The favored theory is that all the moons are relics of a collision between Pluto and another large Kuiper belt object billions of years ago. Credit: NASA, ESA, and M. Showalter (SETI Institute).

The existence of P5 is another useful piece of information for the New Horizons team as their spacecraft streaks toward its 2015 encounter at Pluto/Charon. The moon, currently designated P5, looks to be between 10 and 25 kilometers across, residing in a 93000 kilometer circular orbit that is evidently co-planar with the other four satellites. With five moons, Pluto is likely home to a good deal more debris that we haven’t yet found, a factor in working out the safest trajectory for the spacecraft. We’ll be watching Pluto carefully up to and beyond the New Horizons encounter.

A Not So Quiet Hibernation

Meanwhile, New Horizons, the hero of this piece, continues its relentless journey, now almost 24 times as far from the Sun as our own planet and once again in a state of hibernation. The ‘deep cruise’ phase of the mission lasts until the encounter operations begin to accelerate in the summer of 2014, with closest approach to Pluto on July 14, 2015. Even though most of its subsystems, including science instruments and flight electronics, are turned off, the Student Dust Counter (SDC) is to be left on during the hibernation period, measuring dust impacts as the spacecraft pushes deeper into the system than any dust detector has ever been sent before.

Yes, the Voyagers can measure dust impacts, but they do so with their plasma wave instrument rather than through an actual dust detector. Both spacecraft have been detecting micron-sized impacts for years, noted because the impact of a dust particle causes it to be vaporized and heated to a plasma of electrons and ions. The resulting plasma cloud creates a voltage pulse in the plasma wave receiver. Researchers can count the impacts over a period of time and learn much about the density of the impacting particles in the outer interplanetary medium.

We need to learn about dust in the outer system, of course, because as we push deeper and move faster small impacts become much larger factors, risking the very life of the mission if we’re talking about interstellar flight speeds. Interestingly, the Voyager dust measurements show the number density of impact particles to be more or less the same for both Voyager 1 and 2, a noteworthy item given that Voyager 2 is much closer to the ecliptic than Voyager 1. That suggests the outer system dust both spacecraft are running into is likely cometary in origin, or at least that it does not originate from a planetary source.

But back to New Horizons. The latest report from the spacecraft team now tells us that the Solar Wind Around Pluto (SWAP) and Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instruments will also remain active during hibernation, representing a major upgrade from the instruments on the Pioneer and Voyager spacecraft. The report quotes Matthew Hill (JHU/APL) on their ability to measure charged particle radiation from the solar wind and elsewhere:

“It’s been more than 30 years since we’ve had a spacecraft venture beyond Saturn, and it’s the first time we’ve had observations from this region while having supporting measurements both farther out [from Voyager 1 and 2] and closer to the Sun [missions at Mercury, Earth and Saturn]. Events associated with solar flares and coronal mass ejections that propagate through the solar wind plasma can now be observed throughout the heliosphere as never before. With solar activity on the rise, the timing is great to have these state-of-the-art New Horizons instruments observing the heliosphere.”

The report is calling the new observations ‘enhanced science’ — the use of SWAP and PEPSSI during hibernation had not been part of the original mission schedule — and the hope is that it will provide a welcome additional dataset as we continue our study of the heliosphere. With the Pluto/Charon encounter ahead, we have much to look forward to, but even after the encounter, we’ll eventually be using the James Webb Space Telescope’s infrared capabilities to probe this icy realm. Who knows how many more moons of Pluto, or Kuiper Belt bodies beyond, we’ll have uncovered by then?


Comments on this entry are closed.

  • Christopher L. Bennett July 13, 2012, 10:06

    While I wouldn’t have guessed Pluto had so many moons, it doesn’t really startle or confuse me. It seems intuitive that there’s just a lot more stuff out in the Kuiper Belt, a lot more small objects to fall into orbit of other objects (or collide with them and splash off moon systems), than there are way down here in the vicinity of Sol and Jupiter.

  • djlactin July 13, 2012, 10:34

    The increasing number of Plutonian (?) moons suggests (to me at least) a ring system. NH beware!

  • ljk July 13, 2012, 12:40

    Imagine what we would already know about Pluto if Voyager 1 had been sent there to flyby in 1988 as originally planned, instead of flying by Titan in 1980 and hoping for a break in the clouds.

    As I recall with Cassini when it finally started showing Titan’s surface, scientists were initally confused about what they were seeing. A few glimpses from Voyager 1 even if it could see through breaks in the clouds would have been just as confusing at the time.

    And yes, I know that Voyager 1 actually did see the moon’s surface at certain wavelengths crudely, but that was years after its flyby through the Saturn system.


    SWAP and PEPSSI – Someone on the New Horizons team obviously had fun naming the instruments.

  • stephen July 13, 2012, 12:43

    If Pluto had a ring, would we have detected it by now? What are the parameters for how small a body can have a ring, and how long it would last? Maybe, instead of a ring of dust and/or ice, it might have dozens of tiny moons in very close orbits, some or all co-orbitals. How long would that situation last?
    Considering that we only found Pluto 80 years ago, maybe Pluto didn’t even exist, per se, a few hundred or a few thousand years ago! Maybe it didn’t obtain its moons until a hundred years ago. I have no idea if that’s likely?
    As soon as you mentioned PEPSSI, I wondered if the PepsiCo be persuaded to do a sponsorship and publicity campaign?

  • tom baty July 13, 2012, 15:15

    Does this mean we are the PEPSSI generation???? Speaking of names..nothing of importance…. but wonder if anyone has heard of any suggested names for these newly found moons.

  • william July 13, 2012, 17:43

    Speaking about the new fifth moon the Pluto, do you Paul, happen to know if there has been any adjustment in the trajectory for the New Horizons craft so that it may avoid/close encounter this are any of the other moves that they have found around this planet?
    As an aside, are you Paul going to be attending the Houston symposium?

  • Paul Gilster July 13, 2012, 19:58

    william, I know of no trajectory changes to New Horizons at this point. And yes, I do plan to be in Houston for the symposium.

  • Laurel Kornfeld July 13, 2012, 23:32

    The New Horizons team is still searching for additional moons and rings that could require a trajectory change.

    I too am fascinated with the outer planets, and I consider dwarf planets to be a subset of planets regardless of the controversial decision by four percent of the IAU. Pluto-Charon would be a binary dwarf planet system. Interestingly, the IAU definition makes no allowance for binary planet systems because by definition, neither planet in a binary system has “cleared its orbit” of its companion.

    What is the Houston symposium, and when is it happening?

  • andy July 14, 2012, 8:44

    Wonder if there is a similar system of circumbinary satellites at (90482) Orcus, a.k.a. the “anti-Pluto”. On the other hand, it is not so clear whether Vanth formed from a collision (as is thought to be the case with Charon), or if it is a captured object.

  • Paul Gilster July 14, 2012, 9:07

    Laurel Kornfeld writes:

    What is the Houston symposium, and when is it happening?

    Last year the 100 Year Starship conference was held in Orlando. This year, with the award having been given to Mae Jemison’s Dorothy Jemison Foundation for Excellence (and Icarus Interstellar was a partner in the winning proposal), a second symposium will be held in Houston in September. You can get all the details here:


    There are a lot of articles in the database here about the 100 Year Starship concept, but the site above should answer most questions. Here’s my report from last year’s conference:


  • Denver July 14, 2012, 10:02

    Names for P4 and P5?

    Why, there is only one choice, Pixie and Dixie, giving us Pix, Dix, Nix, Hydra and Charon. If we have to stay within Roman myth, Typhon and Cerberus.

  • Ken_Space July 14, 2012, 11:19

    In response to stephen, the 2nd S (Science) in PEPSSI is surely pointless, so I assume was deliberately put in, perhaps because Pepsi wouldn’t give them any (or enough) money for it. I wonder if NASA would be open to such sponsorship?
    The next comment (tom baty) seems to link it (inadvertantly?) with potential names for P4 & P5 – I assume the IAU wouldn’t want to go down that path!

  • ljk July 16, 2012, 17:35

    Fifth Moon Discovered Orbiting Pluto

    Image Credit: NASA, ESA, Mark Showalter (SETI Institute)

    Explanation: A fifth moon has been discovered orbiting Pluto. The moon was discovered earlier this month in images taken by the Hubble Space Telescope in preparation for the New Horizons mission’s scheduled flyby of Pluto in 2015.

    Pictured above, the moon is currently seen as only a small blip that moves around the dwarf planet as the entire system slowly orbits the Sun. The moon, given a temporary designation of S/2012 (134340) 1 or just P5 (as labeled), is estimated to span about 15 kilometers and is likely composed mostly of water-ice.

    Pluto remains the only famous Solar System body never visited by a human-built probe and so its origins and detailed appearance remain mostly unknown. [Pluto could have been visited by Voyager 1 had NASA opted to do so in 1988. Instead it decided to flyby Titan, where the hoped-for breaks in the moon’s clouds did not exist. LJK]


  • ljk July 25, 2012, 17:32

    Kavli Meets Kuiper

    Two decades later, three scientists are rewarded for discovering a new body of objects in our solar system.

    By Heather Goss

    AirSpaceMag.com, July 25, 2012

    Nearly 20 years ago, a Massachusetts Institute of Technology professor and his graduate student embarked on a project to find new objects in the outer solar system, motivated by the call to arms behind so many scientific discoveries: “If we don’t do it, who will?”

    Each year, the Kavli Foundation gives awards to scientists making “fundamental contributions” in their field of study. In 2012, David Jewitt, the former MIT professor now at the University of California, his former student Jane Luu, and Michael Brown of the California Institute of Technology were jointly awarded the $1 million Kavli Prize in astrophysics “for discovering and characterizing the Kuiper Belt and its largest members, work that led to a major advance in the understanding of the history of our planetary system.”

    Air & Space spoke with Luu, who has moved on from the world of observational astronomy and now works on laser radar systems at MIT’s Lincoln Laboratory. She told us about studying the Kuiper Belt, what it means for demoted planet Pluto, and what we still hope to learn.

    Full article here:


  • ljk October 18, 2012, 10:16

    Will New Horizons have to make evasive manuevers when it encounters the Pluto system in 2015?


    Maybe it was a good thing that Voyager 1 did not flyby Pluto in 1988 after all, at least for the sake of the probe.

    This all begs the question, just how much debris is there in the Kuiper Belt? Is it like the main Planetoid Belt between Mars and Jupiter where the perceived hazards such as vast fields of dust particles were found to be generally non-existent? Or do these oversized comets generate enough debris to make flying through the belt, especially near a KBO, hazardous for a spacecraft?

    So far the twin Voyagers seem to have avoided such problems, but perhaps we are not deep enough into the Kuiper region yet? And what about the Oort Cloud? Hopefully space is big enough to keep things wide enough apart, but so far we can only see the biggest objects out that way.

    This should be another task of the Innovative Interstellar Explorer (IIE) probe concept: To see if an interstellar vessel can get through the Sol system comet belts unscathed. Interesting if this is a possible reason why we have seen no definite sign of visitors from the rest of the galaxy due to our comet belts inadvertently shielding us from their probes. Is this the situation for all solar systems?

    Or have I seen The Empire Strikes Back too many times? Yes, I know that was planetoid field, but you get the point.

    Maybe it’s those giant worms that live in planetoid craters and snack on passing starships….