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Best Glimpse Yet of Nix and Hydra

Nix and Hydra

Thirty years ago it was all but impossible to tease the presence of Charon out of the Pluto images available to astronomers. Today we’re using ground-based telescopes like the twin Keck instruments on Mauna Kea (Hawaii) to see the far tinier Nix and Hydra, the minute satellites discovered by the Hubble Space Telescope in 2005. Making Nix and Hydra visible results in Pluto and Charon, far brighter objects, appearing as a bright blob in the image at left. The streaks are the result of Pluto’s motion against background stars during the exposure.

Image (above): Nix and Hydra. This image combines all 16 exposures taken at Keck, with the contrast adjusted to show Pluto’s new satellites Nix (left) and Hydra (right) as the small dots in the upper right. Both Nix and Hydra are about 5000 times fainter than Pluto, thus both Pluto and Charon are washed out in the image. The Pluto system moved with respect to the background stars during the one hour of observations, leaving the stars trailed. Credit: David Tholen.

This is precision work indeed — Nix and Hydra are each less than 100 kilometers in diameter. At magnitude 23.5, they are far less bright than Pluto itself (14th magnitude). Moreover, Keck’s adaptive optics (which compensates for the atmospheric turbulence that can blur the light of distant objects) is going to be used for a continuing set of observations. David Tholen (University of Hawaii) has this to say:

“It is our intent to obtain several more images of the Pluto system, hopefully with this same level of quality, so that we can track Nix and Hydra completely around Pluto several times. By making extremely precise measurements of the satellites’ positions, we will determine their masses by detecting the tiny displacements caused by their mutual gravitational attraction. Once the masses are in hand, we’ll be able to say something more definitive about how big these new satellites are.”

Pluto and Charon

Which is useful in itself, but even more valuable in terms of the New Horizons mission, now making its way to the Pluto/Charon system for a 2015 encounter. The New Horizons team needs all the information it can get about the motion of these satellites well in advance of the flyby, and thus far Keck has delivered. The Keck image at left shows Pluto and Charon themselves, the contrast now optimized in ways that wash out Nix and Hydra. We’ll be seeing far better images from the spacecraft, of course, but the improvement in Earth-bound viewing in the past thirty years is still amazing.

Comments on this entry are closed.

  • andy October 19, 2007, 10:33

    Regarding these new images, I wonder whether they further constrain the existence of a possible ring system. (Theory goes that micrometeorite impacts on Nix and Hydra would blast material off the moons, generating a ring system)

  • ljk December 11, 2007, 11:13

    Masses of Nix and Hydra

    Authors: David J. Tholen, Marc W. Buie, William M. Grundy, Garrett T. Elliott

    (Submitted on 8 Dec 2007)

    Abstract: A four-body orbit solution for the Pluto system yields GM values of 870.3 +/- 3.7, 101.4 +/- 2.8, 0.039 +/- 0.034, and 0.021 +/- 0.042 km3 sec-2 for Pluto, Charon, Nix, and Hydra, respectively. Assuming a Charon-like density of 1.63 gm cm-3, the implied diameters for Nix and Hydra are 88 and 72 km, leading to visual geometric albedos of 0.08 and 0.18, respectively, though with considerable uncertainty. The eccentricity of Charon’s orbit has a significant nonzero value; however, the 0.030 +/- 0.009 deg yr-1 rate at which the line of apsides precesses is insufficient to explain the difference in the longitude of periapsis seen in the orbits fitted to the 1992-1993 and 2002-2003 data sets. The mean orbital periods for Hydra, Nix, and Charon are in the ratios of 6.064 +/- 0.006 : 3.991 +/- 0.007 : 1, but we have not identified any resonant arguments that would indicate the existence of a mean motion resonance between any pairs of satellites.

    Comments: 14 pages, 6 figures, 6 tables

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0712.1261v1 [astro-ph]

    Submission history

    From: David Tholen [view email]

    [v1] Sat, 8 Dec 2007 07:26:14 GMT (394kb)


  • ljk February 22, 2008, 0:07

    The Effect of Charon’s Tidal Damping on the Orbits of Pluto’s Three Moons

    Authors: Yoram Lithwick, Yanqin Wu

    (Submitted on 20 Feb 2008)

    Abstract: Pluto’s recently discovered minor moons, Nix and Hydra, have almost circular orbits, and are nearly coplanar with Charon, Pluto’s major moon. This is surprising because tidal interactions with Pluto are too weak to damp their eccentricities.

    We consider an alternative possibility: that Nix and Hydra circularize their orbits by exciting Charon’s eccentricity via secular interactions, and Charon in turn damps its own eccentricity by tidal interaction with Pluto. The timescale for this process can be less than the age of the Solar System, for plausible tidal parameters and moon masses. However, as we show numerically and analytically, the effects of the 2:1 and 3:1 resonant forcing terms between Nix and Charon complicate this picture. In the presence of Charon’s tidal damping, the 2:1 term forces Nix to migrate outward and the 3:1 term changes the eccentricity damping rate, sometimes leading to eccentricity growth. We conclude that this mechanism probably does not explain Nix and Hydra’s current orbits. Instead, we suggest that they were formed in-situ with low eccentricities.

    We also show that an upper limit on Nix’s migration speed sets a lower limit on Pluto-Charon’s tidal circularization timescale of greater than 10^5 yrs. Moreover, Hydra’s observed proper eccentricity may be explained by the 3:2 forcing by Nix.

    Comments: 12 pages, submitted to ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.2939v1 [astro-ph]

    Submission history

    From: Yoram Lithwick [view email]

    [v1] Wed, 20 Feb 2008 21:50:17 GMT (656kb,D)


  • ljk February 22, 2008, 0:10

    On the Origin of Pluto’s Minor Moons, Nix and Hydra

    Authors: Yoram Lithwick, Yanqin Wu

    (Submitted on 20 Feb 2008)

    Abstract: How did Pluto’s recently discovered minor moons form? Ward and Canup propose an elegant solution in which Nix and Hydra formed in the collision that produced Charon, then were caught into corotation resonances with Charon, and finally were transported to their current location as Charon migrated outwards.

    We show with numerical integrations that, if Charon’s eccentricity is judiciously chosen, this scenario works beautifully for either Nix or Hydra. However, it cannot work for both Nix and Hydra simultaneously. To transport Nix, Charon’s eccentricity must satisfy e_C less than 0.024; otherwise, the second order Lindblad resonance at 4:1 overlaps with the corotation resonance, leading to chaos. To transport Hydra, e_C greater than 0.7 R_p/a_C greater than 0.04; otherwise migration would be faster than libration, and Hydra would slip out of resonance. These two restrictions conflict.

    Having ruled out this scenario, we suggest an alternative: that many small bodies were captured from the nebular disk, and they were responsible for forming, migrating and damping Nix and Hydra. If this is true, small moons could be common around large Kuiper belt objects.

    Comments: 11 pages, submitted to ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.2951v1 [astro-ph]

    Submission history

    From: Yoram Lithwick [view email]

    [v1] Wed, 20 Feb 2008 21:50:11 GMT (1769kb,D)


  • ljk March 11, 2008, 15:48

    In 1962, Robert Forward made some very interesting
    speculations about Pluto, back when most people still
    thought it was a planet, if a bit unusual compared to
    the rest of the Sol system:


  • Adam March 12, 2008, 4:58

    Hi Larry

    He wrote that for “Astounding/analog” under a pseudonym. I had read it in one of his much later anthologies and nearly fell over backwards when I found it in one of my old SF magazines in original form. I didn’t realise how old it actually was, and yet very fresh conceptually. Shame no one ran with his idea in story form.

  • Administrator March 12, 2008, 7:16

    Adam, I’ve got that issue around here some place but it would probably take me a week to find it. Can you tell me the pseudonym that Forward used? I am (very) gradually assembling Forward information for a possible biography down the road. Thanks!

  • ljk March 14, 2008, 10:29

    Robert Forward’s pseudonym for that 1962 article was
    George Peterson Field.

  • ljk May 23, 2008, 12:35

    Ejecta Exchange, Color Evolution in the Pluto System, and Implications for KBOs and Asteroids with Satellites

    Authors: S. A. Stern

    (Submitted on 22 May 2008)

    Abstract: We examine the ability of impacts by Kuiper Belt debris to cause regolith exchange between objects in the Pluto system. We find that ejecta velocities from KB impacts are too low to escape from Pluto and Charon. However, ejecta can escape Nix and Hydra, and is capable of covering one another to depths as high as 10s of meters, and Charon and Pluto, perhaps to depths up to several 10s of cm.

    Although Pluto’s annual atmospheric frost deposition cycle will cover such imported debris on timescales faster than it is emplaced, no such masking mechanism is available on Hydra, Nix, and Charon. As a result, ejecta exchange between these bodies is expected to evolve their colors, albedos, and other photometric properties to be similar. We examined the ability of ejecta exchange to work for other Kuiper Belt binaries and found the process can be effective in many cases. This process may also operate in asteroid binary systems.

    Comments: 8 pages, 3 tables, 0 figures

    Subjects: Astrophysics (astro-ph)

    Report number: LPI Contribution 413

    Cite as: arXiv:0805.3482v1 [astro-ph]

    Submission history

    From: S. Alan Stern [view email]

    [v1] Thu, 22 May 2008 15:05:31 GMT (173kb)


  • ljk January 20, 2009, 23:43

    January 17, 2009

    Naming Pluto (Review)

    Written by Ian O’Neill

    Naming Pluto explores the chain of events that lead to Pluto’s naming and in 2007 sees Venetia Phair viewing Pluto for the very first time through a telescope, on her 89th birthday, 77 years after Pluto’s discovery. A wonderful, intimate look into the story behind how Pluto got its name. A review of the short film directed and produced by Ginita Jimenez, distributed by Father Films.

    In recent years, Pluto has seen its status change from being a planet to what many people view as a planetary underclass. The reasons behind this have been set out by the International Astronomical Union (IAU) to cater for the increasing number of Solar System bodies being discovered; the traditional nine planets have had to make room for a growing minor planet population.

    Unfortunately, Pluto was at the front line as it inhabits a region of space dominated by the gas giant Neptune, plus thousands of other Kuiper belt objects. Although the mysterious body lost its planetary status (as it does not have the ability to “clear its own orbit”), it has taken the title of “dwarf planet” and now has an entire class of object named in its honour: “Plutoids”.

    However, the recent tumultuous history of the traditional “9th planet” has not impacted the fascination we have for Pluto. It has, and always will be, viewed with intrigue and wonder.

    Full article here: