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— CH. 1 · DISCOVERY AND NAMING —

Hydra (moon)

~4 min read · Ch. 1 of 5
5 sections

  • On the 15th of May 2005, astronomers using the Hubble Space Telescope spotted faint points of light near Pluto. These images captured two new moons that had never been seen before. Max J. Mutchler identified one moon on the 15th of June 2005 while Andrew J. Steffl found the other on the 15th of August 2005. The team announced their findings publicly on the 31st of October 2005 after verifying archival data from 2002. They gave provisional designations S/2005 P 1 and S/2005 P 2 to these celestial bodies. Scientists named the larger of the pair Hydra after a nine-headed serpent from Greek mythology. This name choice honored Pluto's former status as the ninth planet in our solar system. The International Astronomical Union officially approved the name on the 21st of June 2006 through IAU Circular 8723. The initials N and H for the two moons were selected to honor the New Horizons mission. Features on this moon must relate to legendary serpents or dragons according to naming conventions.

  • Hydra measures approximately 42 kilometers along its longest axis and about 30 kilometers across its shortest dimension. Its surface displays a neutral spectrum similar to other small moons orbiting Pluto. Water ice covers much of the terrain, creating a highly reflective exterior. The geometric albedo reaches 83 percent, making it one of the brightest objects in the outer solar system. This high reflectivity suggests the presence of relatively pure water ice without significant darkening. Micrometeorite impacts may constantly refresh the surface by ejecting darker material away. The spectral analysis shows a slightly bluish tint compared to the neighboring moon Nix. Crater counting data indicates the surface is roughly four billion years old. Large indentations suggest the moon lost some original mass during ancient impact events. The irregular shape contributes to how sunlight reflects off different facets throughout its rotation.

  • Unlike most large moons that keep one face toward their planet, Hydra rotates chaotically. Its rotational period was approximately 10 hours at the time of the New Horizons flyby. The rotational axis tilted about 110 degrees relative to its orbital plane during that encounter. This sideways spin causes the moon to tumble unpredictably over astronomical timescales. Gravitational pulls from both Pluto and Charon drive this complex motion. The irregular shape creates torques that amplify the tumbling effect. Hydrodynamics simulations show the rotational axis flips regularly as gravity shifts. Most Kuiper belt objects share rapid rotation periods greater than one day. Centrifugal forces could potentially eject surface material if the spin rate increased further. No stable tidal lock exists between this satellite and its primary bodies.

  • Hydra orbits the Pluto-Charon barycenter at a distance of roughly 65,000 kilometers. It remains the outermost known moon in the entire Plutonian system. The orbit lies beyond Kerberos and follows a nearly circular path. All five known moons maintain low inclinations relative to Pluto's equator. An orbital period of 38.2 days keeps it synchronized with other satellites through resonances. A 2:3 orbital resonance connects Hydra with Nix while a 6:11 ratio links it to Styx. These relationships create conjunctions occurring in a 5:3 pattern among the three bodies. A near-resonance with Charon exists at a 1:6 ratio despite a timing discrepancy of 0.3 percent. Tidal damping from Charon gradually circularized what was once an eccentric orbit. Mutual interactions transferred orbital energy away from Hydra over billions of years. The current configuration suggests all moons migrated outward together after formation.

  • The New Horizons spacecraft flew past Pluto on the 14th of July 2015 capturing images of all five moons. Hydra appeared behind Pluto during the closest approach, placing it further from the probe than other targets. Lower resolution resulted because the satellite sat farther away from the spacecraft. The Long Range Reconnaissance Imager estimated dimensions before arrival suggesting a diameter around 42 kilometers. Detailed imagery downlinked on the 15th of July 2015 revealed brightness variations across the surface. A dark circular feature spanning roughly 7 kilometers appeared in early frames. Highest resolution shots taken from 18,000 kilometers distance achieved pixel scales of 90 meters per pixel. Scientists derived final size estimates from these high-quality images confirming previous measurements. Surface composition and reflectivity data confirmed water ice dominance observed earlier. The flyby provided the first close-up view of this distant world.

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Moons with a prograde orbitTrans-Neptunian satellites

Common questions

When was the moon Hydra discovered?

Astronomers spotted the moon Hydra on the 15th of May 2005 using the Hubble Space Telescope. Max J. Mutchler identified one moon on the 15th of June 2005 while Andrew J. Steffl found the other on the 15th of August 2005. The team announced their findings publicly on the 31st of October 2005 after verifying archival data from 2002.

Who named the moon Hydra and when did the International Astronomical Union approve it?

Scientists named the larger of the pair Hydra after a nine-headed serpent from Greek mythology to honor Pluto's former status as the ninth planet in our solar system. The International Astronomical Union officially approved the name on the 21st of June 2006 through IAU Circular 8723.

What is the size and surface composition of the moon Hydra?

Hydra measures approximately 42 kilometers along its longest axis and about 30 kilometers across its shortest dimension. Water ice covers much of the terrain creating a highly reflective exterior with a geometric albedo reaching 83 percent.

How does the moon Hydra rotate compared to other large moons?

Unlike most large moons that keep one face toward their planet, Hydra rotates chaotically with an approximate rotational period of 10 hours at the time of the New Horizons flyby. Its rotational axis tilted about 110 degrees relative to its orbital plane during that encounter causing the moon to tumble unpredictably over astronomical timescales.

Where does the moon Hydra orbit within the Plutonian system?

Hydra orbits the Pluto-Charon barycenter at a distance of roughly 65,000 kilometers remaining the outermost known moon in the entire Plutonian system. An orbital period of 38.2 days keeps it synchronized with other satellites through resonances including a 2:3 orbital resonance connecting Hydra with Nix.

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All sources

33 references cited across the entry

  1. 1webPlanet and Satellite Names and DiscoverersInternational Astronomical Union (IAU) Working Group for Planetary System Nomenclature (WGPSN)
  2. 2encyclopediahydriaOxford University Press
  3. 3journalResonant interactions and chaotic rotation of Pluto's small moonsM. R. Showalter et al. — 3 June 2015
  4. 4journalComposition of Pluto's small satellites: Analysis of New Horizons spectral imagesJason C. Cook et al. — 15 November 2018
  5. 5journalThe Small Satellites of Pluto as Observed by New HorizonsH. A. Weaver et al. — 18 April 2016
  6. 6journalThe Pluto system: Initial results from its exploration by New HorizonsS. A. Stern et al. — 15 October 2015
  7. 7webHydra By the NumbersNASA — 21 November 2017
  8. 8journalThe Positions, Colors, and Photometric Variability of Pluto's Small Satellites from HST Observations 2005–2006Stern, S. A. — 2006
  9. 9journalPhase Curves of Nix and Hydra from the New Horizons Imaging CamerasA. J. Verbiscer et al. — 2018
  10. 10bookChasing New Horizons: Inside the Epic First Mission to PlutoAlan Stern et al. — Picador — May 1, 2018
  11. 11webPluto's Icy Moon HydraBill Keeter — 5 May 2016
  12. 12webHydra Emerges from the ShadowsNASA — 15 July 2015
  13. 13webNew Horizons 'Captures' Two of Pluto's Smaller MoonsJohns Hopkins University Applied Physics Laboratory — 21 July 2015
  14. 14webPluto's Moon HydraMatt Williams — Universe Today — 13 July 2015
  15. 15webNASA's Hubble Reveals Possible New Moons Around Pluto31 October 2005
  16. 16webIAU Circular No. 862531 October 2005
  17. 17webIAU Circular No. 872321 June 2006
  18. 18webPluto's New Moons are Named Nix and HydraCain, Fraser — Universe Today — 22 June 2006
  19. 19webNaming of Astronomical ObjectsInternational Astronomical Union
  20. 20journalA giant impact origin for Pluto's small moons and satellite multiplicity in the Kuiper beltS. A. Stern — 23 February 2006
  21. 21journalObliquity evolution of the minor satellites of Pluto and CharonA. C. Quillen et al. — January 2017
  22. 22webPluto's Small Moons Nix and HydraSimon Porter — 5 October 2015
  23. 23bookPluto & CharonCodex Regius — CreateSpace Independent Publishing Platform — 2016
  24. 24arxivOn the Early In Situ Formation of Pluto's Small SatellitesM. Y. Woo et al. — 6 March 2018
  25. 25webNASA's Hubble Finds Pluto's Moons Tumbling in Absolute ChaosKaren Northon — 3 June 2015
  26. 26webDPS 2015: Pluto's small moons Styx, Nix, Kerberos, and Hydra UPDATED
  27. 27webPluto's Moons – Five Satellites of PlutoTim Sharp — 23 February 2016
  28. 28journalNew Constraints on Additional Satellites of the Pluto SystemSteffl, A. et al. — 2006
  29. 29arxivThe Effect of Charon's Tidal Damping on the Orbits of Pluto's Three MoonsS. A. Stern et al. — 2008
  30. 30webAstronomers Describe the Chaotic Dance of Pluto's MoonsKenneth Chang — 3 June 2015
  31. 31journalForced Resonant Migration of Pluto's Outer Satellites by CharonW. R. Ward et al. — 2006
  32. 32webHow Big Is Pluto? New Horizons Settles Decades-Long DebateNASA — 13 July 2015
  33. 33journalOrbits and Masses of the Small Satellites of PlutoSimon B. Porter et al. — July 2023