Hydra (moon)
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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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.