Hydra

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Pluto and its large moon, Charon, appear overexposed in this Hubble Space Telescope image obtained in 2006 to confirm the existence of the two small moons Nix and Hydra (at right).

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Hydra

moon of Pluto

Also known as: P1, S/2005 P1

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July 17, 2026

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Pluto and its large moon, Charon, appear overexposed in this Hubble Space Telescope image obtained in 2006 to confirm the existence of the two small moons Nix and Hydra (at right).

Hydra is one of Pluto's five known moons, discovered in 2005 using the Hubble Space Telescope. It is much smaller than Pluto's largest moon, Charon, with a diameter of approximately 36 km. Hydra orbits Pluto outside of Charon's path in a nearly circular orbit. Its orbital period is about 38.2 Earth days, and it is more reflective than Charon. Along with Nix, Kerberos, and Styx, Hydra's orbital periods are in a 6:5:4:3 ratio with Charon, suggesting a stable dynamic resonance between the moons. Unlike most moons, Hydra is not in synchronous rotation with Pluto.12

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Christening Pluto’s Moons
Hydra
In Greek legend, Hydra was a monster with nine (or so) heads—the number varies among sources. With Kerberos (Cerberus), the Hydra was said to guard the entrance to the Underworld. The Roman poet Statius calls Hydra "the comrade of the cruel beast Kerberos (Cerberus)."…
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In Greek legend, Hydra was a monster with nine (or so) heads—the number varies among sources. With Kerberos (Cerberus), the Hydra was said to guard the entrance to the Underworld. The Roman poet Statius calls Hydra "the comrade of the cruel beast Kerberos (Cerberus)."…
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Pluto
Pluto’s moons
Pluto’s other four moons—Hydra, Nix, Kerberos, and Styx—are much smaller than Charon. All four are elongated. They revolve around Pluto outside Charon’s path in nearly circular orbits (like Charon) and in the same orbital plane as Charon. The orbital radius of Hydra is about 64,721 km (40,216 miles);…
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Pluto’s other four moons—Hydra, Nix, Kerberos, and Styx—are much smaller than Charon. All four are elongated. They revolve around Pluto outside Charon’s path in nearly circular orbits (like Charon) and in the same orbital plane as Charon. The orbital radius of Hydra is about 64,721 km (40,216 miles);…
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Hubble Space Telescope
…was also used to discover Hydra and Nix, two moons of the dwarf planet Pluto.
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…was also used to discover Hydra and Nix, two moons of the dwarf planet Pluto.
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New Horizons
Nix, Kerberos, and Hydra—were found to be spinning very rapidly, unlike most other satellites in the solar system, which keep one side facing the planet they orbit.
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Nix, Kerberos, and Hydra—were found to be spinning very rapidly, unlike most other satellites in the solar system, which keep one side facing the planet they orbit.
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