small moon · moon of the dwarf planet Pluto
Hydra
The outermost known moon of Pluto, a small, bright body of nearly pure water ice that spins rapidly and chaotically.
- Distance from Earth today
- 5.35 billion km35.73 AU · light: 4 h 57 min
- Distance from the Sun today
- 5.33 billion km35.64 AU
- As of
- 2 November 2026position from NASA/JPL orbital elements
Where it is today
Top view of the plane of Earth’s orbit, compressed distance scale. Open on the big map with the time slider →
Facts
- Diameter
- 37.0 km
- Mass
- 0.0300 × 10¹⁸ kg
- Density
- 1,210 kg/m³
- Mean distance from the planet
- 65,200 km
- Orbital period
- 38.2 days
- Orbital eccentricity
- 0.0090
- Orbital inclination
- 0.3°
Source of the figures: JPL SSD – Planetary Satellites (public domain).
Discovery: 2005 – Harold A. Weaver, S. Alan Stern et al. (Hubble Space Telescope)
Description
Hydra is the outermost of Pluto's known moons, found with Nix in Hubble images from 2005. New Horizons showed it to be an irregular, elongated body whose surface is very bright and rich in water ice. Like Nix, it rotates quickly and chaotically, tugged by the changing gravity of the Pluto–Charon pair. Its name refers to the nine-headed serpent of Greek myth, and the initials of Nix and Hydra were chosen as a nod to New Horizons, the mission that would later visit them.