Hydra
Moon· orbits Pluto
Hydra is the outermost known moon of Pluto — a small, irregular body with a notably bright, crystalline water-ice surface, also tumbling chaotically.
Open in 3D viewSee Hydra in the live, interactive 3D map
Key facts
Mass (est.)
3.00 × 10¹⁶ kg
0.000000408 Moon masses
Radius
18.5 km
1% of our Moon's radius
Surface gravity
0.00584 m/s²
0% of Earth's
Orbit around Pluto
38.2 Earth days
Science
Physical
- Mass (estimated)
- 3.00 × 10¹⁶ kg · 0.000000408 Moon masses
- Radius
- 18.5 km · 1% of our Moon's radius
- Density
- 1,130 kg/m³ · 1.1× water
- Surface gravity
- 0.00584 m/s² · 0% of Earth's
- Escape velocity
- 14.7 m/s · 0% of Earth's
Orbit around Pluto
- Distance from Pluto
- 71,991 km
- Orbital period
- 38.2 days
- Eccentricity
- 0.009
- Inclination
- 0.30 °
Rotation
- Rotation
- Chaotic — like Pluto's other small moons, Hydra tumbles with no fixed rotation period, driven by the changing Pluto–Charon gravity field (Showalter & Hamilton 2015). Not tidally locked.
Composition
- Bulk composition
- A bright, crystalline water-ice surface; small and irregular.
Radiation & temperature
- Insolation
- 0.00079 S⊕ · 1/1,266 of the sunlight Earth receives
- Geometric albedo
- 0.83
- Equilibrium temperature
- 30 K (-243 °C)
Discovery & history
- By
- the Hubble Space Telescope Pluto Companion Search Team (Hal Weaver, Alan Stern and colleagues)
- Method
- Photographic plates
- Name
- The Hydra, the nine-headed serpent of Greek myth that guarded an entrance to the underworld.
Hydra was found together with Nix in Hubble images in 2005, as scientists searched for moons and ring hazards ahead of New Horizons. The outermost of Pluto's small moons, it is an elongated body that tumbles rapidly end over end. Little is known of it beyond the 2015 flyby.
Images
Images from open archives · credit shown per image