Hydra
The Outermost Companion
Pluto's outermost known moon, tumbling rapidly with a 10-hour day
🌙 Mind-Blowing Fact
Hydra is Pluto's outermost and largest small moon, and it spins incredibly fast - completing a rotation in just 10 hours despite being 5.5 billion km from the Sun!
What is Time on Hydra?
What is Time on Hydra?
Hydra (discovered in 2005) is Pluto's outermost known moon and the largest of the four small moons.
The 38-Day Orbit
- One orbit = 38.20 Earth days - About 5.5 weeks
- Rapid rotation: ~10 hours per rotation!
- Outermost known moon - Farthest from Pluto
- Tumbles chaotically - Rotation axis wobbles
Rapid Rotation
Despite being far from Pluto, Hydra spins fast:
- One rotation every ~10 hours
- Much faster than its orbit
- Not tidally locked
- Chaotic tumbling like other small moons
The Largest Small Moon
Hydra is the biggest of Pluto's small moons:
- Size: ~55 km × 40 km × 30 km
- Irregular, elongated shape
- Bright surface (similar albedo to Nix)
- Fresh water ice composition
The Outer Frontier
At Hydra's distance:
- Pluto appears smaller but still visible
- Charon's influence weaker
- Less frequent eclipses
- Most distant known member of system
A Day in the Life
The Outer Sentinel
You've traveled to Hydra, Pluto's outermost known moon, 64,700 km from Pluto itself.
"We're at the edge," your pilot says. "Beyond Hydra, there might be smaller moons we haven't found yet. But this is the frontier."
You land on the rapidly spinning moon. A full rotation takes just 10 hours - you can watch the stars wheel overhead at a visible pace.
"Pluto looks small from here," you observe. The dwarf planet is just 1.9 degrees across - still impressive, but a far cry from Charon's view where Pluto dominates the sky.
"Hydra is the largest of the small moons," your pilot explains. "55 kilometers long. And bright - almost as reflective as Nix. All four small moons probably formed from the same impact that created Charon. They're siblings, born from violence, dancing together for billions of years."
You watch Pluto slowly drift across the tumbling sky, knowing that beyond Hydra lies only darkness and the possibility of undiscovered companions.
Thought Experiments
Could there be more moons beyond Hydra?
Possibly! New Horizons searched for additional moons and found none, but very small moons (under 1-2 km) could exist beyond Hydra. They would be extremely hard to detect from Earth. Future missions or better telescopes might find more tiny companions in Pluto's outer reaches!
The Science of Time on Hydra
Hydra: The Outer Sentinel
Discovered in 2005 by Hubble (same observation that found Nix), Hydra is named after the nine-headed serpent from Greek mythology.
Key characteristics:
- Largest of the small moons (~55 km longest axis)
- Outermost known moon of Pluto system
- Bright surface (albedo similar to Nix ~0.5-0.6)
- Very fast rotation (~10 hours)
- Chaotic tumbling due to Charon's influence
Formation theory: All four small moons (Styx, Nix, Kerberos, Hydra) likely formed from debris ejected during the giant impact that created Charon billions of years ago. They're the leftover fragments from that ancient collision.
Stability: Despite the chaotic orbital environment created by Charon's mass, all four small moons maintain stable orbits in near-resonances with each other, creating a gravitational ballet around Pluto.