Hydra (moon) facts for kids

Hydra is the farthest moon of Pluto. It was found along with Nix in June 2005. The Hubble Space Telescope's Pluto Companion Search Team discovered it. This team included scientists like Hal A. Weaver and Alan Stern.

The first pictures showing Hydra were taken on May 15 and May 18, 2005. Scientists Max J. Mutchler and Andrew J. Steffl found the moons independently. The discovery was announced on October 31, 2005. This happened after older pictures from 2002 confirmed the moons were there. Hydra was first called S/2005 P 1.

Hydra

Artist conception of Hydra (foreground), Pluto and Charon (background), and Nix (bright dot center left)
Discovery
Discovered by	Hubble Space Telescope Pluto Companion Search Team
Discovered in	June 2005
Orbital characteristics
Semi-major axisa	64 780 ± 90 km
Eccentricity	0.005 ± 0.001
Orbital period	38.206 ± 0.001 d
Inclinationb	0.22° ± 0.12°
Is a moon of	Pluto
Physical characteristics
Mean diameter	61 − 167 km
Mass	5×1016 − 2×1018 kg
Mean density	unknown
Rotation period	unknown
Axial tilt	unknown
Albedo	0.04 − 0.35 (assumed)
Apparent magnitude	22.9 to 23.3 (measured)
Surface temp.	33-55 K
Atmosphere	none
a Relative to the Pluto-Charon barycenter. b Relative to Pluto's equator (also Charon's orbital plane).

Hydra's Orbit Around Pluto

Hydra travels around the center of mass of the Pluto system. This center is called the barycenter. It orbits in the same flat path as Charon and Nix. Hydra is about 65,000 km away from this center.

Unlike Pluto's other moons, Hydra's path is not perfectly round. It is only nearly circular. Its orbit takes about 38.2 days to complete one full trip.

Size and Appearance of Hydra

Discovery images of Hydra.

Scientists have not measured Hydra's exact size yet. But they can guess its size based on how bright it looks. If Hydra reflects light like Charon (35 percent), it would be about 61 km wide. If it reflects light like very dark objects in the Kuiper belt (4 percent), it could be about 167 km wide.

When Hydra was first found, it seemed about 25 percent brighter than Nix. This made scientists think Hydra was about 10 percent bigger. But later observations from the Hubble Space Telescope showed Hydra was still a little brighter. Hydra looks grey, just like Charon and Nix. However, Pluto itself has a reddish color.

Visiting Hydra with New Horizons

The New Horizons spacecraft flew past Pluto in 2015. It also visited Hydra during its journey. This mission helped us learn more about Pluto and its moons.

How Hydra Got Its Name

The name Hydra was officially announced on June 21, 2006. It was also given the formal designation Pluto III. Hydra is named after a monster from Greek mythology. This monster guarded the waters of Pluto/Hades's underworld.

The Solar System
Sun • Heliosphere	Planets ☾ = moon(s) ∅ = rings		Mercury	Venus	Earth ☾	Mars ☾
			Jupiter ☾ ∅	Saturn ☾ ∅	Uranus ☾ ∅	Neptune ☾ ∅
	Dwarf planets		Ceres	Pluto ☾	Haumea ☾	Makemake
			Eris ☾
	Small Solar System bodies	Asteroids (minor planets)	Groups and families: Vulcanoids · Near-Earth asteroids · Asteroid belt Jupiter Trojans · Centaurs · Neptune Trojans · Asteroid moons · Meteoroids · Pallas · Juno · Vesta · Hygiea · Interamnia · Europa
			See also the list of asteroids.
		Trans- Neptunians	Kuiper belt – Plutinos: Orcus · Ixion – Cubewanos: Varuna · Quaoar · Huya
			Scattered disc: Sedna
		Comets	Periodic comets and non-periodic comets Damocloids · Oort cloud
See also the list of solar system objects

Images for kids

• 

  Hubble image of Pluto's moons (annotated)

See also

In Spanish: Hidra (satélite) para niños