Hydra

MOONMoon of Pluto

ILLUSTRATION FROM DENSITY 1.21 G/CM³ · RADIUS 18.5 KM · SHADING, NOT THE PHASE

[

OVERVIEW

] MOON

Hydra is a moon of Pluto. It orbits Pluto at a mean distance of 65 200 . It has a mass of 3×10¹⁶ and a mean radius of 18.5 . It was discovered in 2005.

WHAT IT IS: Moon of Pluto
HOW FAR: 65 200
HOW BIG: 18.5
ONE ORBIT TAKES: 38.2

[

CATALOG DATA

] 24 FIELDS

The raw catalogue values, as the source published them, with the error bars where one was published — sorted by what each set of figures is about rather than by the order they happened to be stored in.

WHAT KIND OF THING IT IS

What the object is in itself, rather than how it happens to look from here.

Mean density: 1.21 ±0.19

THE ORBIT

The shape of the path, where the object sits on it, and the instant those figures were true of. An orbit is a fit to observations and drifts away from the real one either side of its epoch.

Orbits: Pluto
Orbital period: 38.2
Eccentricity
Inclination: 0.3
Longitude of ascending node: 114.3
Argument of periapsis: 139.3
Mean anomaly: 335
Element epoch: 2451545.0000
Elements referred to: the planet's equator
Reference plane pole (right ascension): 132.99
Reference plane pole (declination): −6.16
Nodal precession period: 14
Mean anomaly fitted to the ephemeris: 336.95
Mean anomaly rate: 9.49417
Apsidal precession rate: 0
Nodal precession rate: −0.0704019
Agreement with the ephemeris: 0.83

WHEN IT WAS FOUND

Who first saw it, from where, and by what method.

Discovered by: H.A. Weaver, S.A. Stern, M.J. Mutchler, A.J. Steffl, M.W. Buie, W.J. Merline, J.R. Spencer, E.F. Young, L.A. Young

OTHER NAMES AND NUMBERS

The identifiers other catalogues file this object under, for anyone joining this data to theirs.

Also known as: Pluto III
JPL satellite code: 903
Wikidata item: Q102701
Wikipedia article: Hydra (moon)

WHERE THESE FIGURES CAME FROM

Which catalogue, paper or model each of the fields above was read from, where the source named one.

Ephemeris: PLU060

[

ALSO KNOWN AS

] 1 DESIGNATIONS

Common name Hydra

[

CITE THIS PAGE

] DR1

This page is live and its numbers move when an upstream refreshes, so the citation names the release the rows belong to rather than the day you read them. The release itself is frozen, republished as a bundle, and is what a data-availability statement should point at.

PLAIN TEXT

SpaceCatalog. Hydra [catalogue entry]. Data release DR1 (2026-08-21). https://spacecatalog.org/object/hydra

BIBTEX

@misc{spacecatalog_hydra,
  author       = {{SpaceCatalog}},
  title        = {{Hydra}},
  howpublished = {SpaceCatalog, data release DR1 (2026-08-21)},
  year         = {2026},
  url          = {https://spacecatalog.org/object/hydra},
  note         = {Catalogue entry, data release DR1 (2026-08-21). Values from JPL Planetary Satellite Data.}
}

The values above were taken from JPL Planetary Satellite Data — No licence stated; citation requested. Citing this page does not replace citing that catalogue.

That catalogue asks for this acknowledgement: Solar System Dynamics. (Downloaded 2026, September 13). Planetary Satellite Mean Elements; Planetary Satellite Physical Parameters; Planetary Satellite Discovery Circumstances. https://ssd.jpl.nasa.gov

[

RELATED OBJECTS

] 4 LINKED

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CONSTELLATION