Hydra
A confirmed moon in the Pluto system.
Physical measurements
- Mean radius
- 18.5 ± 1 kmmeasured · Source estimate; uncertainty is the source-reported standard error when available.
- GM · gravitational parameter
- 0.002 ± 0.0003 km³ s⁻²measured · Source estimate; uncertainty is the source-reported standard error when available.
- Mass
- ≈ 2.9966e+16 ± 4.4949e+15 kgderived · GM/G; independent standard uncertainties combined in quadrature. No uncertainty is invented when the source omits GM uncertainty.
- Mean density
- ≈ 1.21 ± 0.19 g cm⁻³derived · JPL derives density from GM and volume-equivalent radius.
Orbit descriptions
Mean elements describe an average orbit. They are not positions for a chosen date.
PLU060 · equatorial plane · 2000-01-01.5 TDB
- Semimajor axis
- 65,200 km
- Sidereal period
- 38.2 days
- Eccentricity
- 0.009
- Inclination to reference plane
- 0.3°
- Argument of periapsis
- 139.3°
- Mean anomaly
- 335°
- Ascending node
- 114.3°
- Apsidal / nodal precession periods
- — / 14 years
- Reference-plane pole RA / Dec
- — / —°
- Plane tilt to parent equator
- —°
- Reference center
- Not specified in this summary sourceThe summary table identifies a parent system but does not explicitly specify the reference center for each solution. Do not infer a physical-primary center, especially for Pluto’s circumbinary moons.
Orientation
No supported orientation model in the admitted PCK source.
Identity & availability
- Stable catalog ID
- hydra
- JPL / NAIF satellite ID
- 903
- Names & designations
- Hydra · S/2005 P1
- Discovery / rediscovery years
- 2005H.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
No validated dated trajectory admitted in catalog 1.0.0. Mean elements are not ephemerides.
No image or global texture admitted in this catalog release.No environment measurements admitted in this catalog release.
Sources & evidence
- JPL discovery circumstancestable tr[464] (zero-based); pluto / Hydra · References: IAUC 8625
- JPL confirmed small-body satellitesdata[325].sat; primary=134340 · References: Weaver et al. (2005), IAUC 8625
- JPL physical parameters#sat_phys_par; Pluto / Hydra / NAIF 903; mean radius · References: 3
- JPL physical parameters#sat_phys_par; Pluto / Hydra / NAIF 903; GM · References: PLU060
- NIST CODATA 2022Table of universal constants, Newtonian constant of gravitationVerified numeric transcription; see acquisition notes in the download.
- JPL physical parameters#sat_phys_par; Pluto / Hydra / NAIF 903; density · References: *
- JPL mean orbital elements#sat_elem row ID=459; Pluto / Hydra / NAIF 903 · References: 6
Full catalog, uncertainty and source manifests · All Pluto moons · Moon model limits