Space · Moons
Hippocamp
A moon of Neptune — Neptune's smallest known moon — likely a fragment of Proteus, knocked off by impact.
Quick facts
Parent planet
Diameter (mean)
17 km
Mass
4.6 × 10¹⁵ kg
6.3e-08 Moon masses
Mean orbital radius
105,283 km
Orbital period
0.95 Earth days
Discovery year
2013
Discoverer
Mark R. Showalter (Hubble Space Telescope)
Naming origin
Half-horse half-fish creature of Greek mythology
Surface conditions
Hippocamp is the smallest known moon of Neptune (17 km across), found by Mark Showalter in 2013 through long-exposure Hubble imaging. Its position close to Proteus and the size of Pharos crater on Proteus suggest Hippocamp formed from material ejected when Proteus was struck by a comet — the only known case of a moon-from-moon-debris formation.
Missions and observations
Every Neptune-system mission has had an opportunity to image or characterize Hippocamp. The list below is the Neptune-system mission catalog; specific Hippocamp encounters are documented in mission archives.
| Mission | Year at Neptune | Status |
|---|---|---|
|
Voyager 2 NASA |
1989 | Completed |
Naming etymology
The hippocamp was a half-horse, half-fish creature of Greek mythology that drew Poseidon's chariot through the sea. Adopted by the IAU in 2019.
Methodology & sources
Diameter, mass, and orbital parameters from JPL Solar System Dynamics — Physical Parameters. Discovery year and discoverer from the JPL Satellite Discovery Circumstances. Naming etymology from the IAU Working Group for Planetary System Nomenclature. Stylized SVG hero composed from NASA / JPL imagery as visual reference; no photographs are reproduced.