Overview
Hermippe, also catalogued as Jupiter XXX and initially designated S/2001 J 3, is one of Jupiter's many small irregular satellites. It is a retrograde moon — meaning it orbits opposite to Jupiter's rotation — and is classed with other distant, captured bodies rather than the large regular moons that formed with the planet. Hermippe's modest size and distant, inclined orbit identify it as part of the family of debris thought to have originated from a captured and subsequently fragmented progenitor.
Orbital characteristics
Hermippe follows an elongated, inclined track around Jupiter. Key orbital parameters are known with reasonable precision from repeated observations: an average orbital radius near 21,182,000 km, an orbital period of roughly 629.8 days, an inclination of about 151° to the ecliptic (around 149° relative to Jupiter's equator), and a moderate orbital eccentricity of approximately 0.229. These values place it well outside the regular satellite system and inside the range occupied by the Ananke group.
- Mean distance: ~21,182,000 km
- Orbital period: ~629.8 days
- Inclination: ~151° to the ecliptic — see orbital inclination and ecliptic
- Eccentricity: ~0.229 — see eccentricity
Physical characteristics
Hermippe is small and non-spherical, with an estimated diameter of about 4 kilometres. Like many distant Jovian satellites it is dark and faint, reflecting only a small fraction of sunlight; detailed composition and surface properties are not well constrained by current observations. Its irregular shape and low mass mean it has no atmosphere or geological activity, and it behaves dynamically as a minor fragment within Jupiter's gravitational field.
Discovery and naming
The moon was discovered in 2001 by a team from the University of Hawaii led by astronomer Scott S. Sheppard. The provisional label S/2001 J 3 was assigned on detection, and the International Astronomical Union (IAU) approved the name Hermippe in August 2003. The name comes from a figure associated with Zeus (Jupiter in Roman myth), following the convention of using names related to Zeus/Jupiter for the planet's satellites.
Ananke group and origin
Hermippe belongs to the Ananke group, a collection of retrograde, irregular moons that share similar orbits between roughly 19.3 and 22.7 million kilometres from Jupiter and inclinations near 150°. Members of this group are generally non-spherical and are believed to be remnants of a single captured object that broke apart under tidal forces or collision. The grouping is identified by clustering in orbital elements rather than by precise compositional matches.
Observation and scientific importance
Because Hermippe is faint and distant, studies rely on sensitive ground-based telescopes and careful astrometric follow-up to refine its orbit. Continued monitoring of such moons improves understanding of capture processes, collisional histories in the outer Jovian system, and the population statistics of small bodies around giant planets. For additional context on retrograde satellites and irregular moon groups see resources on non-spherical satellite shapes and observational methods. Institutional and survey pages that discuss discovery campaigns and follow-up observations include material from the discoverers' teams and observatories referenced by the discoverers (University of Hawaii, Scott S. Sheppard).