Orthosie (moon of Jupiter)

Discovery and naming

The satellite was identified in 2001 during surveys for faint Jovian companions performed by astronomers at the University of Hawaii. The discovery team is associated with several subsequent finds of small outer moons; further information about the group that made the discovery can be found via the discoverers' institutional listing at discovery team. The International Astronomical Union approved the name Orthosie in 2003, drawing from Greek mythology—Orthosie is one of the Horae, daughters of Zeus and Themis. For background on the name’s linguistic and mythological roots see the entries for Greek myth, the Horae, Zeus and Themis.

Orbital characteristics

Orthosie is an irregular, retrograde satellite: it orbits Jupiter in the direction opposite to the planet’s rotation. Key orbital parameters include:

• Average orbital distance (semi-major axis): ~20,568,000 km
• Orbital period: ~602.619 days
• Inclination: ~142° to the ecliptic (about 143° to Jupiter’s equator)
• Eccentricity: ~0.2433

These values show that Orthosie follows an elongated, inclined orbit well outside Jupiter's large regular moons. Explanations of terms such as the ecliptic and orbital eccentricity are available at ecliptic and eccentricity.

Group membership and origin

Orthosie is grouped with the Ananke family, a collection of retrograde, irregular moons that share similar orbital distances and inclinations. The Ananke group occupies orbital radii roughly between 19.3 and 22.7 million kilometres and typically has inclinations near 150°. Members of this family are thought to be fragments of a single captured and subsequently disrupted parent body, an asteroid-like object that broke up after capture by Jupiter's gravity. For an overview of the family see Ananke group.

Physical properties and significance

Orthosie is small and non-spherical, with an estimated diameter of about 2 kilometres; like other tiny outer moons, it is too small to have enough gravity to pull itself into a round shape (non-spherical). Its surface composition is not well constrained from Earth-based observations, but many irregular Jovian satellites are dark and likely composed of rock and possibly ice mixed with dust. Because it is faint and distant, detailed study requires large telescopes or spacecraft observations.

Context and comparison

Orthosie exemplifies the class of irregular satellites that populate the outer regions of giant-planet systems. Unlike the Galilean moons, which are large, regular, and formed by accretion in Jupiter's circumplanetary disk, Orthosie and its fellow Ananke members are smaller, more numerous, and believed to be captured objects. Studying such moons helps astronomers reconstruct capture events, collisional histories, and the dynamical evolution of Jupiter's satellite system.

Further reading

• Discovery details and surveys: discoverers
• Orbital frame references and inclination context: ecliptic
• Astronomical definitions: orbital eccentricity
• Mythological naming sources: Greek, Horae, Zeus, Themis
• Group classification: Ananke group
• Shape and physical notes: non-spherical