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| Elara | |
|---|---|
| Name | Elara |
| Discoverer | Charles Dillon Perrine |
| Discovered | 1905 |
| Mean radius | 43 km |
| Orbital period | 259 days |
| Satellite of | Jupiter |
| Magnitude | 16.2 |
Elara is a small irregular prograde satellite of Jupiter discovered in the early 20th century. It is a member of a dynamical group of Jovian moons and is notable for its moderate size among the outer, non-synchronously rotating satellites. Observations from ground-based observatories and spacecraft have constrained its orbit, rotation, and surface properties, providing context within studies of Solar System formation and satellite capture.
Elara was discovered by Charles Dillon Perrine at the Lick Observatory in 1905 during a period of active telescopic surveys following discoveries such as Himalia and Pasiphae. The naming followed the convention of naming Jovian moons after characters associated with Zeus in Greek mythology, consistent with earlier namings like Io, Europa, and Ganymede. The choice aligned with the International Astronomical Union practices later codified in the 20th century alongside naming decisions for satellites of Saturn and Uranus.
Elara orbits Jupiter at a semi-major axis of roughly 11.7 million kilometers, placing it among the inner members of the group often associated with Himalia. Its orbital inclination is moderate relative to Jupiter's equator, and its eccentricity is modest compared with retrograde irregular satellites such as Sinope and Carme. The orbital period is on the order of 259 days, and Elara participates in long-term dynamical interactions influenced by perturbations from Jupiter and the Sun, similar to effects studied for Pasiphae and Ananke. Studies of secular resonances among Jovian irregulars often reference groups including Elara, Himalia, and Leda.
Elara has an estimated mean radius of about 43 kilometers, making it substantially smaller than the Galilean satellites such as Callisto and Europa but larger than many outer irregulars like Carme and S/2003 J 12. Its visual magnitude near 16.2 in typical opposition conditions places it within detection limits of large ground-based telescopes including the Very Large Telescope and instruments at Palomar Observatory. Photometric lightcurve analyses constrain its rotation period to a range compatible with irregular shapes, and comparisons are drawn with rotation states of Himalia and Elara's groupmates.
Spectral observations in visible and near-infrared wavelengths suggest a neutral to slightly reddish spectrum for Elara, comparable to D-type and C-type minor bodies found in the outer asteroid belt and among Jupiter trojans such as 624 Hektor. Surface albedo estimates are low, resembling the dark surfaces of Phoebe and Nereid more than the bright, icy surfaces of Enceladus or Miranda. The geology is presumed heavily cratered and primitive, with regolith properties analogous to irregular satellites and small asteroids studied by missions like NEAR Shoemaker and Hayabusa for analog comparison. No outgassing or endogenic activity comparable to Io has been detected.
Elara is widely interpreted as a captured object rather than having formed in situ with the Galilean satellites, consistent with capture scenarios proposed for irregular satellites of giant planets including models involving gas drag in a primordial circumplanetary disk or three-body interactions akin to mechanisms inferred for captured objects such as Trojans of Jupiter and captured Centaurs. Dynamical clustering with Himalia, Lysithea, and Leda supports hypotheses of a collisional breakup of a larger progenitor, paralleling fragmentation models applied to asteroid families like the Eos family and events such as the disruption that produced Shoemaker–Levy 9. Isotopic and compositional parallels remain uncertain due to limited spectroscopy.
After discovery at Lick Observatory, follow-up astrometry at observatories including Yerkes Observatory, Mount Wilson Observatory, and later facilities refined Elara's orbit. Photometric and spectral campaigns using instruments on the Keck Observatory and the Hubble Space Telescope improved constraints on its size and surface properties. While no dedicated spacecraft mission has targeted Elara, data from missions to the Jovian system—most notably Voyager 1, Voyager 2, and Galileo—stimulated interest in irregular satellites and informed planning for future missions such as proposals within the ESA and NASA communities that consider broader surveys of outer satellites. Ongoing surveys with wide-field facilities like the Sloan Digital Sky Survey and the Pan-STARRS project continue to monitor Elara and related group members.
The name draws from mythological traditions tied to Zeus and echoes classical naming conventions used across planetary science alongside names such as Selene, Daphne, and Calypso. Elara's inclusion in catalogs and ephemerides used by institutions like the Minor Planet Center and the Jet Propulsion Laboratory reflects its role in celestial mechanics studies and public outreach materials produced by museums such as the Smithsonian Institution and planetariums including the Hayden Planetarium. References to Elara appear in educational resources, atlases of the Solar System, and in datasets curated by NASA and international observatories.
Category:Jupiter moons Category:Irregular satellites