Elara (moon)

Lore & Background

Elara is named after Elara, one of Zeus's lovers and the mother of the giant Tityos. It did not receive its present name until 1975; before then, it was simply known as Jupiter VII. It was sometimes called 'Hera' between 1955 and 1975. Elara orbits Jupiter at an average distance of 11,703,130 km in 258.65 days, at an inclination of about 30° to the ecliptic, in a prograde direction and with an eccentricity of 0.196. Its orbit is continuously changing due to solar and planetary perturbations. Elara belongs to the Himalia group, a prograde group of moons orbiting between 11 and 13 million km from Jupiter at inclinations between 27 and 30°, and eccentricities between 0.11 and 0.24. Elara has a diameter of about 80 kilometers, with a measured albedo of about 4.6%, making it the eighth largest moon of Jupiter. It is about 2.6% of the size of Europa and half the size of Himalia, making it the second largest member of the Himalia group. Like the other members of the Himalia group, the satellite appears gray (color indices B−V=0.66, R−V=0.36), which is typical for C-type asteroids. A measured low beam value of 0.79 ± 0.03 indicates that Elara has considerable surface roughness.

Reader's Guide

Elara's significance lies in its status as the second largest irregular satellite of Jupiter and a key member of the Himalia group. Its discovery by Charles Dillon Perrine in 1905, just one day after his discovery of Himalia, highlights a period of active moon hunting at Lick Observatory. The moon's naming history—from Jupiter VII to 'Hera' and finally Elara in 1975—reflects evolving conventions in satellite nomenclature. As part of the Himalia group, Elara likely originated as a captured heliocentric asteroid that later broke apart, providing clues about the capture processes of irregular moons. Its physical characteristics, such as its gray color typical of C-type asteroids and considerable surface roughness, help astronomers understand the composition and history of these outer moons. The inconclusive rotation period studies in 1987, contrasted with a possible 9.596-hour period found in 2020, illustrate the ongoing challenges in observing small, distant satellites. The New Horizons spacecraft captured Elara in several LORRI images from a distance of 8 million km in February and March 2007, adding to its legacy as a target of exploration.

Did You Know?

• Elara was discovered by Charles Dillon Perrine on January 6, 1905, the day after he discovered Himalia.
• Elara did not receive its present name until 1975; before then it was known as Jupiter VII, and was sometimes called 'Hera' between 1955 and 1975.
• The New Horizons spacecraft captured images of Elara from a distance of 8 million km in February and March 2007.

Discovery and the Long Road to a Name

Charles Dillon Perrine, working at the Lick Observatory, spotted this small body on January 6, 1905, just one day after the announcement of Himalia's discovery. Despite the initial sighting, confirming the object's existence proved difficult; unfavorable weather conditions kept astronomers waiting until February 21 before the discovery could be verified. For decades afterward, the moon carried no personal name at all. It was catalogued simply as Jupiter VII, and during a stretch from 1955 to 1975 it was occasionally referred to as "Hera." It was not until 1975 that it finally received the name Elara, drawn from Greek mythology, where Elara was one of Zeus's lovers and the mother of the giant Tityos. The naming convention reflects the broader pattern of assigning mythological figures to Jupiter's numerous satellites, but Elara's particular story—a late-arriving name after seventy years of anonymity—marks it as a somewhat belated member of the Jovian family.

Orbital Mechanics and the Himalia Family

Elara traces a prograde path around Jupiter at a mean distance of roughly 11.7 million kilometers, completing one revolution in approximately 258.65 days. Its orbital plane is tilted about 30 degrees relative to the ecliptic, and its eccentricity of 0.196 means the distance to Jupiter varies noticeably over each orbit. These parameters are not fixed; solar and planetary gravitational perturbations continuously nudge the orbit over time. Elara is a member of the so-called Himalia group, a cluster of prograde irregular satellites that share a similar orbital corridor between 11 and 13 million kilometers from Jupiter, with inclinations in the 27-to-30-degree range and eccentricities spanning 0.11 to 0.24. As the second largest body in this group, Elara sits just behind its namesake, Himalia, in terms of size. The shared orbital architecture among group members strongly suggests a common dynamical history, linking Elara's present trajectory to the broader population of captured bodies that orbit the gas giant in this distant, inclined region.

A Gray, Rough World of Modest Dimensions

Elara measures roughly 80 kilometers across, making it the eighth largest moon in Jupiter's system and about 2.6 percent the size of Europa. It is approximately half the diameter of Himalia, cementing its position as the second largest member of the Himalia group. Its surface reflects only about 4.6 percent of incident light, a low albedo consistent with dark, carbon-rich material. Color measurements (B−V = 0.66, R−V = 0.36) paint a uniformly gray appearance, the same palette typical of C-type asteroids found in the main belt. A low beam value of 0.79 ± 0.03 reveals that the surface is far from smooth; considerable roughness scatters light in ways that distinguish it from a featureless sphere. Determining how Elara spins has proven elusive. A 1987 photometric campaign yielded no clear rotation period because the light curve was too flat and the small brightness wiggles lacked periodicity. Researchers speculated the moon might be nearly spherical, viewed pole-on, or rotating so slowly that the signal was undetectable. It was not until 2020 that observations suggested a possible rotation period of about 9.6 hours.

Captured from the Asteroid Belt and a Glimpse from New Horizons

Elara almost certainly did not form in Jupiter's vicinity. The prevailing view is that it began life as a heliocentric asteroid orbiting the Sun, was later gravitationally captured by Jupiter, and may represent a fragment of a larger body that shattered after capture. This interpretation fits neatly with the shared orbital signatures of the entire Himalia group, whose members all trace similar inclined, eccentric, prograde paths. Direct imaging of Elara from a spacecraft remains rare. In February and March 2007, the New Horizons probe, then on its long cruise toward Pluto, happened to pass within about 8 million kilometers (5 million miles) of the moon. Its LORRI camera captured several images of Elara during that window, offering one of the few spacecraft-based glimpses of this distant, gray satellite. Beyond that brief encounter, Elara remains largely unexplored, known primarily through ground-based photometry and the orbital clues that tell the story of its capture.